chemical engineering lap

Project Situation:

Harrington Heights Laboratories (HHL) has been hired by Organics R Us (ORU) to test incoming tankers of organic mixtures to verify their composition before accepting the shipment. This week there are samples from two tankers, but the paperwork has been mixed up. Both tankers have 3:2:1 mixtures by weight, but it is unclear to which components the 3:2:1 refers.

I. Introduction

The basis for gas chromatographic separation is the distribution of a sample between two phases. One of these phases is a stationary bed of large surface area, and the other phase is a gas which percolates through the stationary bed. The stationary phase is either solid or liquid, liquid being more versatile.

The basic parts of a gas chromatograph system are:

A. Carrier Gas: The function of the carrier gas is to provide transportation for the sample molecules as they travel from the inlet through the column to the detector. A pressure regulator is used to assure a uniform pressure to the column inlet, and thereby a constant gas flow rate. At a given temperature, this constant rate of flow will elute components at a characteristic time (The retention time). The carrier gas should be chemically inert, pure, readily available, inexpensive, and give little or no signal with the detector used. The most common carrier gases are helium, nitrogen, hydrogen, and argon.

B. Injection port (sample inlet): Gas, liquid or solid samples can be introduced into the system for analysis. Gas samples are introduced with the help of a gas-tight syringe or, preferably, a precision gas sampling valve. Liquid samples are usually introduced with help of a hypodermic syringe into a heated chamber where they instantaneously evaporate. Solid samples can also be introduced using a capsule type syringe.

C. Column: The heart of the gas chromatograph is the column in which the separation takes place. Packed columns are filled with porous particles consisting of either a solid adsorbent (adsorption columns) or a relatively inert support coated with the stationary liquid partition columns). The function of the stationary phase in the column is to hold the different components of the gas in the column varying lengths of time, depending on their chemical and physical properties.

D. Temperature Control: The temperature of the column should be uniform throughout because partition coefficients vary with temperature. Better separations are usually achieved at lower temperature. Faster separations require a high temperature.

E. Detector: The detector indicates the presence and measures the amount of component in the column effluent. The two most popular detectors are thermal conductivity cell (TC) and flame ionization detector (FID). The FID detector, which will be used in this lab, measures ions formed during combustion of organic compounds in a hydrogen flame.

II. Procedure

The gas chromatograph (GC) will be warmed up and ready to go when you arrive in lab. Do not adjust anything on the instrument. The lab assistants will give you specific instructions on how to run the GC. Results are captured by the computer software and should be recorded in your lab notebook.

You will be given a standard sample of known composition, with three components. You will also be given two “unknown” samples (from the ORU tanker shipments) which contain the same three components but with different mixture compositions.

First you will run the standard sample of known composition to collect data needed to calibrate the instrument response for each component. Do 6-8 repeat runs of the standard sample, with each group member taking a turn to inject the sample and record the data.

Repeat the procedure for the unknown samples, again with multiple runs for each sample.

III. Calculations

The recorder-integrator will determine the area under each peak and report those values on the printout. The relative areas of different peaks are a measure of the relative compositions of species in the sample mixture. To determine mixture compositions, we first run a standard (i.e. a mixture of known composition) through the GC and calculate a calibration or correction factor. One compound in the mixture is selected as a reference compound and is assigned a correction factor of 1. Correction factors for the other components are calculated using the following equation: