Gas chromatography electron-capture detection of urinary methylhippuric acid isomers as biomarkers of environmental exposure to xylene

Methylhippuric acid isomers (MHAs), urinary metabolites of xylenes, were determined, after clean-up by C18-SPE and esterification with hexafluoroisopropanol and diisopropylcarbodiimide, by GC with ECD detection, on an SPB-35 capillary column (30 m, 0.32 mm I.D., 0.25 mu m film thickness, beta = 320). S-benzyl-mercapturic acid was used for internal standardization. Chromatographic conditions were: oven temperature 162 degrees C, for 14.2 min; ramp by 30 degrees C/min to 190 degrees C, for 3.5 min; ramp by 30 degrees C/min to 250 degrees C, for 4 min; helium flow rate: 1.7 ml/min; detector and injector temperature: 300 degrees C. The sample (1 mu l) was injected with a split injection technique (split ratio 5:1). MHA recovery was > 95% in the 0.5-20 mu mol/l range; the limit of detection was < 0.25 mu mol/l; day-to-day precision, at 2 mu mol/l, was Cv < 10%. Urinary MHAs were determined in subjects exposed to different low-level sources of xylenes: (a) tobacco smoking habit and (b) BTX urban air pollution (airborne xylene ranging from 0.1 to 3.7 mu mol/m(3)). Study (a) showed a significant difference between urinary MHA median excretion values of nonsmokers and smokers (4.6 mu mol/l vs. 8.1 mu mol/l, p < 0.001). Study (b) revealed a significant difference between indoor workers and outdoor workers (4.3 mu mol/l vs. 6.9 mu mol/l, p<0.001), and evidenced a relationship between MHAs (y, mu mol/mmol creatinine) and airborne xylene (x, mu mol/m(3)) (y = 0.085 + 0.34x; r = 0.82, p < 0.001, n = 56). Proposed biomarkers could represent reliable tools to study very low-level exposure to aromatic hydrocarbons such as those observed in the urban pollution due to vehicular traffic or in indoor air quality evaluation. (C) 1999 Elsevier Science B.V. All rights reserved.