Elsevier

Microchemical Journal

Volume 76, Issues 1–2, February 2004, Pages 161-171
Microchemical Journal

Development of a new method for the separation of vanadium species and chloride interference removal using modified silica capillaries-DIN-ICP-MS

https://doi.org/10.1016/S0026-265X(03)00158-9Get rights and content

Abstract

A new on-line method for the separation of vanadium (IV) and vanadium (V) as well as for the removal of ClO+ mass spectral interference on vanadium determination by quadrupole-ICP-MS has been developed. The sample introduction system consists of a modified fused silica capillary coupled to a direct injection nebuliser (DIN), between the solvent delivery system and the ICP. Fused silica capillaries were treated with different anion and cation exchanger reagents and were tested for the retention of Cl and the separation of vanadium ions at μg l−1 levels. A suitable strong anion exchanger functional group (3-aminopropyltrimethoxy silane) was selected. Chlorine anions were retained in this anionic capillary and the separation between V(IV) and V(V) was possible in the pH range 2–4. The selections of instrumental ICP-MS conditions for the minimisation of the ClO+ interference were carefully considered. Factors affecting the chromatographic separation such as sample pH, sample flow rate, effect of methanol in the mobile phase and length of the capillary for the separation were optimised. The proposed methodology provides a simple and rapid method for vanadium speciation. A relative detection limit of 12 l−1 (i.e. absolute detection limits of 120 pg) for V(IV) based on peak height measurements was obtained. The relative standard deviation for V(IV) was 2.4% for a 10 μl injection (n=6).

Introduction

The chemical and physical properties of a metal species depend very much on oxidation state, hence the accurate determination of each species is important to evaluate the potential risk of some metals. The importance of the simultaneous determination of vanadium in different oxidation states can be attributed to a number of reasons. (i) Environmental. Vanadium is emitted into the environment from vanadium refineries, iron and steel industries, and chemical industries. Among the latter are the phosphate industries, which can be a major source of vanadium pollution [1]. Due to the increase in production by these industries, there has been concern about increases in the environmental levels of vanadium; a phenomenon that needs to be monitored closely. (ii) Biological. Vanadium is an essential trace element, with specific physiological functions. However, numerous reports have warned of the carcinogenic and other toxic effects of vanadium, resulting from excessive industrial exposure [1]. Studies have shown that vanadium, when administrated as vanadate, is physiologically more active in fish, than other species of vanadium [2]. (iii) Industrial. Vanadium metal is present in alloys and other industrially important materials. It can be used as a catalyst in numerous industrial processes, e.g. the Stretford, Unisulf, Shafer, and Sulfolin processes [3]. In these processes (and in alloys) vanadium is capable of existing simultaneously in its different valence states. For a thorough understanding of the chemistry involved in these processes, the determination of vanadium in its different oxidation states is essential.

The significance of vanadium speciation is that the two oxidation states, V(V) and V(IV) have different nutritional and toxic properties. Therefore it is important, from the analytical point of view, to develop sensitive, selective, rapid and cost-effective methods for quantitative speciation, even when the element is present in trace amounts. Although many studies have been devoted to metal speciation, little work on the speciation of vanadate (VO3) and vanadyl ions (VO2+) has been reported.

Several methods have been developed for the determination of vanadyl, V(IV) and vanadate, V(V), generally based on a two-step procedure including measuring one species and obtaining the other by difference after total vanadium measurement [4], [5]

Speciation of vanadium by spectrophotometric detection using 2-hydroxyacetophenone oxime has been reported [4]. The two species were simultaneously determined at 1–3 μg ml−1 for V(IV) and 1–5 μg ml−1 for V(V).

Chromatographic separation was effective for metal species once a proper detector had been selected. Hyphenated techniques of chromatography coupled with atomic spectrometric detection [6], inductively coupled plasma-atomic emission spectrometric detection [7], or ultraviolet absorbance detection [8], [9] after appropriate derivatization have also been reported. Patel et al. [6] determined vanadate and vanadyl species in aqueous standards and in a model yeast system by using a flow-injection system that incorporated a strong anion exchange resin and was coupled to flame atomic absorption spectrometry (AAS). Only V(V) could be quantitatively identified at the ppb level. The same workers have also investigated the toxicological effects of vanadium by dosing yeast cells with the two stable species of vanadium. Yeast cells dosed with concentrations above 200 ppm of the toxic V(V) species did not grow, whereas those with the less toxic V(IV) were able to tolerate a higher concentration. Hirayama et al. [7] applied a two-column system consisting of a chelating functionalised immobilised silica gel (ethylendiamine-chelating immobilized silica gel-bonded, ED-CIS and ethylendiamine-triacetate-bonded, ED3A-CIS) and inductively coupled plasma-emission spectrometry for the pre-concentration and speciation of vanadium ions. Under suitable conditions, each species was adsorbed onto a differently prepared (ethylenediamine-bonded and ethylenediaminetriacetate-bonded) silica gel columns, from which they were separately eluted and detected by ICP-AES. The detection limits, reported only for V(V), was 60 pg ml−1. Due to the selectivity of the detection mode, no significant interferences were observed. This technique does, however, require a considerable time for each analysis. Sugijama and Hori [10] used air-segmented continuous-flow analysis based on a catalytic reaction to determine vanadium species. Komarova et al. [8] proposed the anion-chromatographic separation of the V(IV) and V(V)- EDTA complexes, with subsequent conductometric detection. Unfortunately, no baseline resolution of the V(IV) and V(V) peaks was achieved. A further disadvantage of this method is the use of conductivity as detection mode, since it is almost impossible to detect the vanadium species in complex inorganic sample matrices in this way. The method has therefore only limited application. Later, Jen and Yang [9] achieved the simultaneous speciation determination of V(IV) and V(V) as EDTA complexes by ion-pair liquid chromatography with UV detection. Lewis et al. [11] studied the speciation of trace metals by coupling ion chromatography with direct current plasma atomic-emission spectrometry. The operating conditions for the chromatographic speciation of V(V) and V(IV) were optimised. The concentrations determined for both vanadate and vanadyl ions were 1 ppm (without pre-concentration) and 0.05 ppm (with pre-concentration). However, most of these methods are far from ideal and still do not satisfy the requirements for routine analysis. They often suffer from disadvantages such as incomplete derivatisation, being time-consuming or requiring expensive and sophisticated instruments, and complicated operating procedures.

Within the last decade, ICP-MS has proved ideally suited as an alternative approach for the determination of vanadium in various matrices. The inductively coupled plasma mass spectrometer is a trace element detector with unique analytical capabilities. Liquid chromatography (LC) has been coupled satisfactorily to ICP-MS for vanadium speciation [12].

Despite its reputation for being a relatively interference-free technique, ICP-MS suffers interferences from polyatomic ions, particularly those derived from matrix acids, e.g. chloride, sulfate and phosphate. The anion most commonly considered to be problematic is chloride. The determination of vanadium by ICP-MS is very troublesome if the sample matrix contains large amounts of Cl, since 35Cl16O+ overlaps with the most abundant isotope of V (51V, 99.7%).

In order to achieve accurate and reliable analytical results, pre-concentration and matrix-analyte separation are needed when the analyte concentration is too low to be determined directly by MS, or when the matrix elements interfere with the determination. Interferences arising because of the presence of chloride ions have been alleviated in a number of ways. The addition of a molecular gas to the nebulizer gas flow [13], [14], [15], the introduction of a small amount of an organic solvent [13] and size-exclusion chromatography [16] have all been used successfully.

The direct injection nebulizer (DIN) is an ideal sample introduction system for routine ICP analysis for applications where sample volume is limited or high sample throughput is required. The coupling DIN-ICP-MS used in combination with an on-line separation method is ideally suited for vanadium speciation. The use of micro-columns containing chelating ion exchangers [17] or activated alumina [18] has been reported for speciation studies. To date, such methods have not been applied to vanadium speciation in biological or environmental samples. The objective of this work is to evaluate the capability of modified anionic capillaries connected directly to the DIN nebulizer of the plasma instrument as a novel approach to on-line vanadium speciation.

Previous papers have reported methods developed for interference removal [19], [20], [21] and speciation studies [22], using modified capillaries coupled to DIN-ICP-MS. The method described here has the merit of providing simultaneously the separation of vanadium species and the removal of ClO+ mass spectral interference coupled with the high sensitivity provided by the DIN-ICP-MS system and shorter analysis times.

Section snippets

ICP-MS

Studies were conducted on a VG PlasmaQuad 2 inductively coupled plasma mass spectrometer (VG Elemental, Winsford, Cheshire, UK) The typical operating conditions are shown in Table 1. The nickel sampler and skimmer cones (VG Elemental, Cheshire, UK) had orifices of 1.0 mm and 0.4 mm, respectively. The nickel cones showed no signs of erosion when water or methanol were nebulised through the DIN (as other workers found [23]) so they were used for the present work. The ICP-MS system was optimised

Selection of ICP-MS conditions for vanadium determination

An important limitation of ICP-MS in vanadium determination is the formation of polyatomic species that typically come from precursors in the Ar support gas, entrained atmospheric gases (N and O) or from the sample matrix. Table 2 shows the potential mass spectral interferences encountered in vanadium determination

As discussed above various workers have suggested that ClO+ polyatomic interference on 51V signal can be reduced, though not removed, by careful setting of the instrumental parameters

Conclusions and future work

Vanadium speciation is an attractive and difficult field of research in analytical chemistry because V(IV) and V(V) have many complex forms in aqueous solution that change in accordance with the solution pH and their concentration.

A new sample introduction system for the separation and determination of VIV and VV species using silica-bonded anion-exchange capillaries and ICP-MS has been developed in this work. Attractive features of this novel method include the rapid and quantitative

Acknowledgements

The authors would like to thank the financial support of the European Commission through funding the SMT-project ‘MOSIS – Modified Sample Introduction System’ (SMT4-CT98-2233).

References (29)

  • T. Kawashima

    Anal. Chim. Acta

    (1992)
  • T.V. Komarova et al.

    Anal. Chim. Acta

    (1991)
  • J.-F. Jen et al.

    Anal. Chim. Acta

    (1994)
  • M. Sugiyama et al.

    Anal. Chim. Acta

    (1992)
  • S. Hjerten

    J. Chromatogr.

    (1985)
  • P.C. Cole et al.

    Anal. Chim. Acta

    (1983)
  • S.A. Abbasi

    Anal. Lett.

    (1987)
  • T.G.F. Hudson

    Monographs of Toxic Agents

    (1974)
  • S. Vermaire et al.

    Ind. Eng. Res.

    (1988)
  • G.V.R. Murthy et al.

    Analyst

    (1989)
  • B. Patel et al.

    Analyst

    (1990)
  • K. Hirayama et al.

    Analyst

    (1992)
  • V.D. Lewis et al.

    J. Chromatogr. Sci.

    (1989)
  • M.J. Tomlinson et al.

    J. Anal. At. Spectrom.

    (1994)
  • Cited by (0)

    View full text