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 programme. 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 programme
and the U.S. Department of Agriculture's pesticide data programme.
Since its inception in 1963, the pesticide residue programme 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 programme
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 programme 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 programme is similar to the larger United States (US) Food and Drug Administration (FDA) programme 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 programme tested 37 different commodity types of fresh food and 14 different commodity types of processed
food. These two programmes contrast to the US Department of Agriculture (USDA) pesticide data programme (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 programmes are derived from nontargeted sources (5), as opposed to those in the PDP (6), the results obtained through
the Connecticut programme are thought to be more representative of those in the larger FDA programme.
Figure 2: Comparison of the Connecticut, FDA, and PDP monitoring programmes for pesticide residues in food.
From 1990 through 2005, the results obtained from the Connecticut programme closely matched those obtained in the larger FDA
pesticide residue monitoring programme (Figure 2). During this timeframe, the FDA programme analyzed 167,215 samples (5);
the Connecticut programme separately analyzed 4871 food samples (3). The Connecticut programme analyzed only about 3% (2.91%)
of the total samples in the FDA programme. 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 programme and 2.8% (5) for
the FDA programme, but statistically different (P = <0.001; z = 5.114). These results imply that the sampling design in Connecticut closely parallels the larger programme 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 programme 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 programme 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 programmes were dissimilar and that the analytical
methodology used in the two studies was likely dissimilar.
Perspectives in Modern HPLC: Michael W. Dong is a senior scientist in Small Molecule Drug Discovery at Genentech in South San Francisco, California. He is responsible for new technologies, automation, and supporting late-stage research projects in small molecule analytical chemistry and QC of small molecule pharmaceutical sciences. LATEST: Seven Common Faux Pas in Modern HPLC