Unlike in thin-layer chromatography, the mobile phase is an inert gas (for example helium), or an unreactive gas (for example nitrogen). The stationary phase is a liquid coating the walls of the column. The stationary phase is chosen to be as close to the polarity of the solute as possible.
The gas chromatograph uses a column (flow through narrow tube). The different constituent chemicals from the analyte are carried up the column by the mobile phase at different rates. The rate is dependant on the chemical and physical properties of the chemicals with the stationary phase. This causes each constituent to exit the column at a different time.
You may be wondering why such complex, and often expensive techniques need to be employed, well fear not I hear your worries and I'm going to illustrate the value of gas chromatography, thin-layer chromatography and mass spectrometry with a couple of case studies.
1. FBI's analysis of EDTA in blood at O.J. Simpson trial:
Agent Martz and the FBI forensics team used a combination of the aforementioned techniques to detect the presence (or in this case absence) of EDTA in a blood sample from socks and a watch. EDTA is a preservative for blood. If found in high levels it would indicate that the blood samples had been tampered with. The high level of precision, and ability to remove background peaks, of the mass spectrometer meant that this case was where it really established its role as a reliable form of evidence.
2. Analysis of Fire Debris for ignitable liquids:
In January 2002 there is a fire. One person dies in the fire, and the police decide to bring in a forensics team to investigate. The result is a piece of vital evidence, showing the fire could not have been an accident. The forensics team, using Gas-Liquid Chromatography and Mass Spectrometry find gasoline present on weathered debris. The presence of ignitable liquids (where there should be none) is fairly damning.
Anyway thank you for reading, until next time people.
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