Determination of metal ions using ion chromatography and indirect amperometric detection

Abstract

A method for determination of metal ions subsequent to ion chromatographic separatkn has been devlsed. The method Is bared on indirect amperometric detection whereby a decrease In an oxidation current, proportional to the concentration of metal ion, is monitored. In this particular work, an easlly oxldlred band is added postcolumn. Upon complexatbn with the metal ions, the ligand is rendered electroinactlve at the preseleted potentlal. The fact that the ligand Is oxidized at low anodlc potentials results In a selective detection system.The separation of metal species using liquid chromatography and their subsequent detection remains a challenging field of research. Separation may be achieved by using normal-phase (1, 2) or reversed-phase (3-11) chromatography, subsequent to a suitable derivatization process, or ion exchange chromatography (12)(13)(14)(15)(16)(17)(18)(19)(20). In recent times excellent selectivity has been attained by using ion exchange chromatographic procedures wherein the metals are separated as charged complexes (17-20).Spectrophotometric (I-@, electrochemical (&IO), and atomic absorption spectrometry (16, 17) are amongst the methods of detection that have been investigated. Spectrophotometric (W-vis) and electrochemical (EC) detection have attracted most interest since they offer multielement analysis capabilities. These detection methods are usually employed in conjunction with a derivatization step for determination of metals. For UV-vis detection, the purpose of the derivatization is to produce a metal species, usually a complex, with a high extinction coefficient, enabling sensitive detection.Derivatization prior to electrochemical detection may be used to enhance the electrode process as well as to overcome problems associated with the electrochemical reduction of dissolved oxygen.With the above requirements in mind the dithiocarbamate ligand (I shown below) has proven particularly useful as a(1) dil niocar bamate f dtc derivatizing agent for metal ions prior to separation and detection. The most convenient method described achieves derivatization of the metal ions, in situ, by including the ligand in the chromatographic solvent (3)(4)(5)(6)(7)(8)(9)(10)(11). This allows for direct injection of metal ions. Subsequent to in situ derivatization the metal complexes are separated by using reversed-phase chromatography and detected by using either UV-vis or EC detection. The method is limited in that, in some cases, the instability of the metal complexes on-column mitigates against development of a successful analysis procedure. Other major drawbacks of this method are found in the detection stage. To enable rapid in situ formation an excess of dithiocarbamate must be present in the eluent. The free dithiocarbamate ligand absorbs in the UV region of the spectrum (21) causing high backgrounds with UV-vis detection. The free ligand is also oxidized electrochemically at relatively low positive potentials (22) according to the following:and therefore, monitoring of metal oxidation processes that ...

Indirect detection methods for capillary separations

“…Electrochemical detection has been shown to have virtually the same level of sensitivity as fluorescence when used with CZE (28)(29)(30). The principle of indirect electrochemical detection has been demonstrated with LC (31,32) and with ion chromatography (33,34). Although the former employed Buffer composition: 0.5 mM quinine sulfate dihydrate, 100 mM SDS in 10% (v/v) methanol solution, pH 6.8.…”

“…Peaks: A, 2-propanol; B, 1-propanol; C, 2-butanol; D, 1-butanol; E, 2-methyl-1-propanol. charge displacement of hydroquinone (31) or 2,5-dihydroxybenzoic acid (34), the latter required the complexation of metal ions to an electroactive ligand (dithiocarbamate). Once the electroinactive complex was formed, the consumption of the reagent was monitored electrochemically.…”

Separation and Determination of Thorium in Uranium Mineralizations Using a Mixed Ion Exchange Column by Ion Chromatography

“…A similar elution order was reported in the case of cation exchange chromatography as uranium forms weaker cationic species with the frequently used eluents like hydrochloric and nitric acids (Nelson et al, 1964). Nowadays, stationary phases having both cation and anion exchange capacities are found to be more useful in separating heavy and transition metal ions (Hojabri et al, 1987) because the separation is controlled by the concentrations of free metal ions and the various insitu metal complexes formed, which are in an equilibrium with each other (Park et al, 1993). One such mixed ion exchanger namely IonPac CS5A has been widely used for the separation of lanthanides and transition metal cations in several matrices (Cardellicchio et al, 1997;Perna et al, 2002).…”

Separation Behavior of U(VI) and Th(IV) on a Mixed Ion Exchange Column Using 2,6-Pyridine Dicarboxylic Acid as a Complexing Agent and Determination of Trace Level Thorium in Uranium Matrix Employing High Performance Ion Chromatography

“…Ion exchange separation of metal ions can be affected by adding weak organic acids in the mobile phase, which reduces or alters the effective charge of the metal ions by forming either cation, neutral or anion complexes and the nature and stability of the metal complexes would decide the method of ion exchange (anion or cation) separation. Nowadays, stationary phases having both cation and anion exchange capacities are found to be more useful in separating heavy and transition metal ions [29] because the separation is controlled by the concentrations of free metal ions and the various insitu metal complexes formed, which are in an equilibrium with each other [30]. One such mixed ion exchanger namely Ion Pac CS5A has been widely used for the separation of lanthanides and transition metal cations in several matrices [31][32][33].…”