A total isocyanate method based on derivatization with MAMA and either FL/UV or UV/UV detection has been developed.91,98 MAMA absorbs very strongly in the UV at 256 nm. Even though this is not a particularly selective wavelength, the intensity of the absorbance is so great that nearly all the absorbance of a MAMAderivatized isocyanate at 256 nm comes from the MAMA group. This results in very small compound-tocompound variability of UV responses (Sr 7-14%)62,98, which facilitates identification. Secondary absorbances (such as 366 nm) are relatively weak but highly selective, so that UV/UV ratios are very constant and diagnostic (Sr 3-8%).62,98 FL compound-to-compound variability is relatively high (Sr 55-59%).62,98 But, as in the case of tryptamine, the inherent selectivity of FL detection makes FL/UV identification attractive, especially given the substantially better sensitivity than a method relying on UV/UV identification. The NIOSH method under development using the MAP reagent relies on FL/UV detection for identification. Like MAMA, MAP contains an anthracene group as the chromophore/fluorophore so that the absorbance and fluorescence characteristics of the two reagents are very similar. MAP derivatives appear to have somewhat lower compound-to-compound variabilities in both UV (Sr 3.5%) and FL (Sr 33%) responses than the corresponding MAMA derivatives.62 Instead of using two one-dimensional detectors in series, some methods utilize multidimensional detectors for identification. It has been found that photodiode array (PDA) detection, which provides an entire UV spectrum of a chromatographic peak, is useful in identifying MOPP-derivatized isocyanates.95 The developers of MDHS 25/2 now advocate the use of the PDA detector to strengthen identification.86 The disadvantage of the PDA is that it is somewhat less sensitive than standard UV detectors. Several researchers have investigated the mass spectrometer (MS) as a detector for derivatized isocyanates.99,100,101,102 The MS has the potential to go beyond simply identifying a compound as a derivatized isocyanate. It can provide considerable information about the structure of the compound and serves as an important research tool. The primary disadvantages of using HPLC/MS for typical sample sets are the expense of the instrument and the expense and nonroutine nature of its operation. g. Quantification Once a chromatographic peak has been correctly identified as being isocyanate-derived, it must be quantified. For methods used to determine monomeric isocyanates only, identification is generally based on retention time and analytical standards exist that enable direct construction of a calibration curve for quantification. The major factor in choosing a derivatizing reagent/detector combination is LOD. The LODs for reagent/detector combinations used in NIOSH and OSHA methods are given in Table II. With regard to NIOSH and OSHA methods limited to measuring monomer, NIOSH 2535103 uses the nitro reagent with UV detection to determine TDI and HDI. This reagent/detector combination provides relatively poor sensitivity. OSHA 42104 and OSHA 47105 use 1-(2-pyridyl)piperazine (PP) to derivatize TDI, HDI, and MDI with detection by either FL or UV. FL detection of PP derivatives provides fairly good sensitivity. Methods for quantifying non-monomeric isocyanate species can be divided into those that use the bulk product for calibration, such as Miles (Bayer) Method 1.4.3,106 and those that quantify every isocyanate derivative in the chromatogram based on the response of the derivatized monomer (e.g., MDHS 25/249 and NIOSH Method 552279). Miles Method 1.4.3 uses known amounts of derivatized bulk isocyanate product to construct a calibration curve.106 The areas of the largest peak or several largest peaks in the chromatogram are plotted against the concentration of the product injected. Using this calibration curve, the peak areas in real samples can be correlated with a quantity of bulk isocyanate product. Taking this a step further, the total isocyanate concentration in the sample can be calculated using the known or measured isocyanate content of the bulk 1/15/98
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