Interfacing flow injection analysis (sequential injection analysis) and electro-thermal atomic absorption spectrometry determination of trace-levels of Cr(VI) via on-line pre-concentration by adsorption in a knotted reactor and by liquid-liquid extraction

Two time-based flow injection pre-concentration procedures are described for the analysis of ultra-trace-level concentrations of metal ions, as exemplified for the determination of Cr(VI). While the first approach exploits pre-concentration by on-line formation and extraction into MIBK of the non-polar complex formed of Cr(VI) ions with ammonium pyrrolidine dithiocarbamate (APDC), the second one is based on formation of the same complex compound and adsorption of it in an incorporated knotted reactor which is subsequently eluted by a discrete volume of ethanol. In both cases, the detection of the analyte in the eluent is executed by electrothermal atomic absorption spectrometry (ETAAS). Using a newly developed syringe pump based FI-system, FIAlab-3500, the present communication discusses and emphasizes the performance of these on-line pre-concentration/separation procedures. Particularly, the interface between the FI-system and the detector ensuring reliable, effective and robust operation of the automated combination is detailed, because this calls for an intelligent coupling of the continuously operating FI-system with the discontinuously working ETAAS instrument. In this context, a unique application of the liquid-liquid separator used is described. The operational characteristics obtained were as follows. For the FI-system with liquid-liquid extraction the enhancement in sensitivity of Cr(VI) for 99 s sample aspiration at a flow rate of 5.5 ml min(-1) was 18-fold as compared to direct introduction of 40 mu l samples, yielding a detection limit (3 sigma) of 3.2 ng l(-1). The sample frequency was 10.9 h(-1). By employing the adsorption procedure the enhancement in sensitivity of Cr(VI) was 19-fold as achieved after pre-concentration for 60 s at a sample flow rate of 5.0 ml min(-1) as compared to direct introduction of 40 mu l samples, yielding a detection limit (3 sigma) of 6.7 ng l(-1). The sample frequency was 13.8 h(-1). The procedures were applied for the assay of Cr(VI) in a NIST reference material with good recoveries in both instances. (C) 2000 Elsevier Science B.V. All rights reserved.