Evaluation of Sampling Methods for Measuring Exposure to Volatile Inorganic Acids in Workplace Air. Part 1: Sampling Hydrochloric Acid (HCl) and Nitric Acid (HNO₃) from a Test Gas Atmosphere

Abstract

Historically, workplace exposure to the volatile inorganic acids hydrochloric acid (HCl) and nitric acid (HNO(3)) has been determined mostly by collection on silica gel sorbent tubes and analysis of the corresponding anions by ion chromatography (IC). However, HCl and HNO(3) can be present in workplace air in the form of mist as well as vapor, so it is important to sample the inhalable fraction of airborne particles. As sorbent tubes exhibit a low sampling efficiency for inhalable particles, a more suitable method was required. This is the first of two articles on "Evaluation of Sampling Methods for Measuring Exposure to Volatile Inorganic Acids in Workplace Air" and describes collaborative sampling exercises carried out to evaluate an alternative method for sampling HCl and HNO(3) using sodium carbonate-impregnated filters. The second article describes sampling capacity and breakthrough tests. The method was found to perform well and a quartz fiber filter impregnated with 500 μL of 1 M Na(2)CO(3) (10% (m/v) Na(2)CO(3)) was found to have sufficient sampling capacity for use in workplace air measurement. A pre-filter is required to remove particulate chlorides and nitrates that when present would otherwise result in a positive interference. A GSP sampler fitted with a plastic cone, a closed face cassette, or a plastic IOM sampler were all found to be suitable for mounting the pre-filter and sampling filter(s), but care has to be taken with the IOM sampler to ensure that the sampler is tightly closed to avoid leaks. HCl and HNO(3) can react with co-sampled particulate matter on the pre-filter, e.g., zinc oxide, leading to low results, and stronger acids can react with particulate chlorides and nitrates removed by the pre-filter to liberate HCl and HNO(3), which are subsequently collected on the sampling filter, leading to high results. However, although there is this potential for both positive and negative interferences in the measurement, these are unavoidable. The method studied has now been published in ISO 21438-2:2009.

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

“…This incident demonstrated the advantage of using two impregnated filters in series as a safeguard in case of leakage, the total recovery on the two filters being very near 100% for both acids. The low recoveries obtained during the test gas chamber experiments (11) were not reproduced with the generation and sampling device used for these experiments, and no explanation can be given here for what happened during the test gas chamber experiments.…”

“…The breakthrough experiments described, which were carried out in parallel with those conducted in the test gas atmosphere of IFA, (11) were performed using known amounts of the volatile acids that were introduced into the sampler instead of sampling controlled test atmospheres containing known air concentrations of the acids.…”

“…For HCl and HNO 3 , the results obtained during the experiments in the test gas chamber had raised the question of the possible displacement of one acid by another during sampling when the concentration of the acids sampled was very high. To check this effect, the acids were first sampled from a mixed solution at two levels, the lowest (about 1200 µg of each acid) thought to cause no problem and the highest (about 2300 µg of each acid) having shown poor recovery during the test gas chamber experiments (11) (3rd sampling exercise). The same experiment was then repeated while sampling the higher amount of the two acids sequentially (4 hr for each acid, one at a time) with HCl added first followed by HNO 3 and then conducted with the two acids added in reverse order.…”

“…(11) A higher amount of Na 2 CO 3 is therefore required to overcome this limitation; this can be provided by using either two quartz fiber filters impregnated with 500 µL of 5% Na 2 CO 3 in series, or by using one quartz fiber filter impregnated with 500 µL of 1 M Na 2 CO 3 (10% Na 2 CO 3 ). In this study, the first option was selected as it releases fewer carbonate ions during sample extraction that can interfere with fluoride measurement, depending on the configuration of the ion chromatography system used.…”

“…The MétroPol method was one of the methods that was evaluated in these exercises, the results of which are presented in a companion paper. (11) During that series of tests, there was some evidence of breakthrough at the highest concentrations of acids in the air tested (mainly for HCl) when using the previous MétroPol 009 method (with a single impregnated quartz fiber filter). There was also some apparently anomalous data that suggested that recovery of HNO 3 was unacceptably low at the lowest concentrations tested, which was of special concern when considering the use of this method to check the compliance with the short-term occupational exposure limit value.…”