Controlled thermal degradation for the identification and quantification of amine N-oxides in urine

Studies of amine N-oxides in urine are important for the evaluation of occupational exposure to amines. These thermolabile compounds are difficult to handle by either gas or liquid chromatography, so a device for controlled thermal degradation has therefore been developed. It consists of a short precolumn with shut-off valves at both ends and an aluminum block for heating, and it was connected to the injection port of a gas chromatograph. After injection of amine N-oxides onto the precolumn and thermal degradation, the degradation products were allowed to enter the analytical column. Trimethylamine N-oxide (TMAO) and triethylamine N-oxide (TEAO) were investigated. Their thermal degradation patterns could be used for identification and quantification in aqueous solutions and in urine. Linear calibration graphs based on degradation product peaks (trimethylamine and O,N,N-trimethylhydroxylamine from TMAO and diethylamine and triethylamine from TEAO) were obtained for concentrations up to 500 ppm. Detection limits in aqueous solutions were 0.2 ppm (ca. 1 ng) for TMAO and 1 ppm for TEAO and the precisions were 6% and 9%, respectively. In urine, similar values were obtained for TEAO. The detection limit for TEAO corresponds to the expected concentration in urine after an 8-h exposure to air containing 0.8 mg/m3 of triethylamine.