An overview of the chemistry, manufacture, environmental fate and detection of carbofuran - Carbofuran and Wildlife Poisoning: Global Perspectives and Forensic Approaches - page 11

technique do not generally require any other pre-treatment or modifi cation. Since carbofuran is ther-
mally labile (i.e., sensitive to heat), any residues present in a sample could pyrolysise (i.e., ther-
mally decompose) when injected into instruments that utilise heat during the analytical process,
for example a GC. More specifi cally, the carbamate group (see Figure 1.1) can degrade within
the injection port, which is often set from 200 to 350°C. HPLC systems do not use heat at the sample
injection point and column heating temperatures are generally low, perhaps up to 40ºC. Hence,
HPLC is generally considered the preferred option for isolating carbofuran and its metabolites or to
differentiate it from related compounds.
1.5.1.2 Principles of gas chromatography with mass spectrometry
As is the case with HPLC, GC also utilises a mobile (gas) and a stationary (solid/liquid) phase.
However GC/MS systems can provide higher selectivity than other GC detectors, and can be used
to positively confi rm the identity of an analyte in a single 'determination' step. This is because GC/
MS systems have very well developed/extensive mass spectral 'libraries' that can be extremely use-
ful for identifi cation and characterisation of unknown compounds. GC does however rely on the
compound of interest being volatile at up to 300ºC, which is an important limitation/consideration.
As such, certain thermally sensitive analytes may fi rst require derivatisation in order to be detected
by GC/MS. Derivatisation is the chemical modifi cation of the analyte(s) to improve detection and/
or separation (Harris 2007). A chemical agent is used to react with the compound of interest to form
a product that is more thermally stable and often more volatile than the original compound (Maurer
1999; Park, Pyo and Kim 1999). This ensures that the compound/sample can be detected using
GC-based techniques; the more volatile the compound, the better it is able to move through the GC
column and into the detector.
The analyte (or sample mixture) is injected in solution and rapidly vaporised at the injection
port. The latter is the fi rst point of contact between the sample and the column, usually a fi ne coil of
silica capillary tubing, often several metres (e.g., 15 to 30) long, which is held within its own tightly
controlled heating compartment. The sample is heated in the injection port and swept through the
column held within the oven compartment by a carrier gas (the mobile phase), which is usually
helium (Grob and Barry 1995). The column is maintained at either a fi xed temperature, or, a series
of increasing temperatures can be applied. Depending on the analyte(s) of interest, and the thermal
stability of the GC column, the column temperature can be as high as 350ºC.
The eluted compound molecules are bombarded with electrons at a kinetic energy of 70 eV (elec-
tron ionisation, EI). Because the electron kinetic energy of 70 eV is much greater than the ionisation
energy of the molecules, impact with the high-energy electron stream can remove the electron from
the compound of interest with the lowest ionisation energy (Harris 2007). The resulting ion, which
then has one unpaired electron, is referred to as the molecular ion, and is denoted as [M +. ]. This ion
generally has so much extra internal energy that it readily breaks into fragments (Harris 2007). Since
these fragments, often referred to as m/z ion fragments, are produced with predictable frequency
from any one compound, the combination of fragments (and their masses), and the proportion in
which they are produced, can be used as a highly compound-specifi c way to identify the presence
of a specifi c analyte.
A chromatogram (refer to Figure 6.5, Chapter 6) and mass spectrum (see Figures 1.15 and 1.16)
are generated as these fragments are detected. These analytical instruments tend to have several
analytical modes. In 'scan' mode, the detector will detect, for example, all masses with an m/z value
(i.e., the mass to charge ratio) between 100 and 500. The instrument can then create a spectrum, or
graph, whereby the x-axis of the mass spectrum corresponds to the m/z value of the ion fragment,
whilst the y-axis corresponds to its (relative) abundance (refer to Figures 1.15 and 1.16). A low
abundance of parent ion mass [M +. ] is often observed in GC/MS because the parent ions readily tend
to break apart. The most abundant fragment is commonly known as the 'base peak'.