Send Orders of Reprints at reprints@benthamscience.net 288 Current Analytical Chemistry, 2013, 9, 288-295 Comparative Study of Two Different Speciation Methods for the Determi- nation of Hexavalent Chromium in Water Samples Using Electrothermal Atomic Absorption Spectrometry M. G. Kostakis, #. N. Pasias, V. L. Borova, A. K. Panara and $. S. %homaidis * Laboratory of Analytical Chemistry, Department of Chemistry, University of Athens Panepistimiopolis, Greece Abstract: The aim of this study was to compare two different widely-used methods for the determination of hexavalent chromium (Cr(VI)) in water samples by Electrothermal Atomic Absorption Spectrometry (ETAAS). Both methods are based on the complexation - reaction of Cr(VI) with an organic complexation reagent, which is then extracted and precon- centrated in organic solvent. In the first method, ammonium pyrrolidine dithiocarbamate (APDC) is used as complexation reagent, whereas 1,5-diphenylcarbazide (DPC) is used in the second method. The speciation methods were optimized and validated. Both methods were applied for the determination of Cr(VI) in the same multi-level groundwater samples (0.060 – 42 μg/L, n=13) and the results were compared statistically. Beside the comparison of the two extraction methods (APDC, DPC), the samples were also analyzed by Reagent Free Ion Chromatography (RFIC) with conductivity detector and statistical comparison was also performed. Paired t-test was applied and the results indicated that there was no statisti- cally significant difference between the three methods. Useful conclusions about the analytical performance of these widely-used-in-routine-labs methods were drawn. The selectivity of Cr(VI) determination was significant for both meth- ods. The DPC method had lower limit of detection than APDC, however the APDC method was more robust than the DPC method. Both methods are appropriate for the determination of Cr(VI) in different ground water samples at sub-μg/L levels. Keywords: Hexavalent chromium, 1,5 Diphenylcarbazide, ammonium pyrrolidinedithiocarbamate, ETAAS, uncertainty, water samples. 1. INTRODUCTION Chromium is a relatively common element with an aver- age concentration of 100 ppm in the earth's crust, primarily as chromite (FeCr 2 O 4 ) and krokoite (PbCrO 4 ). The oxidation states of practical importance are III and VI. Cr(II) also ex- ists but is immediately oxidized to Cr(III) in the presence of oxygen. At physiological pH, Cr(VI) exists as an oxyanion (CrO 4 ) 2- that resembles sulfate and phosphate oxyanions [1, 2]. Based on this oxyanion molecular mimicry, chromate is therefore transported into and accumulated in cells instead of sulfate or phosphate. Hexavalent Cr compounds usually exist as oxides or oxohalides. At and above physiological pH (>6), chromate ions predominate (CrO 4 ) 2- , whereas between pH 2 and 6, the dichromate species (Cr 2 O 7 ) 2- and (HCrO 4 ) 1- exist [3, 4]. Trivalent chromium readily forms low-spin oc- tahedral coordination compounds, complexes, and chelates [3]. It can bind six ligands and the binding stability for many such ligands is very strong, rendering it inert to substitution, due to its half-filled outer electron shell [4]. The chemical behavior and biological activity of Cr(III) and Cr(VI) species are very different. Cr(III) seems to have an important role on the metabolism of glucose in the blood- stream together with insulin, while Cr(VI) has toxic and car- *Address correspondence to this author at the Nikolaos S. Thomaidis, Labo- ratory of Analytical Chemistry, Department of Chemistry, University of Athens, Panepistimiopolis Zografou, 15771 Athens, Greece; Tel: +30 210 7274317; Fax: +30 210 7274750; E-mail: ntho@chem.uoa.gr cinogenic effects due to its strong oxidizing potential and easy permeation of biological membranes. Thus, hexavalent chromium damages the cell macromolecules, as proteins and the DNA [4]. There are many references concerning the risks from chromate exposure. In 2007, the U.S. National Toxi- cology Program completed 2-year studies of cancer in F344/N rats and B6C3F1 mice given Cr(VI) (sodium di- chromate dihydrate) in drinking water [5, 6]. There was clear evidence of carcinogenic activity in both male and female rats (squamous cell neoplasms of the oral cavity) and mice (neoplasms of the small intestine). Therefore, there is a great need for validated routine methods for chromium speciation in water samples using simple instrumentation, like atomic absorption spectrometers that could be found in routine labo- ratories. Generally, speciation methods are divided into two main categories [7]: 1. Chromatographic methods. The species are separated in a chromatographic system and they are detected by the respective detector. For chromium speciation, these methods are based on liquid chromatography coupled to element specific detectors. In general, these techniques require simple pretreatment and are generally fast, but a more advanced instrumentation is needed, which is not always available. 2. Non - chromatographic methods. The species are firstly separated off-line with a separation procedure, like solid 1875-6727/13 $58.00+.00 © 2013 Bentham Science Publishers