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Improving the reproducibility and cost efficiency of the analysis of dithiocarbamates - PS2529

Description
Dithiocarbamate pesticides are widely used in agriculture and residues are frequently detected in UK-produced and imprted fruit and vegetables.

Dithiocarbamates are relatively unstable and rapidly degrade to CS2 if the samples are homogenised at room temperature. Samples requiring dithiocarbamate analysis are not homogenised but cut into segments prior to analysis. Using this segmentation procedure losses due to degradation are reduced but imprecision due to sub-sampling are significantly increased. One replicate sub-sample (comprising 2-3 segments) can contain a residue (expressed as CS2) above the MRL whilst another replicate sub-sample drawn from the same primary sample may not contain a detectable residue. Consequently MRL exceedances could be missed and samples containing a mean residue below the MRL may be reported as an MRL exceedance. The Analytical Sub-Group of the Pesticides Residue Committee have stated that this situation is unacceptable and action is required to find a solution and thus provide more acceptable results.

Preliminary experiments have demonstrated that the addition of dry ice during the comminution of frozen samples will improve the homogeneity of sub-samples but will not prevent losses of dithiocarbamates pesticides occurring. The consensus opinion of the ASG is that an analysis, which underestimates the mean residue concentration but gives improved precision, is more acceptable. Before cryogenic processing can be introduced it will be necessary to demonstrate that analysis of samples containing incurred residues (rather than spiked residues) does not give significantly lower mean results compared to the segmentation method.

To evaluate the stability of incurred dithiocarbamate residues during storage of samples (-20 oC) that have been processed cryogenically, samples will be re-analysed at regular intervals after the original analysis.
The variability of the distribution of incurred residues in lettuce (a problematic commodity), will be evaluated by the analysis of the individual halves of 10 lettuce heads taken from a sample found to contain a high concentration of incurred residue during screening.

If cryogenic sample processing can be implemented for dithiocarbamates the following benefits could result;

1) More reproducible results at lower cost. The need for an additional sample segmentation procedure will be eliminated and costs reduced. Most samples for the PRC programme are cryogenically processed so one or two of the sub-samples could be used for dithiocarbamte analysis removing the need for additional sample processing.

2) Because the results are more reproducible the total number of replicate analyses could possibly be reduced.

3) If the residues are relatively stable in frozen cryogenically comminuted sub-samples then it may be possible to provide a archive sub-sample in cases where results are disputed. This is not possible with the current segmentation procedure because the precision is not sufficient to allow meaningful comparisons of results generated in different laboratories.
Objective
Dithiocarbamate pesticides start to degrade on contact with enymes and chemicals released from damaged cells of fruits and vegetables. This occurs when samples are comminuted at room temperature. In order to minimise degradation, segments (typically 2-3 per sample), are taken from a 1-2 kg sample and analysed immediately. This approach provides such poor precision the results of duplicate analysis can be very different (e.g. less than the reporting limit and greater than the MRL for sub-samples of the same primary sample). Therefore the mean of 2 or 4 replicate analyses are reported. The Analytical Sub-Group of the PRC have stated that this is unacceptable and have requested that action is taken to address the problem.
The proposal is to evaluate cryogenic milling; it is expected that losses (perhaps 50-60% for some dithiocarbamate compounds) may occur but much better precision (1) compared to the segmentation procedure will result. If the precision is considered acceptable and the mean results from cryogenic sample processing and segmentation are not statistically different, then it should be possible to implement the use of cryogenic sample processing for the analysis of dithiocarbamates in the PRC programme. If this proves to be the case then cost benefits would result because the need for additional sample processing by segmentation would no longer be required (as most samples are already comminuted in the presence of dry ice for the analysis of other pesticides).
It is proposed that each sample will be analysed in duplicate by each of the two processing methods. One sample will be treated as normal and 4 sub-samples (50g) will be prepared by segmenting (2 will be analysed immediately and 2 will be retained for analysis if considered necessary). The remaining sample (~1 -2 kg) will be cryogenically milled and 2 randomly selected sub-samples (50g) will be analysed using GC-FPD (2).

There was a request from PSD to consider the use the LC-MS/MS method, reported by Brewin et al (3) for the analysis of ethylene bisdithiocarbamates (EBDC) fungicides, instead of the GC-FPD method. The LC-MS/MS method is not considered to be appropriate because;
1) it only includes the analysis of ethylene bisdithiocarbamates (EBDC) fungicides and not all classes of dithiocarbamates.
2) the method is much more complex than the CS2 method and thus would significantly increase the cost of the proposed project.
3) the GC-FPD determination method should provide sufficient precision to demonstrate that most variability occurs during the sample processing step.
However, If in the future the LC-MS method could be simplified to reduce costs and extended to include other classes of dithiocarbamates (e.g. via the CRL laboratories) then its application should be considered especially as the [-SCSNHCH2CH2NHCSS] chain is less volatile than CS2 and unlikely to suffer the same losses during cryogenic sample processing.

The Specific Objective is:

1) To evaluate the application of cryogenic sample processing for the analysis of dithiocarbamates, in an attempt to improve the precision and reduce the cost of this analysis.


Project Documents
• Final Report : Improving the reproducibility and cost efficiency of the analysis of dithiocarbamates   (233k)
Time-Scale and Cost
From: 2006

To: 2007

Cost: £46,518
Contractor / Funded Organisations
Central Science Laboratory
Keywords
Analysis              
Analytical Chemistry              
Analytical Quality Control              
Food Quality              
Food Safety              
Fruit              
Lettuce              
Monitoring and evaluation              
Pears              
Pesticide Residues              
Public Health              
Residues Testing              
Fields of Study
Pesticide Safety