28 February / March 2018
Utilisation of LC/MSMS (QTRAP) and Polarity Switching for the Quantitative Analysis of Over 300 Pesticides in Crude QuEChERS Extracts from Various Fruit and Vegetable Matrices
by Laura M. Melton, Michael J. Taylor, Emily E. Flynn, Kirsty B. Reid, Katie J. Viezens, Devanshi S. Vyas, Andrew T. Wilson Science and Advice for Scottish Agriculture (SASA), Roddinglaw Road, Edinburgh, UK, EH12 9FJ
Science and Advice for Scottish Agriculture (SASA) is an Official UK laboratory that on behalf of the Scottish Government, participates in statutory UK and EU annual surveillance programmes that monitor UK and imported food & drink for residues of pesticides and their metabolites.
LC/MSMS has been a front-line tool in the laboratory for many years but with older instrumentation it was not possible to generate reliable data for all pesticides in a single run.
A highly sensitive LC/MSMS instrument has been commissioned. The instrument is capable of simultaneous acquisition of hundreds of MRMs incorporating fast polarity switching without compromising data or generation of sufficient data points across each peak for quantitation. However, due to the excellent robustness of the system, it has been possible to modify the generic QuEChERS procedure and omit the clean-up step. Consequently, we directly carry out quantitative analysis of crude extracts.
The above features are demonstrated with examples of quantitative surveillance data, UK and EU Proficiency Test data and system performance data.
Introduction
EU legislation requires member states to carry out post-approval surveillance monitoring of pesticides in food and feed to ensure that residues do not exceed Maximum Residue Levels (MRLs) [1]. A MRL is the highest level of a pesticide that is legally tolerated in or on food and feed. MRLs are not safety limits but the highest level expected when pesticides are used in accordance with good agricultural practice. The regulation covers pesticides currently and previously used in agriculture in or out with the EU. For pesticides not listed, a default MRL of 0.01 mg/kg is applied. The co- ordinated EU multi-annual control programme sets out for each member state the minimum number of samples to test and which specific pesticide and crop combinations to test [2]. Member states appoint official laboratories to
carry out this statutory work.
Gas Chromatography coupled with Tandem Mass Spectrometry (GC/MSMS) and Liquid Chromatography coupled with Tandem Mass Spectrometry (LC/MSMS) have been front- line tools in official pesticide laboratories for many years. Tandem mass spectrometry provides the required specificity to detect and quantify hundreds of pesticides in complex matrices. As well as the specificity, the instrumentation must also have adequate sensitivity to comply with the regulations.
Official control laboratories have implemented various multi-residue sample preparation approaches to include many pesticides with different physicochemical properties such as the Quick, Easy, Cheap, Efficient, Rugged and Safe (QuEChERS) extraction technique [3] followed by GC and/or LC coupled with MS. Methods must comply with Method Validation and Quality Control Procedures for Pesticide Residues Analysis in Food and Feed SANTE guidelines [4]. The aim for laboratories is to generate SANTE compliant data for all pesticides in as few analytical runs as possible. With older instrumentation it was not possible to generate reliable data for all pesticides in a single run. Samples often had to be analysed using several different approaches in order to achieve the required detection limits of 10 parts per billion (ppb), to cover all pesticides and to provide sufficient identification points to confirm positive residues. Testing Laboratories must be accredited to international standards (ISO 17025:2005) and as part of this must demonstrate proficiency by participating in
several analytical proficiency tests each year.
In this work, a highly sensitive Sciex 6500 (QTRAP) LC/MSMS instrument was used to improve the workflow. The instrument is capable of simultaneous acquisition of hundreds of Multiple Reaction Monitoring (MRM) experiments incorporating fast polarity switching without compromising data and ensuring generation of sufficient data points across each peak for quantitation. The instrument has excellent robustness allowing modification of the QuEChERS procedure, in order to directly analyse crude extracts without the need for a clean-up step.
The above features will be demonstrated with examples of quantitative surveillance data, UK and EU Proficiency Test data and system performance data.
Experimental Sample preparation
Fruit and vegetable samples submitted as part of the 2016 UK monitoring programme were cryogenically milled using solid carbon dioxide and a R23 vertical cutter mixer from RobotCoupe (UK), Middlesex, UK and stored at -20°C until required for analysis. 10 g of sample were extracted using citrate QuEChERS extraction method [5]. Q-sep QuEChERS salts were purchased from Restek (Thames-Restek), High Wycombe, UK. The dispersive SPE clean-up step was omitted. Sample extracts in acetonitrile with equivalent to 1 gml-1
matrix were filtered
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