Determination of Gas Phase Isocyanates Using Proton Transfer Reaction Mass Spectrometry

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We report the development of proton transfer reaction-mass spectrometry (PTR-MS) methodology for on-line monitoring, using a direct reading instrument, of several typical gas phase isocyanates, including isocyanic acid (ICA), ethyl isocyanate (EIC), phenyl isocyanate (PhI), hexamethylene diisocyanate (HDI) and toluene diisocyanate (TDI). In the study, ICA and MIC were generated by thermal degradation of urea and 1,3-dimethylurea, respectively, while the other isocyanates were generated by liquid and gas permeation techniques. Comparative, reference measurements were made by sampling isocyanate atmosphere using impinger flasks containing the reagent di-n-butyl amine (DBA) then determining the resulting DBA derivatives by liquid chromatography (LC)-MS/MS. Reproducible measurements were obtained, with a <10 %drift in PTR-MS responses during a 12 month period. Further, there were linear correlations (R2>0.99) between the data acquired by the PTR-MS technique and air sampling followed by LC-MS/MS for all tested isocyanates in the concentration range 2–100 ppb, with a PTR-MS detection limits in the low ppb range. For all isocyanates except EIC, BIC, HDI and IPDI, the protonated molecular ions were the most abundant ions in the PTR mass spectra. Overall, the results show that the developed method enables sensitive, time-resolved measurements of airborne isocyanates to be acquired over several weeks.