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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-1. The precision for 10 replicate determinations at the 5 micrograms L-1 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 micrograms L-1. The method was successfully applied to the speciation of vanadium in river water samples.

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A preconcentration and determination method for humic and fulvic acids at trace levels in natural water samples was developed. Cloud point extraction was successfully employed for the preconcentration of humic acid (HA) and fulvic acid (FA) prior to the determination by using a flow injection (FI) system coupled to a spectrophotometric UV-Vis detector. The quantitative extraction of HA and FA within the pH range 1-12 was obtained by neutralization of the anionic charge on the humic substances with a cationic surfactant, hexadecyltrimethylammonium bromide (CTAB). This generated a hydrophobic species that was subsequently incorporated (solubilized) into the micelles of a non-ionic surfactant polyethylene glycol, tert-octylphenyl ether (Triton X-114). The FI method for HA and FA determination was developed by injection of 100 microl of the extracted surfactant-rich phase using an HPLC pump with spectrophotometric detection at 350 nm. A 50 ml sample solution preconcentration allowed an enrichment factor of 167. The limit of detection (LOD) obtained under the optimal conditions was 5 microg l(-1). The precision for ten replicate determinations at 0.2 mg l(-1) HA was 3.1% relative standard deviation (RSD), calculated from the peak heights. The calibration using the preconcentration system for HA and FA was linear with a correlation coefficient (r2) of 0.9997 at levels near the detection limits up to at least 1 mg l(-1). The method was successfully applied to the determination of HA and FA in natural water samples (river water).