Anion mobilization from aqueous media by ion associate extraction into supercritical carbon dioxide with on-line detection by flame atomic absorption spectrometry

Abstract

The nebulizer assembly of a flame atomic absorption spectrometer was modified to operate with conventional flows corresponding to ⩽2.2 l min^–1 of decompressed supercritical carbon dioxide (SC-CO₂) extractor eluate. Supplemental heating of the inlet air to 200 °C resulted in a flow injection system–on-line detector which provided approximately a 10-fold improvement over conventional flow injection operation. The detector was used to evaluate the feasibility of mobilizing/purging anions (chromate, selenate or selenite) from aqueous media by adding an alkylammonium halide directly to the SC-CO₂ mobile phase. At 30.6 MPa and 80 °C, SC-CO₂ solublized sufficient tetrahexylammonium bromide (THAB) to effect the efficient purging (93%) of selenate (60 µg) from aqueous solution during a 40 min extraction. At 24.1 MPa and 50 °C, there was little difference in the recoveries (66 versus 62%) using THAB or tetrabutylammonium dibutyldithiocarbamate (TBADBDTC) as pairing reagent. Selenite or chromate was also efficiently mobilized with either reagent. Mathematical models of the declination in AAS response with time suggested that whereas selenite (half-life, t_1/2= 3.6 min) was mobilized at a rate that was not appreciably different from cation–dithiocarbamate complexes, chromate was mobilized appreciably more slowly (t_1/2= 19.5 min) under identical extractor operating conditions. The Cr and Cu in an aqueous suspension containing chromated copper arsenate wood preservative were simultaneously mobilized using a mixture of THAB and TBADBDTC. Despite appreciable interactions with other components of the sample, 89 and 98%, respectively, of the Cr and Cu burdens were purged from the sample during a 45 min extraction using a decompressed flow rate of 2.2 l min^–1.