Comparison of QuEChERS sample preparation methods for the analysis of pesticide residues in fruits and vegetables

Abstract

This article describes the comparison of different versions of an easy, rapid and low-cost sample preparation approach for the determination of pesticide residues in fruits and vegetables by concurrent use of gas and liquid chromatography (GC and LC) coupled to mass spectrometry (MS) for detection. The sample preparation approach is known as QuEChERS, which stands for “quick, easy, cheap, effective, rugged and safe”. The three compared versions were based on the original unbuffered method, which was first published in 2003, and two interlaboratory validated versions: AOAC Official Method 2007.01, which uses acetate buffering, and European Committee for Standardization (CEN) Standard Method EN 15662, which calls for citrate buffering. LC–MS/MS and GC–MS analyses using each method were tested from 50 to 1000 ng/g in apple–blueberry sauce, peas and limes spiked with 32 representative pesticides. As expected, the results were excellent (overall average of 98% recoveries with 10% RSD) using all 3 versions, except the unbuffered method gave somewhat lower recoveries for the few pH-dependent pesticides. The different methods worked equally well for all matrices tested with equivalent amounts of matrix co-extractives measured, matrix effects on quantification and chemical noise from matrix in the chromatographic backgrounds. The acetate-buffered version gave higher and more consistent recoveries for pymetrozine than the other versions in all 3 matrices and for thiabendazole in limes. None of the versions consistently worked well for chlorothalonil, folpet or tolylfluanid in peas, but the acetate-buffered method gave better results for screening of those pesticides. Also, due to the recent shortage in acetonitrile (MeCN), ethyl acetate (EtOAc) was evaluated as a substitute solvent in the acetate-buffered QuEChERS version, but it generally led to less clean extracts and lower recoveries of pymetrozine, thiabendazole, acephate, methamidophos, omethoate and dimethoate. In summary, the acetate-buffered version of QuEChERS using MeCN exhibited advantages compared to the other tested methods in the study.

Introduction

In 2003, Anastassiades et al. described the “quick, easy, cheap, effective, rugged and safe” (QuEChERS) method for the multiclass, multiresidue analysis of pesticides in fruits and vegetables [1]. The authors questioned the typical conditions previously used for pesticide residue analysis, and through extensive experimentation and novel use of MgSO₄ for salting out extraction/partitioning and dispersive solid-phase extraction (d-SPE) for cleanup, they devised a highly streamlined sample preparation method with excellent results for a wide range of pesticide analytes in many types of foods [1]. Unlike many previous methods developed for traditional chromatographic detection systems (e.g. UV/vis absorbance, fluorescence, element-selective detectors), the QuEChERS approach takes advantage of the wide analytical scope and high degree of selectivity and sensitivity provided by gas and liquid chromatography (GC and LC) coupled to mass spectrometry (MS) for detection. GC–MS and LC–MS(/MS) have become the main analytical tools in most pesticide monitoring laboratories to meet world standards, thus the streamlined features, practical benefits and excellent results provided by the QuEChERS sample preparation approach combined with GC–MS and LC–MS/MS have helped lead to the great popularity of QuEChERS concepts. At the time of writing, there are more than 10 companies marketing QuEChERS products and the original paper [1] has been cited in the literature >210 times according to the ISI Web of Knowledge citation index [2].

A limited number of GC-amenable pesticides was evaluated in the original QuEChERS study and although this version has been demonstrated to yield excellent results for hundreds of pesticides in dozens of commodities [1], [3], [4], [5], subsequent experiments showed some pesticides gave lower stability and/or recoveries depending on pH of the matrix [3], [6], [7]. The original authors of the QuEChERS approach realized that buffering at pH-5 during extraction gave the optimum balance to achieve acceptably high recoveries (>70%) for certain pH-dependent pesticides (e.g. pymetrozine, imazalil, thiabendazole) independent of the fruit/vegetable matrix [6], [7]. Lehotay et al. modified the method to use relatively strong acetate buffering conditions [6] and Anastassiades et al. chose to use weaker citrate buffering conditions [7] in terms of ionic strength. Both versions of these methods went through extensive interlaboratory trials entailing ≈50,000–100,000 data points for dozens of pesticides at fortified and incurred at different levels in different matrices and using different types of GC–MS and LC–MS/MS conditions and instruments. Both methods successfully met statistical criteria for acceptability from independent scientific standards organizations, with the acetate-buffering version becoming AOAC Official Method 2007.01 [8] and the citrate-buffering version being named European Committee for Standardization (CEN) Standard Method EN 15662 [9].

The QuEChERS approach is very flexible and it serves as a template for modification depending on the analyte properties, matrix composition, equipment and analytical technique available in the lab. The template is also very rugged in that high recoveries will be achieved for many pesticides in many matrices even if different ratios and types of sample size, solvent, salts and sorbents are used in modifications. The ruggedness characteristics of the QuEChERS approach have been thoroughly evaluated in the original [1] and subsequent publications by the originators [3], [4], [5], [6], [7], [8], [9], [10], [11], [12], [13], [14], [15], [16], [17], [18]. In multiclass, multiresidue pesticide analysis, the sample preparation method inherently necessitates broad analytical scope which makes it impossible to obtain a high degree of cleanup without reducing recoveries for some pesticides. However, greater cleanup can be achieved by using different sorbents in d-SPE if the application has reduced analytical scope.

Reviews of QuEChERS are starting to appear in the literature [15], [19] and the original method has evolved into a flexible template for modification in several applications. In addition to pesticide residue analysis in foods [1], [3], [4], [5], [6], [7], [8], [9], [10], [11], [12], [13], [14], [15], [16], [17], [18], [19], [20], [21], [22], [23], [24], [25], [26], [27], [28], [29], [30], [31], [32], [33], [34], [35], [36], [37], [38], [39], [40], [41], [42], [43], [44], [45], [46], [47], [48], [49], [50], [51], [52], [53], [54], [55], [56], [57], [58], [59], [60], [61], [62], [63], [64], [65], [66], [67], [68], [69], [70], [71], [72], [73], [74], [75], [76], QuEChERS concepts (including d-SPE) have been used for acrylamide [77], [78], clinical [79], [80], veterinary drug residue [81], [82], [83], [84], [85], [86], [87], [88], [89], [90], food quality [91], supplement testing [92], perfluorinated compounds [93], [94], polycyclic aromatic hydrocarbons [95], alkaloids [96], environmental [97], [98], [99], [100] and mycotoxin [101] analytical applications. Mol et al. developed a universal sample preparation approach for all kinds of chemical contaminants in foods and feeds and QuEChERS concepts contributed to their proposed approach [102].

Due to the great flexibility of the QuEChERS approach, there are so many permutations that vendors of QuEChERS products have difficulties in providing products to meet all the demands (weighing powders in the lab is time-consuming and has higher potential for contamination). The primary application of QuEChERS is for multiclass, multiresidue analysis of pesticides in fruits and vegetables and as part of a training exercise in the USDA lab, we decided to conduct a comparison study to determine if one of the three QuEChERS approaches that have been evaluated among multiple labs [3], [8], [9] gave more suitable performance for the food commodities in this study. Not only would we compare trueness and precision of results, but also evaluate analyst performance and matrix co-extractives in terms of their amount, effects on quantification and analyte detection interferences. Due to a recent worldwide shortage of acetonitrile [103], we also decided to conduct additional experiments to ascertain if ethyl acetate could be substituted without other changes in the method.