Effects of humidity and filter material on diffusive sampling of isocyanates using reagent-coated filters

Diffusive sampling of methyl isocyanate (MIC) on 4-nitro-7-piperazinobenzo-2-oxa-1,3-diazole (NBDPZ)-coated glass fibre (GF) filters is strongly affected by high relative humidity (RH) conditions. It is shown that the humidity interference is a physical phenomenon, based on displacement of reagent from the filter surface. In this paper, this drawback has been overcome by changing the filter material to the less polar polystyrene divinyl benzene (SDB). A series of experiments was performed to compare the analyte uptake on the two filter materials for different sampling periods and analyte concentrations at both low and high RH conditions. Additionally, the materials were investigated as well for passive sampling of ethyl (EIC) and phenyl isocyanate (PhIC) with NBDPZ and 1-(2-methoxyphenyl) piperazine (2-MP) as an alternative derivatising agent. Using 2-MP, the mean GF/SDB response ratios were determined to be 1.02 for MIC (RSD: 6.1%) and 1.03 for EIC (RSD: 6.8%), whereas PhIC could only be determined on SDB filters. Using NBDPZ as reagent, the negative influence of high humidity disappeared when SDB filters were used instead of GF filters. Even at low RH conditions, sampling with SDB material generally resulted in a higher analyte uptake than with GF filters. The GF/SDB response ratios were independent of sampling time or analyte concentration and were determined to be 0.70 (RSD: 4.7%) for MIC, 0.84 (RSD: 4.5%) for EIC and 0.95 (RSD 5.4%) for PhIC, meaning that the NBDPZ diffusive sampler based on SDB can be used at all humidity conditions without any restrictions.