Classifications of Evidence Presentation

06CH_Forensic_Gaensslen.pdf

Home >Law homework help >Criminal homework help > Classifications of Evidence Presentation

1Forensic Science and Criminalistics

Associated Press

Learning Objectives After reading this chapter, you should be able to do the following:

▪ Define forensic science and how it contributes to a case, as well as explain the CSI Effect and the scientific method.

▪ Summarize the history of forensic science and contributors to the field.

▪ List and describe some forensic science specialties.

▪ Identify the elements of a forensic investigation, how physical evidence can be produced, and forensic analysis.

▪ Describe the work and work product of a forensic scientist.

▪ Describe the U.S. court system, and the key rulings on physical evidence admissibility through expert testimony.

▪ List and discuss major issues in forensic science today.

149

6Trace and Materials Evidence

Alice S./BSIP/SuperStock

Learning Outcomes After reading this chapter, you should be able to

▪ Briefly explain the probability of chance duplication and the chain of custody with trace evidence.

▪ Explain the structure of hair and how it is collected and analyzed.

▪ Describe natural and human-made fibers and how they are collected and analyzed.

▪ Discuss the structure of glass and its collection and analysis.

▪ Summarize paint structure and how it is collected and analyzed.

▪ Explain the importance of soil and its collection and analysis.

▪ Describe the limitations a trace analyst should observe when presenting the results of trace evidence analysis.

Section 6.1Probability and the Chain of Custody

Introduction While there can be many types of trace evidence, this chapter will cover some of the most common types, including hairs, fibers, glass, and soil. Trace evidence by definition relates to something small. It encompasses evidence such as particles of glass from a broken win- dow embedded in the sole of a suspect’s shoes, or particles of glass in a victim’s wounds that must be compared to a broken windshield on an automobile suspected of being involved in a hit-and-run accident. It also includes a few hairs from a victim found on a suspect’s jacket, or a few fibers from the suspect’s jacket found on the victim’s clothing. It is worth noting that while the term trace does refer to a small quantity, the term means that the forensic lab is capable of analyzing small quantities of a substance. It doesn’t necessarily mean that you always find the evidence in small quantities in casework.

While this type of evidence can be important to the solution of a crime, care must be taken in its use. Trace evidence is for the most part class evidence, also known as circumstantial evi- dence. It can help point officers in the right direction in their investigations—for instance, by exonerating an innocent person—but it is not on its own usually enough to close a case. Also, remember that because the materials being dealt with are small, extreme care must be taken to avoid loss or contamination of the evidence.

Although forensic scientists strive to devise methods applicable to small quantities of this kind of evidence, evidence could be present in larger amounts. For this reason, we also refer to this category as materials evidence. Many types of transferred evidence fall into this broad category. One of the features that justifies placing it into the same category is that the methods used to analyze trace and materials evidence are similar. One principal technique is micro- scopy—the use of microscopes. There are several types of microscopes used for different purposes in examining trace evidence. You are probably familiar with the “biological” micro- scope, a compound microscope often used to look at biological specimens. These microscopes usually have one eyepiece. There are also stereoscopic binocular microscopes, which have two eyepieces for “stereo” vision and provide views of specimens at various magnifications. A variant of the light microscope called a polarized light microscope (or PLM) allows the same visualization as a regular light microscope but also the measurement of more sophisticated optical and physical properties of specimens. Another type of microscope is the comparison microscope. This instrument uses two connected optical bridges and allows a person to view two different specimens, side by side in the viewer, simultaneously. It is ideal for doing micro- scopic comparisons and can be used for hairs and fibers as well. Finally, there are electron microscopes. They employ a beam of electrons rather than a beam of light and permit very high magnifications of specimens. There are some limited uses for “scanning” electron micro- scopes in forensic work. For example, they may be used to confirm the identity of gunshot residue particles. We will discuss comparison and electron microscopes in Chapter 9. Here, we’ll cover the concept of probability of duplication and the importance of maintaining the chain of custody for these types of evidence.

6.1 Probability and the Chain of Custody Trace evidence was briefly described in Chapter 1, defining the scope of criminalistics. It is often used in forensic analysis because of the Locard exchange principle (see Chapter 1.4),

Section 6.1Probability and the Chain of Custody

and it can be used to tie a suspect to a victim or crime scene, a victim to a crime scene, or—in lucky cases—tie the suspect, victim, and crime scene together simultaneously. In the same way, trace evidence can indicate disassociations. While this sounds simple in principle, most often this type of evidence is circumstantial, defined only in terms of class rather than of indi- vidual characteristics. In other words, the evidence will relate to a group of similar objects rather than an individual object at a scene or a specific suspect. This evidence will only indi- cate proof of a point at issue in a court case, rather than provide conclusive proof. Oftentimes, interpretation of trace evidence comparisons comes down to the probability of chance dupli- cation. What is the likelihood that two people share hair characteristics? How many jackets were made of a particular cloth in a particular color? This information must be determined by forensic or other scientists, in some manner, for presentation to a judge or jury. The trace analyst is generally unable to walk into court and say, “This is the jacket, to the exclusion of all others.” However, the analyst can provide the circumstantial basis for the evidence that points to a suspect, while warning the trier of fact that there could be others. A guilty verdict can be achieved if the buildup of this circumstantial evidence can lead the judge or jury to conclude beyond a reasonable doubt that the evidence points to the suspect instead of another indi- vidual. Trace evidence can also be exclusionary. A trace analyst can use a comparison sample taken from a known source to tell if the known was not the source of the evidence. This finding is important because it is definite. If a textile object is excluded as the source of a fiber, this conclusion is absolute.

It is often difficult to determine how common or rare an item really is. Sometimes, the research has just not been done. In other cases, it cannot be done. For example, if carpet fiber from a car trunk is taken as evidence, an analyst can find out how many carpets of that kind and color were installed in a certain make, model, and year of car. What likely cannot be determined is the number of vehicles still in use and where they are located. If the crime occurred in Los Angeles, the national number matters less than the greater Los Angeles number. Investigators can go to the department of motor vehicles and find out how many of these vehicles are regis- tered in the county, but their distribution among the population remains unknown, and it is difficult to know if the vehicle from which the fiber came is registered in Los Angeles. So, while some frequencies may be given to the court, there is uncertainty in them. Often, several kinds of trace are present in a case and can provide good circumstantial evidence. Therefore, trace evidence of several types can be used to assist in the association of a suspect with various aspects of the crime scene, rather than depending on one item alone to convince a judge or jury that a suspect was at the scene. The logic is that it is unlikely for several different varieties of trace evidence to match exemplars by chance, all at the same time and in the same case.

As noted with other types of evidence, the chain of custody is crucial to the successful pros- ecution of a case. Additionally, the chain of custody allows the scientist and everyone else

Think About It

Since trace evidence does not generally allow a scientist to reach an individualization con- clusion, should we dismiss it in favor of evidence that will lead to an undisputable conclu- sion about the inclusion or exclusion of a suspect? Why or why not?

Section 6.1Probability and the Chain of Custody

who handled the evidence to demonstrate that there was most likely no contamination of the evidence or loss of important evidence and that the evidence was not changed in some detrimental manner. How might this be done with trace evidence? This can be accomplished by maintaining the chain of custody, which starts with proper collection, documentation, and preservation.

Collection The location and collection of the evidence is important in any case. During the examination of large items, trace materials may be located and can be used as evidence. Fibers or hairs on clothing or soil, glass or paint on the ground or elsewhere in a yard—these can help link the suspect to the victim or crime scene. Some of this evidence can easily be picked up using tweezers, while some, such as paint on a wall, must be scraped, taking care to remove the sample all the way to the wall surface. This type of evidence can also tie remote crime scenes or transport mechanisms to the victim or suspect. Conversely, if there is contamination of the evidence in some manner, the evidence could lead to an erroneous conclusion by the analyst.

When locating and collecting trace evidence, the crime scene technician must be aware of the possibility of loss or contamination and take a few precautions. First, he or she must wear proper garb when on the scene. This clothing—sort of like “space suit” garb—should be worn to prevent hairs or fibers from being left on the evidence by the investigator. Also, the investigator should wear nonobstructive headgear so as not to shed hairs onto the evidence. Second, when processing multiple items, victims, or suspects, the items of collected evidence should never share the same space and should not be handled by the investigator at the same time in the same protective clothing. Any postincident contact that transfers hairs and fibers between the victim and suspect will render the evidence useless in court. Lastly, if it appears there is trace evidence on some larger object, such as a bedspread, the entire object should be packaged as evidence. If large items are collected, they should be handled gently and pack- aged in plastic, or if biological fluids are present, paper containers of appropriate size should be used. Individual hairs and fibers that are collected can be stored in druggist folds or small coin envelopes, which will help prevent loss of this evidence. The druggist fold containing the evidence is often packaged in a sealable secondary container, such as a ziplock bag.

Generally speaking, there are two approaches to the collection of trace evidence items. The first involves collecting in its entirety whatever object has the trace on it. For instance, a crime scene technician would collect a bedspread that has hairs or fibers on it, a shoe that has soil on it, or a bicycle that has a paint smear on it from being hit by a car. The second approach involves collecting the trace itself, and not the object on which it is deposited. It is always bet- ter to collect the intact item, but sometimes it isn’t possible. What if a paint smear were on the side of a building? Proper documentation—notes, sketches, photography—precedes collec- tion and packaging, as discussed in Chapter 2.

Preservation Packaging evidence was also discussed in Chapter 2; it helps preserve the evidence. It is always important to package evidence correctly so that the item is not contaminated or

Section 6.1Probability and the Chain of Custody

compromised in any way and so that it cannot work its way out of the package. For example, a single hair placed in a paper envelope could fall through the paper envelope seams at the end if it is not sealed with evidence tape. Investigators and crime scene technicians must also remember that trace evidence alone does not have much value. The lab needs a com- parison specimen—a specimen of the supposed source of the evidence. If the trace item is a hair, the lab will need known hairs, such as from the victim or suspect. If the item is glass, the lab needs a specimen of the item from which the glass is thought to have come, if possible.

Once an item of evidence arrives at the forensic science laboratory, it will be handled in a manner that will also prevent loss and contamination. Some laboratories use clean rooms, which are rooms set aside to handle trace evidence that are kept free of possible contaminants, such as fibers and other materials, for processing. They may use small vacuum cleaners fitted with filters to collect the evidence for packaging. Or they may use tape to collect the evidence. In this method, called tape-lifting, a piece of wide adhesive tape is placed on the surface of the item of evidence, and any hairs, fibers, or other debris will stick to the tape. The tape will be repeatedly placed on the item in a pattern to collect all possible trace evidence from the object. The tape can then be folded over upon itself and effectively sealed against contamination. The tape can be observed under a microscope, and any items that appear to be of probative value (meaningful in the case at hand) can be collected from the tape lift. This is accomplished by using a scalpel to slit the tape next to the item, removing that item using forceps, mounting the item on a microscope slide, and pressing the tape closed to reseal it with no contamination or loss of evidence.

Generally, hairs, fibers, glass fragments, soil, paint particles, and other nonbiological trace items can be packaged in ziplock containers. These can be labeled and sealed with evidence tape. They are also transparent, so you can see what is in them without opening them. Often, the ziplock bag will be a secondary container. That is, the trace itself will be first folded up in a paper container, which is then placed into the ziplock bag. The whole concept is to contain the trace items or material, keep it together, not allow it to move around or transfer to other surfaces or get out of its container, and make it reasonably easy for the forensic analyst to open the container, sample the material, and examine it.

Remember, trace evidence is by nature small and easily transferred. There are examples in many court jurisdictions of defendants convicted of crimes only to find out later that trace evidence had been mishandled and the verdict was wrong because of contamination. Simi- larly, there have been cases where the evidence was lost during collection or handling and could not be used during the case. Hairs and fibers have been seen in a number of cases

imageBROKER/Superstock Tape-lifting is one way to prevent losing very small pieces of evidence. What do you think might be some benefits and drawbacks to this method compared to others?

Section 6.2Hair

in which contamination or loss was an issue. Proper handling, packaging, and documenta- tion will help establish that nothing has occurred that will compromise the evidence prior to analysis and prevent the evidence from being used in the courtroom because of a technicality.

Probability of chance duplication and the chain of custody can impact the significance of trace evidence in a case and in court. Each of the major types of trace evidence will be discussed with these concepts in mind. This will provide you with the proper methods to collect and preserve each type of trace evidence, as well as analysis methods to help determine the probability of chance duplication of evidence. This can help either exclude the suspect, crime scene, and/ or victim from having contributed to the evidence or include them as possible contributors.

As with the other disciplines in forensic science today, the various areas of trace analysis are represented in the OSACs under the NIST, which have been previously discussed. The Materi- als (Trace) subcommittee is under the Chemistry/Instrumental Analysis section of the OSACs (https://www.nist.gov/topics/forensic-science/osac-organizational-structure).

6.2 Hair One of the most common types of trace evidence is hair. Hair is found on mammals and is an outgrowth of the epidermis (skin) composed of keratin and other proteins. It appears as a long thin filament and can serve as a covering for the animal on which it is found. Most hairs can be easily differentiated from one another; the microscopic structure of a human hair can be used to distinguish it from any other species’ hair. Since humans continually shed hairs through the day, it is a type of evidence that a suspect might not know was left at a crime scene. Of course, the case could be made that since everyone sheds hair, anyone with access to a crime scene could have left it. The examiner’s analysis is necessary to determine whether a hair could have come from a suspect or the suspect can be excluded as a hair depositor, based on the analysis.

Structure and Growth Phases of Hair Examination of a hair will reveal that there are three layers. The outermost layer is called the cuticle. It is composed of keratin-filled scales. These are very tough and give hair the ability to retain its structure and be resistant to chemical action. Because of these two characteristics, hair may be found at crime scenes for a long period of time. The scales overlap toward the tip of the hair, so if no root is present, the scientist can determine which end is the root and which end is the tip. This outer layer also has characteristics that allow an analyst to determine if a hair has originated from a human or an animal. As seen in Figure 6.1, the scales that make up the cuticle in animals are regular and, in many cases, appear to take up most of the circumfer- ence of the hair. The cuticle of humans is irregular in shape. The scales are rough and are not seen as a pattern. This is a helpful characteristic to use in the preliminary determination of whether the hair is human.

https://www.nist.gov/topics/forensic-science/osac-organizational-structure

Section 6.2Hair

Figure 6.1: Hair samples

As can be seen, the outer layer of hair is made up of scales. The scales in human hair are irregularly shaped, while those in other animal hair, such as cat hair, are more uniform in shape. What other similarities and differences can you see between these two hair samples?

a) Science Picture Co/Superstock; b) POWER AND SYRED/SCIENCE PHOTO LIBRARY

Hairs can be presumptively characterized according to species using microscopy. Ideally, a comparison microscope is used to compare hairs found at the crime scene with hairs from the victim or suspect or to compare unknown hairs with known hairs to see if a particular type of animal might have left them behind at the scene. Knowing the possible species of origin for a hair can be useful if there is evidentiary value to knowing whether that hair came from a cat or dog instead of a human. In many cases, the trace evidence can tie a suspect to a victim or crime scene because something he or she leaves behind is not part of the victim’s lifestyle. Imagine the suspect owns a cat. At the scene of the crime, cat hair is found only near the vic- tim and in no other part of the residence. Law enforcement officers investigate and find out that the victim was allergic to cats. The hair could then add value to the case by helping tie a suspect to the crime scene.

The cuticle can also be used to determine if hair has been dyed. Hair dyes will coat the cuticle cells but will not be taken up by the second layer of hair. This means that if an analyst looks at a hair sample and the outer layer appears pigmented, or painted, then the hair was dyed. However, if enough time passes between a crime and the collection of hair samples, the dye may have worn away. This should be noted in the results, if the analyst found other similar characteristics between suspect and evidence hair.

The second layer of the hair is known as the cortex. This layer is composed of roughly hour- glass-shaped cells called cortical cells. These cells contain the pigments that give hair its color. There are thought to be two colors of pigment that give hair its color. One is a dark brown pig- ment, and the other is a yellowish-colored pigment. Though it has not been proven, there may be a third pigment involved that could be reddish in color (Preedy, 2012). The combination of

a) Human hair b) Cat hair

Section 6.2Hair

pigments in different proportions results in the hair colors that occur in nature. White hair is simply devoid of pigment.

The final layer present is the medulla. In animals, this can be seen as a shaft running the length of the hair. In humans, the medulla can run the length of the hair; be broken into reg- ular-length pieces, known as an interrupted medulla; be broken into irregular-length pieces, known as a fragmented medulla; or may even be absent. Using a micrometer, which is a mea- suring device integrated into a microscope, a simple measurement and reference to known values can tell whether a hair is probably human or animal. This is known as the medullary index. To calculate this number, a scientist will measure the diameter of the hair and the diam- eter of the medulla. The medullary diameter is divided by the hair diameter to come up with a fraction. If the fraction is less than ⅓, it is most likely a human hair. If the fraction is greater than ½, it is likely to be animal hair. The layers of hair can be seen in Figure 6.2.

Figure 6.2: Layers of hair

Hair has three layers, with the cuticle as the outermost layer, the cortex as the middle layer, and the medulla as the deepest layer.

Adapted from ttsz/iStock/Thinkstock

Figure 6.3 shows an entire strand of hair and the three areas along its length. The first, wid- est section of the hair is the root. It is located below the surface of the scalp in the hair follicle and is where all growth takes place. The root is connected to the follicle through capillaries entering the bulb. Nutrients are fed into the root through the capillaries. As was mentioned in Chapter 5, hair can be tested to determine drug use. The drugs get into the hair through these capillaries. Once in the hair, they are bound to proteins within the hair and remain trapped until that section of hair is cut.

Cuticle

Cortex

Medulla

Section 6.2Hair

Figure 6.3: Components of a hair strand

The hair shaft is the visible part of a hair strand, while the root is under the skin, within the follicle.

Adapted from ttsz/iStock/Thinkstock

The shaft of the hair takes up most of the length and is the visible portion of the hair above the scalp. The tip of the hair is the terminating end and the narrowest part of the entire hair. Even the tip of the hair can be used to assist in analysis. Should there be color variations, the tip can be used to determine if a hair was bleached and the real color is coming back. If time has passed since the hair was colored, the tip end will typically remain colored, while the newer growth, toward the scalp, will be the person’s natural color.

Head hair usually grows at a rate of about 1 cm, or about ½ an inch, per month. Other body hair grows at different rates (Alaiti, 2011). Each hair goes through a growth cycle, and this growth cycle normally ends in the hair being shed. When hair is actively growing, the fol- licular cells are tightly against the root of the hair. The capillaries feeding the root are firmly attached, and the hair is hard to remove. During this anagen phase, growth is almost continu- ous. Periodically, the growth of the hair starts to slow. The root begins to shrink in on itself, and the hair becomes loose in the follicle as the follicular cells shrink away from the root. This is the catagen phase of hair growth and development. It normally lasts from 2 to 3 weeks, until growth completely stops. When growth has ceased completely, the hair enters the telo- gen phase. At this phase, the root of the hair has completely pulled away from the follicle, and the hair is loose in the follicle. It can easily slide out of the follicle and be shed. This shedding of the loose hair will normally trigger the resumption of a new anagen phase, and the hair will be replaced. Therefore, observation of the root end of a hair can be very telling. If the root end

Hair shaft

Hair root

Follicle

Capillaries

Section 6.2Hair

is shriveled, it can be deduced that the hair fell out on its own. If the root is fully rounded with follicular cells attached to the base of the hair, it can be deduced that the hair could have been pulled or forcibly removed. Hair with the root attached is important for analysis and must be included in sample collection.

Collection and Preservation Hair can be an important piece of evidence and should be collected carefully, as even one strand with a root can change the outcome of a case. As with every other type of trace, there are two types of hairs that must be collected: evidence hairs and standards. You could also call these “unknowns” and “knowns.”

Evidence Hairs The first type of hair that should be collected is evidence hair, which is taken from the crime scene. These specimens are of unknown origin. Evidence hairs can be collected in different ways, depending on the source of evidence. Many hairs are a part of sexual assault cases. In those cases, evidence hairs might be found on the victim, a suspect, his or her clothing, or even the location where the assault took place. If evidence is located on a bed, the entire bedding should be collected. The analyst will search the sheets and find not only hairs but also possible fibers and body fluids. The victim’s clothing can be collected at a hospital or clinic. The victim should gently be instructed by medical personnel to disrobe while standing on clean, white butcher paper. Often, two layers of paper are used to prevent contamination from the floor or carpet. The clothing should be packaged separately in paper bags. These will allow air to circulate, preventing the degradation of any body fluids and stains that may be present along with the hair or fiber evidence.

Evidence can be found on and collected from the victim as well. Often, the suspect’s head and pubic hairs can be found on the victim’s body, head, or pubic region. Standard sexual assault kits have historically contained paper envelopes for hair evidence and clean combs for use in collection. One comb is for the pubic region and one for the head. Any loose hairs found may belong to the suspect. Any of these hairs found while combing should be packaged in the appropriate envelope and sealed to be sent to the laboratory. After combing and collecting the potential evidence hairs, knowns must be collected. Some protocols ask for the known hairs (from either a victim or suspect) to be forcibly pulled using a tweezers, while others specify that they can be cut, close to the skin. These pulled or cut strands are obtained and packaged in the prelabeled, sealable paper envelopes that sexual assault evidence-collection kits pro- vide. These sealed envelopes are in turn sealed back into the kit box for additional security. (More about these kits in Chapter 10.)

Extreme care must be taken in the evidence-collection process so that victim and suspect clothing are not in the same area at the same time. Investigators may take the precaution of not collecting this type of evidence from a victim and a suspect in the same location for this reason. Mixing clothing of a victim or a suspect together in the same container is bad practice. Hairs can be transferred from one surface to another. So a hair that was attached to under- wear, for example, could end up attached to outerwear by the time the analyst looks at it. The location of the hair might suggest different interpretations about how it got there.

Section 6.2Hair

Standards When comparing hairs, an analyst must have standards (knowns or exemplars). These come from the victim and the suspect, if one has been identified. The analyst may also call for elimi- nation hair standards. These are sometimes called alternate knowns. They are collected from family members or roommates living in the same home as the victim or from others who may have had occasion to shed hair in the same location. They are necessary because those who have free access to the dwelling will shed hairs that may be collected by the evidence technicians. Standard hairs must originate from the same area of the body as the evidence hairs. This is because head hairs are different from pubic hairs, which in turn differ from facial hairs and body hairs. Generally about 25 to 50 known hairs, which represent the scope of characteristics in the individual, will be needed for comparison. These should all be pulled hairs so that their origin is known and the evidence technicians will have roots to aid in com- parison, although some collection protocols say that hairs cut close to the skin are acceptable. Additionally, they should be pulled from different parts of the area being sampled in order to get a representative sample over the whole head, pubic region, or other area. For example, hairs pulled from different portions of one’s head might contain different subpopulations of hair. As we are about to discuss, the analyst needs to have a true picture of the variation in a person’s known hairs before making any comparison to the evidence hairs. You might be wondering: Why 25 to 50 hairs? Why not 10, or 20, or 200? There is no good answer to this question. For years, hair analysts did not completely agree among themselves on the number of known hairs that should be collected for comparison. The latest recommendations from the Scientific Working Group for Materials Analysis (2005) call for 50 strands of head hair and 25 strands of hair from any other body region and as of mid-2018 were being reviewed by the Materials (Trace) Subcommittee of NIST. The logic behind this thinking is that enough known hairs must be collected to reveal any intra-individual variation so that this can be taken into account in the comparison. Head hairs from the same individual show more variability than do hairs from other body regions.

Analysis and Comparison Most analysis of hairs is completed using microscopy, in which the structural characteristics of the hair are analyzed under a microscope. The analysis consists of a comparison between known and evidentiary hairs. This type of analysis will only provide results that are of a class nature rather than individuality, and this type of evidence was routinely considered cir- cumstantial in nature until nuclear DNA analysis became available. Information that can be gleaned through microscopy includes the possible race of the contributor, area of the body the hair originated from, color, length, diameter, whether the hair was dyed, amount of curl, and presence and type of medulla. In all, a microscopist trained in the comparison of hair can look at nearly 60 characteristics. They can be directly compared to the hair of a suspect or victim for similarity. After analysis, an assessment is made that estimates how common the char- acteristics might be and how many people in the population are expected to share the char- acteristics, assuming there were no discrepancies between the knowns and the unknowns. Any discrepancy would show that the knowns were excluded as a source of the unknowns. In nonexclusion cases, the estimates are based on population studies of people’s hair. Generally, there is a sizable likelihood that characteristics can be shared. An analyst cannot ever say a hair specifically came from an individual based on microscopical comparison analysis. The characteristics can, however, be used to exclude possible suspects as possible depositors. In addition, an analyst can list the shared characteristics and let the jury know the estimated

Section 6.2Hair

probability that another person besides the suspect has them. Keep in mind that estimates like this are subjective, often based on the examiner’s experience. There are not any large, sys- tematic studies on the frequency of hair structure characteristics in the general population.

Individualizing Hair There are some things hair analysis cannot determine, such as age or gender. (However, nuclear DNA analysis can determine gender; we will discuss DNA in detail in Chapter 11.) Briefly, there are two types of DNA in the body: nuclear and mitochondrial. Nuclear DNA is the one that is usually discussed and the one that is almost always meant by “DNA typing” or “DNA profiling.” This DNA is found in the cell’s nucleus. Mitochondrial DNA, just as its name tells you, is found in mitochondria. These are structures in the cell that contain the machinery for the cell to process fuel to make energy. Mitochondrial DNA is very small in comparison with nuclear DNA, and it is inherited only from one’s mother. Two regions of mitochondrial DNA exhibit differences among people and can be analyzed for those differences.

The differences are not sufficient to allow individualization. They simply put people into a group. In this way, mitochondrial DNA is like trace evidence. It is never unique to one person. However, if it differs from a known specimen, then we know for sure the known was not the source.

Hair shafts do not contain nucleated cells, and therefore they do not have nuclear DNA. This includes all shed hairs. It is possible to type mitochondrial DNA in hair shafts, and in addi- tion, as mentioned above, if a hair is pulled out, the root will have some skin cells attached. Those skin cells make it possible to do nuclear DNA typing. Mitochondrial DNA typing is by no means a routine lab procedure. There are only a few labs that can do it, and it is only done in cases in which every other option has been exhausted. Not every hair case is going to involve mitochondrial DNA analysis; in fact, the vast majority of such cases do not.

Because there has been some misleading testimony from analysts in the past, some people have suggested that microscopical analysis of hair be dropped and that we do DNA analysis or nothing. Many forensic scientists do not agree with that thinking. There is a lot of important information that can be gleaned from microscopical hair comparisons that can be of assis- tance to a case investigation. As long as it is appreciated that hair cannot be individualized by microscopical comparison (nor by mitochondrial DNA analysis), there should not be any major interpretation problems. A source of known hairs can be unequivocally excluded as a source of questioned hairs. As noted earlier in the chapter, trace evidence is sometimes said to have “exclusionary” value. That means that exclusion is the only absolute conclusion that can be reached after comparison. Failure to exclude does not mean individualization. It just means “included” in whatever group of people share all the features found.

Think About It

There are some who think that if we have the ability to individualize hairs using DNA, then we should go to that step immediately and skip the preliminary microscopic analysis. Do you agree? Why or why not?

Section 6.3Fibers

6.3 Fibers The use of fibers as evidence is very similar to that of hairs. You can think of hair as a type of animal fiber. It is a class type of evidence, and its value depends on how common the fiber might be. White cotton fibers, for instance, would be of lesser value since white cotton is used in many fabrics and is widely available to the public. Fibers come in many types but can be reduced to two major groups: natural fibers and human-made fibers.

Natural Fibers Natural fibers come in two classes, those from animals and those from plants. Animal fibers are the easiest to deal with, since they are generally hairs, which have already been discussed. Microscopy can be used to identify the possible animal. Wool is one of the most used of the animal fibers in textiles. It appears as a clear thread, but you can see the cuticle, which helps identify it. It can also be seen in nearly any color, since it takes many dyes. Other animal fibers are used in fur coats, blankets, or brushes; or they may come from the family pet. Silk, another animal fiber, looks similar to many human-made fibers, but occlusions in the fiber itself can serve to differentiate it.

Plant fibers can originate from many different plants. Cotton, as mentioned, is the most com- mercially used of the plant fibers. It is easily differentiated from other plant fibers under the microscope, because the form of the fiber always appears as a flat, twisted ribbon. Hemp, made from the Cannabis sativa plant, is gaining widespread use again. While previously used to make twine and rope, it is now being seen in paper products, jewelry, and clothing. Its natural appearance is rougher than cotton and appears flat but not twisted. New technol- ogy has allowed manufacturers to remove the coarseness of the fibers while retaining the strength, creating fine cloth that does not stretch. Flax fibers are used to make linen and have been used to make cloth for thousands of years. Since the linen fibers are strong, they are sometimes blended with other materials, such as wool, to make a stronger cloth. Ramie is the last of the plant fibers that are popularly used. This Asian grass has been used for several thousand years and produces cloth of a very fine, almost silky, feel, which is stain resistant and not prone to stretching.

Human-Made Fibers Human-made fibers have only been used since the industrial age began. The first human- made fibers were processed wood. Cellulose, the main component of wood, is a naturally occurring polymer (a large chemical composed of simpler chemical units). The cellulose was treated chemically to form a viscous pulp and fed through a machine with very small holes, which allowed the pulp to be formed into long threads. This is called the extrusion process. These newly formed threads could be twisted together to form thicker threads and used for a variety of purposes, including clothing. The first of these semisynthetic fibers was called rayon. What followed was a revolution in the manufacture of clothing. Today many of our fibers are completely synthetic or manufactured from petroleum products. Nylon was the first of these human-made polymers. The purpose of developing these new materials was to increase durability and replace some natural products, such as silk. Since then, many new

Section 6.3Fibers

materials have been created and are used in nearly every part of our lives. Synthetic fibers now account for nearly half of all fibers used in manufacturing. Fibers can be made in any shape, because they are formed by forcing the material through a spinneret. Some modern fibers are round or oval, but others are lobed, star shaped, or triangular. The manufacturer has the ability to vary the shape of the fibers as desired, which can add information to the sci- entist’s analysis and help differentiate fibers that are made of the same chemical compound.

Collection and Preservation The collection of fibers is similar to that of hairs, as discussed above. As always, care must be taken to ensure that the evidence is not lost, incomplete, or contaminated. In cases in which fibers are part of a larger item (article of clothing, carpet, etc.), the entire item should be col- lected and submitted. This may more often be the case with knowns—items believed to be the source of questioned fibers. Small quantities of fiber evidence can be collected in paper druggist folds or coin envelopes, labeled, and sealed. The paper container can then be placed into a ziplock bag for additional security.

Fiber evidence is like any other type of trace or material evidence. The analysis consists pri- marily of comparison between unknowns and knowns. Investigators must remember, there- fore, to collect appropriate knowns. It is necessary in some cases to collect alternate knowns, if there is more than one potential source.

Analysis and Comparison There are two aspects to the analysis: identification and comparison. Microscopy can be used to look at and identify many fibers. Obviously, questioned and known fibers have to be of the same type if the comparison is to continue. If they are not, the knowns are excluded as a source of the questioned specimen. Many fibers can look very similar because the extrusion process helps maintain uniformity, but the fibers themselves can have many shapes that will be noted under the microscope. Fortunately, there are several instrumental methods of analy- sis that can help in the comparison of fibers found at the crime scene with knowns. Among these, we can mention specialized microscopy techniques, infrared spectrometry, and pyroly- sis gas chromatography.

Earlier, we talked about specialized light microscopes. One of them was a polarized light microscope (PLM). This is an especially useful tool for looking at and comparing fibers, because it is capable of measuring some special properties of the fibers. One of those proper- ties is called the refractive index (RI). The RI is calculated by finding the speed of light as it passes through a material and dividing the speed of light in a vacuum by that number. This will result in a number greater than 1. Chemicals and materials have characteristic refractive indices. Unfortunately, some have the same RI, so this measurement may not be conclusive. Fibers can actually provide three pieces of information that help with characterization of the fiber. Those are the RI parallel to the length of the fiber, the RI perpendicular to the fiber, and the difference between those two refractive indices. Because of the alignment of the poly- mers in the fibers, light passing through the fiber sees two different chemical patterns. This is dependent on whether the light is passing through the fiber’s length or through its cross section. The RI can be different between this parallel (lengthwise) direction and the perpen- dicular (cross-sectional) direction. This difference is actually a characteristic and helps in

Section 6.3Fibers

the identification of the fiber. It is called the birefringence. It is worth noting that these PLM measurements are nondestructive to the fibers.

Another technique useful for fiber comparison is infrared spectrometry, which gives a graphical readout for comparison known as an IR spectrum. This technique plots the absorp- tion characteristics of chemicals in the infrared (IR) region of the electromagnetic spectrum. It was developed primarily to identify chemical compounds. When used for that purpose, the technique needs a nearly pure sample, but it can identify the chemical to the exclusion of all others. This means that a sample placed in the instrument can be analyzed and a graph generated of the way the sample absorbs IR light. That graph can be compared to standard graphs. If the sample matches a standard, it can be identified as that chemical represented by the matching graph. The technique can also be used to compare more complex chemical materials, like fibers. Here, the resulting IR spectrum does not lead to the identification of a specific chemical compound. But if fibers are chemically identical, they will have identical IR spectra. Unfortunately, matching fibers by IR does not allow an analyst to identify that par- ticular questioned fiber in any particular known, such as the jacket the suspect was wearing. Again, it is a class characteristic, and knowing the prevalence of the fiber in general will allow weight to be given to the evidence. A rare fiber may carry more weight than a polyester fiber, since polyesters are a common type of synthetic fiber used today.

Fibers may also be compared by a process called pyrolysis gas chromatography (PGC). We introduced gas chromatography in Chapter 5 as an analytical technique for certain types of toxins and drugs in the body. In this pyrolysis version of the test, a gas chromatograph (GC) is used in combination with a pyrolyzer. The pyrolyzer heats the fiber until it turns into a gas. This breakdown will result in vaporous products that can be passed through the GC and graphed. The graph can be compared to standards. The fiber can be identified through its pyrolysis products; in addition, questioned and known fibers can be compared based on their PGCs. If they are chemically identical, they will have identical PGCs. One disadvantage to this test, however, is that the sample used is destroyed.

Individualizing Fibers As with other trace, an analyst can typically identify fibers. Then, based on a comparison between known and questioned specimens, it can be concluded either that they do not match (an exclusion) or that they do match (an inclusion). An exclusion is absolute. An inclusion requires more explanation. As we have noted, we usually do not know exactly how common or rare a certain fiber is. Some fibers, like white cotton, are so common that they have almost no evidentiary value. But in other cases, there may be ways of estimating that a fiber type is relatively uncommon, adding to its value as evidence.

Think About It

In a criminal investigation, an analyst finds that both the questioned and known fibers are of the same type of nylon fiber, are of the same color and RI, and cannot be distinguished by PLM, IR, or PGC. What can the analyst conclude from this analysis?

Section 6.3Fibers

There is one way evidence made from cloth can be better individualized, though it is rare. In some accidents, pieces of clothing are ripped from the victim and attach to an object such as an automobile. In a few instances, this piece of cloth can be fitted back to the original item of clothing. We will get into this more in Chapter 8, but this is called a secondary fracture match, or physical match. Fracture matches involve some rules. The piece must fit into the original in the same way pieces fit in a jigsaw puzzle. Patterns in the piece must match patterns in the original, and the chemical composition must be the same. If all of these factors are met, the scientist may be able to say that the piece came from the original larger item. This type of evidence is highly individual and can serve to be very powerful in court. As stated, however, this type of evidence is very rare to find. Direct physical matching is sometimes possible with glass or plastic, as we will discuss a bit more in the next section.

If you have read or heard about the Wayne Williams case in Atlanta, Georgia, you may know that there were several types of fibers and animal hairs found on the victims that could be tied to the Williams household. This case is a well-known one, in which circumstantial evidence, consisting exclusively of hairs and fibers, was used to prosecute and convict someone of murder.

Case Illustration: State v. Wayne Bertram Williams In the greater Atlanta area on July, 1979, the bodies of two Black teenage boys were found after they had been reported missing. By 1981 there were six additional victims, and law enforcement was sure a serial killer was at work. Since all the victims were Black, there was a hate-crime racial element to the investigation as well. In all, there were 30 murdered Black children and young men on the list of victims at one time or another. Some were taken off the list because the cause of death or gender did not match the pattern. Some of the bodies were found in waterways, and police believed the perpetrator was killing his victims, then dumping them into rivers. All the victims ultimately attributed to this murder case were Black males who died from asphyxia and either were young or looked young. The Georgia Bureau of Investigation handled these cases. However, as the case and the publicity heated up and pressure mounted on law enforcement to solve it, FBI agents and some FBI Laboratory examiners joined in the investigation. The recovered bodies had several types of fibers on them, and these fibers appeared on several victims. There were also dog hairs on some of the bodies. The forensic lab examiners realized that these fibers might be the key to the whole case.

Law enforcement officers eventually began to stake out bridges over rivers in the Atlanta area, in the hope that they could pick up some clue as to the identity of this serial murderer. In May 1981 they got lucky. While they were positioned under a bridge one night, they heard a splash, followed by a vehicle driving away from the bridge. They pursued the vehi- cle and stopped the driver. It was a local Black man, Wayne Williams, on whom the inves- tigation then immediately focused. A short time after the stop, another body that fit the pattern was recovered from the river. Armed with this information and data from previous victims, police were able to obtain a search warrant for Williams’s home and his vehicles.

Fibers and dog hairs were recovered from the Williams home and from vehicles that Williams had owned and operated. These were compared with the evidentiary fibers from the bodies of the murder victims. Table 6.1 shows many of the associations that were found.

(continued on next page)

Section 6.3Fibers

Case Illustration: State v. Wayne Bertram Williams (continued)

Based on the table, you may notice how many associations were found between victims and various fiber and hair sources associated with the Williams’s home. Prosecutors picked what they believed were the strongest of the cases, and ultimately, Williams was tried for the murders of two young men, Payne and Cater. The fiber matches were circumstantial, and the analysts could not say that the fibers on the body came specifically from the items in the Williams home. But one of the sources from the carpet in Williams’s bedroom helped with the case. Studying cross sections (slices at right angles to the length) of these fibers showed they were quite unusual. Investigators were able to determine that they had been made by the Wellman Corporation. This type of fiber was not included in many of the car- pets they manufactured, and it had only been manufactured for a short time. This informa- tion permitted the experts to put some numbers on the commonness/rarity of Williams’s bedroom carpet. These were estimates based on available data. From the estimates, they could figure out about how many carpets were manufactured that had these fibers. Then, assuming that the carpets were evenly distributed in 10 southwestern states, they could estimate how many would be found in Georgia and in Atlanta. From this, the chances of randomly selecting a house with a room containing carpet with this particular fiber could be calculated—it worked out to be about 1 in 7,790. This information did not individualize Wayne Williams or his residence directly. However, coupled with the rest of the associative fiber and hair evidence, it was sufficient for the jury to convict Williams.

Reflect On It The unique carpet fiber helped prosecute Wayne Williams as the murderer. If you were the expert presenting this evidence to the jury, how would you explain the probability of dupli- cation of this carpet fiber?

Table 6.1: Associations between fiber and dog hair from Williams’s home and the victims

Name of victim Alfred Evans

Eric Middle- brooks

Charles Stephens

Lubie Geter

Terry Pue

Patrick Baltazar

Joseph Bell

Larry Rogers

John Porter

Jimmy Payne

William Barrett

Nathan- iel Cater

Violet And Green Bedspread Williams’s Bedroom

X X X X X X X X X X X X

Green Carpet Williams’s Bedroom

X X X X X X X X X X

Dog Hairs Williams’s Dog

X X X X X X X X X X X

Yellow Blanket Williams’s Bedroom

X X X X X X

Blue Rayon Fibers Debris From Williams’s Home

X X X X X

Trunk Liner 1978 Plymouth

Carpet 1979 Ford

Carpet 1970 Chevrolet

X X X X X X

Additional Items From Williams’s Home, Automo- biles Or Person

Yellow Nylon Ford Trunk Liner

Yellow Nylon White Polyester Backroom Carpet Ford Trunk Liner

Kitchen Carpet

White Polyester Backroom Carpet

Yellow Nylon White Polyester Head Hair Glove Jacket Pig- mented Polypro- plyene

Yellow Nylon Porch Bedspread

Porch Bed Spread

Blue Throw Rug

Glove Backroom Carpet Yellow Green Synthetic

Source: FBI Special Agent Harold Deadman, FBI Law Enforcement Bulletin. May 1984.

Section 6.4Glass

6.4 Glass Glass is an important part of our daily lives. It is in homes and offices, packaging for food and drink, and automobiles. It is nearly impossible to think of a place where glass cannot be found. It would appear, therefore, that glass will be found as evidence in many types of cases. Indeed, it can serve to link a burglar to the scene of a break-in. It can tie a victim of a hit-and-run to a broken windshield or headlight from a suspect vehicle. It can connect a broken bottle covered in suspect fingerprints to the glass in the head wound of the victim struck by the glass bottle. All of these cases can utilize the glass as evidence. As with hair and fibers, glass evidence will most often be class evidence rather than individual evidence. Even though this is the case, cir- cumstantial evidence can build up enough to help a judge or jury come to a verdict in a case.

Structure of Glass Very simply, glass is a hard, brittle material that is considered amorphous in its composition. This means that the arrangement of atoms and molecules in the substance is random. Amor- phous material does not have a definite structure like a crystal does. Glass is made of silicon oxide (sand), which is heated until it melts and then poured to form flat pieces that can be used as windows. It can be blown or extruded to form bottles or formed into dishes, plates, and other items used daily. It can be poured and cooled to make untempered glass, or plate glass, used in doors and windows. The unfortunate aspect to plate glass is that when broken, it can form shards, which can be dangerous. When glass is poured and tempered during the cooling process, it makes glass plates that are stronger. When broken, this glass will fracture into hundreds of cubic pieces, which are less dangerous than those produced by an untem- pered product. This type of glass can be used as a safety product, such as in the side and rear windows of cars.

Glass can also be layered with thin polycarbonate sheets between the layers. This laminated glass has a variety of uses, including the front windshields of automobiles. If made thick enough with many layers of glass and polycarbonate materials, it can be used as bulletproof glass. There are hundreds of types of glass in existence, including exotic lead crystal and fine porcelain used in china, but there are a few that are commonly found and are typically seen as evidence.

Collection and Preservation Collection of glass evidence depends on what analysis is to be completed. All glass evidence should be packaged to prevent loss and breakage. This can be best accomplished by placing the glass into plastic or paper bags to prevent loss and then placing the bags into a box to prevent further breakage. If a fracture match is requested, all glass should be submitted for analysis so the window or glass piece can be reassembled. This is done for untempered glass. In cases where tempered glass is involved or the glass cannot be reassembled, only a few rep- resentative pieces of the window or glass object are needed.

Analysis and Comparison Known and unknown glass specimens can be compared as to color, fluorescence, thickness, and surface features. Forensic scientists have also traditionally used two properties to provide

Section 6.4Glass

information about the characteristics of glass. Those are refractive index and density. There are other tests available that mainly measure the elemental composition of glass, such as inductively coupled plasma mass spectroscopy. Instruments for determining elemental com- position are expensive, and only the largest full-service labs typically have them. Such analy- sis does not really offer better information than the classical tests and often results in the sample that is tested being destroyed during analysis.

Refractive Index Because glass is amorphous, no matter how light passes through it, the light moves through exactly the same random composition. This means that glass only has one RI. RI is a physical property that can be determined by dividing the speed of light in a vacuum by the speed of light as it passes through any other medium—in this case, glass. This is important, because evidentiary samples of glass can be any shape or size. The analyst has only to place the glass in the measuring device and record the RI of the sample. No special orientation is necessary for the test. One common type of testing for RI is known as the immersion method of analy- sis. The sample of glass for testing is immersed in a fluid that has an RI similar to glass. This fluid has been characterized, and its RI is known at many temperatures. (RI can change with temperature, and liquids are more sensitive to this change than solids.) The glass fragment of interest is observed under a microscope fitted with a hot stage. The hot stage allows sensi- tive control of temperature. When the glass fragment and the liquid in which it is immersed have identical RI, the fragment will seem to disappear. The analyst then looks up the RI of the suspending oil at that temperature, and that is the RI of the glass.

The analyst must perform the same test on the glass standard (known). If the RI is the same, the analyst can say the evidence glass could have originated from the same glass as the stan- dard. If the RI of the evidence and standard are different, the analyst will say the evidence could not have originated from the known standard glass. As with other trace evidence, it is easy to exclude a specimen, but if the evidence and glass give the same characteristics, ana- lysts can only say that they may have come from the same source and give an estimate of the commonness or rarity of glass with that characteristic.

Today scientists have an instrument that helps simplify this analysis. The Grim 3 serves to automate many of the processes involved with this analysis, through the use of computerized testing and calculation of the results of the analysis in a controlled instrumental environment. This makes determination of the RI of the evidence very repeatable, simple, and digital.

Density Density is a physical characteristic of chemicals, compounds, and mixtures. It is the mass of the item per unit of volume; it is determined by dividing the mass (weight) and volume (amount of space something takes up). Density is expressed in grams per cubic milliliter in the metric system (or pounds per cubic foot in the English system). Each element and chemi- cal or mixture will have a density. Because many items can have the same density, this is only a class characteristic and cannot be used to individualize something. See Table 6.2 for the density of some common items.

Section 6.4Glass

Table 6.2: Density of common items

Item Density (g/ml)

Wax 0.80

Water at room temperature 1.00

Alcohol 0.82

Lead 11.35

Mercury 13.95

Gasoline 0.72

Air 0.0013

Rubber 0.96

Iron 7.20

Glass 1.5

Some of these examples of differences in density within various items can be seen in everyday life. If a piece of wood is placed in a container of water, it floats. This is because the density of the wood is less than the density of the water. Any object will float in a substance of greater density, such as oil floating in water. If the density of two substances is the same, they will remain suspended.

In the test for density, two chemicals and mixtures of the chemicals have classically been used. They are layered in a tube so that the denser chemical is on the bottom and each layer above decreases in density. Two identical tubes are made up in this fashion. The glass from the crime scene is dropped into one tube. It will sink until it reaches a layer that has the same or greater density. It will stop at that layer and sink no further. The standard is dropped into the second tube. It will also sink until it reaches a layer of equal or greater density. If both samples of glass sink to the same level, they both have the same density, which is calculated based on the mixture of the chemicals involved. This density-gradient method allows us to measure density similarity or difference between glass fragments without necessarily know- ing what the density is. Using a density meter, however, it is easy to then determine the den- sity of a liquid in which a glass particle remains suspended.

Think About It

Using Table 6.2, you can determine whether one substance might float on top of another by comparing the densities. For instance, will gasoline float on or sink in water? Will lead float on or sink in mercury? Now consider a rubber-like piece of evidence that is found in a case. Would comparing its density to water be enough to identify it? If not, what other tests could you do to confirm what this evidence is made of?

Section 6.4Glass

RI and density are somewhat related; as a result, analysts sometimes just measure RI. But keep in mind that many specimens of glass will share common RI and density.

Elemental Analysis Determining the elemental composition of known and evidentiary glass is another way of comparing them. A number of fairly complicated instrumental methods are available for these determinations. One such test is inductively coupled plasma analysis with mass spectrometry. This test provides an elemental analytical profile of the glass sampled. Another procedure uses scanning electron microscopy coupled with X-ray fluorescence or energy dispersive X-ray analysis. Other procedures are based on atomic spectroscopy. Think of this spectros- copy as a way of distinguishing chemical elements based on the way different ones absorb or respond to the absorption of energy. Unfortunately, the results of elemental analysis com- parisons will not give a more conclusive answer about the origin of the glass than common testing, and the techniques destroy the sample.

Individualizing Glass One way of thinking about the forensic approach to glass (and other trace item) comparisons is that the analyst sets out to show that the specimens are different. If they are different, this conclusion is reported. If the analyst cannot show that they are different, then they have the same characteristics with respect to the properties compared. That does not mean they share a common origin. There are too many specimens that share the properties that are measured. An estimate of the commonness or rarity of the characteristics found in a nonexclusion case may be provided if the data are available to determine it.

The only way to individualize glass samples is by using fracture matching (direct physical matching). In this case a piece of glass must be taken from the suspect that is big enough to piece into the broken window or other glass item. All of the glass from the broken object must be submitted so it can be put together again. In glass, as with the fabric discussed earlier, if there is a lock-and-key fit between the evidence piece and the original broken glass object, it can be determined that the piece did originate from the object. But these jigsaw-fit matches are even more convincing with glass or plastic, because they are solid and fracture in com- pletely random ways.

However, there are some issues of concern with physical matching. First, all pieces of the broken object must be packaged and submitted to the laboratory. The specimens should be protected against further breakage and loss. This test only serves to individualize the glass to the object with a proper fit. If known glass is lost or left at the scene and the analyst cannot fit the evidentiary piece into the known, the test would be inconclusive at best, and other tests must be run. A second issue is that the pieces from a broken safety glass window cannot be reassembled. There are way too many pieces, and they are too similar in shape and form. This means that physical matching is not attempted with broken automobile windows or other cases that involve safety glass.

It is possible to find out information about impacts to glass objects like windows. If the win- dow is not tempered, then it can be reassembled; if the outside is known, the glass can be

Section 6.4Glass

examined at the impact point to see from which direction the impact originated. If the point of impact itself is visible, it can be easy, as in Figure 6.4. The impact zone will often look like a crater with a smaller opening on one side that expands toward the other side; the impact came from the side that has the smallest hole. If the impact site is destroyed, looking at the side of the glass may give an indication. If glass is broken but does not shatter, one can discern two types of fracture lines within the break area: radial and tangential. As shown in Figure 6.5, radial cracks radiate out from the point of impact, while tangential cracks connect radial cracks. If you view a radial crack line from the side, characteristic lines are curved along the side of the glass where the break happened. This curve starts parallel to the glass surface and curves through the glass so it is perpendicular to the opposite surface. The parallel portion indicates the side of the impact. The perpendicular lines show the side opposite. There is a so-called 4R rule to help remember radial cracks: Ridges on Radial cracks tend to be at Right angles to the Rear (side opposite the impact).

Figure 6.4: Impact zone on glass

Usually the impact zone looks like a crater and can help determine the direction from which the glass was struck.

iStockphoto/Thinkstock

Section 6.4Glass

Figure 6.5: Radial and tangential glass fractures

Radial cracks extend from the impact zone and can help determine which side of the glass was hit. Tangential cracks connect radial cracks. If glass has been hit multiple times, glass fractures can help determine the first impact zone.

Adapted from 3quarks/iStock/Thinkstock

Lastly, if glass has been struck more than once and it does not shatter, the order of impacts can sometimes be found. This can become complex if there are many impact points, but to start simply, remember that a fracture line will stop at an existing fracture line. Working backward, it is possible to find the impact where the fracture lines of other impact points stop. This will be the first impact. Most of the time, the order and direction of impact will not be useful in the case. However, should this be an issue, all of the glass must be packaged and carefully handled. It is very important to prevent subsequent breakage of the glass during handling and transport. These procedures have been used to figure out things like whether a gun- shot through a windshield came from inside or outside a car, and sometimes the order of the impacts. We call these patterns “patterns for reconstruction,” and we will have more to say about them in Chapter 8.

Radial fracture lines

Tangential fracture lines

Section 6.5Paint

6.5 Paint Paint as trace evidence is found in cases involving hit-and-run accidents between motor vehi- cles or between a vehicle and a person. It is also used in tool mark cases, as paint scrapes off the tool and can become embedded in the mark. Conversely, a screwdriver used to pry open a door that is painted white, for example, could end up with a small fragment of white paint on its prying surface. This may seem like good evidence. Unfortunately, there is an issue when using paint as evidence. Paint formulations are common to a large number of motor vehicles and are used on a large number of tools, other everyday items, and surfaces. This means a large suspect pool must be considered when any evidentiary paint sample is found at a crime scene. It will take some work to make paint usable as evidence. On the other hand, paint, like most types of trace evidence, can be used to quickly exclude a specimen that does not match a known.

Composition of Paint Paint is more complex to analyze than most samples because it contains both organic and inorganic chemicals. The techniques used for analyzing these classes of chemicals are quite different. When both components are analyzed, however, a thorough profile of the samples can be obtained. From this, it may be possible to derive a possible manufacturer and color of automobile or compare a mark left by a tool to a tool coating.

Simple paints are composed of several components. They can be composed of an inorganic pigment or a combination of pigments that give a certain overall color. The binder (organic polymer) holds the color in place on the object being painted, and a solvent keeps the other components in suspension for application. Once the paint is applied to a surface, the solvent evaporates as the paint dries. At that point, the pigments and binders are left for analysis. Automotive, tool, and many structural paints are made in a similar way. There are other coat- ings that can sometimes be found in cases, as well. Varnish is a clear coating for wood. It is composed of organic compounds that dry to form a protective layer over the wood. Stains are similar to varnish except there is an added pigment for color. Artists’ paints can be of many types, including oil based, acrylic, watercolor, and others. Typically, these will not be an issue in forensic criminal cases, though it could be of concern in the art authentication field. Here, we will discuss the methods used to characterize paint that are found in most forensic cases.

There are several types of coating materials that can be utilized as evidence. Most often, the evidence will come from a tool or automobile. With automobiles, the application of the coat- ing materials leaves layers on the sample surface, which can be analyzed by the scientist. Next to the metal is an electro-coat material. This is applied directly to the metal and is used for rustproofing and to prepare the object for further coating materials. On top of the electro-coat is a primer coat. The primer can be many different colors and is used as additional rustproof- ing and to further prepare the object for the real color coating material. The third layer is the actual color coating. After application of this coat, IR treatment will help dry the paint, and during this process, the polymers in the binder are able to interact, forming a hard film that will protect the paint from damage and chipping. The final coat is a clear coating designed to protect the paint from chipping and sun damage. Together, analysis of these four coatings and utilization of the International Forensic Automotive Paint Data Query (PDQ) database for paints can give the scientist an idea about the manufacturer, model, color, and year of manu- facture for the suspect’s vehicle.

Section 6.5Paint

Having knowledge of the composition of the paint components used on newly manufactured cars is obviously a big help in a paint case. Keep in mind, though, that the database is only good for original factory paint. If a vehicle has been repainted at some point, the paint com- position may not be in the database.

Tools may not get the same complete painting treatment, but the coatings present on a tool can be characterized in a very similar manner. In this case, however, the best that can be said is that the paint and primer from the scene is similar to the coating on the tool in ques- tion—and all other tools from the same company are coated in the same manner. There is no database for tool coatings at this time.

Collection and Preservation Collection of paint samples should be completed in the same manner as other trace evidence collection. If they are found on a larger object, such as clothing, the object should be carefully folded and sealed in an appropriate paper or plastic sealable container. If the sample is a chip that can easily be collected and might be lost, it should be placed in a paper drug- gist fold and then sealed in a coin envelope or ziplock bag. If the sample is a smear rather than a discrete chip, the object con- taining the smear should be submitted. The goal should always be to collect as complete a sample as possible for analysis in the laboratory. Known standards will need to be obtained for comparison. Paint samples should be collected by scraping all the way to the metal on any object thought to be involved in the case. If possible, multiple samples should be taken and packaged separately in a manner similar to the sample. Remember that layer structure is important in paint compari- sons, so investigators must be sure to collect all the layers of paint and coating when they collect known specimens. For both evidence and standards, remember that chain of custody is as important here as in any other case. Specimens must be sealed in their containers so they cannot escape, and the containers must be properly labeled.

Analysis and Comparison In the laboratory, the most obvious first test is a visual comparison of the evidence to the standard. Microscopically, the analyst should be able to see all of the layers that make up the paint chip. If a standard does not appear to be similar to the evidence in general class characteristics like color and texture, the evidentiary specimen can be eliminated as having come from the known. If the sequence of colors matches, further testing will be completed. Something to consider during collection is that if a vehicle has been involved in an accident, it may have been repaired and repainted; this is the reason to take multiple standards from a vehicle. The samples can be compared to find the original paint characteristics, which can then be compared to the repaired area.

Vadim Ghirda/AP Images Since paint is so common that it is hard to use as evidence. However, a car paint database helps narrow information down on automobiles.

Section 6.5Paint

If the color layer sequence matches between evidence and standard samples, other analyses can be completed. Layers of the paint specimen can be sampled one at a time to make sure that each layer in the known is the same as the corresponding layer in the questioned sample. Since there are many different formulations of paints of the same color, this will help establish similarity between the evidence and standard or eliminate the suspect if the paint is of the wrong type. Another simple test is solubility of the paints. An analyst might add a chemical such as acetone and observe whether the sample dissolves, which can help characterize the layers further. If everything still compares favorably between standard and evidence, instru- mental analysis can be done. However, as with other trace evidence, these analyses still only provide class characteristics and cannot provide individualization.

Several different tests can be used, depending on the equipment available in the laboratory completing the work. Infrared spectrophotometry, which we discussed earlier in connection with fiber comparisons, can be used to characterize the way light in the infrared frequencies is absorbed by the sample. This identifies the type of paint and can show that two specimens have identical organic chemical composition but cannot individualize it to a specific car. If the scientist wants to determine the inorganic components of the paint, X-ray fluorescence can be performed. This test determines which elements are present in the sample and, if used quan- titatively, gives approximations of the relative amounts of each element. Different inorganic elements and compounds are used as pigments to give the paint its color. PGC, which was also discussed in connection with fibers, can be used to generate a pyrogram of the samples. Here, as with fibers, a sample is placed in the pyrolysis unit attached to a GC. The pyrolysis unit is heated and the sample contained within is pyrolyzed, meaning it is turned into vapor. The components of the vapor are graphed after passing through the GC and can be compared to standards. If evidence and standard samples give the same graph, the samples could be the same compound or mixture of compounds.

This chemical analysis, though complex and time consuming, will not confirm the identity of a particular vehicle or tool. However, it will allow the PDQ database to narrow the search to a make, model, year, and color of car, if the paint is original manufacture. After that, statistics come into play. How many of that particular type and color of vehicle are in the area? Are there any other circumstances that could help, such as obtaining a partial license plate num- ber that could improve the probabilities of having the correct suspect vehicle? Another factor that can improve the statistics involves repainting of the vehicle. Some owners will repaint to repair their car or to change the color for aesthetic purposes. This repainting could increase probabilities because there is additional information, in the form of more layers of paint to compare. The more times a vehicle was repainted, the fewer vehicles one would expect to be in that category.

Think About It

The PDQ system can be used to give the analyst information about a possible make and year of automobile involved in an accident. Suppose the paint evidence indicates the auto- mobile was repainted at some time. In other words, during analysis the analyst finds an additional layer of paint not applied by the manufacturer in the evidence paint chip, which is also present in the suspect paint chip standard. Does this help or hinder the inclusion or exclusion of a suspect? Why or why not?

Section 6.6Soil

Individualizing Paint Normally, it is not possible to say that a particular sample of paint came from a particular source. Once again, only a secondary fracture match would be conclusive. In that case a paint chip from the crime scene or victim would need to fit exactly into a void on the suspected item of origin like a jigsaw piece. Though rare, this has been seen.

6.6 Soil One type of trace evidence that is perhaps often misunderstood and misused is soil. This material is seen throughout the world as the layer of dirt on the earth’s surface, made up of any decomposing material that happens to have been deposited in a certain area. Therefore, it can be composed of literally anything, from natural materials to human leftovers. Soil can also include local pollen that can point an expert towards a specific location. Soil does not change much over a short dis- tance. Indeed, while there are many differ- ent colors and compositions of soil, the likelihood that in any particular jurisdic- tion you will be able to differentiate the local soil is remote. So what can be done with soil? If collected, it could link a pos- sible crime scene to a victim and suspect. It may demonstrate that there is a remote scene involved in a crime. It could also serve as an alibi for a suspect. The use and usefulness of soil will depend greatly on its collection and preservation and two aspects of the soil itself: the ability to be transferred easily and the uniqueness of the soil at or near the crime scene.

Collection and Preservation Evidentiary soil can be found in a number of places, such as trapped in the tread of tires, on the soles of shoes, on clothing, or on tools. Samples from these items can be collected in a manner similar to every other type of trace evidence, in containers such as small plastic or cardboard boxes that can be sealed. It can also be placed in small metal tins that can be sealed or envelopes that will not leak the sample. Soil can also be packaged in a clean glass vial.

Sometimes, soil is associated with a cast of a tire or footwear impression, which we will dis- cuss in Chapter 8. The soil from these casts can be saved for possible future comparison. Known standards are a different consideration. Standards should be collected at the crime scene. Then, using the crime scene as a central starting point, standards should also be col- lected in all directions leading out from the crime scene and up to 100 yards away. Sometimes it is suggested to use the four compass points and collect some knowns at each point 25 feet away and 100 feet away. These standards, while numerous, will allow the scientist to

imageBROKER/SuperStock Dirt is everywhere, which may make it seem unimportant to the scene of the crime. But because soil is distinctive to certain areas, it can be a helpful tool of analysis.

Section 6.6Soil

determine if there is something unique about the crime scene or if it looks the same as all the other soil in the area. Common sense should be used when collecting a number of knowns, though. For example, if you were on a white sand beach, you wouldn’t need lots of knowns. As with every other type of evidence, care must be taken to preserve chain of custody, and as with all trace evidence, contamination of the sample and standards must be avoided during handling and packaging.

Analysis and Comparison If a suspect indicates that he or she had been nowhere near the crime scene, but had been somewhere else and picked up the soil from there, then the second area must also be tested. In all, this would seem to involve a huge amount of analysis. Fortunately, the first step in the analysis is easy, and can rapidly exclude a suspect or serve to include them and require fur- ther analysis. Samples of the soil should be placed in an oven and dried to the same humidity level. Then, the color and texture should be observed. While there are well over 1,100 differ- ent soil colors worldwide, fewer than 100 are very common (Saferstein, 2014). If the soil is evaluated from both the crime scene and the suspect and they are different in terms of color and texture, the analysis is complete—until a new suspect is found. Different color and tex- ture of the soil excludes the knowns as a source of the evidence specimens.

If the color and texture are the same, additional analysis can be performed. Soil can be passed through a sieve to separate components by size. The fractions can be weighed to establish a particle size distribution profile and observed macroscopically to see if the components are similar. Inclusions can consist of materials left by humans, such as bits of plastics or manu- factured products, cigarette butts, and other litter. If there is no similarity, the analysis can end. If the same materials are present, even further analysis can be performed. Microscopic analysis under low magnification can reveal components such as animal hairs, parts of plants, or animal waste that are present. Chemical treatment of the soil, such as the use of peroxide solutions, followed by microscopic analysis can reveal the presence of minerals or diatoms. These microscopic components of soil can show differences in soils that appear similar to the naked eye. Should all testing be completed by the scientist, and if everything appears similar, the scientist can try to determine how likely it is that the soil from the suspect is from the crime scene and what the estimated probability is that it could have come from another area with similar characteristics.

In rare cases during collection of evidence, investigators may find something at the crime scene that is different from the soil in the surrounding area. It could be directly related to the suspect in some way. Should that happen, the probability of tying the suspect to the crime scene and victim would increase. Soil cannot be individualized at the present time. Recently, there have been some techniques involving molecular genetics tried on soil to see whether a microbiological profile could be developed that would discriminate soils, since soils are full of small organisms such as bacteria and fungi. So far, these techniques have not succeeded in showing that soils can be individualized.

Section 6.7Reporting and Court Testimony Considerations

6.7 Reporting and Court Testimony Considerations The forensic trace examiner has a great responsibility in reporting as well as in courtroom testimony. The prosecution will try to use the testing and conclusions to convince the jury that a suspect had to have been at a crime scene because trace evidence such as fibers that were similar to the suspect’s clothes were found at the scene and glass particles that appeared to match the broken window at the entrance to the scene were found embedded in the suspect’s shoes. Simultaneously, the defense will argue that fibers like cotton are found everywhere and the glass is so common that nothing can be said about it.

It is important that trace examiners stick with the conclusions that are defensible on a scien- tific basis. Earlier in this chapter, we mentioned some analysts who went beyond the limits of possible interpretation of evidence in nonexclusion cases, and people were wrongly convicted as a result. It is essential that the limitations of the similarities in knowns and unknowns be made clear. In fact, the Department of Justice and FBI are actively working to address errors made in hair analysis testimony and reports prior to 1999 (FBI National Press Office, 2015). Current thinking, for instance, is that “match” is not a good word to use in reports or testi- mony. Even though forensic scientists understand that “match” does not mean common ori- gin, jurors can interpret it in that way. Today examiners are more inclined to use phrases like “consistent with one another in the properties compared” or “failure to exclude.”

Sometimes, there is information or data that make it possible to say something about how common or rare you might expect an evidentiary trace item to be. We saw this with the bed- room carpet in the Wayne Williams case from Georgia. But often, the data do not always exist to enable meaningful estimations of rarity or commonness.

We have also noted that a case involving two or more types of trace that associate a suspect with a victim or scene can be more persuasive than just a single type. But analysts are careful not to be drawn into definitive probability statements or calculations while they are testify- ing. As a rule, we do not have the data to compute meaningful probabilities in trace cases.

Think About It

The composition of soil makes it difficult to analyze. Do you think this is the most difficult trace evidence to analyze in a case? Why or why not?

Think About It

Since we are often not able to provide individualizing conclusions for trace evidence, does this mean it is not important in court? Why or why not?

Conclusion

Conclusion Trace evidence is easily transferred between suspects, victims, and the crime scene. This transferred evidence tends to remain where it was located at the point of contact. Multiple transfers of different types of trace evidence can increase the probability that the suspect is the person who committed the crime. The use of trace evidence will generally not constitute proof of the case a majority of the time. However, it can provide important circumstantial evidence, which can add to other evidence in the case. It can also very quickly exclude a ques- tioned specimen from a known and exonerate a suspect if the trace is different.

Unfortunately, trace evidence is not without problems. Because it is small, it can easily be lost. Because it is easy to transfer, it can easily be contaminated. With proper care during collec- tion, hairs, fibers, paint, soil, and glass can help make a case.

Key Ideas

• Trace evidence can be any type of material, but the most common types of trace evidence in casework are hairs, fibers, glass, paint, and soil.

• Trace evidence cannot in general be individualized. There are some narrow exceptions—involving physical matches—to this generalization.

• When collecting known hairs, a sufficient number must be collected to enable the analyst to see any intraindividual variations.

• When collecting evidentiary specimens of trace materials, larger items containing trace specimens should be submitted intact if possible.

• The location of trace evidence on items may be important and should be recorded prior to collection.

• Trace evidence should be packaged in containers that truly contain the specimen; fibers, hairs, and small samples should be packed in secondary containers for added security.

• In some unusual instances, pieces of glass or paint or torn pieces of textiles may be amenable to physical matching.

• Known paint specimens from painted surfaces must be collected so all the layers are present in the specimen.

• Known soil specimens should be collected at and around the area of the crime scene in sufficient quantity to allow the analyst to see how much variation exists in the soil in the area.

Critical-Thinking Questions

1. How would you go about properly collecting and preserving different trace evidence (such as hair, fibers, and soil) from a carpeted area of a crime scene?

2. You are given different hair samples to analyze from a crime scene. These hairs could be human, cat, or dog hair. How would you go about analyzing and comparing these samples?

3. A broken tempered glass window is found at a crime scene. Can this be pieced back together?

4. If you are not reassembling a window, what tests will be used to analyze the glass pieces?

Conclusion

5. If a paint chip is found to have come from a repainted automobile, is it more individ- ual than a chip that has come from an automobile that has not been repainted?

6. After determining the color layer sequence of a paint chip and comparing it to the standard, you find that the two do not match. What would you do next?

7. You find soil at a crime scene that has some individualizing characteristics but appears to be as hard as concrete. How useful will this soil be in helping identify the suspect?

8. In court, the prosecution tries to get you to embellish your results to help the case. What should you do?

Key Terms birefringence A property of certain mate- rials, including some fibers, in which refrac- tive indices in two perpendicular directions are different.

cortex The outer layer of a hair shaft, lying just beneath the cuticle.

cuticle The outermost layer of a hair shaft, made up of overlapping scales, like the shingles on a roof.

density A physical property of materials determined by measuring the mass of a material and then determining the volume of displacement of that material. Dividing the mass by the volume gives the density in grams per milliliter.

elimination hair standards Also called alternate or alibi knowns; known hair speci- mens from an alternative possible source.

fibers Natural or human-made filaments that can be used as class evidence in crime scene investigations.

glass A hard, brittle substance composed of sand plus trace elements or compounds that are added to change its characteristics.

hair The outgrowth of the epidermis in mammals consisting of three layers—the cuticle, cortex, and medulla—that can be used as class evidence in crime scene investigation.

infrared spectrometry A type of instru- mental chemical analysis in which the absorption of light in the infrared region of the spectrum by a chemical compound or material is determined.

International Forensic Automotive Paint Data Query (PDQ) A database of original manufacturer automobile paints and their chemical and physical properties.

human-made fibers Fibers made from chemically modified natural polymers or from chemically synthesized polymers.

medulla The innermost layer of a hair shaft, containing the hair pigment, among other components.

microscopy The use of a microscope to do an observation or an analysis.

natural fibers Fibers found in nature, such as cotton.

paint Coating materials used in many ways. These materials can be composed of inor- ganic pigments and organic binders, which can be analyzed and compared to attempt to relate an object to a crime scene.

physical match A jigsaw fit match between physical objects that have been broken or severed in a random way. Also called a frac- ture match.

Conclusion

pyrolysis Vaporization of a solid material, including fuels, caused by heat. In the con- text of gas chromatography, a solid is vapor- ized for analysis. In the context of combus- tion or fire, a solid fuel must be pyrolyzed before it can burn, because combustion takes place in the vapor state.

pyrolysis gas chromatography (PGC) A process in which a pyrolyzer is attached to a gas chromatograph and a sample is con- verted from a solid to gaseous products by heat and then analyzed, using the separating capabilities of the gas chromatograph.

questioned sample Also referred to as evi- dence; any object or material that is going to be used to try to connect or dissociate a suspect to a crime scene or victim of crime.

refractive index (RI) A physical prop- erty of materials that can be determined by dividing the speed of light as it passes through the material being tested by the speed of light in a vacuum.

soil The material near the earth’s surface made up of decomposed and decomposing materials that can be used as trace evidence to connect a suspect to a crime scene.

standard Also referred to as knowns or exemplar; items with a known origin, col- lected for comparison to evidence.

tape-lifting A process by which wide tape is repeatedly used on a piece of evidence to remove and contain trace materials such as hairs, fibers, and other debris that can be important to a case.

trace evidence Evidence that can be easily transferred between a scene, victim, and/or suspect and that can be very small.

Web Resources A story about two imprisoned individuals who have been exonerated by DNA analysis who had hair evidence in their original cases: http://www.washingtonpost.com/local/crime/justice-dept-fbi-to-review-use-of-forensic -evidence-in-thousands-of-cases/2012/07/10/gJQAT6DlbW_story.html

FBI “Forensic Human Hair Examination Guidelines”: http://www.fbi.gov/about-us/lab/forensic-science-communications/fsc/april2005/ standards/2005_04_standards02.htm

CSI and trace evidence: http://www.forensicsciencesimplified.org/trace

FBI document on “Hairs, Fibers, Crime, and Evidence (Part 1)”: http://www.fbi.gov/about-us/lab/forensic-science-communications/fsc/july2000/ deedrick.htm

FBI document on “Hairs, Fibers, Crime, and Evidence (Part 2)”: http://www.fbi.gov/about-us/lab/forensic-science-communications/fsc/july2000/ deedric3.htm

An article on fiber analysis in the “Atlanta Child Murders”: https://www.fbi.gov/news/stories/-serial-killers-part-5-wayne-williams-and-the-atlanta -child-murders

http://www.washingtonpost.com/local/crime/justice-dept-fbi-to-review-use-of-forensic-evidence-in-thousands-of-cases/2012/07/10/gJQAT6DlbW_story.html

http://www.fbi.gov/about-us/lab/forensic-science-communications/fsc/april2005/standards/2005_04_standards02.htm

http://www.forensicsciencesimplified.org/trace

http://www.fbi.gov/about-us/lab/forensic-science-communications/fsc/july2000/deedrick.htm

http://www.fbi.gov/about-us/lab/forensic-science-communications/fsc/july2000/deedric3.htm/

https://www.fbi.gov/news/stories/-serial-killers-part-5-wayne-williams-and-the-atlanta-child-murders

Conclusion

FBI “Forensic Paint Analysis and Comparison Guidelines”: http://www.fbi.gov/about-us/lab/forensic-science-communications/fsc/july1999/ painta.htm

A video about trace evidence: https://video.nationalgeographic.com/tv/the-great-american-manhunt/ trace-evidence?_ga=2.213974996.295395316.1528490049-342747364.1528490049

Scientific Working Group for Materials Analysis documents for forensic trace examination and training: http://www.asteetrace.org (click on Resources, then SWGMAT)

http://www.fbi.gov/about-us/lab/forensic-science-communications/fsc/july1999/painta.htm

https://video.nationalgeographic.com/tv/the-great-american-manhunt/trace-evidence?_ga=2.213974996.295395316.1528490049-342747364.1528490049

http://www.asteetrace.org

_GoBack