Flow injection on-line preconcentration of low levels of Cr(VI) with detection by ETAAS: Comparison of using an open tubular PTFE knotted reactor and a column reactor packed with PTFE beads

A flow injection (FI) on-line sorption preconcentration procedure utilizing a packed column reactor and combined with electrothermal atomic absorption spectrometry (ETAAS) is proposed for the determination of low levels of Cr(VI) in water samples. Polytetrafluoroethylene (PTFE) beads packed in a mini-column is used as sorbent material.

The complex formed between Cr(VI) and ammonium pyrrolidine dithiocarbamate (APDC) is sorbed on the PTFE beads, and is subsequently eluted by an air-monosegmented discrete zone of absolute ethanol (35mul), the analyte being quantified by ETAAS. The preconcentration procedure using the proposed column significantly enhances the preconcentration efficiency as compared with the preconcentration approach incorporating an open tubular PTFE knotted reactor (KR).

Comparing the two procedure for equal surface sorption area, the advantages of using a packed column are observed in terms of limit of detection, enrichment factor and retention efficiency. With a preconcentration time of 60 s, and a sample flow rate of 5.0 ml l-1, the enrichment factor (30.1) and the retention efficiency (24.1%) were doubled, yielding a detection limit (3sigma) as low as 8.8 ng/l.

The sample frequency was 16.7 s/h. The concentration efficiency was 8.38 and the precision was 1.05% at 0.5 mug/l of Cr(VI). The proposed column has been applied successfully to the analysis of natural water and synthetic seawater. Its performance was verified by the analysis of two certified Cr(VI)-reference materials and by recovery measurements on spiked samples.