Simple sensitive solid-phase extraction of paraquat from plasma using cyanopropyl columns

We report an inexpensive, sensitive paraquat quantitation method which is simple to perform. First, 5 mL of blanks, standards, or patient plasma are applied to 1-mL cyanopropyl extraction columns equipped with 15-mL reservoirs. The samples are drawn through the columns under vacuum, followed by a rinse with 15-20 mL of 0.1M NH4OH. Paraquat is eluted with 0.8 mL 0.1M HCl, which is then neutralized with 25 microL concentrated NH4OH. Sodium dithionite reagent (0.23M in 4M NaOH) is added (100 microL) and the color produced is measured by absorbance difference (A395-A460). The assay is linear up to at least 4.35 microM paraquat. The lower limit of quantitation is 0.23 microM. Lipemic and icteric sera do not affect the method, but easily visible hemolysis elevates the concentrations measured by up to 0.7 microM, independent of paraquat concentration. Equimolar amounts of diquat with paraquat, at paraquat concentrations from 0.4 to 4.0 microM, elevate apparent paraquat concentrations by 0.08-0.28 microM. At 0.632, 1.92, and 4.06 microM paraquat, within-run coefficients of variation (CVs) were 6.27, 7.23, and 2.14%, and between-run CVs were 6.82, 8.42, and 4.43%, respectively.