An improved flow analysis–ion chromatography method for determination of cationic and anionic species at trace levels in Antarctic ice cores

Abstract

A method was developed for the quantitative determination of cations and anions in Antarctic ice cores at μg L −1 and sub-μg L −1 levels by ion chromatography (IC), after ultra-clean decontamination procedures. Strict manipulation and decontamination procedures were used in sub-sampling, in order to minimise sample contamination. Na +, NH 4 +, K +, Mg 2+ and Ca 2+ were determined by 12-min isocratic elution (H 2SO 4 eluent). Contemporaneously, in a parallel device, F −, MSA (methanesulfonic acid), Cl −, NO 3 − and SO 4 2− were analysed in a single 12-min run with multiple-step elution using Na 2CO 3/NaHCO 3 as eluent. Melted ice samples were pumped from their still-closed containers (polystyrene accuvettes with polyethylene caps), shared between the two ion chromatographic systems, online filtered (0.45 μm Teflon membrane) and pre-concentrated (anions and cations pre-concentration columns) using a flow analysis system, thus avoiding uptake of contaminants from the laboratory atmosphere. Sensitivity, linear range, reproducibility and detection limit were evaluated for each chemical species. Anion or cation detection limits ranged from 0.01 to 0.15 μg L −1 by using a relatively small sample volume (1.5 mL). Such values are significantly lower than those reported in literature for almost all the components. These methods were successfully applied to the analysis of cations and anions at trace levels in the Dome C ice core. The composition of the atmospheric aerosol for the last 850 kyr was reconstructed by high-resolution continuous chemical stratigraphies. Concentration trends in the last nine glacial-interglacial climatic cycles were shown and briefly discussed.