Liquid-solid extraction (LSE) cartridges containing an intermediate strength, primarily tertiary amine, anion exchanger are mounted on a vacuum manifold and conditioned with appropriate solvents. LSE disks may be used instead of cartridges if all quality control criteria are met. A 100-mL sample is extracted and the analyte is eluted with 8 mL of acidic methanol. After addition of a small volume of methylene chloride as a co-solvent, the dimethyl ester of endothall is formed within 30 min with modest heating (50'C). After addition of salted reagent water, the ester is partitioned into 8-10 mL of methylene chloride. The extract volume is reduced to 1 mL with nitrogen purge for a concentration factor of 100. The extract is analyzed by GC/MS or GC/FID with a megabore capillary column.