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…
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…