Developments in Gas Chromatography - Forensic Applications of Gas Chromatography - page 71

group. Addition of the trimethyl-silyl group is the most popular route for
silylation derivatisation.
Addition of the trimethyl-silyl group is accomplished by use of spe-
cific silylating reagents; these include N,O -bistrimethylsilyl-acetamide
(BSA), N,O -bis-trimethylsilyl-triluoroacetamide (BSTFA), N -methyl- N -
trimethylsilyl-trifluoroacetamide (MSTFA) and N -trimethylsilylimidazole
(TMSI). ( Note: Trimethylchlorosilane [TMCS] is often used as a catalyst to
increase the reactivity of the derivatising reagents. For example, it is typical
to use the combined BSTFA + 1% TMCS derivatising reagent; the addition
of TMCS is to ensure that difficult to derivatise samples are fully derivatised
prior to analysis by GC.) Addition of the t -butyldimethylsilyl group is done
using the derivatising reagent N -methyl- N -( t -butyldimethylsilyl)triluoro-
acetamide (MTBSTFA).
The resultant derivatised molecule is also less thermally labile, which
results in better resolution of analyte peaks. An example of its application is
the derivatisation of Δ 9 -tetrahydrocannabinol (Δ 9 -THC), the active compo-
nent of cannabis. In this case, silylation is used to derivatise an active hydro-
gen on Δ 9 -THC as shown in Scheme 7.1 using BSA as the derivatising reagent.

7.2.1.2 Acylation
As with silylation, acylation produces a resultant molecule that is more vola-
tile and less polar than the underivatised, or parent, analyte. This process of
acylation is affected by the reaction with acyl derivatives or acid anhydrides
(Figure 7.2).
Typical acid anhydride acylating agents include trifluoroacetic acid
(TFAA), pentafluoropropionic anhydride (PFPA), heptafluorobutyric anhy-
dride (HFBA) and heptafluorobutrylimidazole (HFBI). These reagents add
functional groups that are electron 'rich' (e.g., contain oxygen and fluorine);
therefore, they are sensitive to detection using the electron capture detec-
tor (ECD). Acylating reagents are very good at reacting with highly polar
functional groups that contain active hydrogens (e.g. -OH, -SH and -NH),
converting them into esters, thioesters and amines, respectively. As in the
silylation process, the resultant derivatised molecule is also less thermally
labile, which results in better resolution of analyte peaks. An example of its
application is the derivatisation of metoclopramide . In this case, acylation is
used to derivatise an active hydrogen on the amide group of metoclopramide,
as shown in Scheme 7.2 using HFBI as the derivatising reagent.

O O O

CF 3 C 2 F 5 C 3 F 7

Figure 7.2 Acylation using the acyl group.