Carrier Gas & Flow Regulation | Gas Chromatography

The mobile phase is a gas (helium, nitrogen or hydrogen) which can be purchased from industrial sources or generated on-site in the case of nitrogen and hydrogen since the flows are relatively small (1 to 25 ml/min depending on column type).

The carrier gas should not contain traces of water or oxygen because these are very deleterious to the stationary phase. Typically, a filtering system containing a drying agent and a reducing agent is used between the gas source and the chromatograph.

In gas chromatography, the nature of the carrier gas does not significantly alter the partition coefficient K between the stationary and mobile phase. However, the viscosity of the carrier gas and its flow rate have an effect on the analyte dispersion in the column (Van Deemeter equation) thus affecting the efficiency and sensitivity of detection. The pressure at the head of the column (several tens to hundreds of kPa) is stabilised either mechanically or through the use of an electronic device ensuring that flow rate (linear velocity of the gas) in the column is at its optimal value. When a temperature programme is used during analysis, the viscosity of the mobile phase is increased thus increasing the resistance to carrier gas flow. It is, therefore, desirable to correct the pressure to compensate for this effect.

In a chromatogram contains a peak for a compound that is not retained on the stationary phase, it is possible to calculate the mean linear velocity ū of the carrier gas in the column. It is also possible to determine ū0 at the outlet of the instrument at atmospheric pressure p0 by putting a flow meter at the end of column. The ratio between these two velocities is equal to the compression factor J, which is related to the pressure differential between the inlet and outlet, P/P0 (P is the pressure at the head of the column).