Residuals of pesticides widely used in agriculture are monitored in foods for human consumption through the use of statutory
Maximum Residue Levels (MRLs). Regulation EC 396/2005 adopted in the European Union sets MRLs for over 500 pesticides in over
300 food commodities, many at a default value of 0.01 mg/kg (the typical routine analytical method limit of determination).
Thus, food safety laboratories must curb costs and turnaround times (often to <48 hours) while testing a wide array of foods
for a large number of pesticide residues at concentrations levels 10 lower than 0.01 mg/kg. This is most often achieved using
multiresidue methods with a combination of LC–MS-MS and/or GC–MS techniques to determine pesticide residues in a single generic
solvent extract of the sample.
One such example is the QuEChERS (Quick, Easy, Cheap, Effective, Rugged and Safe) procedure, which is based on acetonitrile
extraction and dispersive solid-phase extraction. After the QuEChERS extraction, a solvent exchange is made to facilitate
the GC injection. Although the QuEChERS extraction technique provides a fast turnaround time for a large number of samples
with small sample volumes in the range of 10 g, the heavy matrix load of QuEChERS extraction requires increased robustness
of the GC inlet system and increased selectivity from the MS-MS analyser. Described here is a high-quality, low-level analysis
of pesticides in produce samples using the Thermo Scientific TSQ Quantum XLS Ultra GC–MS-MS system.
Figure 1: Comparison of U-SRM and standard SRM for pentachloroanisole and isodrin in wheat at 10 ppb level. Top: The chromatogram
in U-SRM SRM (Q1 FWHM at 0.1 Da). Bottom: The same sample in standard mode (Q1 FWHM at 0.7 Da).
For most of the pesticide compounds included in this method, the complete list of the compounds with their respective SRM
transitions have been downloaded from the Pesticides Method Reference CD (provided with the manual, P/N 120390) into the instrument
acquisition method. Each transition has been determined for optimal sensitivity and selectivity, with the complete list documented
for TSQ Quantum XLS Ultra™ users.
Over 400 pesticides have been monitored in several matrices, such as wheat, black currants, and cucumber. The results of the
most challenging pesticides in terms of activity and response are highlighted, showing calibration curves, repeatability and
ion ratio stabilities.
The TSQ Quantum XLS Ultra is able to perform SRM with a higher mass resolution (0.1 Da) setting to allow for better selectivity.
Not all pesticides in all matrices benefit from a higher mass resolution setting, but depending on the matrix and the compound
analysed, there can be a significant improvement on the signal-to-noise ratio. Some examples are shown in the Advanced GC-MS/MS
Experiment section of this application note.