Project: GC-MS
Introduction
Gas chromatography is a technique used to separate mixtures of gases. The gases are injected into a column, which (in our case) is a long, hollow tube with a non-polar liquid coating on the inside. They are pushed through the column with a carrier gas, He in our instrument. The column is initially held at low temperature (a little above room temperature) and ramped at a defined rate up to 250 degrees C. The combination of the temperature ramp and the non-polar column allows compounds to be separated based on boiling point and polarity. GC can be paired with many different techniques, and is often used with electron-ionization mass spectrometry. This type of mass spectrometer breaks molecules apart into fragments, sorts the fragments by mass, and counts the number of fragments of each mass. The specific spectrum (or distribution of fragments) is unique for every chemical compound and is based on the stability of each bond in the molecule. In this way, the identity of a compound can usually be deduced from a mass spectrum alone, although techniques such as nuclear magnetic resonance spectroscopy and Fourier-transform infrared spectroscopy can greatly aid in identification.
You will conduct liquid phase microextraction to analyze the volatile constituents of the powdered samples. You should identify the major flavor constituents and comment on whether or not they may be harmful at the approximate levels present.
The general order for GC-MS sampling is: (1) run solvent blanks on the GC-MS until the instrument is clean, (2) prepare a single sample using liquid-phase microextraction, (3) run the sample, and (4) repeat 2-4 as necessary. You should obtain a clean blank before preparing your samples. For GC-MS, a passing blank will have a baseline less than 50,000 counts that is stable and peak-free. You may see an increase in the baseline at temperature above 250$^{\circ}$C; this is normal and still considered passing. If the blank is dirty you will not be able to tell what is from your sample and what is contamination!
This is a qualitative analysis.
Understanding Electron Ionization Mass Spectrometry
Safety
- This lab uses small amounts of methanol (SDS). Methanol can cause blindness and absorbs through the skin. Wear gloves and goggles when working with methanol, and use it in a hood.
- Microliter syringes may be very sharp!
- Although samples are over-the-counter products, you should avoid ingesting and/or contacting them with your skin.