A test-paper has been developed that will enable up to 0·10 p.p.m. v/v of tolylene di-isocyanate (TDI) vapour to be determined in a 5-litre sample of air drawn through a circle of the paper 1 cm in diameter at the rate of 1 litre per minute. In the presence of tolylene di-isocyanate a reddish grey stain is produced, development of which is complete within 15 minutes of the end of sampling. The intensity of the stain is compared with artificial standards made by coating paper with specially formulated pigment suspen sions. The 2,4-and 2,6-isomers react equally.

The test-papers are produced by treating Whatman No. 1 filter-paper with a solution of sodium nitrite, 2-hydroxy-11H-benzo[a] carbazole-3-carboxy-p-anisidide (Brenthol GB), ammonium acetate and diethyl phthalate in methanol. They are stable for at least 3 months when stored in the dark.

This method is simpler than existing “wet” methods and can be carried out by less skilled operators.