MDHS 25 Revisited; Development of MDHS 25/3, the Determination of Organic Isocyanates in Air

Isocyanates (NCO) are potent sensitizers and the largest cause of occupational asthma in the UK. Probably, MDHS 25/3 is the method most commonly used worldwide for the determination of organic NCO in air. This method uses an electrochemical (EC) detector to quantify oligo-NCO using derivatized NCO monomers as calibrants. This paper gives the results of a validation exercise using this method to quantify industrially used monomeric and oligo-NCO formulations. An expanded uncertainty of approximately 56% was found for seven formulations spiked onto two sampler types at four spiking levels. No differences were found between the analytical results obtained from spiked filters or spiked impinger/filter samplers. Also presented is work on the validation of the EC/UV ratio approach used in MDHS 25/3 to identify NCO-derived peaks. A total of 58 industrially used non-monomeric NCO formulations were studied. These formulations gave 138 peaks that were identified using MDHS 25/3 as oligo-NCO MP derivatives. One of these peaks had an EC/UV ratio outside the range given in MDHS 25/3. Work describing the use of an internal standard to improve the variability of the EC detector is reported. Finally solutions to practical problems encountered during long-term sampling using an impinger are given.