Walnuts have high fat content and it's possible that some apolar pesticides stay in an emulsion layer which forms at the interface of the aqueous layer and the acetonitrile layer. Moreover I think that fats are poorly soluble in acetonitrile.

Are your apolar pesticides that exhibit the lowest recovery rates ?

it would be a clue to this hypothesis.

In order to remove some fat at the first step of Quechers method to adress this issue and before cleanup step , you could use "freezing out" method by storing first step solution (just after the extraction with acetonitrile and before add salts) at -20°C , then remove the solid fat layer and go on further steps

I am looking for a large number of analytes but the lowest recoveries I have found come from the compounds: fenpyroximate, quinoxyfen, etoxazole, hexythiazox, chlorpyriphos, buprofezin, clofentezin, and cyprodinil.@Alain Delvaux.

No. As a matter of fact QUETCHERS cannot cleanup samples with less than 25% water content. Nuts contain less than 10% water and 50-70% lipid matrixes. In fact it is rather a cleanup problem. Please try the low temperature cleanup method described in

ISSN 1615-9306 · JSSCCJ 40 (11) 2315–2xÓÓ (2017) · Vol. 40 · No. 11 · June 2017 · D 10609. Seems quite reliable.

@ William Beckham

Hello,

There can be many reasons for poor recovery performance. Nevertheless, for the pesticides I know (chlorpyriphos and cyprodinil), it seems to me that they are relatively stable under Quechers conditions (pH 4-5, acetonitrile-water). To find out if the particularly high lipid matrix is the cause of poor recovery yields, you can examine the solubility data for pesticides in water and/or the water/octanol partition coefficient and relate them to the respective recovery yields. Otherwise I found an article detailing the conditions for extracting pesticides in high-fat matrices (peanuts) using an "adapted" Quechers method : https://pubs.acs.org/doi/abs/10.1021/jf902988b?src=recsys&journalCode=jafcau

If neither freezing-out nor this adapted method or the use of a thorough clean-up (SPE,dSPE (C18, Z-Sep,...) do not work, you can always "correct" yields with the addition of deuterated pesticides (before extraction of course).