USP 561 Pesticide screen

[betz.jason]

This is a question just out of curiosity... Is there anyone working with the new USP 561 monograph regarding "General Method for Pesticide Residue Analysis"? Its an FDA regulated pesticide screen containing ~100 analytes and metabolites. I am currently running them on GC/MS and have produced quality results but looking to optimize sample prep extraction and clean up as well as instrument method. I would love to discuss this topic with anyone else involved

[Camisotro]

At my new job we have both a GC-MS and an LC-MS/MS, and we're looking into method development for USP 561 analysis. Still in the planning stages, but we have some interest in doing as much as possible by LC (especially ones that would otherwise require derivatization on GC) and then leaving to GC only the ones that won't co-operate with LC at all.

Any thoughts on North American sources for multi-component pesticide mixes that would be well-suited for a USP 561 method?

[betz.jason]

I wish you the best of luck on this whole analysis. Looking at 98 total peaks (analytes and metabolites) for each injection times 25 injections (on average) is nothing but a headache. I had no choice but to run everything on a GC/MS and in order to keep good separation and resolution my run time is approx 58 min. Over an hour per injection with running complex matrices it is impossible to keep good linearity.

We have conversed with a good number of reference standard suppliers regarding pesticide mixes that are appropriate to this application but have had no luck. We order everything through Accustandard (theres a number of metabolites that do not have reference standards... I called USP for assistance with no luck, they referred me to a contact at the FDA since they now regulate this USP analysis, and they also were no help). You can request an exact weight standard through Accustandard, then quantitatively transfer the neat material into a volumetric flask using appropriate solven and boom you have you're concentration (after accounting for %purity). I make an Intermediate mix containing ALL analytes at a level 20 times their EPA tolerance level (listed in the USP 561 monograph) and this becomes my spiking solution as well as my highest calibration standard. I make serial dilutions from there to produce a 6 point calibration curve. I can send you my material if you are interested. I am leaving my current job in a month and honestly this is the number 1 analysis I will be happy to wipe my hands clean from. I can't begin to tell you the frustration I've encountered from this analysis.

[Camisotro]

We have conversed with a good number of reference standard suppliers regarding pesticide mixes that are appropriate to this application but have had no luck. We order everything through Accustandard (theres a number of metabolites that do not have reference standards... I called USP for assistance with no luck, they referred me to a contact at the FDA since they now regulate this USP analysis, and they also were no help). You can request an exact weight standard through Accustandard, then quantitatively transfer the neat material into a volumetric flask using appropriate solven and boom you have you're concentration (after accounting for %purity). I make an Intermediate mix containing ALL analytes at a level 20 times their EPA tolerance level (listed in the USP 561 monograph) and this becomes my spiking solution as well as my highest calibration standard. I make serial dilutions from there to produce a 6 point calibration curve. I can send you my material if you are interested. I am leaving my current job in a month and honestly this is the number 1 analysis I will be happy to wipe my hands clean from. I can't begin to tell you the frustration I've encountered from this analysis.

Oh my, well that is very interesting to hear. I noticed that a lot of pesticide mixes in AccuStandard's catalogue are in organic solvents such as toluene, ethyl acetate, hexanes and DCM which are inappropriate for reverse-phase LC-MS. Which is too bad because they do have a number of mixes that come very close to being entirely composed of a subset of USP 561.

Suppose I were to order one of these mixed standards in toluene/DCM/ETAc/hexanes, and dilute it 100-fold into methanol as my intermediate standard. If I then dilute those into aqueous (or mostly aqueous) standards in order to have a proper injection solvent for LC-MS... should that work out fine, or am I asking for trouble?

I would certainly be interested in seeing your material providing that it is not a corporate secret or anything of that nature!

[betz.jason]

Camisotro: This sounds like it would work to me. Just check the solubility of your solvents before making any dilutions. Diluting 100 fold should be fine for running on an LC/MS since it will be more sensitive. I would dilute a couple volatile early eluters (Methamidophos or Acephate) and inject this to be sure you can see low concentrations. I encountered a lot of retention time shifts after making a mix of all the compounds, maybe reacting with each other in solution (I was not able to determine this). Using a reverse-phase type analysis should elimate possibility of shifting peaks.

You can purchase "neat" material from Accustandard. Request "exact weight" on your order form and they will give you a weight in grams up to 4 decimal places. 99% of the standards are soluble in Methanol so just rinse and quantitatively transfer to a volumetric flask and calculate your concentration based on the purity. In doing this you'll have a stock solution of each analyte in Methanol around 1000 ppm. Make your mix and serially dilute to make you're calibration curve.

[Camisotro]

Turns out Accustandard sells through Chromatographic Specialties in Canada and they charge 40-80% more on most supplies. Anyway we're ordering some standards to test out.

I see there is no PM feature implemented on this forum... if you'd like to contact me with details of your method and experience, I made a 1-week disposable forwarding address (to avoid spambot e-mail scraping): droodoussah@dunflimblag.mailexpire.com

[martinmeng]

This is a question just out of curiosity... Is there anyone working with the new USP 561 monograph regarding "General Method for Pesticide Residue Analysis"? Its an FDA regulated pesticide screen containing ~100 analytes and metabolites. I am currently running them on GC/MS and have produced quality results but looking to optimize sample prep extraction and clean up as well as instrument method. I would love to discuss this topic with anyone else involved

Hi Betz.Jason, I am assigned this project to set up the exact testing by GC/MS. I however would have to start from scratch. The GC/MS is not installed yet, but will be in a month. Do you mind leave me you contact information so that we can discuss in more details in the future? My e-mail is menggl@yahoo.com. Thanks a lot.

[Camisotro]

We have a small set of single compound standards here, and we've purchased a database of pesticide MRM transitions (with RTs for set methods & columns as well) in order to put together a dynamic MRM method for eventually as many of the list as we can get.

Comments so far - pretty much any synthetic pyrethroid is just not working. Cypermethrin, permethrin, deltamethrin, fenvalerate, flucythrinate, cyhalothrin. Either it doesn't come up using the transitions in the literature/database, or the signal is very weak, or it appears on one transition but not the other. Fortunately all the above are looking pretty good on GC-MS. In contrast all six of the natural pyrethrins are working for us quite well on LC-MS/MS, and almost impossible to find on GC-MS.

Everything else in the database that we've tried is working. There are just some peculiarities. With dichlofluanid, the 332.97->223.95 shows a peak with a shoulder that elutes first, but the -> 123.02 peak is missing the same shoulder. When I reduce the injection from 1 uL to 0.2 uL, the peak with both transitions becomes accordingly smaller, but the shoulder only reduces in size by about 20%.
So I decided to discard the 223.95 transition completely. Instead my confirmation peak will be an isotopic peak, 334.97-> 123.02 which comes out nicely. My theory is that dichlofluanid is reacting/degrading in solution and the product fragments in-source back to the original MH+ mass; however since one side of the molecule was modified, it no longer shows the same product ions.