Pesticides are like Siblings – some get along well and some don’t – No. . .Really?

My colleague, Joe Konschnik and I have been asked by many food chemists out there about how much they can trust their pesticides mixes after they combine them into one single larger mixture for calibrating their instrument, or monitoring the accuracy and precision of their method – and rightfully so.  Well, like siblings, sometimes they get along well and sometimes they don’t.  As with many things we purchase there’s an expiration date on the label.  Through much testing and experience, companies can predict when our meat will spoil, when our canned goods are no longer safe and when our kid’s car seats need to be replaced.

If you’ve ever purchased a reference standard or certified reference material (CRM), you’ll notice there’s an expiration date on the label.  These dates tell you how long the components in that solution are stable and the date by when it should be used.  However, once you crack open the standard ampule, or other container, and especially when you mix its contents with any other CRM, chemical or matrix, that expiration date no longer applies.  All CRMs, which are those reference materials manufactured according to the strict ISO 17034 standard, are required to remain stable only until the container is opened for use.

To illustrate what happens when you combine the ten mixes contained in RESTEK’s multiresidue pesticides LC kit (Catalog #31971) let’s look at a recent study we did at Restek where over 200 pesticides were spiked into then extracted from celery matrix.  You can see the featured application here.

For this analysis we spiked all of the components of the LC Multiresidue Pesticide Kit  into the celery matrix.  The LC Multiresidue Pesticide Kit contains 204 pesticides that are grouped into 10 different vials for long-term stability.

Organophosphorous, Carbamate, Uron, and Organonitrogen pesticides are all represented in these mixtures.  However, once all ten vials are mixed, some pesticides in the mix begin to degrade AND matrix can further accelerate this degradation.

While CRM suppliers may have a lot of data regarding the stability of a single, or multi-component reference standard, very little data exists for what happens once they are mixed together.  Joe suggested we try an experiment to find out what happens and I agreed so here’s what we did.  For this experiment, the pesticides were solvated in 90:10 Water:ACN (containing celery matrix).  They were measured against a matrix matched calibration curve and a stable internal standard to determine precision, accuracy and concentration during the evaluation.  Over the testing period the samples were stored on a Peltier cooled autosampler (4 °C).

After a single day, 14 residues had lost 20% of their signal and after the entire eight day trial, more than 50 residues had lost more than 20% signal.  To compare the effect of matrix on the degradation, a matrix free study was performed with the same concentrations and solvents.  After eight days, only 19 residues lost 20% signal, showing that even a simple matrix like celery can hasten degradation.

This observation showed us our calibration curves and standards should be made daily.  The interactions with the pesticides themselves and the interactions of the pesticides with the matrix required it.  While this meant more work at the beginning of each day, we see this as a best practice for how to obtain data with the greatest integrity.

To clarify, this doesn’t mean having to crack open a new ampule each day, but simply using new aliquots from the vials provided with your ampules.  Upon opening each ampule, the contents should always be transferred to the vial provided and properly stored following the guidance in your CoA for future use.

We suggest, based on this information, it would seem appropriate for each lab to determine their own laboratory specific quality assurance procedures for handling and measuring the stability of their pesticide mixtures prior to and during analysis.  The decision whether or not to reuse a mix should be based upon a study which determines the stability of the compounds of interest in specified matrices.  This will ensure the best practices for handling reference materials for their intended purpose.

…by the way, May 2nd is National Brothers and Sisters Day according to National Today https://nationaltoday.com/national-brothers-sisters-day/, so don’t forget to reach out to thank, congratulate, tease, hug or otherwise irritate that special sibling of yours.  And. . .good luck getting along with your sibling(s)!!!

2 Responses to “Pesticides are like Siblings – some get along well and some don’t – No. . .Really?”

  1. Derek says:

    Did you try with the mixture dissolved in just ACN? The presence of the water is driving a hydrolytic reaction. Most pesticides are designed to rapidly degrade (so that there isn’t a residue problem with the parent chemical) and water and oxygen are the primary drivers of those reactions. Also like you say some things do not play well and get together and create something completely different. Good information to have out there.

  2. Landon Wiest says:

    Hi Derek, thanks for your question. Having been at many conferences presenting this data, I have given your question some thought and multiple people have asked similar things. Sorry for the long turnaround time. In terms of general storage, ACN would be the way to go, however, when diluting and creating calibration curves, it is good chromatographic practice to introduce your sample in as similar a solvent as the initial solvent of your method. In this case, the sample is good for chromatography, but as you noted in your question, many pesticides are designed to degrade in the presence of water. When separating with a reversed-phase mechanism, more water typically makes for a better chromaotgraphy, especially with early eluting peaks like cyromazine. The conclusion we realized after having multiple conversations internally and with customers is that you need to test the suitability of your system on a regular basis as determined by your organization or even your experiment. Make sure that your samples are fresh. If the analytes your are interested are degrading significantly over a day, then you might have to make your calibration standards every day. Time consuming, but at least you then have confidence in your data.

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