Investigation of measurement errors in sampling of aerosols of hazardous workplace materials

Status:

completed 12/2013

Aims:

Aerosols of Semi-Volatile Organic Compounds (SVOC) origin represent a big challenge to industrial hygienists due to toxicological and sampling issues. Especially problematic is the sampling of hazardous semi-volatiles that appear in both particulate and gaseous phases in the workplace. More attention has to be given to these dynamic aerosol systems as the particle-vapor distribution is considerably depending on the sampling conditions. This study was carried out to address the issue of semi-volatile aerosol particle evaporation both theoretically and experimentally as well as to provide experimental data that show the extent of evaporation losses that might bias the workplace risk assessment.

Activities/Methods:

Aerosols of several SVOC substances (tetradecane …eicosane) and particle diameters were generated, diluted with particle free nitrogen and monitored for particle evaporation losses.

The aerosols were quantified using on-line and off-line methods. The on-line approach to analyze SVOC aerosols was to account for the total mass (TM) with a Flame Ionization Detector (FID) coupled with a heated transfer line. The particle mass (PM) was calculated from the particle diameter measured using an optical particle sizer, while the vapor mass (VM) was calculated as the difference between the measured total and particulate mass (VM=TM-PM). The off-line samplers comprised of a glass fiber filter for the separation of the particle phase and XAD2 adsorber for the quantification of the remaining vapors.

In order to study SVOC particle evaporation an experimental apparatus comprising of a Sinclair-LaMer aerosol generator, a flow tube and the above mentioned aerosol sampling system was set up inside a temperature controlled chamber. Particle evaporation in the flow tube was assessed using the on-line method and compared with a theoretical diffusion based computer model. Parallel measurements using the on-line and off-line methods were conducted to quantify for the amount of PM evaporated from filters.

Results:

Flow tube measurements have shown that droplets of SVOC particles can evaporate completely within 3 seconds as was the case for tetradecane or remain unchanged (octadecane), depending on substance volatility, vapor saturation, particle diameter and sampling temperature. The experimental and model data were found to be in a good agreement for the flow tube experiments with less than 20% deviation for 80% of the data points. Comparisons between the on-line method and the off-line filter-adsorber sampling method have shown that both methods had similar results for TM. The off-line method tended to lose a significant amount of particle mass due to evaporation, especially for small particles.

Conclusions: Our results show that aerosol particle evaporation can be predicted with good accuracy and that the off-line method can be biased for SVOC particle measurement. The only comparable result that was independent of sampling method is for TM. The off-line method systematically returns lower PM and higher VM values, a clear indication for particle evaporation loss. This effect is also expected to influence the workplace risk assessment; sampling using filters only can significantly underestimate the real workplace concentration.

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