Selective Spectrophotometric Determination of Trace Amounts of Cadmium in Soil and Sediment Samples Using a Green Aqueous Biphasic Extraction.

A green extraction spectrophotometric method was presented for the determination of trace amounts of cadmium in soil and sediment samples. This method is based on the selective extraction of cadmium as its iodide complex by aqueous biphasic extraction composed of polyethylene glycol (PEG) and sodium sulfate, and a subsequent sensitive determination by spectrophotometry using 2-(5-bromo-2-pyridylazo)-5-[N-n-propyl-N-(3-sulfopropyl)amino]phenol. This extraction method is simple and environmentally benign, since the organic solvents used for the traditional liquid-liquid extractions are replaced by the non-toxic polymer and inorganic salt. Cadmium can be selectively extracted from soil and sediment samples into the PEG-rich upper phase by an aqueous biphasic system containing potassium iodide and sulfuric acid. The proposed method was applied to the determination of cadmium in certified reference materials for soil and river sediment. The obtained results were in good agreement with the certified values.

On-line redox derivatization liquid chromatography for selective separation of Fe(II) and Fe(III) cyanide complexes using porous graphitic carbon.

On-line redox derivatization HPLC was applied for the analysis of Fe(II) and Fe(III) cyanide complexes. The HPLC system used consisted of two C18 silica columns treated with trimethylstearylammonium chloride and a small column packed with porous graphitic carbon (PGC) placed between them. The PGC column treated with sodium sulfite completely reduced the Fe(III) cyanide complex to the Fe(II) complex, while the Fe(II) cyanide complex was converted to the Fe(III) complex by the PGC column treated with hydrogen peroxide. On the other hand, the oxidation states of the other metal cyanide complexes were not affected by PGC. Selective separations of the Fe(II) and Fe(III) cyanide complexes from the other metal complexes were demonstrated by on-line redox derivatization HPLC using the oxidized and reduced PGC column as the redox derivatization unit, respectively. The method was successfully applied for the analysis of Fe(II) cyanide complex in food grade salt.

Multistep pH-peak-focusing countercurrent chromatography with a polyethylene glycol-Na2SO4 aqueous two phase system for separation and enrichment of rare earth elements.

Multistep pH-peak-focusing countercurrent chromatography was developed for separation and enrichment of rare earth metal ions using a polyethylene glycol-Na(2)SO(4) aqueous two phase system (ATPS) and pH stepwise gradient elution. Metal ions in a sample solution are chromatographically extracted in a basic stationary phase (polymer-rich phase of the ATPS) containing a complexation ligand such as acetylacetone at the top of the countercurrent chromatography (CCC) column. After the sample solution is introduced, the mobile phases of which the pH values have been adjusted with buffer reagents are delivered into the column by stepwise gradient elution in order of decreasing pH. Each metal ion is concentrated at a pH border formed between the zones of different pH in the CCC column through extraction with a complexing agent into the stationary phase at the front side of the border (basic region) and back extraction into the mobile phase at the back side of the border (acidic region), moving toward the outlet of the column with the pH border. Mutual separations of La(III), Ce(III), Nd(III), Yb(III), and Sc(III) were achieved by the present method using five step pH gradient elution, and each rare earth metal ion was effectively enriched at each of the five pH borders. The mechanism for formation of pH profile of the column effluent and the potential of this technique for preparative scale separation are also discussed.

Evaluation of the thermal effect on separation selectivity in anion-exchange processes using superheated water ion-exchange chromatography.

The thermal effect on retention and separation selectivity of inorganic anions and aromatic sulfonate ions in anion-exchange chromatography is studied on a quaternized styrene-divinylbenzene copolymer anion-exchange column in the temperature range of 40-120 °C using superheated water chromatography. The selectivity coefficient for a pair of identically charged anions approaches unity as temperature increases provided the ions have the same effective size, such that the retention of an analyte ion decreases with an increase in temperature when the analyte ion has stronger affinity for the ion-exchanger than that of the eluent counterion, whereas it increases when it has weaker affinity. The change in anion-exchange selectivity with temperature observed with superheated water chromatography has been discussed on the basis of the effect of temperature on hydration of the ions. At elevated temperatures, especially in superheated water, the electrostatic interaction or association of the ions with the fixed ion in the resin phase becomes a predominant factor resulting in a different separation selectivity from that obtained at ambient temperature.

Reversed-phase ion-pair liquid chromatographic method for determination of reaction equilibria involving ionic species: exemplification of the method using ligand substitution reactions of ethylenediaminetetraacetatochromium(III) ion with acetate and phosphate ions.

A reversed-phase ion-pair liquid chromatographic method is presented for the determination of reaction equilibria involving ionic species of the same charge sign as reactant and product compounds. It has been demonstrated that ion-exchange chromatography or reversed-phase ion-pair chromatography is a useful tool for the determination of equilibrium constants of chemical reactions involving ionic species such as metal complexation reactions. Previous work with these methods has been based on the assumption that the limiting retention factors of the reactant and product species are constant independent of concentration of the chemical species (X) in the mobile phase, which reacts with the analyte compound. However, when all the reactant and product species are ions of the same charge sign as that of the species X, it is virtually impossible to apply these methods to the equilibrium constant determination because the retention factors of both the reactant and product species may depend on the concentration of X. In this study, an alternative approach was developed that estimates the limiting retention factors of ionic species from the dependence of the retention factor on the ionic strength of the mobile phase. Ligand substitution reactions of ethylenediaminetetraacetatochromium(III) ion with acetate and phosphate ions were used as model reactions to test this method. The equilibrium constants determined by this method are in good agreement with those obtained by a UV-visible spectrophotometric method.

On-column electrochemical redox derivatization for enhancement of separation selectivity of liquid chromatography use of redox reaction as secondary chemical equilibrium.

An on-column electrochemical redox derivatization for enhancement of high-performance liquid chromatography (HPLC) separation selectivity is presented using electrochemically modulated liquid chromatography (EMLC) and porous graphitic carbon (PGC) as the packing material. PGC therefore serves two purposes: it acts both as a chromatographic stationary phase and as a working electrode. The capability of on-column electrochemical redox derivatization was evaluated using hydroquinone and catechol as model compounds. By manipulation of the applied potential, hydroquinone and catechol will migrate as equilibrium mixtures, hydroquinone and p-benzoquinone and catechol and o-benzoquinone in the potential region of 25-125 mV and 150-200 mV (vs. Ag/AgCl), respectively. These redox reactions can be used as secondary chemical equilibria so that the corresponding equilibrium mixtures elute as single peaks and their retention times can be controlled by alterations in the potential applied to the PGC stationary phase. Homogeneity of the redox activity of the PGC stationary phase applied potential was also demonstrated.

Measurement of mobile-phase volume in reversed-phase liquid chromatography and evaluation of the composition of liquid layer formed by solvation of packing materials.

The mobile-phase volumes (Vm) in reversed-phase liquid chromatography (RPLC) with alkyl-bonded silica, defined as the difference between the total volume of eluent in the column (V0) and the volume of the eluent solvent layer formed by solvation of the bonded phase (VL), are determined by the method derived from the eluent electrolyte effect on the retention of ionic analytes. The validity of the Vm values obtained is evaluated by comparing them with the retention volumes of various organic compounds and inorganic ions, which have been suggested as unretained markers, and those obtained from a linear dependence of the logarithmic retention factor on the carbon numbers of homologous series. From the results obtained, it has been concluded that the solvated liquid phase on a column packing material should be assigned to a part of the stationary phase and the method developed for determination of the Vm value based on the ion partition model gives the most reasonable value as the mobile-phase volume in RPLC. The volume and the solvent composition of the solvated liquid phase on C1, C8, and C18 bonded silica are estimated, and the effects of organic modifiers and the physicochemical structures of the packing materials on these values are discussed.

Preconcentration and determination of cadmium by GFAAS after solid-phase extraction with synthetic zeolite.

The solid-phase extraction (SPE) method for the preconcentration of trace amounts of cadmium using synthetic zeolite A-4 and its determination by graphite furnace atomic absorption spectrometry (GFAAS) was investigated. The preconcentration conditions, such as the optimum pH range of the sample solution for the adsorption of cadmium and the kind of acid solution for dissolving the cadmium-adsorbed synthetic zeolite A-4, as well as the measurement conditions for the determination of cadmium by GFAAS, e.g., the ashing and atomizing temperature, were investigated. Quantitative recovery of cadmium onto zeolite A-4 from the sample solution over the pH range 2.0 - 9.0 was achieved by the batch method. After the solid-phase (cadmium-adsorbed zeolite A-4) was separated from the sample solution by a membrane filter, it was dissolved in 2.0 cm(3) of 2.0 mol dm(-3) nitric acid. An aliquot of the resulting solution was injected into the graphite furnace. In GFAAS measurements an alternate gas (Ar, 90%; O(2), 10%) was used as a sheath gas, and the ashing temperature and atomizing temperature were 400 degrees C and 1600 degrees C, respectively. The detection limit (3 sigma) for cadmium was 0.002 microg dm(-3). The relative standard deviation at 0.010 microg dm(-3) was 3.5 - 4.5% (n = 5). The proposed method has been successfully applied to the analysis of trace cadmium in environmental water samples.

On-line redox derivatization liquid chromatography using double separation columns and one derivatization unit.

A new on-line redox derivatization technique using double separation columns and one redox derivatization unit was presented for enhancement of separation selectivity of HPLC. This on-line redox derivatization HPLC system consisted of two separation columns and one redox derivatization unit placed between them. The redox reaction proceeds in the derivatization unit so that an analyte compound migrates as its original form in the first column, while as its oxidized or reduced form in the second column. The retention of the analytes is controlled by the lengths of the two separation columns in this system. We adopted a small column packed with porous graphitic carbon (PGC) as a redox derivatization unit and two C18 silica columns treated with hexadecyltrimethylammonium chloride as separation columns. The redox activity of PGC and the efficiency of the on-line redox derivatization HPLC system for enhancement of separation selectivity were investigated using EDTA complexes of some metal ions. Original untreated PGC and PGC treated with hydrogen peroxide completely oxidized Co(II)-EDTA and converted it to Co(III)-EDTA, while the other metal complexes eluted as their original oxidation states throughout the system. Selective separation and determination of cobalt in a reference copper alloy by the developed method were demonstrated.

Evaluation of the surface charge properties of porous graphitic carbon stationary phases treated with redox agents.

The effect of treatment of porous graphitic carbon (PGC) stationary phases with hydrogen peroxide and with sodium sulfite on the retention behavior of analyte compounds has been investigated using benzene, aromatic sulfonate ions, and benzyltrialkylammonium ions as model compounds. It is shown that the retention times of the cationic analytes are increased by treating the PGC column with the reducing agent, while decreased by treating it with the oxidizing agent. On the other hand, the retention times of the anionic analytes are decreased by treating the column with the reducing agent, while increased by treating it with the oxidizing agent. The effect of the redox treatment on the retention of benzene is negligibly small. The investigation of the ion-exchange property of the PGC packings have shown that PGC has anion-exchange property and the anion-exchange capacity is decreased by treating PGC with the reducing agent, whereas it is increased by treatment with the oxidizing agent. This means that the modification of the retention selectivity of the PGC stationary phases with redox treatment can be interpreted in terms of the change of the surface charge. The mechanism of chemical modification of the PGC stationary phase with redox treatment is discussed on the basis of the experimental results obtained on the ion-exchange capacity and the redox activity.