Evaluation of Sampling Methods for Measuring Exposure to Volatile Inorganic Acids in Workplace Air. Part 2: Sampling Capacity and Breakthrough Tests for Sodium Carbonate-Impregnated Filters

Abstract

In France, the MétroPol 009 method used to measure workplace exposure to inorganic acids, such as HF, HCl, and HNO₃, consists of a closed-face cassette fitted with a prefilter to collect particles, and two sodium carbonate-impregnated filters to collect acid vapor. This method was compared with other European methods during the development of a three-part standard (ISO 21438) on the determination of inorganic acids in workplace air by ion chromatography. Results of this work, presented in a companion paper, led to a need to go deeper into the performance of the MétroPol 009 method regarding evaluation of the breakthrough of the acids, both alone and in mixtures, interference from particulate salts, the amount of sodium carbonate required to impregnate the sampling filter, the influence of sampler components, and so on. Results enabled improvements to be made to the sampling device with respect to the required amount of sodium carbonate to sample high HCl or HNO₃ concentrations (500 μL of 5% Na₂CO₃ on each of two impregnated filters). In addition, a PVC-A filter used as a prefilter in a sampling device showed a propensity to retain HNO₃ vapor so a PTFE filter was considered more suitable for use as a prefilter. Neither the material of the sampling cassette (polystyrene or polypropylene) nor the sampling flowrate (1 L/min or 2 L/min) influenced the performance of the sampling device, as a recovery of about 100% was achieved in all experiments for HNO₃, HCl, and HF, as well as HNO₃+HF and HNO₃+HCl mixtures, over a wide range of concentrations. However, this work points to the possibility of interference between an acid and salts of other acids. For instance, interference can occur through interaction of HNO₃ with chloride salts: the stronger the acid, the greater the interference. Methods based on impregnated filters are reliable for quantitative recovery of inorganic volatile acids in workplace atmosphere but are valuable only in the absence of interferents.

Keywords:

• breakthrough
• impregnated filter
• interference
• volatile inorganic acids

ACKNOWLEDGMENTS

The authors want to thank Dr. Dietmar Breuer (IFA), Alan Howe (HSL), and Edmond Kauffer (INRS) for their comments on all the results during the study, with special thanks to Alan Howe for his very careful and critical review of the draft manuscript and his help for the formulation of results and conclusions in English.