Determination of complex mixtures of airborne isocyanates and aminesPart 4.† Determination of aliphatic isocyanates as dibutylamine derivatives using liquid chromatography and mass spectrometry

Abstract

An LC–MS method is presented for the determination of aliphatic isocyanate [hexamethylene diisocyanate] (HDI), isophorone diisocyanate (IPDI), biuret and isocyanurate] adducts of HDI as their dibutylamine (DBA) derivatives. The method is based on sampling in midget impinger flasks containing 10 ml of 0.01 mol l^–1 DBA in toluene (see parts 1–3 of this series†). The excess reagent and the solvent are removed by evaporation. The enriched samples are then analysed using LC–electrospray MS. Quantification of HDI–DBA and IPDI–DBA was effected by monitoring the molecular ion [MH]⁺. Linear calibration graphs were obtained in the range 50–500 nmol l^–1 with correlation coefficients of 0.9965–0.9997. The RSD for samples spiked at a concentration of 1 nmol l^–1 was 1.7 and 2.9% for the two IPDI–DBA isomers (n = 8) and 1.2% for HDI–DBA (n = 8). On injecting 4 µl of the 0.5 ml sample, the instrumental detection limit was about 20 nmol l^–1, which corresponds to about 0.1 µg m^–3 for HDI and 0.2 µg m^–3 for IPDI for a 15 l air sample.

No degradation was observed after storage of the derivatives for up to 4 months in acetonitrile or toluene. No losses were seen when the derivatization reaction took place in the presence of interferents such as morpholine, hexamethylenediamine, isophoronediamine and phenol corresponding to concentrations of 10 ppm in air for a 15 l air sample. The influence of water was studied by spiking with volumes corresponding to a relative humidity of 80% at 30 °C in 15 l of air. The reaction rates of HDI and IPDI with DBA were found to be 2–3 times faster than those for 1-(2-methoxyphenyl)piperazine.