product. There are several advantages to this approach. It requires the integration of at most a few peaks in the sample chromatogram. Since identification is based on retention time, only a single detector is needed. Finally, it does not depend on all isocyanate species in the sample eluting from the HPLC as identifiable peaks. The disadvantage of this approach is that it does not enable accurate quantification of total isocyanate group in all environments. For example, this approach cannot be used to quantify the isocyanate species produced by thermal decomposition of polyurethane. Also, even when an isocyanate product is being used, this approach cannot take into account newly formed isocyanate species that are not present in the product. The approach assumes that the major components in the bulk are the major components to which exposure occurs. This is probably a reasonable assumption for spraying relatively slow-curing isocyanate products. The more reactive the product and the greater the time between initiation of curing reactions and exposure, the more likely that the composition of the isocyanate species to which exposure occurs will differ significantly from the composition in the bulk product. Total isocyanate methods that attempt to quantify every isocyanate species in a chromatogram typically use two detectors for identification; only one detector is required for quantification. Nevertheless, the method LOD is generally limited by the LOD of the less sensitive detector, since below that LOD a compound cannot be properly identified. A possible exception to this is for isocyanate compounds that are present in the bulk product. Analysis of a relatively high concentration of the derivatized bulk product enables evaluation of each chromatographic peak using both detectors. Derivatized isocyanates can then be identified in real samples if their retention times match those of compounds identified as isocyanates in the bulk product, even when the levels are below the LOD of the less sensitive detector. In addition to LOD considerations, an important aspect of quantification of total isocyanates is accuracy. To quantify compounds for which analytical standards are not available, the detector response factor for the unknown derivatized isocyanate species must be the same as that of the derivatized monomer. This is achieved by choosing a derivatizing reagent/detector combination such that nearly all the detector response is attributable to the derivatization reagent label and that response does not change from compound to compound. Whereas a certain amount of compound-to-compound variability in detector response is tolerable for compound identification, especially when dealing with very selective detectors, compound-to-compound response variability in the detector used for quantification translates directly to errors in quantification. For MDHS 25,78 which quantifies MOPP-derivatized isocyanates by EC, two studies have found compound-tocompound variability of the EC response for model compounds to be Sr of 26%97 and 28%62, respectively. Also, a study of MOPP-derivatized urethane oligomers found that the EC response is not proportional to the number of derivatized isocyanate groups. Instead, it was found that the EC response increased as the size of the oligomer increased.96 Methods employing the tryptamine reagent use FL and EC detection; FL is recommended for quantification. Studies have found the compound-to-compound variabilities by FL in the range of Sr 13-26%.62,82,97 The sensitivity for detection of tryptamine-derivatized isocyanates by FL is fairly good. Methods employing the MAMA and MAP reagents use UV and FL detection. The very small compound-tocompound variability in UV response for MAMA- and MAP-derivatized isocyanates, coupled with the fairly good sensitivity for UV detection around 256 nm, makes quantification of MAMA and MAP derivatives by UV arguably superior to other reagent/detector combinations. The sensitivities of detection for MAMA and MAP derivatives by FL are better than any other reagent/detector combination in common use for isocyanate determination. Unfortunately, the compound-to-compound variability is unacceptably high for quantification of derivatized isocyanates for which standards are unavailable. Fluorescence detection can be used to quantify 1/15/98
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