CHM 221-02, 03 – Gas Chromatography

Overview

Gas chromatography (GC) is an important aspect of chemical analysis of mixtures. The method allows separation of individual substances which have different degrees of attraction to the substrate of the packing material in the column, called the stationary phase. Depending on the detector, further analysis may be done on the identity of the substances. The retention time (time for the substance to completely pass through or elute from the column) is the variable obtained with any detector, and cal be used to compare to the retention time for compounds of known structure. A mass spectrometer (MS) is a common detection system in research labs. A mass spectrometer will provide specific structural information about the substance for further identification.

Method

The method allows analysis of very small amounts of material, typically a microliter (mL), even though the vial of the mixture contains far more. The sample is injected with a microsyringe, either manually or by an automated system containing a rack of sample vials.

A column must be chosen which will distinguish among the attractive forces among the possible substances in the mixture. These columns have a (proprietary) solid packing material, and the sample is injected, then vaporized into an inert gas stream (the mobile phase),  which passes through the column and then to the detector system. [NOTE: Column packing is chosen based on the recommendations of the manufacturer of that column packing for the substances of interest supplied by the lab analysts.] The stronger the interaction of the substance with the solid packing material (stationary phase), the longer the retention time for that substance.

The amount of a substance can be obtained by determining the area of (integrating ) the peak from the substance on the chromatogram.

Instructions –  Lab Report

1. Read the “Lab Handout” on Moodle provided for this reaction. [Note: There are some typos with “CG” instead of “GC” in several places!]
2. Your report will be to submit the answers to the “Lab Questions” , given below.

Lab Questions

NOTE: Use complete sentences in to be eligible for full credit, unless the question is presenting a calculation. Each question should have at least 3 sentences for a response, if an explanation or description is expected.

1. Complete the calculations for the % of each of components A and C in the chromatogram on p. 5 of the lab handout. The value for % B is already shown in the example on p. 4.

2. The analysis of the results of the chromatogram in Fig 4 is described on p.5 of the lab handout. Briefly summarize the assumptions and prior information needed in to identify the unknown, according to this description.

3. The following chromatogram shows the effect of changing temperature on the results of a chromatogram of the sample in each case:

Briefly explain why you think the retention times change as they do with temperature. What does this show about the experimental conditions needed for a good chromatographic separation and analysis?

4. The following chromatogram has included the results from the mass spectroscopy results (the mass spectrum). Infrared spectroscopy (IR) is another type of detector which is often used and which would provide additional, structural information about the substance eluting at each time (peak).

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If you did not have such additional detectors, but only had the retention time, what might you do in the lab to further characterize the mixture you are interested in? This would include taking advantage of the information from the retention times of known compounds, such as you see in Figure 4 of the lab handout.  Note: This is a general question about the approach you would use, not specifically  to the compounds in Figure 4.

Think of what we have studied so far this semester, and suggest two or three approaches you would try in to confirm the identity of each substance in the mixture. Assume you have enough sample to do macroscale analyses or procedures.

Note: This question requires at least 6 sentences for a response in to be eligible for full credit.