Method for analysis of methyltetrahydrophthalic acid in urine using gas chromatography and selected ion monitoring

A method for the determination of methyltetrahydrophthalic acid (MTHP acid), a metabolite of methyltetrahydrophthalic anhydride (MTHPA) in human urine, was developed. The investigated MTHP acid was obtained by hydrolysis of a commercial MTHPA mixture, composed of three major isomers. These were synthesized and identified as 3-methyl-delta 4-tetrahydrophthalic anhydride, 4-methyl-delta 4-tetrahydrophthalic anhydride and 4-methyl-delta 3-tetrahydrophthalic anhydride. The urine was worked up by a liquid-solid extraction technique using C18 sorbent columns. Esterification was performed with methanol and boron trifluoride. The derivative in toluene was analyzed with capillary gas chromatography and selected ion monitoring. Deuterium-labeled MTHP acid was used as internal standard. The intra-assay precision for the overall method was between 4 and 8% in the range 3-110 ng/ml and the inter-assay precision was between 4 and 7% in the range 30-110 ng/ml. The total recoveries of the MTHP acid at 19 and 190 ng/ml were 94 and 97%, respectively. The total detection limit for the three isomers was < 6 ng/ml. Analysis of urine from a worker exposed to MTHPA makes it reasonable to assume that the method may be used for biological monitoring of MTHPA exposure.