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Quadrupole GC-MS System for Pesticide Analysis in Tea
Technology performs fast, cost-effective pesticide analysis
by Eric Phillips, James Chang, PhD, David Steiniger, and Anton Mayer
Pesticides are intended for use on food crops to prevent, destroy, and control pests, which can be chemical, biological (such as a virus or bacterium), antimicrobial, or disinfectant.1 As a result, pesticide residues can be found in agricultural products like tea leaves and, due to their potential toxicity, can be harmful to human health. Pesticide consumption can cause a wide range of long-term health problems, including damage to the nervous and reproductive systems, birth defects, and, in some cases, cancer. Short-term health effects include dizziness, headaches, fatigue, memory impairment, visual disorders, and vomiting, as well as skin, eye, and respiratory tract irritation.
To protect consumers and ensure product quality, multi-residue pesticide monitoring in complex matrices like tea leaves is an ongoing global requirement for regulatory agencies, contract laboratories, and industrial laboratories. It is important that the maximum residue limits (MRLs) of pesticides allowed in food and drink products are not exceeded.
In the United States, the Environmental Protection Agency sets limits on the levels of pesticide residue that can remain in food and feed products without posing a risk to human health. These pesticide residue limits are known as tolerances and apply to both domestic and imported food.2 The U.S. Food and Drug Administration (FDA) is charged with enforcing regulations regarding imported and domestic foods shipped in interstate commerce in the United States. The FDA also carries out pesticide monitoring to gain an increased understanding of particular pesticide/commodity or food product combinations and, ultimately, to ensure consumer safety. The Office of Plant and Dairy Foods prepares an annual summary and detailed analysis of the residue data obtained. This information is made available to food producers and suppliers through the Center for Food Safety and Applied Nutrition.3 In the U.S., pesticides are also subject to the requirements of the Federal Insecticide, Fungicide, and Rodenticide Act and the Federal Food, Drug, and Cosmetic Act.
In addition, the World Health Organization (WHO) has developed the WHO Pesticide Evaluation to coordinate pesticide testing and evaluation, safeguarding public health.4 In March 2007, the WHO and the Food and Agriculture Organization of the United Nations (FAO) signed a Memorandum of Understanding to administer a joint program for pesticide management. The International Code of Conduct on the Distribution and Use of Pesticides, originally adopted by the FAO in 1985 and revised in 2002, promotes pesticide management practices that minimize potential health and environmental risks.1 This code gives a shared responsibility to governments, industry, trade, and international institutions.
Pesticide analysis poses a number of challenges for laboratories and operators due to the wide-ranging chemistries within the contaminants.
Despite the fact that food products are traded across international borders, pesticide regulations vary across countries. These differences led delegates at a conference of the FAO to adopt an International Code of Conduct on the Distribution and Use of Pesticides, establishing voluntary standards of pesticide regulation for different countries.5 In the United States, the FDA has since developed a number of guidelines to control and monitor the import of food and agricultural products, including the Automatic Detention of Raw Agricultural Products for Pesticides legislation, implemented in 2006.3
Pesticide analysis poses a number of challenges for laboratories and operators due to the wide-ranging chemistries within the contaminants. Multi-residue pesticide analysis methods are pushed to ever greater complexity as the number of regulated pesticides expands. Most analytical methodologies focus on improving the analytical method for enhanced analysis, which is a significant development given the growing need for robust analytical methods for multi-residue analysis. The bottleneck in such methods is the sample preparation stage, however, which can be lengthy, labor intensive, and error prone, limiting sample throughput. In addition, sample preparation can consume large amounts of solvent, increasing costs for analysis and disposal, generating considerable waste, and contaminating the sample. A key factor in improving sample throughput and providing a robust analysis begins with the sample extraction and concentration.
Other methods for pesticide analysis include liquid chromatography-mass spectrometry (LC-MS) using the AMR 3705-95 procedure.6 Because this procedure involves approximately 20 cleanup steps, however, the number of samples that can be analyzed is limited to around eight to 10 per day. There are also limitations associated with sample preparation techniques used for pesticide analysis by gas chromatography (GC). One of the most commonly used techniques, Soxhlet extraction, uses expensive, high-purity organic solvents such as acetone and methylene chloride, generating high costs for laboratories. As the risk of interference during pesticide analysis increases with the complexity of the matrix studied, accurate and efficient sample preparation becomes necessary.
This article discusses how triple quadrupole GC-MS with solid phase micro extraction (SPME) technology can be used to perform fast and cost-efficient analysis of pesticides in complex tea leaf matrices, while reducing the use of solvents, thereby providing a more environmentally sensitive sample preparation method. Proof of concept is demonstrated for pesticide analysis in heavy matrix without the need for typical laboratory extractions, and the results justify further method development.
The method outlined below analyzes a list of pesticides by tandem mass spectrometry, saving a significant amount of time and money for laboratories and complying with industry regulations.
For this experiment, SPME (Supelco) was used to screen and confirm pesticides on the Thermo Scientific TSQ Quantum XLS triple quadrupole GC-MS using multiple reaction monitoring (MRM). This method minimizes solvent consumption and extraction time.
Dry tea leaves were homogenized with a blender and 1 g added to 20 mL volatile organic analysis vials. The dry tea was spiked with standards for the development of a curve, and 3 mL of 1% MeOH in deionized water saturated with NaCl was added. The vial was then capped, swirled gently, and placed on the autosampler (see Figures 1a and 1b, p. 30).
The SPME technology was programmed to incubate and agitate the sample and expose the SPME fiber to the vial headspace. This eliminated the need for traditional extractions, improving laboratory productivity by enabling fast sample preparation. Due to the high levels of aroma, flavors, and polyphenols in tea, however, the detection system must be able to deal with the volatile matrix.
After a designated period of time to reach equilibration, the fiber was inserted into the inlet of the GC to begin pesticide analysis. A matrix blank was analyzed in full scan to show the interfering compounds (see Figure 2, p. 31). Using MRM, triple quadrupole GC-MS separated the interfering volatile compounds from those of interest and provided the sensitivity required to reach the required MRLs for all of the evaluated pesticides. In this example, pesticide levels were examined in relation to Japanese MRLs, some of the most stringent in the world.
As the risk of interference during pesticide analysis increases with the complexity of the matrix studied, accurate and efficient sample preparation is necessary.
Results and Discussion
Calibration curves were developed for the 28 compounds listed in Table 1 (above). For these compounds, the correlation coefficient of the calibration curves was r2>0.995. Two or more transitions were used for each compound, and confirming ions were found to be within limits set by the European Union. An additional 28 compounds that do not currently have MRLs for tea in Japan were analyzed in the standard mix. The additional compounds were used to show applicability of the technique in a heavy matrix. Initial calculations show that laboratories using this technique can expect cost savings of approximately $158,000 per year from accelerated sample preparation and, ultimately, increased pesticide analysis throughput.
As demonstrated in the above experiment, detecting low pesticide levels in tea leaves using SPME as an extraction method requires MRM functionality of the TSQ Quantum XLS. This was due both to the sensitivity requirements and to the matrix that is forced into the headspace. Additionally, it removes the need for typical laboratory extraction methods, reducing the amount of time needed for pesticide analysis and increasing sample throughput. Adopting this method gives food safety laboratories the potential to increase cost savings. Furthermore, the calibration curves showed at least 0.995 or greater r2 correlation coefficient values for the 28 compounds analyzed in the tea matrix. The MRLs achieved were in accordance with stringent Japanese regulations.
- Food and Agriculture Organization of the United Nations. International Code of Conduct on the Distribution and Use of Pesticides. FAO. Available at: www.fao.org/docrep/005/ Y4544E/y4544e00.htm. Accessed November 22, 2010.
- U.S. Environmental Protection Agency. Pesticides and food: what the pesticide limits are on food. EPA. Available at: www.epa.gov/opp00001/food/viewtols.htm. Accessed November 22, 2010.
- U.S. Food and Drug Administration Compliance Program Guidance Manual. Chapter 04—Pesticides and Chemical Contaminants. FDA. Available at: www.fda.gov/downloads/Food/GuidanceComplianceRegulatoryInformation/ComplianceEnforcement/ucm073186.pdf. Accessed November 22, 2010.
- World Health Organization. WHO Pesticide Evaluation Scheme: “WHOPES.” Available at: www.who.int/whopes/en/. Accessed November 22, 2010.
- Willson HR. Pesticide regulations. In: Radcliffe EB, Hutchison WD, eds. Radcliffe’s IPM World Textbook. St. Paul, Minn.: University of Minnesota; 1996. Available at: http://ipmworld.umn.edu/chapters/willson.htm. Accessed November 22, 2010.
- Nathan EC III, Demario D, Westberg GL, et al. Analytical method for the determination of DPX-JE874 and cymoxanil residues in various matrices. DuPont Report No. AMR 3705-95 Revision 1. Wilmington, Del.: E.I. du Pont de Nemours and Co.; 1998.