Metal speciation by supercritical fluid extraction with on-line detection by atomic absorption spectrometry

A silica flame-in-tube interface is described for the sensitive detection, by AAS, of As, Cd, Cu, Mn, Pb, Se, or Zn in supercritical (SC) fluid extractor eluate. In operation, analyte metal in aqueous medium is derivatized by in situ complexation with tetrabutylammonium dibutyldithiocarbamate (TBADB-DTC) and the product complex is mobilized into SC-CO2. The extraction process is equally efficient whether the mobile phase is presaturated with complexing agent during a static preequilibration stage of the process or this reagent is solubilized dynamically as the mobile phase traverses a saturation vessel placed in series with the extraction vessel. SC-CO2 extractor eluate is nebulized into a diffused flame maintained within the upper region of the flame tube and the optical tube of the interface. Optimal flame conditions maintained with separate flows of O2 and H2 to the base of the flame tube are slightly reducing for aqueous and SC-CO2 mobile phases but slightly oxidizing for a methanolic mobile phase. For each analyte element, limits of detection were subnanogram to low picogram if standard was flow injected into the mobile phase. These sensitivities permitted differences in the rates of mobilization of analyte metal from different matrices to be explored as a technique for probing the interactions of the analyte metal with the matrix. A portion of the total Zn burden of fresh bovine liver slurry was rapidly mobilized in the absence of complexing agent, and the remainder was solubilized more rapidly than the Zn in a freeze-dried standard reference material of this tissue.