Controlling pesticide and other residues in herbs and spices 47
(which do not fluoresce appreciably). For compounds having photo-ionisable functional
groups, the photoconductivity detector is especially advantageous over UV detectors.
It has been well studied and used by FDA and other laboratories for residue analysis.
The electrochemical detector is also under study for its potential to improve detection
of electro active functional groups.
The thin layer chromatography (TLC) technique is based on partitioning a pesticide
between a solvent and a thin layer of adsorbent, which is usually silica or alumina
oxide that has been physically bonded to a glass or plastic plate. Samples are applied,
dissolved in a solvent, as spots or bands at one edge of the plate and the plate is then
placed in a tank containing a solvent. The solvent migrates up the plate by capillary
action, taking the pesticide with it, and depositing it at a given distance on the plate.
The time required for TLC plate development ranges from a few minutes to several
hours depending on the pesticide, the solvent, and the adsorbent. Following complete
development, the plate is removed from the tank and the spots or bands left by the
migration of the solvent are detected using any one of several techniques available
such as visualisation under UV light, using reagents to produce colours resulting
from chemical reaction specific for the pesticide/reagent combination. Amounts of
pesticide can be judged semi-quantitatively by comparison with standards that are
developed on the same plate as the unknowns. As a separator technique, TLC is much
less efficient than either GC or HPLC because the resolution separated by TLC is
approximately less than one-tenth of that found using a packed GC column to produce
the same separation. Consequently, TLC as a separator technique has largely been
replaced by GC and HPLC. On the other hand, interest exists in using TLCS to
develop rapid, semi-quantitative methods.
For regulatory agencies like the FDA and the FSIS, the monitoring methods must
provide results in a cost-effective, timely, reliable, and verifiable manner. These
methods should also identify as many pesticides as possible in a range of food
commodities because these agencies are responsible for monitoring all foods for all
pesticides to keep the products containing higher levels from reaching the market.
Analytical methods must also be able to detect pesticides at or below tolerance
levels, and endure interfering compounds such as other pesticides, drugs, and naturally
occurring chemicals. They should be insensitive to such environmental variations as
humidity, temperature and solvent purity as well. There are different classes of methods
that are used by the regulatory bodies, each method selected based on the need of the
monitoring, type of sample, and sensitivity required. They are multi residue, single
residue and semi-quantitative methods.
Multi-residue methods (MRMs) are designed to identify a broad spectrum of
pesticides and their toxicologically significant metabolites simultaneously in a range
of foods, and mostly meet the method needs of regulatory agencies. They are sensitive,
precise, and accurate enough, and are economical or affordable. In addition, an MRM
may detect, but not measure, a particular pesticide or metabolite, and also record the
presence of unidentified chemicals, known as an unidentified analytical response
(UAR). MRMs involve steps of preparation, extraction, cleanup, chromatographic
separation, and detection. All MRMs used today in the USA are based upon either
gas chromatography (GC) or high-performance liquid chromatography (HPLC) as
the determinative step, while thin layer chromatography (TLC) is also used by several
agencies in Europe. The basic weakness of MRMs is that they cannot detect every
pesticide. For example, of the 316 pesticides with tolerances, only 163 could be
analysed with FDA’s five routinely used MRMs. Another weakness is that some