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Pilot project for rapid screening of pesticides using time of flight mass spectrometry - PS2528

The screening of pesticide residues and their transformation products in food is not only time consuming and costly but it is also limited in scope because it requires a combination of GC-MS (/MS) and LC-MS (/MS) techniques used in conjunction with a pre-defined list of target analytes. In addition, there is an increasing trend to analyse more pesticides by LC-MS rather than GC-MS. This is as a result of the development of a new, more generic, acetonitrile-based extraction procedure. Although GC-MS is, and will remain, an essential technique for comprehensive screening of pesticide residues, this proposal focuses on the further development of the capabilities of LC-MS.

The current LC-MS/MS approach for detecting residues has been employed for many years despite the knowledge that not all (i.e. those suitable for LC-MS) pesticide residues present in the sample will be detected. There is therefore a high risk that pesticide misuse is being missed due to incomplete target compound lists. In addition there is also an issue that strategic data regarding changing patterns in both legitimate and illegal use of pesticides cannot currently be captured. Liquid chromatography coupled to Time of Flight Mass Spectrometers (LC-TOF-MS) has been proposed as a technique for overcoming these issues.

TOF-MS does not require a pre-defined list of target pesticides to be programmed into the instrument. This means that any compound, which elutes under the chromatographic conditions, gives sufficient response with the MS parameters employed, and is soluble in the extraction solvent, should be detected. Therefore, this pilot project proposes to evaluate the use of accurate mass LC-TOF-MS to maximise the number of analytes screened in a single analysis. The actual improvements that can be realised will depend up on a number of factors, which will need to be evaluated, including; the TOF-MS parameters for all target compounds, associated quality control procedures and data handling procedures.
Preliminary investigations in the use of LC-TOF-MS for the analysis of pesticides at µg/kg concentrations in crude extracts have indicated that this approach can provide sufficient selectivity and sensitivity. Although TOF instruments remain less sensitive than triple quadrupoles operated in Multiple Reaction Monitoring mode, their sensitivity is independent of acquistion range - allowing high resolution, full range spectra to be obtained without compromising detection limits.

If the use of TOF technology could be implemented to screen PRC samples then a number of significant benefits could arise;

1) In theory TOF-MS should be able to detect any pesticide that is amenable to liquid chromatography and ionisation in LC-MS. In practice, LC-TOF-MS will not be able to detect all pesticides but it should be possible to screen several hundred different pesticides in a single analysis. If the scope of the monitoring is increased and fewer pesticide residues are missed then the risk assessments can be demonstrated to be more sound.

2) In cases where chromatograms contain unidentified peaks (e.g. new pesticides), accurate mass data can be extracted from high resolution, full range spectra allowing chromatographic peaks to be mass measured. The ion masses can be matched against database entries to identify candidate compounds, although unequivocal confirmation of identification would also require analysis of the reference standard material or fragmentation data from other MS techniques.

3) Should the misuse of a pesticide residue become known then it might be possible to retrospectively search existing chromatograms for this compound to rapidly establish the timescale and frequency of misuse without the need for additional analysis. Such information could then be used to determine the most appropriate and cost effective course of action (e.g. a decision not to undertake additional surveillance).

4) The fast acquisition rate of TOF-MS is compatible with fast chromatography, thus analysis times could be decreased. Laboratories could then analyse more samples per instrument to provide some reductions in cost and in turnaround times.

It is proposed that a short term pilot project (outlined in this document) is undertaken to assess the potential advantages of LC-TOF-MS in the context of screening samples for the UK monitoring programme on pesticide residues in foods. If the preliminary results are promising then further R&D funding would be required to realise the full potential of the technology.

Liquid Chromatography (LC) coupled to tandem quadrupole mass spectrometry is commonly employed to analyse samples for a pre-defined list of pesticides. This approach fails to take into account other non-targeted pesticides that may also be present in the sample. The extent to which pesticide residues in food are underestimated is not known and is of concern to consumers. The only way to demonstrate that foods are safe is to develop analytical methods capable of determining the presence of all pesticide residues. However, the diverse physico-chemical properties of pesticides will inevitably limit the scope of any multi-residue method. LC-TOF-MS has the ability to detect a very large number of pesticides (potentially several hundreds) with good sensitivity and selectivity but only if the pesticide is soluble in the extraction solvent and can be ionised in the source.

The objective of this project is to assess the advantages, limitations and application of LC-TOF-MS to pesticide residues analysis. Pesticides will be chromatographed, ionised and the exact mass of ions measured with very high accuracy. The retention time and spectra from any individual peak in the chromatogram of an unknown sample can be compared against a library of ‘target analytes’ to confirm the identity of the pesticide. The abundance of ions in the spectra can be compared with calibration standards to estimate the concentration of the residue. In the case of samples containing matrix peaks then peak deconvolution will precede spectral matching and detection.

If the mass spectrum of an unknown peak does not match the target analytes in the library the accurate mass values can potentially be used to calculate the elemental composition of the compound detected. Potentially this can be used with the isotope pattern to match against thousands of compounds contained in chemical databases to obtain a possible identity for the unknown analyte causing this response. This identification is not an objective in this project as other information from MS/MS fragmentation studies is likely to be required to assign a definitive identity. The main objective is to attempt an optimisation of LC-TOF-MS conditions enable the screening for at least 50 pesticides at a concentration of ca 0.05 mg/kg in a single fast analysis.

The use of LC-TOFMS has been reported for the rapid screening of multiple pesticides (1), the analysis of emerging contaminants (2), and the determination of unknown pesticides in tomatoes (3). The University of Almeria (who have experience with TOF-MS for the analysis of pesticides and who are assisting the newly appointed CRL laboratory (Valencia Spain) for multi-residue analysis of pesticides in fruit and vegetables) have expressed an interest in collaborating/exchanging information with CSL. The Veterinary Medicines Directorate have funded a 3-year CSL project on the use of LC-TOF-MS for the determination of veterinary drug residues in food. This means that operator familiarisation and experience with the instrument etc. can be shared between projects.

The Specific Objective of this project is:

To perform a preliminary evaluation of the potential of LC-TOF-MS for the screening of multi-pesticide residues (50 pesticides selected from the current PRC LC-MS/MS multi-residue list) in one target matrix by the end of August 2007.
Project Documents
• Final Report : Pilot project for rapid screening of pesticides using time of flight mass spectrometry   (338k)
Time-Scale and Cost
From: 2006

To: 2007

Cost: £40,083
Contractor / Funded Organisations
Central Science Laboratory
Agriculture and Food Chain              
Analytical Chemistry              
Food Safety              
Mass Spectrometry              
Monitoring and evaluation              
Pesticide Residues              
Public Health              
Residues Testing              
Fields of Study
Pesticide Safety