Pesticides are like Siblings, They Don’t Always Get Along part 2 – now with the GC mix!

Last year my colleague Landon published his blog about pesticide multiresidue LC mix stability in a celery matrix and in a solvent. Today, I’d like to show you what happens when you do this with the GC multiresidue pesticide kit.

On day 0, I prepared stock solutions from all 9 ampules of the GC multiresidue pesticide kit. These ampules contain between 8 – 40 pesticides in toluene and are formulated to be stable before opening and mixing. In my experience, they last a long time in the freezer as long as they are stored separately. The stock solution (containing the nine ampules mixed from the kit) was spiked into the celery matrix at 100 ppb level (final concentration) and split into three aliquots. The second set of samples was prepared from the stock solution in acetonitrile, also at 100 ppb/pesticide and split into three aliquots. Samples were also spiked with the internal standard triphenyl phosphate and the samples were analyzed the first day. After analysis, they were split into three groups based on storage: freezer, fridge, and autosampler tray. Samples were reanalyzed every 2-3 days up to 9 days and then on day 17. Samples were recapped after each analysis to prevent evaporation losses and septa contamination.

Figure 1: Comparison of the number of pesticide residues with a loss greater than 20% of the signal

We were keeping a tally of samples that lost more than 20%, using the peak area ratio of the pesticide peak and internal standard peak (Figure 1).

There were a few unexpected results:

•  The difference between solvent and matrix-based samples was minimal
•  Fridge samples had the most pesticides with sample loss
•  Freezer samples did not fare better than samples left in the autosampler.

In the end, I have to agree with Landon’s conclusions – prepare your stock solutions the day of analysis if you want to avoid overestimating the concentrations of your pesticide residues because even storing them in a freezer won’t stop the degradation.