Evaluation of the SureSpot direct-reading instrument for the determination of polymeric MDI aerosols

Recent studies in our laboratories have focused on the reliability of direct-reading instruments for the determination of airborne isocyanate concentrations. The evaluation of airborne isocyanates is complicated because these substances exist as diisocyanate monomers and polyisocyanate oligomers, both in the vapor and aerosol phases. The studies showed that a number of direct-reading instruments, including the SureSpot test kit, do not allow an accurate determination of isocyanates in the aerosol phase. Using a test chamber, concentrations of various commercial polymeric methylenediphenyl diisocyanates (PMDI) were generated in the aerosol phase to examine the correlation between the SureSpot monitor and two laboratory methods. The results obtained with the SureSpot and laboratory methods correlated poorly. More precisely, the results indicated that the SureSpot direct-reading instrument had a distinctive response to each commercial PMDI and, in addition, it was not accurate for the determination of total PMDI in the aerosol phase. It seemed that the analytical response of the SureSpot is based on a calibration curve only reliable for MDI and not for the determination of PMDIs in the aerosol phase. Further investigation also indicated that the calibration provided by the manufacturer could not be converted into a linear curve over the suggested MDI concentrations range. An appropriate calibration procedure was developed in our laboratories for the SureSpot to accurately determine all commercial PMDIs without great variability.