(2)
Internal wall losses: There can be deposition of the isocyanate species on the internal walls of the sampler where it is not available for subsequent analysis.
(3)
Transmission losses: Losses can occur from isocyanate species passing completely through the sampler.
The relative importance of these problems depends on the particle size, the sampler and inlet geometries, the sampling rate, and the collection mechanism of the sampler. Samplers have been evaluated for aspiration efficiencies and internal losses.47 A number of samplers have been designed with the intent to collect particles with efficiencies that match human inhalation efficiency.48 One of these, the UK Institute of Occupational Medicine (IOM) personal inhalable sampler, has been recommended for isocyanate sampling.49 Problems with poor aspiration efficiency and internal losses may be very important factors in the overall accuracy of the method. A recent study by the International Isocyanate Institute (III) on collection of MDI aerosol by filters and impingers found high variability in the aspiration efficiencies of particles with diameters in the 5 to 30 micrometers (µm) range. 50 This high variability is not inconsistent with what is often found in the sampling of large particles.48 The III study also found that for large particles, a substantial percentage of the aerosol collected by filter samplers was deposited on the filter holder. It has also been shown that losses of relatively large particles can occur on the walls of both the inlet and the nozzle of the impingers.47,51 Assessments of collection efficiencies in isocyanate sampling have often been limited to measuring the relative amount of isocyanate species passing through the sampler. Based on this criterion, reagent-coated glass fiber filters (GFFs) appear to prevent the passage of isocyanate vapors and particles of widely varying sizes.52,53 Recent studies have investigated the mechanisms by which particles pass through impingers.54,55 Impingers have been found to prevent passage of vapors and particles greater than 2 µm in diameter, but allow substantial penetration of particles smaller than 2 µm. 50,56,57 Particles smaller than 2 µm include condensation aerosol (e.g., environments where MDI is heated) and aerosol generated from combustion processes. Two methods have been developed that segregate isocyanate species on collection according to their physical states. Being able to differentiate vapor and aerosol exposures is desirable because vapor and aerosol differ in their extent of penetration and deposition in the respiratory tract. These different types of exposures can result in different health consequences. One method uses a dual filter system, where aerosols are collected on a reagentless front filter and vapors collected on a reagent-coated back filter.58,59 One potential problem with this system is the loss of isocyanate species in the aerosol fraction due to curing reactions occurring between the times of collection and post-sampling derivatization. This problem would be expected to be greater the longer the sampling time and the more reactive the isocyanate system. Another potential problem is the misclassification of semivolatile species, such as monomers, either by adsorption of vapor on the front filter or volatilization of species originally collected as aerosol. Another sampler used for isocyanates that separates vapor and aerosol consists of an annular denuder for vapor collection, followed by a reagent-coated GFF for aerosol collection.60 A limitation of this system is that it is too large for personal sampling. b. Derivatization Once the isocyanate species have been collected, they must be efficiently derivatized. Derivatization of isocyanate species accomplishes two things. First, it stabilizes the isocyanate, which would otherwise be lost
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