lab repot
jasmin-99
ftir_v3_1.docxUsing FTIR-ATR Spectroscopy to quantify the vinyl acetate component of EVA copolymer from a glue stick by the Internal
Standard Method
Objectives:
(1) The principle behind attenuated total reflection (ATR) IR spectroscopy
(2) Get familiarized with the internal standard method
Principles
In attenuated total reflectance (ATR) IR spectroscopy, a special accessory is placed in the sample compartment of FTIR spectrometer. The ATR accessory consists a high refractive index IR-transparent crystal that is arranged in such a way that the IR beam is subject to total internal reflection. There are two variants of ATR accessory, single-bounce (one internal reflection shown in Fig. 1) and multiple-bounce (also known as multiple total internal reflectance or MTIR) configurations. Internal reflection results in a special standing electromagnetic wave (an evanescent wave) which propagates a short distance (~1 µm) away from the surface. The attenuation of the evanescent wave caused by absorption of sample materials leads to a corresponding reduction in the intensity of the main beam. This allows the IR spectrum of material in close contact with the surface of the crystal to be obtained.
Figure 1. Schematic of single reflection ATR system
The method of internal standards is used to improve the precision of quantitative analysis. In the method, an internal standard is added at a known concentration of a substance to each sample to be analyzed. In principle, the internal standard added or present needs to behave similarly to the analyte yet to provide a signal that can be distinguished from that of the analyte. Ideally, any factor that affects the analyte signal will also affect the signal of the internal standard to the same degree. Thus, the ratio of the two signals will has less variability than the analyte signal itself.
The method is often used in quantitative analysis by FTIR spectroscopy, chromatography, mass spectroscopy and atomic emission spectroscopy. They can also be used to correct for variability due to analyte loss in sample storage and treatment, or inconsistency in sample loading. In ATR FTIR spectroscopy, the difference in sample physical states and the contact of the sample with the ATR crystal poses a great challenge to the acquisition of reproducible signal intensity from one sample preparation to another. The use of internal standard method alleviates the challenge.
The calibration curve can be constructed according to the following equation:
In this experiment, we analyze the polyvinyl acetate concentration referring to polyethylene the height of the C-O peak at 1020 cm-1 due to the presence of the vinyl acetate copolymer relative to the height of the C-H peak at 720 cm-1 due to the presence of polyethylene.
Instrumentation:
A Thermo Nicolet 6700 FT-IR equipped with an Smart Orbit ATR accessory. The ATR is based on a diamond platform.
Materials:
(A) Four standards, 12, 18, 25 and 40% poly(ethylene-co-vinyl acetate) in pellet form.
(B) The unknown, an EVA based glue stick.
CAUTION : Do not use your bare fingers to remove a pellet from the bulk standards. The peak at 720 cm-1 corresponds to C-H bond, and if there’s any oils or dirt from your fingers, it will contaminate the standards, or your results. If you touch the sampling area of the ATR accessory with bare fingers, please inform GA, they will clean it with ethanol and a kimwipe. Do not oven tighten or over loosen the ATR accessory, it will damage it.
Suggested Procedures
1) GA will fill the FT-IR with liquid nitrogen. ¾ of the green dewer in enough to fill the FT-IR for 12 hours.
2) Allow detector to cool for 30 minutes.
3) Turn on the FT-IR by switching the black power box “ON,” it is located behind the FT-RAMAN.
4) On the desktop click the icon “OMNIC”
5) It will say “communication failure” click “ok”. This is because the microscope and Ramon are turned off.
6) Click “Expt Set”
7) It will display the collect tab, change the following settings
a. No. of scans : 60 (if time permits 250)
b. Resolution: 6
c. Final format: Absorbance
d. Correction: None
e. Click “Collect background after” and change the time to “100” minutes
8) Next to the collect tab, there is a tab called “bench” click it, and change the following settings
a. Sample of compartment: Main
b. Detector: MCT/A
c. Beamsplitter: XT-KBr
d. Source: IR
e. Accessory: Smart Orbit
f. Window: Diamond
g. Max range limit: 4000
h. Min range limit: 650
i. Gain: Autogain
j. Optical Velocity: 0.9494
k. Aperture: 50
9) Press “Ok”
10) Frist we compensate for the background. Press “col bkg”. And a window will appear, press “Ok”
11) After the background is collected a new window will appear, press “No.”
12) Now its time to measure your samples, press “Col Smp.” Enter sample’s name and press “Ok”
13) The screen below will pop up, do not press OK till you loaded the sample
a. LIGHTLY push the “press” to the right
b. In the middle of the metal disk there’s a diamond (looks like glass), which has a slit in the middle. Place your pellet or unknown on top of the spit.
c. If the press is too low, that when you push it LIGHTLY back it knocks the pellet off, turn the big knob counter clockwise (yellow dot) until its right above the pellet. DO NOT JUST KEEP ON TURNING IT, DAMAGE WILL OCCUR TO THE PRESS.
d. Once the press is on top of the bead, turn the big knob clockwise (blue dot) till your sample is lightly pressured between the diamond and press. DO NOT KEEP ON TURNING IT; IT ONLY NEEDS A LITTLE BIT OF PRESSURE. IF YOU OVER TIGHEN IT, IT MIGHT CRACK THE DIMOND OR DAMAGE THE PRESS.
14) Now press “Ok” on the window that popped up in Step 12
15) After collection a window will appear, press “yes”
16) It will open a new window with all the spectrum(s) you have collected, do not click anything, just save your data (save your data after every run)
a. go to File -> Save As -> Desktop -> Chem 462 Spring 2014 -> Section X -> Group X -> change the name -> SAVE. Where X represents your respective section and group number.
17) Do not close the spectra you produced just repeat 12-16 for all four standards and for the unknown 3 times. Every time you do the unknown change it position.
18) Data collect
a. After all the spectrums are collected, it will be displayed as an overlap.
b. Press “Stack Spe.”
c. It will now display them as separate spectrums
d. Press “View,” then “Display Limit,” and change start to “1200”, and end to “650” and press “Ok”.
e. Press “View” again then “Full Scale”
f. On bottom left hand there is 6 circles. Press the 4th button which is called “Peak Height Tool”
g. Now click on the top of the peaks at “720 cm-1” or “1020-1”. On the bottom left right above the 6 circles a gray box will appear, it will display two heights, “height” and “uncorrected,” record the uncorrected height.
h. Repeat for each spectrum
i. Now click “stack spec” and take a picture (GA will assist, and will provide you a copy). Make sure to record what color is corresponding to what peak.
Questions:
Question 1. As noted in the introduction, ATR crystal poses a great challenge to the acquisition of reproducible signal intensity from one sample preparation to another. Discuss the reproducibility of the peak heights and compare that to that of the ratios for your unknown samples. Do you think that internal standard method is a good way to account for poor reproducibility?
Questions 2. What does the “FT” in FT-IR stand for? In 4 to 5 sentences explain how it works, and what it is used for. What two other commonly used instruments also use “FT.”
Question 3. Describe the major components of the interments. In class we discussed how noise could be reduced, what component of the FT-IR is used to reduce noise, and how does it work?
Question 4. You are measuring numerous amounts of samples; you notice that every sample you’ve tested displayed three unique peaks; two sharp peaks from 2300-2400, and a broad peak from 3000-3700. An example is show in the specta below. You think this is noise. What do theses peaks correspond too (Hint: found in air)? Describe two ways you can try to compensate for this problem.
Evanescent Wave
Sample
ATR Crystal
Reflected Beam
Incident Beam
Evanescent Wave
Sample
ATR Crystal
Reflected Beam
Incident Beam
÷
÷
ø
ö
ç
ç
è
æ
=
ernal
analyte
ernal
analyte
ion
Concentrat
ion
Concentrat
F
Signal
Signal
int
int
