Instrumentation for Gas Chromatography - Forensic Applications of Gas Chromatography - page 8

of a generator is preferred and often adopted within laboratories. Typically,
a generator could be located within the laboratory or an adjoining location
(i.e., a separate room) and connected to a GC or a series of GCs effectively
and efficiently. Gas generators are available for nitrogen, hydrogen and air.
Attached to the gas cylinder (or generator) is a regulator that controls the
pressure of the gas released to the GC as well as indicating the amount of gas
left in the case of the cylinder.

2.3 Sample Introduction

Introducing a sample (or calibration standard) into a GC requires some prior
preparation. Extensive examples of the processes involved are described in
Chapter 8 with a specific focus on forensic analysis. In general terms, how-
ever, the most common method of sample introduction is the split/splitless
injector, which relies on the use of a (precision-made) syringe (Figure 2.4).
Typically, the syringe will deliver precisely 1 μL of sample (or calibration
standard) into the GC sample introduction system.
The syringe can be operated manually either by the scientist injecting the
sample into the GC or by an autosampler (Figure 2.5) in which the syringe is
located. The use of an autosampler is a more robust approach to reproducibly
inject samples into a GC system. Nevertheless, scientists can reproducibly
inject samples (calibration standards) provided they are meticulous and dili-
gent in their use of the syringe. It is typical when using this mode of sample
introduction that an internal standard is added to the sample or standard to
allow for any inconsistency in the operation of the syringe by the scientist or
autosampler. In reality, in a forensic laboratory manual injection would rarely
be used. The added advantage of using the autosampler is that the forensic
scientist is free to perform other tasks while the samples are being analysed.
An important component within the GC sample introduction system is
the injection port. The injection port is (a) heated independently of the GC

PTFE
seal

Steel
liner

Sample

Needle

Fine steel
wire plunger

Plunger

Glass graduated
barrel

Steel locking
nut

Figure 2.4 Syringe for sample introduction.