46 Handbook of herbs and spices
occur in analytical columns. Historically, five detectors have been used. They are the
electron capture detector (ECD), Hall micro-electrolytic conductivity detector (HECD),
thermionic detectors (NPD and AFID), and the flame photometric detector (FPD).
ECD measures the loss of detector electrical current produced by a sample component
containing electron-absorbing molecule(s). This detector is very sensitive for measuring
halogenated pesticides, in the analysis of chlorinated hydrocarbon pesticides
(organochlorines) such as aldrin, dieldrin and DDT. ECD is efficient for the analysis
of poly chlorinated biphenyls (PCBs) as well. The HECD can measure chlorine (and
other halogens), nitrogen, or sulphur. This detector is more selective than the ECD,
though the ECD is more sensitive. The Hall electrolytic conductivity detector also
has improved over the last few years, and has replaced the ECD in those laboratories
where extreme sensitivity is not required. Both the NPD and AFID measure the
presence of nitrogen and phosphorus atoms in the pesticide, with little response
resulting from other types of atoms in the molecules.
Today, the flame photometric detector (FPD) measures sulphur or phosphorus,
and is a rugged, highly stable, and very selective detector, since it does not detect
compounds other than organophosphates and those containing sulphur. The flame
photometric detector is less sensitive for phosphorus than the NPD and less sensitive
for sulphur than the Hall detector. However, it is useful for the analysis of unclean
crude herbal extracts. Conventional mass spectrometers (MS) have also been used by
some pesticide residue laboratories as gas chromatography detectors, and as high-
performance liquid chromatography detectors as well. MS is normally used when
special techniques are necessary to confirm the identity of a particular pesticide,
when conventional detectors cannot detect the pesticide. The use of MS is growing,
especially with the development of the more portable and less costly mass selective
detector (MSD). The MSD and ion trap detector (ITD) may become more routinely
used for pesticide residue analysis, as improvements in their computer software are
made and their scan parameters become more suitable for chromatography.
High-performance liquid chromatography (HPLC) for the analysis of pesticide
residues is a fairly recent technology, but it is becoming the second most frequently
used technique after GC. GC depends upon the volatilisation of the pesticide, whereas
HPLC is dependent on the stationary phases that can selectively retain any molecular
structure; polar, non-polar, ionic, or neutral. Separations can even occur as a function
of molecular size (gel permeation) or chemical derivatisations (synthesis of a chemical
derivative of the pesticide). HPLC is not as efficient as capillary gas chromatography
for separator purposes because the chromatographic peaks are broader, though HPLC
columns are more efficient than packed GC columns when columns of equal length
are considered. HPLC columns usually last longer because they are not subjected to
the extremely high temperatures that GC columns are. The HPLC detectors used for
pesticide residue analysis are the UV absorption, fluorometer, conductivity, and
electrochemical.
Many pesticides absorb UV light at the wavelength of mercury discharge (254
nanometres) and can be detected in very small quantities. Unfortunately, many food
co-extractives do so as well, making this detector nearly useless for trace analysis in
foods. An alternative is the variable wavelength detector, which can be tuned to a
wavelength that is absorbed by the pesticide but not by the food co-extractives. The
fluorometer is a highly sensitive HPLC detector for some pesticides, which is typically
used for pesticides with aromatic molecular structures such as alachlor or paraquat.
This detector, however, has limited application to the detection of most pesticides