Controlling pesticide and other residues in herbs and spices 45
The first step in analysing a spice/herb sample is to chop and grind the sample.
The samples must be handled in such a way as to avoid loss of volatile pesticide
residues and to prevent contamination of the sample with other pesticides or interfering
chemicals. Chopping or grinding followed by blending and mixing are manipulations
designed to produce a homogeneous composite sample from which sub-samples can
be taken, and to disrupt the gross structural components of the sample to facilitate
extracting pesticides from the sample. Once the sample is prepared, extraction is
performed with a solvent to remove the pesticide residue of interest from other
components of the sample. In most analytical laboratories, a solvent such as acetone
or acetonitrile is used to extract pesticides from 250 grams or less of the spice/herb
to be analysed. The solvent is blended with the sample, and smaller amounts are
further homogenised using an ultrasound generator. Salts, such as sodium chloride or
sodium sulphate, can be added to absorb water. Or, additional water is added, so that
the resulting aqueous solution can be partitioned with a water-immiscible solvent in
a subsequent cleanup step.
Extraction times vary from a few minutes to several hours, depending on the
pesticide to be analysed and the sample type. After putting the sample through an
alumina packed column, solvent is added to elute the pesticides off of the packing in
the column. The cleanup step is often a limitation in pesticide residue methods
because it generally consumes a large amount of the total analysis time and restricts
the number of pesticides that are recovered in some cases, as a result of losses in
chromatography, partitioning, and other cleanup steps. Problems that occur during
the extraction process include incomplete recovery and emulsion formation. Incomplete
recovery generally can be remedied by selecting a more efficient solvent. Emulsions
are the production of a third phase or solvent layer that confuses the partitioning
process. They can usually be broken down by adding salt to the sample/solvent
combination.
Super-critical fluids (SCFs) provide a new technique for extracting pesticides.
They are fluids that are more dense than gases but less dense than liquids. They are
not yet used in regulatory methods to analyse pesticide residues in food, but are
gaining favour for their ability to extract a wide variety of chemicals from many
sample types. Solid phase extraction (SPE – also known as accumulator or concentrator
columns) is another technique that can speed up cleanup as well as extraction. The
SPE packing materials or cartridges retain the pesticide. These cartridges also have
the advantages of batch sample processing capabilities, small size, adaptability to
robotic technology, low cost, and ready availability from many sources. SPEs have
the disadvantages of being unproven for many pesticides, inability to handle large
sample sizes, and generally ineffective for extracting water soluble pesticides and
metabolites. SPE is being used by industry and private laboratories, but is not yet
routinely used by regulatory agencies to a significant extent. Some FDA laboratories
use SPE to clean up extracts before the detection step to protect the column used in
high-performance liquid chromatography (HPLC). After a pesticide has been extracted
and isolated from the sample by a combination of the above-mentioned techniques,
it is further separated from other co-extractives, usually by either gas chromatography
or liquid chromatography or, less frequently, by thin layer chromatography.
Gas chromatography (GC) has been a dominant technique for separation, with at
least 40 years of development and refinement. Most multi-residue methods (MRMs)
used by the FDA and USDA and many single-residue methods (SRMs) are based on
GC. In a gas chromatography setup, separation of pesticides and sample co-extractives