Development of an automated multiple extraction procedure coupled on-line to ICP-OES for assessment of bio-accessible trace metal fractions in airborne particulate matter

Abstract

Airborne particulate matter (APM) came into focus of scientific research over the last years due to medical tests showing its detrimental effects on human health. Airborne particulate matter consists of a mixture of various compounds such as miscellaneous organic substances, black carbon (soot), secondary inorganic ions (e.g., sulphate and ammonium), and crustal-derived particles. These groups of substances affect human health to different extents. One fraction that can verifiably be connected to negative effects on human health is the metallic portion. Although toxic elements like arsenic, cadmium and mercury only account for an insignificant fraction of total metal concentration, further insight about this group is required for accurate risk assessment. For a realistic evaluation of the hazard potential emanating from airborne particulate matter, a determination of the total respired metal amounts is insufficient. Additional information about metal fractions that can be mobilized by the human body and are, therefore, bio-accessible is necessary, because only those soluble fractions can be absorbed by the human body and cause toxic effects. Most of the procedures presented in the past employ water as the leaching agent. However, for an estimation of the actual hazard on human health, this information is not satisfactory, since body fluids interacting with the respired airborne particulate matter might exhibit leaching properties that are significantly different to water. Thus, an advanced experimental design should be used to be able to perform a comprehensive determination of the bio-accessible trace metal fraction in APM. Metal contents have been classified as a presumably harmful component of APM due to their possible absorbance into human lung tissue. This holds true primarily for particles with an aerodynamic diameter of less than 10 microns (PM10). An in-vitro physiologically based extraction test using synthetic gastric juice (SGJ) mimicking the conditions of the human digestive tract is suggested in order to appraise the bio-accessibility of inhaled and subsequently ingested metals. In the present study, a dynamic sequential injection procedure as a front end to ICP-OES is employed for on-line determination of bio-accessible trace metals in APM samples. The method is based on a preliminary extraction of samples with SGJ under dynamic conditions and the subsequent on-line ICP-OES measurement of the dissolved fraction of trace metals. The assembly permits an automated successive measurement of three sample replicates in less than 19 minutes. The developed on-line extraction procedure offers increased sample throughput, reduced risk of sample contamination and overcomes re-adsorption processes compared to traditional batch-wise systems. Furthermore, it provides deeper information on the leaching process. The developed procedure was applied to the determination of bio-accessible trace metal fractions under worst-case scenarios in PM10 samples from Palma de Mallorca (Spain) and Vienna (Austria).