Chemistry 439

GC - Inside and Out

Objectives

    1. Learn to install a new GC Column
    2. Learn how to condition a new GC column
    3. Learn to optimize parameters to achieve optimal separation
    4. Learn to create a new method for analysis
    5. Learn to analyze and compare data produced
Gas chromatography is one of the most powerful separation methods used by chemists. Forensic scientists use GC regularly and rely on the data obtained as a definitive answer as to whether a substance is present or not. Therefore, it is important for you to be able to take apart and rebuild a GC as well as to determine the optimal operational parameters for the instrument you are using to analyze your data.

Part 1
You are preparing to run evidence for a very important case that your supervisor has just given you. It is an honor to be in charge of the evidence at this stage of your career, so you must ensure that you handle the samples with care and that the GC you are using to analyze the samples is running superbly. To make sure that the results come out as accurately as possible, you must change the column in the GC. While this will ensure a good separation you also know that the column must now be conditioned and tested before it is ready to run any unknown samples. Using the instrument manuals for instructions, change the column and condition it before making any injections. The actual unknown samples will be analyzed in the arson lab.

Part 2
Once the new column has been fully installed and conditioned it must be tested to ensure that no leaks are detected. Once you are sure that the column is ready for action, standards must be run to make sure that the column is separating properly. You want to achieve the best separation possible so you will also want to experiment with different separation parameters such as flow and carrier gas pressure. A good range for flow rate is between 1.0-6.5 mL/min and the carrier gas pressure should not be below 40 psi. The temperatures should also be adjusted; this includes the initial temps of the injector, column and detector, as well as the temperature gradient and the final temperature of the column. The initial temperature of the injector should be around 200¡C, column 100¡C, oven 260¡C. A typical temperature gradient is 5¡C/min. The temperature of the detector should remain around 260¡C. Varying these parameters will help you know the optimal operation parameters for the GC you are going to run your evidence on. The main objective of optimizing these parameters is to decrease the run time, but to still obtain baseline resolution for all peaks. Using Kovat's Indicies is an excellent way to test the separation ability of your instrument, as well as other volatile standards under the various configurations.

Report
You know that your supervisor wants to be continually updated of your progress in this case. Plus, you want to impress him with your thoroughness and expert analytical abilities. So, after you complete each part you write a full lab report. In part one, you tell your supervisor how you skillfully changed and conditioned the column, making sure to note any problems that you might have encountered. Part two, you report all of the standards and tests that you ran as well as what parameters and method you feel work best and those that didn't work so well, and of course you support your decisions with extensive data.