Stability of Selected Pesticides on Solid-Phase Extraction Disks

The storage stability of 2,4-D (2,4-dichlorophenoxy acetic acid), triclopyr ([(3,5,6-trichloro-2-pyridinyl) oxy] acetic acid), carbofuran (2,3-dihydro-2,2-dimethylbenzofuran-7-yl methylcarbamate), molinate (S-ethyl hexahydro-1 H-azepine-1-carbothioate), and thiobencarb (S-[(4-chlorophenyl) methyl] diethylcarbamothioate) on C₁₈ solid-phase extraction (SPE) disks was determined under three temperature regimes. Water was fortified with either mixtures of the five pesticides at 20 μg L^-1 of each pesticide or with methanol. Storage treatments included storage in water at 4°C, or the analytes were extracted onto the SPE disks and stored at 4°C, -20°C, or 4°C for 1 d followed by -20°C for the remaining storage period. Residue analyses were conducted after 0, 3, 30, 90, and 180 d of storage. All pesticides evaluated were more stable when stored on the disks vs. cold storage in water. Carbofuran was the least stable of the pesticides evaluated with losses ranging from 13 to 100% depending on storage period. The pesticides were most stable on the disks at temperature regimes that included -20°C with losses of ≤20% for 2,4-D, triclopyr, molinate, and thiobencarb for storage periods of 180 d. Storage of the same pesticides in water at 4°C resulted in losses of 25 to 35%.