Analysis of methylhexahydrophthalic acid in human urine

A method for analysis of urinary methylhexahydrophthalic acid (MHHP acid), a metabolite of the highly sensitising methylhexahydrophthalic anhydride, is described. The method is based on a double liquid-solid extraction of the MHHP acid from urine, esterification with methanol and boron trifluoride catalysis, and analysis with gas chromatography-mass spectrometry in the electron impact mode. The detection limit was 3 ng mL–1 and the precision between 2 and 5% in the range 34 to 340 ng mL–1. The method gave similar results compared to another method. Urine stored for five years at –20°C lost between 38 and 86% of the MHHP acid (range 16–85 ng mL–1).