Interlaboratory comparison of pesticide recovery from water using solid-phase extraction disks and gas chromatography

An interlaboratory study was conducted to assess the suitability of C18 solid-phase extraction disks to retain and ship different pesticides from water samples. Surface and deionized water samples were fortified with various pesticides and extracted using C18 disks. Pesticides were eluted from disks and analyzed in-house, or disks were sent to another laboratory where they were eluted and analyzed. Along with the disks, a standard pesticide solution in methanol was also shipped to be used for fortification, extraction, and analysis. The highest recovery from deionized or surface water using shipped disks was obtained for cyanazine (>97%), followed by metalaxyl (>96%), and atrazine (>92%). Although <40% of the bifenthrin, chlorpyrifos, and chlorothalonil fortified in surface water was recovered from shipped disks, recoveries from deionized water were >70%. From in-house eluted disks, bifenthrin and chlorpyrifos were recovered at 118 and 105%, whereas chlorothalonil showed 71% recovery, indicating that poor recovery from surface water was due to loss during shipping rather than low retention by the C18 disks. There was no consistent relationship between recovery from C18 disk and physicochemical properties for the pesticides included in this study. For most of the 13 pesticides tested, there were no differences in recovery between in-house extracted disks and shipped disks, indicating the suitability of disks to concentrate and transport pesticides extracted from water samples.