Directly suspended droplet microextraction coupled with electrothermal atomic absorption spectrometry for the speciation of chromium(III)/chromium(VI)

A novel procedure is presented for the speciation of Cr(III)/Cr(VI) involving directly suspended single droplet microextraction combined with electrothermal atomic absorption spectrometry. In this method, Cr(III) can be extracted with 1-decanol at pH 6.0 due to the formation of a hydrophobic complex, Cr(III)-8-hydroxyquinoline (8-HQ), whereas Cr(VI) remains in the aqueous solution. The Cr(VI) concentration was calculated by subtracting the concentration of Cr(III) from the total chromium concentration after reducing Cr(VI) to Cr(III) by addition of hydroxylamine hydrochloride. Different factors affecting the extraction of Cr(III), such as pH, 8-HQ concentration, stirring rate, extraction temperature and time, and interfering ions, were schematically evaluated. Under the selected conditions, the limit of detection for Cr(III) was 0.03 ng mL−1 with a relative standard deviation of 4.7% (C = 1.0 ng mL−1, n = 5). The calibration curve was highly linear for the Cr(III) concentration range of 0.10–2.0 ng mL−1. The proposed method was validated against certified reference materials of environmental water (GSBZ50027-94, GBW(E)080642), and successfully applied in the speciation of chromium in well and tap water samples.