Determination of water-soluble, acid-extractable and inert fractions of Cd, Pb and Cu in Antarctic aerosol by square wave anodic stripping voltammetry after sequential extraction and microwave digestion

Abstract

A two-step microwave (MW) digestion (ramps to 135°C and to 185°C) was set up for the subsequent voltammetric determination of the inert fraction of Cd, Pb and Cu in Antarctic aerosol (PM10), after a sequential extraction with water (soluble fraction) and dilute HCl (acid extractable fraction) of the filter samples. A digestion mixture of 5.0mL HNO3, 1.0mL H2O2, and 1.0mL HF was used. The amount of H2O2 was adjusted at a compromise value to avoid interferences from organic substances or from the reagent itself. Hydrogen fluoride was used at the maximum quantity suggested in the literature, since there were no interferences due to excess HF with the voltammetric measurement. Our procedure did not provide for a third digestion step with H3BO3, as generally required in order to eliminate excess HF when subsequent spectrochemical analyses are to be carried out. Thus the total digestion time was reduced from ~5h to ~1.5h. The ultrasensitive technique of square wave anodic stripping voltammetry (SWASV) was used and optimized by maximizing the signal-to-noise ratio (frequency 150Hz, amplitude 20mV). The limits of detection (e.g. for atmospheric concentration 0.1–0.3pgm−3 for Cd and 1–5pgm−3 for Pb and Cu) and the repeatabilities (Cd 21–26%, Pb 16–20%, Cu 11–14% as atmospheric concentration) compared favourably with literature reports. Significant aliquots of metals were present in all the three fractions determined and the inert fraction represented 20–74% for Cd, 10–63% for Pb, and 7–33% for Cu, against total contents of Cd 1–19pgm−3, Pb 17–36pgm−3, and Cu 177–429pgm−3. Possibly significant metal fractions could be associated to the crustal origin.