Developments in Gas Chromatography - Forensic Applications of Gas Chromatography - page 78

eophylline/
Paraxanthine

180

68

Abundance
18000

123

m/z 180

53

95

151

eobromine

180

m/z 194

55

67

82 109

123 137

10.0 11.0 12.0

Retention Time (minutes)

13.0 14.0

Caffeine

194

109

55 67

82

42
40 60 80 100 120 140 160 180 200

74 165

Figure 7.8 Extracting xanthines from urine by MEPS. (Reprinted with permis-
sion from SGE Analytical Science.)

7.3 Developments in Column Technology

Column manufacturers are always producing modifications to capillary GC
stationary phases and these developments can be considered as part of the
normal evolution of the technique. This section proposes to look at three spe-
cific developments only: fast GC, two-dimensional GC and the use of ionic
liquid GC columns.

7.3.1 Fast GC
While fast GC has been included in this section under column technology, it
is essential to appreciate that other instrumental developments were required
in order to allow its successful application. These GC instrumental devel-
opments include rapid automated injection, high head pressures and split
flows, accelerated oven temperature ramp rates and fast detection acquisi-
tion rates. In terms of column development, fast GC uses shorter columns
(traditional 30 m column lengths are reduced to column lengths in the range
of 5 to 20 m) and considerably narrower internal diameters (traditional 0.25
to 0.53 mm internal diameter columns are reduced to 0.05 to 0.18 mm inter-
nal diameters) with a stationary phase of choice. In addition, hydrogen is
the preferred carrier gas (due to its high diffusivity and high optimal linear
velocity; see Section 2.2). Figure 7.9 shows the advantage of increased col-
umn efficiency (see Section 3.2.2) obtainable as the column internal diameter