Method development and validation for the determination of sulfites and sulfates on the surface of mineral atmospheric samples using reverse-phase liquid chromatography

Abstract

Earlier studies suggest that SO2 gas reacts at the surface of mineral dust and forms sulfites or bisulfites, which are then converted to sulfates. In order to monitor and quantify the amounts of both sulfites and sulfates formed on the surface of mineral dusts of volcanic and desert origins an accurate and precise reversed-phase liquid chromatography method was developed and validated to extract, stabilize and individually analyze sulfites and sulfates initially present on the surface of dusts exposed to SO2. The method was developed on a 25 mm Restek Ultra Column C18, Particle size: 5 μm, I.D. 4.60 mm column which was dynamically coated with 1.0 mM cetylpyridinium chloride in 7% acetonitrile solution to produce a charged surface as recommended in the literature. Mobile phase used: 1 mM Potassium Hydrogen Phthalate at pH 6.5 at a flow rate of 1.0 ml/min with negative UV–Vis detection at 255 nm in 15 min. The method was validated for specificity, linearity and range, injection repeatability, stability, robustness, limit of detection and limit of quantitation, and sample preparation and extraction reproducibility. The method was adapted for straight sulfite and sulfate quantification: (i) of environmental samples, and (ii) natural samples additionally exposed to SO2 gas in a dedicated laboratory setup. The method was then successfully applied to quantify sulfites and sulfates on natural volcanic and a desert dust samples both collected in the environment and additionally exposed to SO2 gas in the laboratory. The method can be efficiently used to identify sulfites and sulfates on fresh volcanic ash following an eruption, on aeolian desert dust exposed to industrial pollutants, as well as for laboratory investigations of sulfite and sulfate formation on the surface of minerals and natural dusts of different origins.