What is Gas chromatography?
Gas chromatography (GC) is a widely-use and very sensitive chemical analysis technique, particularly suitable for samples that are volatile (easily turned into gases), and that are stable when heated. Examples are residual solvent analysis, blood alcohols, metabolic fatty acids and drugs of abuse analysis.
Use of Zero Air in a GC instrument
The most common usage of Zero Air in GC is to provide oxidant gas for detection. For example, the most common Flame Ionization Detectors (FID) measure the electrical conductivity of a very clean hydrogen / zero air flame to measure the presence of hydrocarbons in the sample.
As a general-purpose hydrocarbon detector, good performance depends on the absence of residual hydrocarbon from sources other than the sample, such as the burner air supply. For this reason zero air is essential for sensitive and reproducible GC-FID analysis.
How does it work?
Gas chromatography works by separating mixtures of compounds in a column (normally a long, hollow coated glass tube with a narrow internal diameter). The column is internally-coated with a substrate (the stationary phase), and through which a gas (the mobile phase) containing the sample mix is passed.
The differential interaction of molecules between the mobile (gas) and stationary phases allows separation of mixtures of compounds based on physical and chemical properties. This causes different compounds to travel through the column at difference velocities and allows mixtures to be fully or partially separated into the constituent parts. The column is suspended in a precisely temperature controlled oven, to allow the balance between run-time and separation to be optimized.
The sample is converted into a gas and introduced into the carrier gas by injecting liquid mixtures into the heated inlet, and hence into the column. On exiting (or eluting) from the column, detection of compounds takes place by suitable detection methods such as flame ionization detection, thermal conductivity, or even mass spectrometry (GC-MS).
Many GC detectors require a source or Zero Air. This may be supplied by a cylinder, or alternatively from a compressed air source equipped with a Zero Air generator.
The key advantage of Zero Air generators over cylinders are: