Instrumentation for Gas Chromatography - Forensic Applications of Gas Chromatography - page 7

Figure 2.3 An example of an in-line trap to remove moisture and oxygen.
by the GC; impurities within the gas supply will appear as either unwanted
peaks or peak deterioration over time within the chromatogram. Therefore,
it is important to use carrier gases with high purity (e.g., 99.9995% purity).
Typical impurities that can occur in the carrier gas are oxygen, water, and
hydrocarbons. However, purchasing a high-purity carrier gas is not the
end of the story. It is possible for impurities (principally oxygen and water)
to become entailed with the carrier gas stream downstream of the supply
(cylinder or generator) due to minuscule leakages in the connector fittings.
One way to reduce their input into the carrier stream is to introduce a trap
in-line between the carrier gas source and the sample introduction system
(Figure 2.3). Typically, a trap is added in-line that has the following sequence:
a molecular sieve (to remove moisture), hydrocarbon trap (removes hydro-
carbons and prevents contamination of the oxygen trap) and an oxygen
scrubber (to remove oxygen).
When installing a trap it should be positioned vertically to prevent chan-
nelling; channelling occurs as a result of the settling of the material within
the trap, leading to the potential for less interaction between the carrier gas
and the trap material.
2.2.2 Electronic Pressure Control Devices
The use of electronic pressure control (EPC) devices incorporating mass
flow controllers maintains a steady flow of carrier gas through the GC. The
use of the EPC acts to minimise or reduce pressure surges as a result of the
sample introduction process (see Section 2.3) that would lead to chromato-
gram baseline disturbances and drift (see Chapter 6). The use of an EPC also
compensates for viscosity changes in the carrier gas resulting from the use of
temperature programming in the separation process (see Section 2.4).
2.2.3 Gas Cylinders or Generators
Traditionally, the use of gas cylinders as the source of the carrier gas (and fuel
gas) was common. However, having multiple high-pressure (e.g., 2000-3000
psig) cylinders in the laboratory environment (albeit chained to a bench or
wall) raises significant potential safety issues. In most cases, therefore, the use