A laboratory comparison of analytical methods used for isocyanates

Monomeric and oligomeric 1,6-hexamethylene diisocyanate (HDI) and isophorone diisocyanate (IPDI) are components of catalyst hardeners in two-part polyurethane coating systems. Exposure to these isocyanates in the collision repair industry has been associated with increased risk for work-related asthma; however their quantitation remains a challenging task. Four analytical methods were compared: modified version of the National Institute for Occupational Safety and Health Method 5525 (NIOSH); liquid chromatography and mass spectrometry (LC/MS) using 1-(2-methoxyphenyl)piperazine (MPP) as the derivatizing reagent; modified version of the Occupational Health and Safety Administration Method 42/PV2034 (OSHA); and modified version of the Omega ISO-CHEK® Method (WA-DOSH). Five levels of a widely used clear coat hardener were spiked onto sampling media. A reference value was calculated by using a hardener NCO titration and manufacturer composition data. Intra- and inter-method variability was determined. All methods measuring NCO-HDI monomer, NCO-IPDI monomer, NCO-IPDI oligomers, and total NCO mass (except OSHA method) compared well against the reference values (slopes ≥ 0.816, R² > 0.90). The NCO-HDI oligomer results for the NIOSH method compared well with the reference values (slope = 1.161, standard error = 0.046 and R² = 0.98, p < 0.001) while WA-DOSH were above (slope = 2.293, standard error = 0.055 and R² = 0.99, p < 0.001) and LC/MS were below (slope = 0.264, standard error = 0.011 and R² = 0.98, p < 0.001) the reference values. The principal challenges associated with comparing methods were: 1) the reporting metrics were not always directly comparable and 2) not all methods reported all isocyanate species of interest. Although this present study provided valuable information, a more extensive investigation is required in order to critically evaluate these methodological differences.