Determination of airborne anhydrides using LC-MS monitoring negative ions of di-n-butylamine derivatives

Abstract

An air sampling method for simultaneous determination of organic acid anhydrides and isocyanates is presented. Air samples are collected in impinger flasks filled with 0.01 M di-n-butylamine (DBA) in a mixture of toluene-acetonitrile (7:3, v/v) with a 13 millimetre glass fibre filter in series. The amount of anhydrides and isocyanates are determined as their amide and urea derivatives using LC-MS. Four anhydrides, maleic anhydride (MA), phthalic anhydride (PA), tetrahydrophthalic anhydride (TA) and cis-hexahydrophthalic anhydride (HA) and 11 isocyanates could be separated in 9 minutes using gradient elution. Anhydride-DBA derivatives in standard solutions were quantified using LC with chemiluminescent nitrogen detection (CLND). Anhydride-DBA derivatives were found to be stable for at least two months when stored in acetonitrile or toluene in the freezer. The yield of DBA derivatives of anhydrides in the 0.01 M DBA in toluene-acetonitrile (7:3, v/v) was in the range of 70->95%. Using MS and negative electrospray ionisation (ES-) linear calibrations curves were obtained for the anhydrides with correlation coefficients ranging from 0.9970-0.9997. The instrumental detection limit for the anhydrides ranged from 10-30 fmol, based on a signal to noise root mean square (RMS) ratio of 3. Monitoring positive and negative ions simultaneously, both isocyanates and anhydrides could be determined as their DBA derivatives in the same chromatographic run. When air samples were collected during thermal degradation of different coated metal sheets both anhydrides and isocyanates were present in the same samples and all the studied anhydrides were found.