monomers. It may also be suitable for quantification of other isocyanate species present below the UV LOD if their UV/FL ratios have been previously determined in a sample containing levels above the UV LOD. Excellent limits of detection have been reported for HPLC/MS, comparable to or better than those of MAMA and MAP derivatives by FL.99,100 A limitation of MS in quantifying isocyanate species for which analytical standards are not available is that compound-to-compound variability in response is likely to be very high. This is because the kinetics of the processes that produce ions for quantification in the MS are greatly influenced by the molecular structures of the compounds. As a result, quantification of non-monomeric isocyanate species based on the monomer response is likely to be very inaccurate. 6. CONCLUSIONS The ability to measure isocyanate-containing substances in air, whether they are in monomer, prepolymer, polyisocyanate, and/or oligomer forms, is important when assessing a worker’s isocyanate exposure. Adverse health outcomes from isocyanate exposure include irritation to the skin, mucous membranes, eyes, and respiratory tract; contact and allergic dermatitis; hypersensitivity pneumonitis; and respiratory sensitization (asthma). Respiratory sensitization (an asthma-like response) is the most common of these health outcomes with a prevalence ranging from 5 to 30% of workers in a variety of industrial processes. Experience has shown that monomeric, prepolymer, polyisocyanate, and oligomeric isocyanate species are capable of producing respiratory sensitization in exposed workers. After sensitization, any exposure, even to levels below existing occupational exposure limits or standards, can produce an asthma-like response that may be life threatening. Accurate and sensitive determination of isocyanates is complex and difficult. The advantages and disadvantages of the various methods must be understood, in order to choose the most appropriate sampling and analytical method for a particular workplace environment. Isocyanates may be in the form of vapors or aerosols of various particle size; the species of interest are reactive and unstable; few pure analytical standards exist; and high analytical sensitivity is needed. In addition, there are numerous points in the sampling and analytical procedures where errors can be introduced. The selection of the most appropriate isocyanate method for a given workplace environment is based upon an evaluation of measurement accuracy, specificity, sensitivity, convenience, simplicity, and speed. These factors must be considered for the entire analytical measurement process including collection, derivatization, sample preparation, separation, identification, and quantification. Unfortunately, the need to measure highly reactive isocyanate species at low levels is many times in conflict with the desire of industrial hygienists and chemists to choose methods that are convenient to use in the field and are easy to run in the laboratory. Table I summarizes NIOSH, OSHA, ACGIH, and United Kingdom HSE isocyanate exposure standards. Table II summarizes NIOSH and OSHA isocyanate methods and method selection for a given workplace environment. The selection of the most appropriate isocyanate method depends upon the isocyanate species, its physical state, its cure rate, the sensitivity required and other factors shown in Table II. This information, which is used to select methods for NIOSH research studies and health hazard evaluations, is provided when employers, industrial hygienists, or laboratories request NIOSH technical assistance on isocyanate methods.
In closing, more research is needed to resolve the limitations of current sampling and analytical methods. Such research is ongoing at NIOSH and elsewhere in government, in academia, and in the private sector. Therefore,
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