Determination of complex mixtures of airborne isocyanates and amines. Part 1. Liquid chromatography with ultraviolet detection of monomeric and polymeric isocyanates as their dibutylamine derivatives

Abstract

An LC–UV method using dibutylamine (DBA) as derivatization reagent for airborne monomeric and polymeric isocyanates is presented. Isocyanates were collected in impingers containing 0.01 mol l^–1 DBA in 10 ml of toluene. Stable isocyanate–DBA (urea) derivatives were rapidly formed in the sampling solution and the reaction was completed within 10 s. After evaporation of the solvent and excess reagent, the urea derivatives were analysed using reversed-phase LC–UV at 240 nm. Linear calibration graphs were obtained for toluene 2,4-and 2,6-diisocyanate (TDI) and methylene diphenyl-4,4′-diisocyanate (MDI) in the range 0.4–20 nmol ml^–1. The overall precision for a DBA solution spiked at concentrations of 540 µg l^–1 of 2,4-TDI, 540 µg l^–1 of 2,6-TDI and 520 µg l^–1 of 4,4′-MDI were found to be 1.1, 5.0 and 3.0%(n= 6), respectively. The detection limit was 0.5–0.8 µg m^–3 for a 15 l air sample. Ratio chromatograms between λ₁= 240 nm and λ₂= 254 nm demonstrated an increased selectivity. Interferences from morpholine, ethanol, phenol, toluene-2,4-and 2,6-diamine, 4,4′-methylenedianiline and water were studied by spiking the sampling solution with 6.2 µmol of each chemical, corresponding to an air concentration of 10 ppm. No losses of the urea derivatives were found. Air samples of TDI were simultaneously collected with impingers containing 9-(N-methylaminomethyl)anthracene (n= 10) and DBA (n= 10). No significant differences between the results obtained with the two methods were observed. Sampling of isocyanates in the work environment based on derivatization with DBA was demonstrated to be a robust method. DBA is especially well suited for the determination of complex mixtures of airborne isocyanates.