ADV06-15_1-412

38 / 413

| 1-800-356-1688 or 1-814-353-1300 | Feedback? E-mail 12
By Julie Kowalski 1, Jack Cochran1, Jason Thomas1, Michelle Misselwitz1 , Rebecca Wittrig2 *, and André Schreiber3
• One fast, simple QuEChERS extraction for a broad range of pesticides.
• Rxi®-5Sil MS and Rtx®-200 column selectivity and orthogonality promote good GCxGC separations.
• Ultra Aqueous C18 LC column retains and gives excellent peak shapes for small polar pesticides.
Pesticide residue analysis of food has traditionally been performed
using GC, but there is increasing use of LC with tandemmass spec-
trometry (MS/MS). LC is favored for polar, less thermally-stable, less
volatile, compounds. GC-MS is preferred for volatile, thermally-stable
species, and pesticides that do not ionize well in electrospray or atmo-
spheric pressure chemical ionization LC sources. With MS, complete
chromatographic resolution of compounds is not always essential, as
selected ions or selected reaction monitoring (SRM) transitions are
used for pesticide identification and quantification. However, data
quality can be improved through better retention and separation of
components, especially for structurally similar pesticides and high-
level matrix coextractives. In the work summarized here, we employed
a comprehensive approach and analyzed QuEChERS extracts of a
variety of foods for pesticides by both GCxGC-TOFMS and LC-MS/MS.
Food commodities were fortified with pesticides and processed
using Q-sep™ QuEChERS extraction salts and dSPE tubes. QuEChERS
(Quick–Easy–Cheap–Effective–Rugged–Safe) is a sample preparation
approach developed by Anastassiades et al. [1] as a simple, rapid,
effective, yet inexpensive, way to extract pesticide residues from
fruits and vegetables, followed by a dispersive solid phase extraction
(dSPE) cleanup of the extract. The foods chosen varied in water, fat,
and pigment content, so the ruggedness of QuEChERS as well as the
performance of GCxGC-TOFMS and LC-MS/MS could be assessed.
Commodities tested were red bell pepper, cucumber, black seed-
less grape, spinach, lemon, raisin, and hazelnut. In this summary, we
report data for the grape and lemon, the least complex and most
complex of the matrices we assessed. Complete results are available
at in the full application note.
Column Selectivity and Multidimensional Techniques
We first assessed the complexity of different commodities by exam-
ining the total ion chromatogram (TIC) contour plots generated by
GCxGC-TOFMS. It is clear from Figure 1 that lemon contains many
more coextractives than grape, as demonstrated by the large num-
ber of intense (red) signals. While it should be possible to analyze
QuEChERS grape extracts for pesticides by one-dimensional GC,
multidimensional techniques (e.g., GCxGC-MS, GC-MS/MS, or LC-MS/
MS) are necessary for samples as complex as lemon. Column selec-
tivity is an important consideration in multidimensional techniques
and the Rxi®-5Sil MS (cat.# 13623) x Rtx®-200 (cat.# 45001) column
combination used here provided orthogonal separations that helped
isolate target analytes from matrix interferences. Column selectivity
is also important in LC-MS/MS methods because coelutions can be
problematic if the analytes share MRM transitions. The Ultra Aqueous
C18 column (cat.# 9178312) used for this work is both selective for
small, polar compounds, showing good retention and peak shape,
and has balanced retention for a large number of compounds that
vary in physiochemical properties. More balanced retention reduces
the number of MRM transitions being monitored at any point in time,
and improves data quality by allowing more time to be spent on a
smaller number of MRM transitions.
Evaluation of a Comprehensive Approach
Good recoveries were obtained for most pesticides in most commodi-
ties as determined by both GCxGC-TOFMS and LC-MS/MS. As shown
in Table I, quantitative results for grape were excellent, but lemon
proved to be a difficult matrix as demonstrated by the fact that 11
pesticides were not detected by LC-MS/MS and two pesticides had
interfering compounds when using the GCxGC-TOFMS method. Given
lemon’s complexity, ion suppression from coelution with coextrac-
tives is likely the cause of the undetected compounds in the LC-MS/
MS analysis. Similarly, coextracted matrix compounds likely caused
the interference that prevented determination of propoxur and ter-
bacil in fortified samples by GCxGC-TOFMS. While recovery results for
most pesticides in most commodities demonstrate successful extract
Comprehensive Pesticide Residue Monitoring in Foods
Using QuEChERS, LC-MS/MS, and GCxGC-TOFMS
1Restek Corporation, 110 Benner Circle, Bellefonte, Pennsylvania 16823, USA
2AB SCIEX, 353 Hatch Drive, Foster City, California 94404, USA
3AB SCIEX Research and Development, 71 Four Valley Drive, Concord, Ontario, Canada L4K 4V8
*Current address: Restek Corporation, 110 Benner Circle, Bellefonte, Pennsylvania 16823, USA