Analyzing 201 Pesticides in Fruits and Vegetables
Multiresidue Method For Pesticide By GC/MSD
by Julie Fillion, Laboratory Services Division, Food Production and
Inspection Branch, Agriculture and Agri-Food Canada, Ottawa, Ontario
(Canada)
In the past few years, pesticide residues in foods have become
a consumer safety issue. To address these concerns, regulatory
agencies have resorted to the use of multiresidue methods to
increase the extent of their monitoring programs both in terms
of scope of analysis and number of samples analyzed.
Multiresidue methods cover a broad scope of pesticides and offer the
advantages of being cost-effective, rapid, sensitive, and sufficiently
accurate for regulatory purposes.
Reducing the Number of Injections
A variety of multiresidue methods exist today. For this
type of analysis, the mass selective detector (MSD) offers
several advantages over specific detectors. Because of its
universality and specificity, the number of injections required
for analysis and the need for confirmation of positive analytes
by a secondary technique are reduced. The MSD also has the
sensitivity required for residue analysis.
The multiresidue method developed in our laboratory allows
for the analysis of 191 pesticides in fruits and vegetables
by GC/MSD, and 10 carbamates by HPLC with fluorescence
detection. Organophosphorous, organochlorine, and nitrogencontaining
pesticides are included in the analysis.
Analysis is performed in selected ion monitoring (SIM) mode,
thereby maximizing sensitivity and reducing interferences.
Positive analytes are confirmed by retention time and ion
ratios. Limits of detection range from 0.02 to 0.2 ppm,
depending on the sensitivity for each compound.
Only 150 mL Solvent Per Sample
Fruit and vegetables samples are extracted with acetonitrile, followed
by a salting-out step and a cleanup on a C18
cartridge. A second cleanup is performed on an
Envicarb(TM) tube coupled
with an aminopropyl cartridge.
The sample preparation steps require only 150 mL of solvent per sample,
and no chlorinated solvents are used. After concentration and addition of
an internal standard, the extract is split for analysis of carbamates by
HPLC (Chaput,1988) and GC/MSD analysis of the 191 remaining compounds.
Improving Quantitation of Late Eluters
Two GC/MSD injections in SIM mode are required in order to cover
all the pesticides included in the analysis. One injection covers
115 compounds, and the second injection covers the remaining 76.
Two to three ions are monitored for each pesticide. The ions are
chosen very carefully to minimize interferences, especially from
closely eluting compounds of interest. The ions for each compound
are acquired for a minimum of 1.0 minute, often in two successive
acquisition groups, starting 0.5 minute before the expected
retention time. Despite the one-hour-long chromatographic runs,
the electronic pressure control allows for good chromatography
throughout the analysis, thereby improving integration and
quantitation of late eluters.
Identification, Quantitation and QC
Pesticides are identified according to their retention time and are
confirmed by detection of their qualifying ions with ratios to the
target ion within expected limits. Quantitation of positive findings
is performed using a single-point calibration with a standard prepared
in blank matrix to compensate for matrix effects (Erney et al, 1993; Erney & Poole, 1993). For most accurate
results, the concentration of the standard must be similar to the
concentration of the compound to be estimated.
Our laboratory operates under the stringent requirements of a
quality system accredited to ISO-25 standard under the Standards
Council of Canada Laboratory Accreditation Program for Pesticide
Residues. Good quality control constitutes an essential component
of our analytical procedures. Analyses are performed by sets
consisting of a blank (used to prepare standards in matrix), a
spike and 10 samples. Two spiking solutions are used alternately
to generate recovery data for all 201 compounds every two sets.
Recovery Data for Every Compound
The custom report software provided with the workstation allows
for calibration (retention time, area count, ion ratios) and
calculation of recovery data for every single compound present
in the standard and spike automatically. Sample results are also
quantitated automatically.
We have written macros to print the extracted ion chromatograms
of all compounds monitored during a run. The chromatograms are
printed in a format set up by the analyst, typically 12 compounds
per page. The analyst can easily verify results simply by looking
at the printout. Recovery data for every compound/matrix combination
is easily monitored and compiled which, without the custom report
software, would represent an enormous task.
A Powerful Technique
This multiresidue method has been used on a variety of fruit
and vegetable samples. It is characterized by a broad scope
of application, good recoveries and sensitivity and low solvent
consumption, coupled with good analytical quality control. It is
ideally suited for the large number of samples received in a
regulatory laboratory.
Envicarb is a trademark of Supelco, Inc.
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