Method Development - Forensic Applications of Gas Chromatography - page 42

extraction (SPME), or automated thermal desorption (ATD). (See Section 8.3
for further information.)
Practical issues, such as the chemical nature of the analytes, the nature
of the matrix, and the concentration of the analytes, should all be consid-
ered. As has been previously mentioned in Chapter 2, split or splitless injec-
tion may be used depending upon the amount of analyte(s) present and the
method of sample introduction. The higher the sample ratio (i.e., 100:1 versus
20:1) the less sample will be introduced into the GC. If too much sample is
introduced to the instrument, this will result in poor peak shape and detec-
tor overload; not enough sample in the GC will result in poor sensitivity of
the method and may mean no detection (therefore, no peaks).

4.3 Influence of the Carrier Gas

Unlike with high performance liquid chromatography (HPLC), where the
mobile phase has a large influence over the separation, the choice of carrier
gas in GC does not influence the decision-making process in the same way.
The most commonly used carrier gases are nitrogen, helium and hydrogen:
All three are inert and will not react with the analytes but are used to carry
the analyte(s) through the instrument to the detector. In HPLC, the analytes
will partition themselves between the mobile and stationary phases depend-
ing upon their affinity for one or the other. In GC, the separation is based
on the boiling point(s) of the analyte(s) and the chemical nature of those
analyte(s). As has been previously explained (in Chapter 2) the decision of
which carrier gas to use ultimately depends on a compromise between cost
and appropriateness, which usually results in the use of nitrogen in the case
of GC-flame ionisation detector (FID) and the use of helium with GC-mass
spectrometry (MS).

4.4 Influence of the Column

In Chapter 2, the choice of GC column was explained considering param-
eters such as the stationary phase choice, the length of the column, the thick-
ness of the stationary phase and the internal diameter of the column. In most
cases, a compromise between peak shape, resolution and run time must be
achieved. For some forensic applications (e.g., the identification and quanti-
tation of diazepam and desmethyldiazepam), baseline resolution and good
peak shape are essential; however, this is not the case when carrying out the
identification of petrol in fire debris.