Significant Improvements in Pesticide Residue Analysis in Food Using the QuEChERS Method


The quick, easy, cheap, effective, rugged, and safe (QuEChERS) sample preparation procedure combined with both gas chromatography–mass spectrometry (GC–MS) and liquid chromatography–mass spectrometry (LC–MS) was adopted in our laboratory for the analysis of pesticide residues in food samples as part of the state of Connecticut's regulatory monitoring program. In 2006, data from a QuEChERS-based sample preparation procedure were compared to data from our previous analytical method. In this article, these results are further compared to those of the U.S. Food and Drug Administration's pesticide residue monitoring program and the U.S. Department of Agriculture's pesticide data program.



Since its inception in 1963, the pesticide residue program in the Department of Analytical Chemistry at The Connecticut Agricultural Experiment Station (CAES) has made major advancements in the analyses of pesticide residues present in food — primarily but not exclusively produce. In 1992, the method of Pylypiw (1) was used in our laboratory to replace our older methods (2) for the extraction of organochlorine and organophosphorous pesticides from food samples. At the time, residues were analyzed by gas chromatography (GC) with element-selective detection. Beginning in 1993, mass spectrometry (MS) was introduced for the confirmation of violative residues. By 1999, all samples were subjected to MS analysis for the presence of pesticide residues (3). In 2006, following the acquisition of a ion trap liquid chromatography–mass spectrometry (LC–MS) system, a direct comparison was made between the Pylypiw method and the then newly published quick, easy, cheap, effective, rugged, and safe (QuEChERS) method (4). In 2011, an orbital trap LC–MS system (Thermo Scientific Exactive Orbitrap) was added to our program and is currently used for the exact-mass confirmation of violative pesticide residues.


Figure 1: Flowchart of 2006 sample extract and analysis.
In 2006, we compared the Pylypiw method (1), which offers petroleum ether extracts that are amenable to GC analysis, with an adaptation of the recently introduced QuEChERS method (4), which offers acetonitrile or toluene extracts that are amenable to both GC–MS and LC–MS. Approximately 181 samples obtained for analyses in the Connecticut program were tested using a paired sample blind study protocol (vide infra). The extracts from the Pylypiw method were analyzed by GC–MS and GC with micro electron-capture detection (ECD), and the QuEChERS extracts were analyzed by GC–MS and LC–MS as outlined in Figure 1.

The Connecticut program is similar to the larger United States (US) Food and Drug Administration (FDA) program in that it tests a wide variety of samples available to the consumer in the market place. The samples tested in these two surveys can be comprised of nearly any type of food offered for sale to the consumer. On an average annual basis from 1990 to 2010 the Connecticut program tested 37 different commodity types of fresh food and 14 different commodity types of processed food. These two programs contrast to the US Department of Agriculture (USDA) pesticide data program (PDP) which, on average, targets 12 fresh and four processed samples per year. Owing to the fact that the results obtained from the Connecticut and FDA programs are derived from nontargeted sources (5), as opposed to those in the PDP (6), the results obtained through the Connecticut program are thought to be more representative of those in the larger FDA program.


Figure 2: Comparison of the Connecticut, FDA, and PDP monitoring programs for pesticide residues in food.
From 1990 through 2005, the results obtained from the Connecticut program closely matched those obtained in the larger FDA pesticide residue monitoring program (Figure 2). During this timeframe, the FDA program analyzed 167,215 samples (5); the Connecticut program separately analyzed 4871 food samples (3). The Connecticut program analyzed only about 3% (2.91%) of the total samples in the FDA program. It is noteworthy that there is not a significant difference in the proportions of pesticide residue–free samples, 63.3% reported by Connecticut and 64.2% reported by FDA (5) (Figure 2), over the 16-year timeframe (1990–2005) when the data are compared using a z-test (P = 0.230; z = 1.200). The average violation rate over the same period was similar, 1.5% for the Connecticut program and 2.8% (5) for the FDA program, but statistically different (P = <0.001; z = 5.114). These results imply that the sampling design in Connecticut closely parallels the larger program of the FDA, that the analytical methodology used in the two surveys was comparable over the timeframe 1990–2005, and that the smaller Connecticut subsample is representative of the larger with respect to samples containing pesticide residues.

From the inception of the USDA PDP study in 1992 and through 2005, the results obtained from the Connecticut program contrasted sharply to those obtained in the PDP study by as much as 38% (Figure 2). During this timeframe the PDP targeted 112,395 samples (6), and the Connecticut program tested 4150 samples. When compared, the percentages of pesticide residue–free samples over the inclusive 14-year timeframe (1992–2005), 62.7% reported by Connecticut and 38.8% reported by the PDP, was not similar nor was it statistically significant (P = <0.001; z =34.117). The average violation rate reported, 3.5% by the PDP (6) and 1.7% by Connecticut, was likewise statistically dissimilar (P = <0.001; z = 6.208). These results suggested that the sampling designs of the two programs were dissimilar and that the analytical methodology used in the two studies was likely dissimilar.