Gas chromatography (GC) is a chromatography technique where the separation of individual components (analytes) from a sample relies on their differing distribution between a mobile and stationary phase. The mobile phase carries the analytes through the stationary phase. In GC, it’s an inert gas (usually helium or nitrogen). The gas must be inert, so it won’t react with the sample to give a false reading The stationary phase is a substance fixed in place to which the sample adsorbs because of the attractive forces that exist between molecules (intermolecular forces).
In GC, it’s a thin layer of a high boiling point (bpt), non-volatile liquid or polymer adsorbed onto an inert solid support or silica particles. GC requires compounds to be in a gas or vapour state limiting GC to gases or substances that vaporise without decomposing when heated. The sample is introduced into the carrier gas stream by injecting it through a seal using a syringe (A). The injection chamber’s high temperature vaporises the sample which then flows along the gas stream into the chromatography column (B). The column is inside an oven where the temperature can be maintained.
When the sample elutes (leaves the column), it reaches a detector (C). The detector sends a signal to the chart recorder which creates a chromatogram (a graph that displays the data obtained by GC) (D). The area of the peaks on the chromatogram is proportional to the concentration of analytes in the sample, so concentrations can be determined, making GC a quantitative method. The time elapsed between the injection and elute is unique for the substance and is called the retention time (Rt), and can be used to identify the component.
A reference sample is analysed and Rt values are compared to confirm the identity of a compound and to prepare a calibration curve for calculations of amounts, making GC a qualitative method. GC is typically coupled with a mass spectrometer (MS) to obtain more accurate identification of analytes. The chemical basis for GC is that different bpt of analytes causes them to reach the detector at different times due to different interactions between mobile and stationary phases. This is due to intermolecular forces (IMF), molecular weight, and the polar/non-polar nature of molecules.
Bpt is the amount of energy required to overcome the IMF and turn a liquid into a gas. Stronger IMF means higher bpt. The three IMF are dispersion forces, dipole-dipole forces and hydrogen bonds. Dispersion forces arise from the random motions of electrons in all molecules. Fluctuations in electron density causes a temporary partial negative charge (dipole) and can repel electrons from a nearby molecule, inducing a partial positive charge. The two dipoles are electrostatically attracted to each other, but are temporary and are the weakest of the IMF.
Dipole-dipole forces is when a molecule has a permanent partial charge on its poles. When an atom covalently bonds to another atom with a different electronegativity, it causes electrons to spend more time with the more electronegative atom. These are stronger than dispersion forces because of permanent dipoles. This asymmetrical distribution of charges in the molecule makes the molecule polar. Hydrogen bonding is an extreme form of dipole-dipole forces where hydrogen covalently bonds to a highly electronegative atom (nitrogen, oxygen, or fluorine).
This creates a higher partial charge than in dipole-dipole forces. Molecules with high molecular weights will have more electrons meaning stronger dispersion forces. Polar molecules will generally adhere to a surface better than non-polar molecules because of the permanent partial charges that exist. If the stationary phase is polar, then polar analytes in the sample will be more likely to adhere to the stationary phase and thus elute slower than non-polar components, which will be more likely to stay in the mobile phase (gas stream).
The more a molecule adheres to the stationary phase (the stronger its IMF) the slower it elutes (a higher Rt value) and vice versa. In forensics, GC (typically coupled with MS) is used to identify and quantify the presence of drugs that may be used for/against a person in a court of law. Once a sample is obtained, it undergoes a screening test, and then a confirmatory test. The screening test uses thin layer chromatography (TLC) to detect substances in the sample that match other substances being used as a control.
TLC can suggest the presence of a substance but it’s not a definitive enough method to be certain. The sample then undergoes a confirmatory test. For example, a substance suspected to be cocaine is weighed and then dissolved into a solvent, such as methanol. If a sample won’t readily dissolve in available solvents, then it will be heated so the components can vaporise This technique is called pyrolysis. A syringe is used to put the solution into a GC machine. A detector will measure the Rt value. If the Rt value matches that of a known sample of cocaine, then it is very likely that the sample has cocaine.
GC is used over other techniques such as TLC or high performance liquid chromatography (HPLC) because: • It’s more sensitive and more accurate when detecting volatile compounds so not require much sample is needed (ug/uL can be used and can measure in ppm and ppb). In forensics, it may not be possible to obtain a large sample. • GC has a higher efficiency (resolves more compounds per unit of time) than HPLC • It’s easier to operate than HPLC so highly skilled personnel are not required, making it suitable for routine forensic analysis