Selective Flow-Injection Quantification of Ultra-trace Amounts of Cr(VI) via On-line Complexation and Preconcentration with APDC Followed by Determination by Electrothermal Atomic Absorption Spectrometry

A rapid, sensitive and selective time-based flow injection (FI) preconcentration procedure is described for the determination of ultra-trace amounts of Cr(VI) via on-line reaction with ammonium pyrrolidine dithiocarbamate (APDC) and formation of the Cr(VI)-PDC complex. The preconcentration is effected by adsorption on the inner wall of a knotted reactor made from PTFE tubing. The complex is subsequently eluted with a monosegmented discrete zone of ethanol (55 mu l), and the analyte is quantified by electrothermal atomic absorption spectrometry (ETAAS). The operations of the FI-system and the ETAAS-detector are synchronously coupled.

Efforts have been made to design a generic and versatile FI-ETAAS system that allows all necessary manipulatory operations to be executed on-line which, in addition to effective entrapment of the analyte in the preconcentration unit, comprise washing the surface of the retained analyte in order to remove possibly interfering matrix components; quantitative dissolution of the retained analyte in the smallest possible volume of eluent to satisfy the volumetric requirements of the graphite tube (platform); reproducible transport of the concentrate to the ETAAS instrument at minimum dispersion; and, finally, a thorough cleansing of all pertinent conduits in the FI-system in order to prevent carry-over between individual samples.

A 19-fold enhancement in sensitivity of Cr(VI) was achieved after preconcentration for 60 s at a sample flow rate of 5.0 mi min(-1) as compared to direct introduction of 55 mu l samples, yielding a detection limit (3 sigma) of 4.2 ng l(-1). The sample frequency was 21.2 samples per hour. The proposed method was evaluated by analyzing drinking water, a NIST Cr(VI)-reference material and synthetic sea water. A major contribution to successful operation of the system can, to a large extent, be ascribed to the incorporation of a newly developed mixing-point component, which is attached directly to the commercially available 8-port rotary valve. (C) 1998 Elsevier Science B.V.