I am having difficulty extracting Guthion from wine in the ppm range using 500mg C-18 SEP. I condition with ethyl acetate followed by 70% grain spirits then 10% grain spirits. I elute the compounds of interest with ethyl acetate. I am using the same method as another lab that obtains close to 100% recovery Guthion.

All of my other organochlorine and organophosphorous compounds are recovered except Guthion and I have no problem seeing guthion in a standard. I use a DB5 UI (but have also had no luck with a DB-17).

Any suggestions?