Custom-made PTFE filters for ultra-clean size-fractionated aerosol sampling for trace metals

Atmospheric deposition of marine aerosols is one of the important sources of dissolved trace metals in the ocean. Analyses of trace metals and stable isotope ratios in size-fractionated aerosol particles using extraction experiments are useful to understand the dissolution behavior and biogeochemical cycles of trace metals in the marine environment. These analyses require sample treatments in clean environment without contamination of target metals from the sampling filter. However, the contamination of target metals from sampling filter is not low enough to determine the fractional solubility (extractive metal/total metal) and the stable isotope ratio of trace metals. In this study, we optimized the acid-washing procedure of hydrophobic PTFE filter for the Sierra-type cascade impactor. The results showed that the filter blanks of the custom-made PTFE filters used for the upper stages (= PTFE-ST) in the cascade impactor were about 10-1000 times lower than those of cellulose filters. In addition, the filter blanks of major ions and water-soluble organic carbon were also decreased by using PTFE-ST. Trace metal concentrations in size-fractionated (7 fractions) aerosol particles with very short sampling periods have been successfully determined except for the backup stage. Thus, aerosol sampling with PTFE filter with only a tiny contamination is a useful method to obtain chemical and isotopic compositions of various elements in marine aerosol particles.