Ronald E. Majors
|
Pesticide residue analysis of food and environmental samples has been performed for over 40 years. Most pesticide methods
are not oriented toward measuring a single pesticide. Instead, residue-monitoring laboratories are geared to perform multiclass,
multiresidue methods to detect a wide variety (in the hundreds) of pesticides potentially in the sample. Because of the wide
range of chemical properties of pesticides (including acidic, basic, and neutral) and the wide variety of matrices (polar,
nonpolar, fatty, waxy, and so forth), the sample initially must be cleaned up using a compatible sample preparation technique
before injection into the chromatographic system. Ideally, a multiresidue method should be fast and easy to perform, require
a minimum amount of chemicals (especially solvents), provide a certain degree of selectivity, and still cover this wide array
of analyte–matrix pairs. Although many sample preparation protocols involve lengthy multistep procedures, if the number of
steps can be minimized by use of a simple sample preparation procedure, reproducibility (precision) and accuracy can be improved
and time can be saved.
For many years, the standard approach for pesticides multiresidue methods for foods and agricultural products involved the
use of organic solvent extraction (usually acetone), followed by water dilution and partitioning into a nonpolar solvent (such
as methylene chloride and petroleum ether). This approach worked fine for nonpolar pesticides but certain polar compounds
such as organophosphorus insecticides and several modern pesticides were partially lost. More sophisticated approaches were
needed and analysts began to experiment with different organic solvents for the initial extractions. Next, the addition of
various salts to the mixtures was found to affect recovery by changing solvent polarity and this became the fashionable approach
(salting out effect). Some of these methods became the standard official multiresidue methods used in government, contract,
and testing labs today.
With environmental and health concerns about the use of chlorinated solvents, workers investigated various non-halogen-containing
solvents and solvent mixtures such as ethyl acetate, acetonitrile, and cyclohexane–ethyl acetate for extraction. For various
reasons, none of these procedures gave an entirely satisfactory set of results, particularly in the area of extraction efficiency
for wide ranges of pesticides. Extraction solvent compatibility with the analytical technique, be it high performance liquid
chromatography (HPLC) or gas chromatography (GC) with and without mass spectrometry (MS) detection was another complicating
problem.
Therefore, new extraction techniques were devised for solid samples, including supercritical fluid extraction, microwave-assisted
extraction, solid-phase microextraction, matrix solid-phase dispersion, and pressurized fluid extraction–accelerated solvent
extraction. Although most of these techniques used less organic solvent than conventional extraction, some were slow and most
of the instrumental techniques were run in a serial manner, were of high cost, and required specialists to develop and troubleshoot
methods. Some of these extraction techniques lacked sufficient selectivity (microwave-assisted and pressurized fluid extraction),
and sample sizes were limited, an important consideration for trace analysis. Supercritical fluid extraction was particularly
problematic with each analyte–matrix requiring a different set of experimental conditions and the technique was susceptible
to varying water content among the samples. Some of these methods involved considerable cleanup of glassware and extraction
vessels before the next use. The need for a simple, rapid, inexpensive, multiclass multiresidue method that provided high quality results with a minimal
number of steps, with reduced reagent use, and required little glassware led Anastassiades and coworkers (1) to develop a
new method for the sample preparation of pesticide residues in fruits and vegetables. They gave the method the name QuEChERS,
which stands for quick, easy, cheap, effective, and safe. The technique has attracted the attention of pesticide laboratories
worldwide. Official methods from AOAC International and the Committee of European Normalization (CEN) are now available. Although
one easily can assemble the necessary materials from a general laboratory catalog to perform QuEChERS — convenient kits have
now been assembled commercially by Sigma Aldrich/Supelco (Bellefonte, Pennsylvania), Restek (Bellefonte, Pennsylvania), and
United Chemical Technologies (Bristol, Pennsylvania).
The purpose of this article is to give an introduction to QuEChERS by providing some background information, typical samples
and types of pesticides handled, quantitative possibilities, and applications examples. We will contrast the technique to
matrix solid-phase dispersion, which has some similarities.
Follow Us
RSS
Bookstore
