36 May / June 2017
LC-MS/MS and GC-MS/MS Multi Residue Pesticide Analysis in Fruit and Vegetable Extracts on a Single Tandem Quadrupole Mass Spectrometer
by Kari Organtini1 1
2 , Gareth Cleland1 , Eimear McCall2
Waters Corporation, Milford, MA, USA Waters Corporation, Wilmslow, UK
Hundreds of pesticides are commercially available and are approved for use on various fruit and vegetable plants to prevent pest infestation and improve shelf-life of fresh produce. Maximum Residue Levels (MRLs) are set at the highest level of pesticide that the relevant regulatory body would expect to find in that crop when it has been treated in accordance with good agricultural practice. In the EU, if a pesticide is not explicitly mentioned in the MRL legislation, a default MRL is used for enforcement. This default value is set to be equal to the limit of quantification (LOQ) achievable with the analytical methods used for analysis. National authorities control and enforce MRLs by testing samples for pesticide residue levels using analytical surveillance programs. These programs check for compliance with MRLs, assess dietary exposure, and check for use of unauthorised pesticides. The food industry also carries out its own due diligence analyses.
Mass spectrometry (MS) coupled with both gas chromatography (GC) and liquid chromatography (LC) is needed to provide comprehensive analysis of a wide range of pesticide residues with sufficient sensitivity to meet global MRL regulations. The use of Quick, Easy, Cheap, Efficient, Rugged and Safe (QuEChERS) sample extraction and clean up has streamlined analytical efficiencies for multi residue analyses [1]. The advantage of ultra high performance liquid chromatography (UHPLC) coupled with tandem quadrupole mass spectrometry (MS/MS) for multi residue pesticide analysis is widely reported [2]. More recently the use of GC-MS/MS utilising atmospheric pressure ionisation (APGC) has been shown to offer significant improvements in performance over EI for challenging pesticides, in terms of selectivity, specificity, and speed of analysis [3,4].
The APGC source ionises compounds using a corona discharge at atmospheric pressure in an APCI-like manner. Therefore, this ionisation mechanism is a much softer technique than classic electron impact (EI) ionisation and produces larger amounts of intact parent ions, especially in the case of
fragile or easily fragmented compounds. APGC ionisation can occur using two mechanisms; proton transfer (wet source) or charge transfer (dry source). In proton transfer ionisation, [M+H]+ ions are formed, whereas in charge transfer ionisation, M+· ions are formed.
In this work, a single workflow for the multi residue analysis of pesticides is demonstrated on a variety of fruit and vegetable samples. Utilising the universal source of Waters Xevo®
TQ-S micro mass
spectrometer allows for LC (electrospray ionisation) and GC (atmospheric pressure ionisation) analyses to be completed on the same tandem quadrupole MS instrument, with less than 30 minutes needed to switch between chromatographic inlets. The performance of the method will be highlighted in terms of sensitivity, repeatability, and linearity for both LC and GC in compliance with the SANTE guidelines (11945/2015) for pesticide analysis [5].
Methods The LC and GC suites of pesticides analysed
in this study (listed in appendix tables) were chosen to cover a wide range of different pesticide classes and chemistries. The multi residue MS/MS methods were generated using the Quanpedia™ database, with separate databases utilised for generation of the LC and GC methods. Each database contains MRMs and retention time information for each compound. When the MS method is generated the MRM function windows are automatically set for each compound. For the LC method, a window of 1 minute was placed around each compound’s expected retention time. For the GC method, a window of 30 seconds was used due to the narrower peak widths exhibited in GC analysis. In addition to the MS methods, the TargetLynx™ software data processing methods and LC inlet method were also generated through the Quanpedia database.
Sample Extraction and Cleanup
Celery, lemon, corn, and kale samples were purchased at a local grocery store. Samples were chosen to be representative of different types of matrix complexity from
, and Simon Hird2
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