Development and evaluation of a method for hexavalent chromium in ambient air using IC-ICP-MS

The challenge to quantify ambient airborne Cr(VI) comes from three interrelated issues: sensitivity, selectivity, and stability. In this study, we developed a sensitive and reliable method to measure Cr(VI) in ambient air by optimizing each step involved in measurement. The enriched isotope was employed to determine the recovery and inter-conversion between Cr species (valences) during sampling, sample storage and chemical analysis. Specifically, ambient particles were collected on a 47-mm mixed cellulose ester (MCE) filter that was pre-cleaned by HNO3 (10% v/v) and pre-treated with NaHCO3 (2 g L−1). 53Cr(VI) and 50Cr(III) isotopes (4 ng each) were spiked on the filter either before or after sampling. Samples were subsequently sonicated in 5 mL HNO3 (pH 4) solution at 60 °C for 40 min, and separated and analyzed for Cr(VI) and Cr(III) by IC-ICP-MS. The method detection limit was 0.08 ng m−3 and the percent difference was <25% for the collocated samples. The recovery of the spiked Cr(VI) and the conversions between Cr(III) and Cr(VI) varied with matrix (clean filter, NIST 1648 PM, and ambient PM), and lower recoveries and higher inter-conversions were found for Cr species spiked before sampling than the post-sampling spikes. These results showed the effects of sampling process and particle matrix on the stability of Cr species. The effects of co-existing PM elements on Cr(VI) recovery and conversion were also examined and discussed. The acid extraction method developed in this study was compared to the ERG carbonate buffer extraction method using 53Cr(VI) isotope spiked NIST 1648 PM. The recoveries of 53Cr(VI) were 90.9% ± 8.8% (N = 11) for our acid extraction method and 89.8% ± 10% (N = 10) for the ERG carbonate buffer extraction, respectively. The ambient Cr(VI) concentrations in an urban area in NJ measured with the developed method are also reported in the manuscript.

► The work reports a sensitive and reliable method to measure ambient airborne Cr(VI).
► The method detection limit for Cr(VI) was 0.08 ng m−3.
► The recovery and conversion of Cr(VI) was affected by sampling and PM species.
► Airborne Cr(VI) were 0.44 ± 0.35 ng m−3 in Paterson and 0.40 ± 0.53 ng m−3 in Chester.