forensic toxicology - analytical techniques - gas chromatography

1. Explain how GC uses the two phases of chromatography. 2. For what types of sample is GC analysis most suited? 3. What is the function of the inlet of the GC? 4. What factor is most important in selection of a column for GC analysis? 5. What factor is most important in sample preparation for GC analysis? How does derivatisation allow this to be achieved? 6. Explain how the three basic components of a mass spectrometer allow this type of analysis to be achieved.

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Last updated: August 13, 2023

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First submitted	August 13, 2023
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what is gas chromatography used for?	what is the mobile phase of gas chromatography?	what is the stationary phase of gas chromatography?	how does separation primarily occur in gas chromatography?
the analysis of compounds that can be vaporised (without decomposition) into a gas state	a carrier gas such as helium or hydrogen	a layer of liquid or polymer contained within a column made of glass or metal	on the basis of differences in boiling point
how must samples be prepared for gas chromatography?		how do we achieve this if the samples do not meet these requirements?
in a liquid or gas state		vaporisation
they need to be volatile at the temperature of the inlet (e.g. 300°C)		extraction into a volatile solvent (e.g. methanol)
		chemical treatment
what is derivatisation?		what does derivatisation allow us to do?
the addition of non-polar groups to a molecule to reduce its boiling point		use gas chromatography on molecules that would otherwise not be possible
what is QuEChERS, and what does it stand for?	what does it involve?	what is SPME?	what does it involve?
a multistep extraction process for sample preparation	initial extraction with solvent (acetonitrile)	solid-phase microextraction	a fibre coated with an extraction phase (e.g. liquid polymer or solid sorbent)
Quick
Easy	partitioning using a salt mixture		it is inserted into a liquid or gas sample & analytes are extracted onto the fibre
Cheap
Effective	cleaning using dispersive solid-phase extraction		it is then inserted into the injector of the gas chromatogram where the analytes are volatised
Rugged
Safe
what is the inlet of gas chromatography?	what are the most common type of inlet in gas chromatography?	how does it work?	which part of the inlet pushes all the sample into the column?
the syringe that injects the sample into the gas flow	split/splitless (S/SL) inlet	it injects the sample into a heated chamber	splitless (split mode is used to analyse only part of the sample (e.g. 10%))
		this causes
		volatisation of the sample
		then
		the carrier gas (e.g. Helium) sweeps the sample into the column
what is the most important parameter to consider in selection of the column for gas chromatography?	what will choosing the appropriate column give you?	what is the most common column chosen for gas chromatography?	what is the purpose of a column oven?
the polarity of the sample	increased separation and resolution	5% phenyl 95% methyl-polysiloxane (5ms column)	to maintain precise temperature control
what is the relationship between the rate at which the sample passes through the column and the temperature of the column?		how does gas chromatography use temperature to separate the sample?
proportional		isothermal analysis (temperature stays the same)
		using a temperature gradient (most common method)
what are the most common types of detector used in gas chromatography?		how do these work?
flame ionisation detector (FID)		electrodes are placed next to a hydrogen or air generated flame at the end of the column
		as organic compounds elute, they are pyrolysed (incinerated) in the flame, forming cations & electrons
		this generates a current between the electrode, that is detected
thermal conductivity detector (TCD)		analyte elution causes a drop in thermal conductivity, that can be detected
mass spectrometer		-
how do we perform data analysis?
using the peak area of a chromatogram