Impurities:

Impurities like as hydrocarbons, oxygen as well as water could and commonly will contribute to unwanted noise levels and perhaps baseline drifts. In further peaks are also seen in the chromatogram. Ideally, (ultra-pure gases (99.9999%) should be used. Simply obtainable gases at 99.995% purity level are commonly satisfactory. Commercial grade gases could be used only if special driers and scrubbers are used to decrease the impurities to a usable or satisfactory level. Water and trace hydrocarbons could be simply removed through installing a 5Aº molecular sieve filter among the gas cylinder and the instrument. After two gas cylinders have been used, the sieve should be regenerated through heating to 300°C for three hours along with a slow flow of nitrogen. Oxygen is harder to remove and needs a special filter, like as BTS catalyst from BASF. The gases which are generally used in GC as carrier gases involve hydrogen, helium, nitrogen and argon. Argon and nitrogen are inert and are simply obtainable at the purity levels needed and thus are the most popular carriers. A few detectors need specific gases and purity levels for optimum performance. For example

Detector                                             Carrier gas

Thermal conductivity                      Helium

Flame Ionization                              Helium or nitrogen

Electron capture                               Very dry nitrogen or argon, 5% methane

The solutes of interest in the sample might also dictate the choice of a particular carrier gas. For instance, in the analysis of hydrogen in air, nitrogen would be the best choice of carrier. Therefore, there is a requirement of flow controller.