Continuous Monitoring of Diisocyanate Monomers
| Principal investigator(s): | James T. Purdham (University of Toronto) |
| Co-investigator(s): | Susan M. Tarlo (University of Toronto) |
| Sponsoring Institution: | University of Toronto |
For more information about this study, please contact James T. Purdham: j.purdham@utoronto.ca
Results
The study was able to evaluate the response of an infrared photoacoustic detector to isocyanate concentrations in a stream of nitrogen, determining the limits of detection and the precision of the measurement. Measurements were made using two different wavelength filters which produced narrow wavelength bands of infrared radiation in a region where isocyanates would give a strong response (centered on 4.5 micrometres (m) and 4.4 m). The limits of detection were found to be 40 parts per billion (ppb) and 25 ppb respectively. The relative standard deviation of measurements was found to be 3-4 percent. These findings indicated that the instrument would not be sufficiently sensitive to allow routine field measurements of exposure, although its use as a leak detection/alarm system was still a possibility as well as certain limited applications in exposure chamber work.
For the purposes of leak detection, the instrument would need to be able to make measurements in air, which contains carbon dioxide, a potentially strong interferent. The instrument has the capability of correcting for both water vapour and carbon dioxide absorption at wavelengths of interest. This correction relies upon measurement of these substances at a wavelength other than wavelength of interest and using scaling factors to determine the degree of correction required at the wavelength of interest. The instrument was calibrated to make these corrections at the isocyanate absorption wavelengths. However, it was found that the carbon dioxide absorption at the measurement wavelength was so large, that even a small error in the carbon dioxide scaling correction resulted in unacceptably large errors in the determination of isocyanate concentration.
Attempts were made to eliminate carbon dioxide and water vapour from the sample prior to entry to the measurement chamber of the instrument, using selective adsorption by molecular sieves. All attempts to do so, however, met with no success. While carbon dioxide and water vapour were successfully reduced to less than one percent of their original concentrations, the isocyanate was also completely lost. Under normal circumstances, this might be an area for further investigation, but coupled with the lack of adequate sensitivity, this would not seem warranted.
One further possible application of the instrument was investigated. In creating known concentrations of diisocyanate in human exposure facilities, diisocyanate vapour is generated by passing a stream of nitrogen through the liquid diisocyanate. Dry nitrogen rather than air is used, because water vapour present in air will react with the liquid diisocyanate causing polymerization. This concentrated vapour is then delivered to the chamber in a stream of dilution air. The final concentration of diisocyanate in the chamber is dependent upon the flow rate of the dilution air, the concentration of toluene diisocyanate (TDI) in the delivery line, and the flow rate through the delivery line. Since the concentration of diisocyanate in the delivery line is relatively high (in the parts per million range), and it is an atmosphere free from carbon dioxide and water vapour, the infrared photoacoustic detector can reliably and accurately measure the isocyanate concentration in the delivery line and this can be used to calculate the chamber concentration. Experiments were performed to confirm this and they demonstrated a very good correlation between the level of isocyanate in the chamber predicted from photoacoustic measurements of TDI concentration in the delivery line and the actual level as determined by an MDA tape sampler. This instrument, therefore, could be a useful addition to the instrumentation used to ensure that concentrations of diisocyanate in human exposure facilities are kept within safe limits. However, since the MDA tape sampler can measure the concentration of diisocyanate directly in the chamber where the person is exposed, the role of the infrared photoacoustic detector would likely be that of backup.
Conclusions
- known concentrations of TDI can be generated by the dynamic system designed and built in our laboratory and can safely be used for isocyanate experiments
- the infrared photoacoustic detector is not sufficiently sensitive for assessment of compliance with current exposure standards, or for direct measurement of diisocyanate concentration within the human exposure challenge chamber
- the infrared photoacoustic detector is unable to reliably measure TDI concentrations in room air, because it is unable to correct with sufficient accuracy for the very large carbon dioxide absorbance at TDI responsive wavelengths
- carbon dioxide and water vapour interference with isocyanate measurements using the infrared photoacoustic detector cannot be eliminated by selective absorption using molecular sieves without loss of TDI
- the infrared photoacoustic detector can be used to measure generated TDI concentrations in nitrogen prior to dilution with air in exposure chamber applications.
The overall conclusion of the project is that the infrared photoacoustic detector is capable only of limited application in the field of diisocyanate measurement.
Objective
Diisocyanate exposure is a very serious occupational health problem. Diisocyanates are respiratory sensitizers and one of the leading causes of occupational asthma. Accurate methods of measurement are therefore essential for these substances. Instrumental methods of measurement, to date, have been limited to colourimetric tape samplers. In this study we evaluated an infrared photoacoustic detector for its ability to measure isocyanate in workplace air and other situations.
Methods
The initial work in the project was devoted to the development of a safe isocyanate vapour generation and delivery system for use in the experiments. The development of the system was a successful by-product of this research project. We were able to demonstrate that known concentrations of isocyanate can be produced and maintained at a stable level over extended periods of time.
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