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Study Notes: The GC Mobile Phase (Carrier Gas)

In GC the mobile phase is commonly called the Carrier Gas. Carrier gases must be chemically inert and include the following.

  • Helium (He)
  • Nitrogen (N2)
  • Argon (Ar)
  • Hydrogen (H2).

The choice of the gas is often linked to the kind of detector chosen. The carrier gas supply is also associated with peripheral equipment such as:

  • pressure regulators
  • pressure gauges, and
  • flow meters.

Additionally, there are various filters and traps in the carrier gas system to remove water, oxygen, hydrocarbons and other impurities from the supply. Oxygen and moisture, for instance, can cause the stationary phase to degrade while hydrocarbons can lead to ‘ghost peaks’ to appear on chromatograms. Molecular sieve traps are often used to remove moisture. The purchased gas must be of very high purity and is usually obtained from a gas cylinder.

A pressure regulator on the gas cylinder as well as a pressure or flow regulator (controller) on the chromatograph controls the flow rate of the gas.

A schematic diagram of a GC showing how the components are connected.

Another example of how the components of a GC come together.

Inlet pressures are usually in the range from 10 - 50 psi (70 - 350 kPa), which lead to flow rates of 1 to 25 mL/min and higher depending upon the type of column being used.

The flow rate is often assumed to be constant if the inlet pressure is constant but flow rates should be measured on the end of the column by such devices as soap bubble flow meters. Good separations will depend upon maintenance of the appropriate flow rate throughout the run.

A picture of a bubble flow meter.

The carrier gas is of lesser importance than the stationary phase as the carrier gas does not interact with the analyte but should have the following characteristics.

  • Compatible with the stationary phase and the sample.
  • Allows good separation of the analytes by the column.
  • Is of the highest purity.
  • Must not contain particulate matter, water or other chemical constituents.
  • Is compatible with the detector used.
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