48 Handbook of herbs and spices
MRMs require a great deal of time to perform, thereby reducing the number of
samples analysed and the speed of analysis. For example, certain foods, such as those
with high concentrations of fats and oils, are difficult to analyse in a timely manner.
Single residue methods (SRMs) are another category that depend on a number of
different techniques and vary widely in terms of reliability, efficiency, throughput
(samples per day), degree of validation, and practicality for regulatory use. Because
SRMs have been developed by the private sector for submission to EPA as part of the
tolerance setting process, a method exists for every pesticide with a tolerance. Most
SRMs, like MRMs, are based on GC using the full array of element specific detectors.
Although less efficient than MRMs, SRMs are necessary to monitor those pesticides
that cannot be detected by MRMs. SRMs are generally not considered adequate for
routine monitoring by the regulatory agencies, though FDA uses them. To monitor
one pesticide with an SRM is considered inefficient when an MRM can measure
many pesticides using the same resources. In addition, SRMs vary widely even for
chemicals of the same class, so a laboratory needs a wide array of glassware, evaporative
devices, chromatography, and detectors to use the SRMs available.
There is a third class of methods, namely the semi-quantitative and qualitative
methods, that range widely in their ability to quantify the chemical present in a sample.
Semi-quantitative methods indicate the range of pesticide residue concentration in a
sample, while qualitative methods show whether or not a particular pesticide exists
above detectable limits. These methods use technologies like thin layer chromatography
(TLC), enzyme inhibition, and immunoassay, all of which can be moved from the
laboratory into the field without losing their ability to detect pesticides. The enzyme
inhibition-based colour reactions make spots and bands of pesticide residues on thin
layer chromatographic plates visible, in order to measure the pesticide residue either
visually or with instruments. Such techniques are being used for cholinesterase-
inhibiting insecticides and photosynthesis-inhibiting herbicides. Because sophisticated
instrumentation is not required they are relatively inexpensive compared to quantitative
methods. The benefits of these methods are their low cost, speed, or ease of use and
more number of samples that could be analysed. Nevertheless, neither FDA nor FSIS
is currently using these methods for pesticides. A drawback of semi-quantitative
methods is that they do not provide the degree of accuracy necessary for enforcement
action, as in a court of law. Violations found by a semi-quantitative method would have
to be verified by a quantitative analytical method – or maybe two.
As techniques are improved by changes in instrument and hardware design, bringing
about more sensitive, selective, and reproducible devices, their costs usually increase,
particularly when automated sample handling and data manipulation are included.
These additional costs translate into higher costs to implement contemporary pesticide
methodologies for varied herbal samples. Supercritical fluid chromatography (SFC)
is a new technique of chromatographic separation used in the regulatory analysis of
pesticide residues in food. With super fluids as the solvent phase, SFC can chromatograph
chemicals that cannot be handled by gas chromatography because of their non-
volatility or thermal instability. Many detectors designed for GC can also be used in
SFC, such as the flame ionisation, the nitrogen-phosphorus, and the atomic emission
spectrometric as well as the UV absorbance detectors. New analytical methods are
needed to expand the range of pesticide analytes that can be detected in plant derived
food products like herbals in a more efficient process.
Some of these advanced technologies include gas or liquid chromatography/mass
spectrometry (or tandem mass spectrometry), solid phase extraction, laser-induced