The reference values were determined by USEPA SW846 (3rd edition) Analysis Method 8141A. Certified valuesAzinphos-methyl (Guthion) . 587 μg/kg Malathion. 2790 μg/kg Chlorpyrifos . 10300 μg/kgMethyl parathion (Parathion, methyl) 2620 μg/kgDemeton-S 667 μg