Abstract
Two new methods for the measurement of atmospheric isocyanates are under development to complement the HPLC-electrochemical method currently used by the Health and Safety Executive (Methods for the Determination of Hazardous Substances, MDHS 25).
The first is a general method for total isocyanate groups, in which sampled isocyanates are converted into urethanes by reaction with propanol in a bubbler. After removal of the excess propanol by evaporation, the urethanes are hydrolysed and propanol is released in an amount proportional to the original NCO groups present. The released propanol is determined by head-space gas chromatography. The method has significant advantages, in addition to its simplicity. Interfering co-pollutants, such as amines or polyols will not react with propanol. Volatile interferents will be removed in the evaporation stage and non-volatile interferents will not partition into the head-space. However, isocyanates react only slowly with propanol relative to amines; for this reason, undiluted propanol in very large excess is used as the reagent, but the presence of large amounts of amine may partly suppress urethane formation.
The second is a specific method for toluene diisocyanate (TDI) vapour, and is based on combining paper tape technology with diffusive sampling. Diffusive sampling offers considerable advantages in simplicity and convenience and enables eight-hour monitoring to be accomplished with tapes designed for short-term pumped sampling. The method uses the commercial 3M OVM badge; the supplied charcoal cloth (for organic vapours) is replaced by pieces of reactive paper tape as used in the SKC isocyanates paper tape monitor. After exposure, the tape can be examined densitometrically or compared with a stain chart.
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