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 all isocyanate species to which a worker may be exposed are present in the product. When isocyanate products are being used, the components are typically undergoing curing reactions with polyols, so new species containing isocyanate groups are being generated. Isocyanate-containing species are also generated during thermal breakdown of polyurethane. Chromatographic retention times are not available to identify these new species. In order to identify all isocyanate species — monomers or oligomers, those present in the bulk product and those newly generated — a means of identification other than chromatographic retention time is necessary. Correct identification of unknown isocyanate species requires that the detection scheme be selective or provide some qualitative information about the species in question. Isocyanate methods generally utilize derivatizing reagents that are responsible for the detectability of the reagent/isocyanate derivative. Total isocyanate methods generally seek to identify all compounds labeled with the derivatizing reagent. Knowledge of the work environment is required to discount any nonisocyanate species that may react with the derivatizing reagent and give a signal in the sample chromatogram. Once these compounds are accounted for, it is assumed that all other compounds in the chromatogram that contain the reagent label are derivatized isocyanates. A single non-selective detector (such as a UV detector) is insufficient for total isocyanate analysis because it provides little qualitative information about the chromatographic peaks. However, two detectors in series provide a considerable amount of qualitative information. The reason for this is that when the responses for each detector are in the detectors' linear operating range, the ratio of detector responses is a constant (i.e., independent of concentration) for any given compound. Therefore, a compound can be identified by that ratio. If the detector responses of derivatized isocyanates are derived primarily from the reagent label, then it is conceivable that all derivatized isocyanates would have similar detector response ratios and could be identified on the basis of those ratios. Several total isocyanate methods operate by this strategy.78,79,82,84 A widely used method for total isocyanate group is MDHS 25.78 It identifies isocyanates derivatized with MOPP based on the ratio of EC and UV detector (242 nm) responses. The EC detector is sensitive and fairly selective,94 but it also has been found to be relatively unstable.82,95 UV absorbance at 242 nm is not selective and not especially strong for MOPP-derivatized isocyanates.62 As a result, a substantial portion of the absorbance of MOPP-derivatized aromatic isocyanates comes from the aromatic portion of the isocyanates rather than from the MOPP-derivatized isocyanate group. Under these circumstances, an oligomeric aromatic isocyanate compound containing more aromatic rings per isocyanate group than the derivatized monomer will have a substantially higher UV response per isocyanate group than the derivatized monomer. This could adversely affect the identification of unknown isocyanate species. This problem was demonstrated in a study involving 2,4-TDI-based urethane oligomers.96 This same study found that the EC detector response for these compounds is not directly proportional to the number of derivatized isocyanate groups. It was concluded that MOPP-derivatized oligomeric isocyanates, particularly oligomeric aromatic isocyanates, are likely to give EC/UV ratios substantially different from the EC/UV ratio of the MOPP-derivatized monomer and therefore not recognizable as isocyanates. Methods based on tryptamine derivatization, including NIOSH 552279 and the method developed by the Ontario Ministry of Labour,82 use FL and EC detection in series for identification. Estimations of the compound-tocompound variabilities of detector responses, as indicated by their relative standard deviations (Sr), vary in different studies (FL Sr 13-26% and EC Sr 18-71%).62,82 ,97 Not only is the compound-to-compound response variability relatively small for FL detection of tryptamine derivatives, but the selectivities of FL and EC detection are much greater than that of UV detection. Overall, FL/EC detection of tryptamine derivatives would appear to give more reliable identification than EC/UV of MOPP derivatives.

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