ANALYST FULL PAPER THE www.rsc.org/analyst Speciation and preconcentration of vanadium(V) and vanadium(IV) in water samples by flow injection-inductively coupled plasma optical emission spectrometry and ultrasonic nebulization Rodolfo G. Wuilloud, a Jorgelina C. Wuilloud a Roberto A. Olsina ab and Luis D. Martinez* ab a Department of Analytical Chemistry, Faculty of Chemistry, Biochemistry and Pharmacy, National University of San Luis, Chacabuco and Pedernera, P.O. Box 375, 5700 San Luis, Argentina b Consejo Nacional de Investigaciones Científicas y Técnicas (CONICET), Argentina Received 4th December 2000, Accepted 14th March 2001 First published as an Advance Article on the web 24th April 2001 An on-line separation, preconcentration and determination system for vanadium(IV) and vanadium(V) comprising inductively coupled plasma optical emission spectrometry (ICP-OES) coupled to a flow injection (FI) method with an ultrasonic nebulization (USN) system was studied. The vanadium species were retained on an Amberlite XAD-7 resin as a vanadium–2-(5-bromo-2-pyridylazo)-5-diethylaminophenol (V–5-Br-PADAP) complex at pH 3.7. Enhanced selectivity was obtained with the combined use of the formation on-line of the complexes and 1,2-cyclohexanediaminetetraacetic acid (CDTA) as masking agent. The vanadium complexes were removed from the microcolumn with 25% v/v nitric acid. A sensitivity enhancement factor of 225 was obtained with respect to ICP-OES using pneumatic nebulization (15-fold for USN and 15-fold for the microcolumn). The detection limit for the preconcentration of 10 mL of aqueous solution was 19 ng L 21 . The precision for 10 replicate determinations at the 5 mg L 21 V level was 2.3% relative standard deviation (RSD), calculated from the peak heights obtained. The calibration graph using the separation and preconcentration system for vanadium species was linear with a correlation coefficient of 0.9992 at levels from near the detection limits up to at least 100 mg L 21 . The method was successfully applied to the speciation of vanadium in river water samples. Introduction The chemical and physical properties of a metal species depend very much on its oxidation state, hence an accurate determina- tion of each species is important for evaluating both the potential risk and benefits of some metals. 1,2 Vanadium has various oxidation states and ionic forms in aqueous solution. It exists in two different oxidation states, V(V) and V(IV), in well aerated natural and industrial waters. 3,4 The significance of vanadium speciation is that the two oxidation states have different nutritional and toxic properties. 5,6 The toxicity of V is dependent on its oxidation state, with V(V) being more toxic than V(IV). 3 Otherwise, the insulin-like properties of V, especially its effects on mitogenesis, suggest that the element plays a role in growth and development. 5 Therefore, the speciation and determination of V are receiving increasing attention in pollution and nutritional studies. Since one of the routes of incorporation of V into the human body is water, 6–8 its determination in this type of sample is very important. The concentration of V in natural water is very low, 9–11 of the order of a few mg L 21 , hence powerful techniques are required and only a few of them show sufficient sensitivity. Neutron activation analysis (NAA) 12,13 has been applied to the determination of V, but it is time consuming and the routine analysis of numerous samples is laborious. This method also requires sophisticated instrumentation which may not be available in most analytical laboratories. Inductively coupled plasma mass spectrometry (ICP-MS) 14,15 is used for the determination of V because of its high sensitivity, high selectivity and high sample throughput. However, the cost of the instrumentation may be prohibitive to many laboratories. Inductively coupled plasma optical emission spectrometry (ICP-OES) or electrothermal atomic absorption spectrometry (ETAAS) are the most commonly used techniques for the determination of traces of V, but the low level of V concentration in water is not compatible with the detection limits of these techniques. In order to achieve accurate, reliable and sensitive results, preconcentrations and separations are needed when the concentrations of the analyte elements in the sample are too low to be determined directly by ICP-OES. Many separation and preconcentration techniques 6,16,17 for the determination of V(V) and V(IV) species have been proposed, including chelation and extraction, precipitation and the use of ion-exchange resins. However, many of these methodologies are performed in batch, thus requiring large sample volumes in order to reach low detection limits. Further, these systems have higher contamination risks and are not practical for application in routine analysis. This situation has been significantly improved by utilizing flow injection (FI) associated with ICP-OES. 18,19 In fact, to date the most dramatic improvements achieved in FI-ICP-OES have been in the field of on-line preconcentration. 20,21 On the other hand, the use of an ultrasonic nebulizer can provide a 5–50-fold improvement in detection limits. 22–24 XAD resins have been used as packing materials in preconcentration columns for FI. 25,26 They have been employed as supports for the immobilisation of chelating agents and metal complexes. 27–29 2-(5-Bromo-2-pyridylazo)-5-diethylamino- phenol (5-Br-PADAP) forms stable complexes with numerous metal ions, 30,31 and is therefore a suitable reagent for V preconcentration on an XAD resin. 32 Recently we have reported a preconcentration FI system for the determination of total V This journal is © The Royal Society of Chemistry 2001 DOI: 10.1039/b009705p Analyst, 2001, 126, 715–719 715