Development of a new system for the speciation of chromium in natural waters and human urine samples by combining ion exchange and ETA-AAS

The nature of ETA-AAS makes the use of on-line devices difficult, so that habitually the separation and/or preconcentration steps must be made by means of discontinuous procedures that are time-consuming. In this work, an original system for the preconcentration and speciation of chromium in natural waters and human urine has been devised, replacing the sample tip of the autosampler arm by two microcolumns packed, respectively, with chelating or anionic ion exchange resins. By using this device, diverse advantages are attained: (a) elimination of the matrix effect, (b) increase of the sensitivity by the preconcentration, and (c) automatization of different steps. Moreover, workings of the rotary valve and of the peristaltic pump have been automated by using an electronic control device. Under the optimum conditions, using a 60 s preconcentration time, a sample flow rate of 2 ml min(-1) and a injection volume of eluent of 40 mul, a linear calibration graph was obtained from 0.3 to at least 6 ng ml(-1) of Cr(III) and from 0.4 to at least 8 ng ml(-1) of Cr(vi). The detection limits (3 (7) for Cr(m) and Cr(vi) were 0.14 and 0.08 ng ml(-1), respectively. For Cr(III) the enrichment factor was 7.4, yielding a retention efficiency of 15%. For Cr(VI) the enrichment factor and retention efficiency were 5.6 and 11% respectively. The proposed method has been successfully applied to the analysis of natural water and human urine samples. Its performance was investigated against certified reference water samples and by recovery measurements on spiked samples.