Roy J. Rando, H.G. Poovey

Tulane University, School of Public Health & Tropical Medicine, Department of Environmental Health Science, 1430 Tulane Ave., New Orleans, Louisiana, USA 70112

The analytical chemistry of airborne isocyanates, which initially focused on colorimetric techniques, has matured with the introduction of various secondary amine reagents coupled with collection and chromatographic determination of the resulting urea derivatives. Recently, research has focused on two issues concerning the isocyanates. The first of these is the measurement of total reactive isocyanate group (TRIG) in the workplace environment. Case reports in the medical literature have suggested that occupational asthma may be induced by exposure to oligomeric and pre-polymeric isocyanates and other forms of TRIG in the absence of sensitization to the parent diisocyanate monomer. Furthermore, various isocyanate oligomers have been shown to be potent respiratory irritants in their own right. Promulgation of various exposure guidelines and regulations for isocyanate oligomers and other TRIG compounds has begun in Europe and in the United States.

The second issue of concern is the characterization of the physical state of airborne isocyanate compounds. Isocyanate compounds exhibit a wide range in volatilities. For example, HDI monomer has a saturated vapor concentration of about 66 ppm at room temperature, whereas HDI-biuret has an apparent saturated vapor concentration of less than 1 ppb. The physical state of airborne isocyanates will affect their pattern of deposition and uptake upon inhalation. In addition, the performance of respiratory protective equipment used for isocyanates is affected by the physical state - e.g., it has been shown that aerosol-bound isocyanate may readily penetrate charcoal respirator canisters, whereas isocyanate vapor is effectively retained with high capacity.

To aid in studying these issues, we have developed a dichotomous TRIG sampler which is shown in Figure 1. The sampler consists of an impactor or cyclone inlet in series with an annular diffusional denuder followed by a treated glass-fiber filter. The denuder walls and the back-up filter are both coated with 9-N-methyl-aminomethyl-anthracene (MAMA) and tributylphosphate. Isocyanate in the vapor phase diffuses to the walls of the denuder during transit and is collected while the respirable fraction of the aerosol phase exiting the inlet pre-separator penetrates past the denuder to be collected by the back-up filter.

Figure 1: The Dichotomous Aerosol/Vapor Isocyanate Sampler.

TRIG collected in the sampler's components is determined by a technique developed in our laboratory using reversed-phase HPLC with a triethyl-ammonium-phosphate buffer /acetonitrile mobile phase. Detection of MAMA-labeled isocyanate-urea derivatives is accomplished by analysis of detection response ratios at dual UV-absorbance wavelengths of 245 and 370 nm and confirmed by fluorescence. Quantitation is done in comparison to standards prepared from the MAMA-urea derivative of the parent diisocyanate monomer.

The performance of the MAMA technique for measurement of TRIG was evaluated for a series of 6 isocyanate- and 5 diisocyanate-monomers, and for 6 isocyanate-containing oligomers/pre-polymers based on TDI, MDI or HDI. For the 11 monomers, the average (s.d.) of UV absorbance response ratios was 10.5 (± 0.9) with a coefficient of variation of 8.3% and a range of 9.1 to 11.9. The recovery of TRIG from the 6 isocyanate oligomer/pre-polymer samples averaged 104 ± 4% in comparison to a reference titration assay using di-n-butylamine / HCl.

The MAMA-coated diffusional denuder is predicted to have a collection efficiency of over 94% for MDI vapor at a sampling flow rate of 2.8 L/min, according to Possanzini's algorithm.(1) In contrast, a 1-µm MDI particle is predicted to have a penetration efficiency of over 99%. The ability of the dichotomous sampler to separate MDI vapor and condensation aerosol in the laboratory is shown in Figure 2. Overlaid on the data is a model based upon the saturated vapor concentration of 4,4'-MDI as reported by Brochhagen and Shal (2). There is excellent correlation between the data and the model.

Figure 2: Separation of MDI Vapor and Aerosol by the Dichotomous Sampler

Dichotomous sampling of test atmospheres of nebulized TDI, HDI, and MDI oligomers in the laboratory showed partitioning of the monomers between vapor and condensed phases. In general, the oligomers did not partition between the phases but rather were found only in the aerosol phase. Workplace dichotomous sampling was conducted during polyurethane paint spraying applications(3) and during the manufacture of flexible polyurethane foam products. Levels of TRIG during the spraying operation averaged 391 ± 154 µg/m3. HDI concentrations averaged only 14 ± 6.5 µg/m3. HDI-biuret was the largest component of TRIG found in these samples and was completely in the aerosol phase. Significant (15 - 26%) proportions of the total HDI were also found in the aerosol fraction of the paint overspray. In the foaming factory atmosphere, TDI (up to 129 µg/m3) and small amounts of apparently low molecular weight TRIG compounds (up to 4.7 µg/m3) were measured in the atmosphere. Most of these materials were found to be in the vapor phase.

These results illustrate the validity of the dichotomous sampler for identifying TRIG and for studying the partition of isocyanate between vapor and aerosol phases. More research is needed to examine these complex phenomena in various work environments.

References 
1. M. Possanzini, A. Febo, A. Liberti, Atmos. Environ. 17 (1983) 2605. 
2. F. Brochhagen, H. Shal, Amer. Ind. Hyg. Assoc. J. 47 (1986) 225. 
3. R. Rando, H. Poovey. Amer. Ind. Hyg. Assoc. J. 60 (1999) 737.

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