An overview of the chemistry, manufacture, environmental fate and detection of carbofuran - Carbofuran and Wildlife Poisoning: Global Perspectives and Forensic Approaches - page 14

%
100

164

149

50

57

131
1 35

122
12 7

103

77
8 1

91

5 1

65 69

1 1 0 22 1

2 0 7

0

50 60 70 80 90 100 110 120 130 140 150 160 170 180 190 200 210 220 230 240 250 260 270 280 290 m/z

Figure 1.15 GC/MS EI full scan mass spectrum of carbofuran

165.1

100

222.1

%

123.0

244.1

0

m/z

100 110 120 130 140 150 160 170 180 190 200 210 220 230 240 250 260 270 280 290

Figure 1.16 LC/MS ESI mass spectrum of carbofuran (in positive ion mode)

The major differences between the mass spectra yielded by each technique (GC/MS with EI or
LC/MS with ESI) is due to the fact that ESI is a much 'softer' ionisation technique than EI (and APCI
in LC/MS), i.e., ionisation in ESI imparts less internal energy to the molecule and less fragmenta-
tion occurs. Hence, there is a much higher relative stability and abundance of the molecular parent
ion in ESI. Increased fragmentation can be generated in LC/MS by controlling what is known as
the 'fragmentor' voltage which is applied to the ion stream before the quadrupole, but after the ion
source. By adjusting this voltage within the method used, greater or lesser parent ion fragmenta-
tion can be achieved. This allows a single quadrupole instrument to begin to gather more detailed
structural information on the compounds being analysed but it falls short of the capability of a
highly selective triple-MS instrument which can select and fragment ions more than once (i.e., select
specifi c fragments and then capture/fragment and mass fi lter those further).

1.5.1.6 Thin layer chromatography (TLC)
Sometimes viewed as an 'old-fashioned' method, especially when compared to some of the ana-
lytical techniques described earlier, thin layer chromatography (TLC) can be used to determine the
number of components present in a given mixture, assess a substance's purity, and establish, in a
preliminary fashion, whether a compound is present in a sample and if further (i.e., more specifi c)
analyses are warranted. TLC is often used for screening because it is rapid, inexpensive and simple
to perform once one knows what to look for. Indeed, a TLC plate costs less than 1 USD and, once a
TLC chamber is set up, running an additional compound is only a matter of operator time, perhaps
10 minutes or so. By contrast, the cost of analysing a sample via GC/MS or LC/MS, for example, is