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Perspective
Sampling and Sample Processing in Pesticide Residue Analysis Steven J. Lehotay, and Jo Marie Cook J. Agric. Food Chem., Just Accepted Manuscript • DOI: 10.1021/jf5056985 • Publication Date (Web): 13 Feb 2015 Downloaded from http://pubs.acs.org on February 18, 2015
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Journal of Agricultural and Food Chemistry
Sampling and Sample Processing in Pesticide Residue Analysis Steven J. Lehotay a and Jo Marie Cook b,* a
United States Department of Agriculture, Agricultural Research Service, Eastern Regional Research Center; 600 East Mermaid Lane; Wyndmoor, Pennsylvania 19038; USA b
Florida Department of Agriculture and Consumer Services, Division of Food Safety; 3125 Conner Boulevard; Tallahassee, Florida 32399; USA * To whom correspondence should be addressed. Tel: 1-850-617-7505. Fax: 1-850-922-9110. E-mail:
[email protected] Disclaimer: The use of trade, firm, or corporation names does not constitute an official endorsement or approval by the USDA of any product or service to the exclusion of others that may be suitable. 1
Abstract
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Proper sampling and sample processing in pesticide residue analysis of food and soil has always
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been essential to obtain accurate results, but the subject is becoming a greater concern as
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approximately 100 mg test portions are being analyzed with automated high-throughput
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analytical methods by agrochemical industry and contract laboratories. As global food trade and
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importance of monitoring increase, the food industry and regulatory laboratories are also
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considering miniaturized high-throughput methods. In conjunction with a summary of the
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symposium, “Residues in Food and Feed - Going from Macro to Micro: The Future of Sample
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Processing in Residue Analytical Methods” held at the 13th IUPAC International Congress of
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Pesticide Chemistry, this is an opportune time to review sampling theory and sample processing
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for pesticide residue analysis. If collected samples and test portions do not adequately represent
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the actual lot from which they came and provide meaningful results, then all costs, time, and
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efforts involved in implementing programs using sophisticated analytical instruments and
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techniques are wasted, and can actually yield misleading results. This article is designed to
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briefly review the often-neglected but crucial topic of sample collection and processing, and put
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the issue into perspective for the future of pesticide residue analysis. We emphasize that analysts
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should demonstrate the validity of their sample processing approaches for the analytes/matrices
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of interest, and we encourage further studies on sampling and sample mass reduction to produce
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a test portion.
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KEYWORDS: sampling, sample comminution, processing, homogenization, blending, milling,
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grinding, theory of sampling, high-throughput analysis, pesticide residues, cryomilling,
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validation, food, soil
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INTRODUCTION At the 248th American Chemical Society (ACS) National Meeting and Exposition, held
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August 10–14, 2014 in San Francisco, California, the ACS Agrochemicals Division (AGRO) co-
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sponsored the 13th IUPAC International Congress of Pesticide Chemistry, which drew 1216
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participants from 53 countries around the world. One of the many well-attended symposia
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organized at the Congress was “Residues in Food and Feed - Going from Macro to Micro: The
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Future of Sample Processing in Residue Analytical Methods.” The opening talk in the session
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by Jo Marie Cook reviewed the topic of sampling and sample theory and described current
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practices, showing examples and results from the Florida Dept. of Agriculture and Consumer
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Services. The speakers to follow, Robert Gooding and Kari Lynn from the agrochemical
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industry, described recent pesticide field trials and inter-laboratory studies involving cryogenic
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milling followed by high-throughput analysis of soil and food sample test portions as little as 75
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mg. The session was concluded with an extensive and lively discussion involving the speakers
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and many experts in the audience.
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Others have recently described their concerns with analytical sampling/processing in
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general applications.1-3 Analysis of samples in several applications, such as elements and food
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composition,4-9 have taken sample comminution into account for many years, and extensive
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measurements of sample homogeneity are required for certified reference materials and
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proficiency test samples involving interlaboratory trials,10,11 Sampling of cereals and grains for
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mycotoxins analysis has also been studied rather extensively to meet real-world analytical
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needs.12-15 In the case of pesticide residue applications, sampling and sample processing have
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been too often neglected, with relatively few studies reported in the literature on the latter subject
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in particular.16-25 Tremendous attention has been given to the development and validation of new
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analytical tools/methods in pesticide studies, but if sampled test portions do not adequately Page 2 of 28 ACS Paragon Plus Environment
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represent the original lot or unit from which they came, all of the costs, time, and efforts
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involved in using sophisticated analytical instruments and techniques cannot produce meaningful
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results, and in fact, they can provide misleading information.
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Our intent in this perspectives article is to capture the main points made in the
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presentations and discussions in the IUPAC symposium, including regulatory aspects, and to
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provide a brief overview of sampling and sample processing in order to convey the importance
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of the subject, which is becoming increasingly crucial as automated high-throughput analysis of
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micro-subsamples is being implemented by the agrochemical industry. As global food trade and
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the importance of pesticide residue monitoring are increasing, the food industry and regulatory
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laboratories are considering utilization of miniaturized high-throughput methods. There is little
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doubt that modern technology and methodology are capable of meeting analytical needs in an
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efficient manner, but great caution and care are required to ensure that sample processing is
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properly done to provide meaningful results that lead to better conclusions and decisions.
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SAMPLING Why should laboratories care about sampling? It’s easy for lab personnel to justify that
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sample collection is not their concern, but quality of work suffers if expensive analyses are
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blindly conducted with no regard for the correctness of the sample and how it was obtained. Too
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often the sampling plan and its manner of execution are thought to be of no importance to
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laboratory personnel, but the final reports are often signed by laboratory staff who become liable
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for the quality of the findings. Just as an analyst cannot hide from the quality of his/her results
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when confronted with quality control (QC) findings, laboratory chemists should take ownership
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of their results with a holistic perspective. They should test the validity and determine
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uncertainties of all aspects in the process, and those responsible for sampling plans and sample
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collection should be held accountable to the same degree of oversight and verification to which
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the analysts are held accountable for their results. The tide of importance in pesticide residue
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analysis is rising, and analytical practices that can be likened to sand castles on the beach will not
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last long.
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As food safety becomes increasingly complex due to far reaching global food supplies,
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pesticide manufacturers are seeking to register pesticides in multiple countries and have their
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residue data accepted by regulators in the European Union, United States, Japan, Australia, Page 3 of 28 ACS Paragon Plus Environment
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Brazil, and many other countries. To demonstrate environmental and human health impacts, as
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well as the anticipated food residues, expensive field trials must be conducted. It is important
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that residue data generated from these studies yields acceptable trueness and precision for use in
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complex risk assessment calculations and statistical analyses. Regulators who enforce maximum
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residue levels (MRLs) must have confidence that the reported analytical results represent the true
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residue concentrations. Millions of dollars of perishable food shipments may be needlessly lost,
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causing much economic hardship to farmers and importers/exporters, if sampling leads to a false
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violation, or environmental and consumer safety are compromised if actual violations are missed. Due to the high stakes involved, food safety regulatory agencies can no longer afford to
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ignore the contribution of sampling to total analytical uncertainty. When data is shared between
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far-reaching organizations, there should be transparency and understanding of what, when,
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where, and how the sample was taken, and the degree of uncertainty in the analytical findings
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needs to be known when making inferences about the population from which the sample was
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taken.
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CURRENT SAMPLING PRACTICES Actually, studies involving pesticide application field trials have shown current practices
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to be acceptably precise (
