GC033 Purity of the carrier gas

The purity of the gas is very important in GC. Impurities such as oxygen and water can chemically attack the stationary phase of the columns and destroy it. Polyester, polyglycol and polyamide phases are particularly susceptible. Water impurities can lead to desorption of other column contaminants, producing high detector background or “ghost peaks”. Trace amounts of hydrocarbons can cause high background or noise signal in most ionization detectors and thus worsen their detection limits.

Several ways to obtain the gas with the desired purities are possible. One of them is to purchase ultrahigh purity gas cylinders, which is very expensive. Another possibility is to use gas generators, especially for hydrogen and air. This possibility is economically feasible, but require and initial high investment and maintenance. The more common practice is to purchase high-purity gases and further purify them. Water and trace hydrocarbons can be removed by using a 5 Armstrong molecular sieve filter before the instrument. Drying tubes are commercially available or can be readily made by filling a 6‐ft by 1/4‐in. column with GC grade 5‐Å molecular sieve. After a couple of gas cylinders, the molecular sieves should be regenerated by heating to 300 ºC for 3 h with a slow flow of dry nitrogen.

Oxygen is harder to remove. Its elimination requires of special adsorbents. Scrubbers for removing oxygen and water are available from most GC supply houses.