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IntroductionFor the purpose of separation of metal ions, either for purification or preconcentration, various kinds of chelating resins have been developed. This has been the subject of numerous papers and reviews related to synthesis, characterization and metal ion uptake studies. [1][2][3][4] The insertion of suitable specific functional groups into solid matrices makes them capable of reacting with metal species under certain favorable conditions to form metal complexes. One of the most used supports is silica gel due to its good mechanical, thermal and chemical stability as well as excellent swelling resistance in different solvents. 4,5 Synthesis and application of silica gel phases loaded with various organic reagents for preconcentration of metal ions have been reviewed by Jal et al. 4 Additionally, several new reagents like dimethylglyoxime, 6 4-aminoantipyrene, 7 5-formyl-3-(1′-carboxyphenylazo) salicylic acid, 8 2-aminothiazol 9 and dendrimer-like polyamidoamine 10 have been recently reported for the chemical modification of silica surface.Unfortunately, the modified silica phases have not been studied for selective extraction and enrichment of vanadium. For this purpose chelating resins 11-14 and several modified polymers [15][16][17] have usually been applied. Determination of vanadium at trace levels is important for monitoring pollution as well as for understanding its geochemical and biological effects. However, when the level of this metal in real samples is very low, a preconcentration/separation step is necessary to improve the precision and accuracy.18 Moreover, the procedures based on sorption principles are very useful for separation of vanadium species as V(IV) and V(V), two main forms present in natural waters, which have different nutritional and toxic properties.In the present study, modified silica gel was used for the solid phase extraction of vanadium. Tetrakis(4-carboxyphenyl)-porphyrin (TCPP) was chosen as a complexing reagent as it forms stable complexes with vanadium. Vanadyl porphyrins occur for example in fossil fuels. 19 The silica surface was modified by physical adsorption and by direct chemical immobilization through formation of an amide bond. The resulting chelating sorbents have been studied for separation and preconcentration of vanadium species. Experimental Reagents and apparatusThe chemicals used were of analytical grade, and all solutions were prepared with deionized water obtained from a Milli-Q system (Millipore).A commercial atomic absorption standard solution (Merck) containing 1000 mg L -1 of V(IV) (VOSO4 in 0.5 mol L -1 H2SO4) was used. Vanadium(V) solutions were prepared by dissolving V2O5 in 1% (v/v) HNO3.5,10,15,20-Tetrakis(4-carboxyphenyl)porphyrin (TCPP) from Fluka was used without further purification.A Perkin Elmer Model AAnalyst 300 atomic absorption spectrometer equipped with a deuterium background correction system utilized the vanadium 318.5 nm line. HGA-800 furnace and pyrolytically coated graphite tubes were employed. The instrumental settings...

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