Environmental Engineering Reference
In-Depth Information
Gas
purification
traps
Fixed
restrictors
Electrometer
Computer
Injection
port
Flow
controller
Regulator
Detector
Figure 10.3
Schematic diagram of a typical GC system (
#
Agilent Technologies, Inc. 2006,
Reproduced with permission, courtesy of Agilent Technologies, Inc.)
supply, (b) flow control, (c) sample introduction and splitter, (d) separation column,
(e) temperature control zones (ovens), (f) detector, and (g) data-acquisition system.
As can be seen from the graph, the analytes are carried by an inert carrier gas through
the injection port, the column, and the detector. The detector measures the quantities
of the analytes and generates an electrical signal. The signal then goes to a data
system/integrator that generates a chromatogram (signal vs. run time). The purpose
of each component, its positioning within the system, and its functioning are further
described below.
Carrier gases are the mobile phase in GC, and as the name implies, their
purpose is to carry analytes through the GC system. Helium and nitrogen are the
most common carrier gases, but other auxiliary gases, such as air and hydrogen
shown in Figure 10.3, may be used in certain detectors. Carrier gases should be
inert and high in purity, because impurities, such as oxygen and moisture, may
chemically attack the liquid stationary phase (polyester, polyglycol, and
polyamide columns are particularly susceptible). These impurities should be
removed using gas purification traps placed between the gas cylinder and the
instrument. The high pressure in the gas cylinder is also reduced to approximately
20-60 psig through a two-stage regulator before it is connected to the GC. The
flow is carefully controlled to ensure reproducible retention times and to minimize
detector drift and noise.
Samples can be introduced either by manual injection or by an autosampler. By
piercing a microsyringe through a septum (a polymeric silicone with high
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