Separation of uranium(VI) and lanthanides by capillary electrophoresis using on-capillary complexation with arsenazo III

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Abstract

The viability of the separation of lanthanides and uranium(VI) in the form of strongly absorbing complexes with arsenazo III (AIII) was studied with the aim to increase the sensitivity of absorbance detection in determination of these metals by capillary electrophoresis (CE). Special attention was paid to the complexation equilibria in the background electrolyte (BGE). On-capillary complexation provided better peak shapes for lanthanides compared to pre-column complexation. While the BGE composition had very little effect on the peak shape of the kinetically inert uranium(VI) complex, it played a crucial role in the peak shapes of the kinetically labile lanthanide complexes. Addition of a second ligand competing with the metallochromic ligand AIII for the metal ions was found to be critical to achieve good peak shape. The nature and concentration of the competing complexing ligand added to the BGE, the pH, and the concentration of AIII were found to exert a strong influence on the separation selectivity, peak shapes and the detection sensitivity. Several carboxylic acids were compared as BGE competing ligands and citrate provided best selectivity and peak shapes. A citrate BGE at pH 4.7 and containing 0.1 mM AIII was used for the separation of uranium(VI) (350 000 theoretical plates) and LaIII (63 000 theoretical plates) while, to separate most lanthanides and uranium(VI), a similar BGE with a lower (0.03 mM) AIII concentration was used. Using hydrostatic sampling (100 mm for 10 s) detection limits of 0.35 μM (49 ppb) LaIII and 25 μM (60 ppb) UO2 were obtained. Using on-capillary complexation, sample stacking was retained for injection times of up to at least 100 s (ca. 30-mm sample plug) without loss of peak shapes for lanthanides or recovery for LaIII. When this process was used, the detection limit for LaIII was reduced to about 5 ppb. Optimal properties of metallochromic ligands for separation and detection of metals by CE are discussed.

Introduction

Numerous examples of separations of both small anions and small cations by capillary electrophoresis (CE) have been reported, and these generally exhibit much superior separation efficiency and shorter analysis times compared to ion chromatography (IC), but with concentration sensitivity usually lower than IC by an order of magnitude or more 1, 2, 3. This is the case even when both techniques are compared using the same detection system, for example indirect photometric detection, which is universal and is consequently frequently used for non-absorbing ions in CE and IC [2]. The main reason for the reduced sensitivity of CE is the much shorter optical path length (usually by two orders of magnitude) of the on-capillary photometric detector in CE compared to a typical photometric detector used in IC. To increase the sensitivity of absorption detection, some analytes can be converted to species with higher absorptivity [4]. If non-absorbing ions can be derivatised to absorbing species, direct photometric detection can be used instead of indirect, which generally results in superior limits of detection 1, 2.

In the area of metal determinations, this approach is utilised when metals are determined as highly absorbing complexes with metallochromic ligands. Numerous photometric methods [5]and liquid chromatography–IC methods with photometric detection [1]exist. In the past several years also CE and a related method, micellar electrokinetic chromatography (MEKC), have been increasingly applied to separations of metal ions in the form of complexes with metallochromic ligands 3, 6. The metallochromic ligands used have included 4-(2-pyridylazo) resorcinol (PAR) 7, 8, 9, 10, 11, porphyrin derivatives [12], 2,2′-dihydroxyazobenzene-5,5′-disulfonate (DHABDS) [13], 2-(5-nitro-2-pyridylazo)-5-(N-propyl-N-sulfopropylamino)phenol (Nitro-PAPS) [14], 2,2′-(1,8-dihydroxy-3,6-disulfonaphthylene-2,7-bisazo)-bisbenzenearsonic acid (arsenazo III) 9, 15, 2-nitroso-1-naphthol-4-sulfonic acid (Nitroso-NSA) [16], 2-(5-bromo-2-pyridylazo)-5-(N-propyl-N-sulfopropylamino) phenol (Br-PAPS) [17], 2-(5-bromo-2-pyridylazo)-5-(diethylamino)phenol (PADAP) 18, 19, (2-(4,5-dihydroxy-2,7-disulfo-3-naphthylazo) phenylarsonic acid), (arsenazo I) 20, 21, and (3,6-bis(2-sulfophenylazo)-4,5-dihydroxy-2,7-naphthalenedisulfonic acid (sulfonazo III) [22]. However, despite the high separation power of CE, often only a few metals (typically one to five metals 7, 8, 9, 10, 11, 12, 15, 16, 18, 19, 20, 21) have been separated, even though most of these ligands form complexes with a broad range of metal ions. Therefore problems in the application of this approach can be anticipated and consequently more research is needed to identify the factors which govern the separation process.

One of the most challenging tasks is the separation of lanthanide metal ions. Separations of lanthanides by CE have been achieved utilising secondary equilibria with an auxiliary complexing ligand [3]. All 14 lanthanides were separated using partial complexation with 2-hydroxyisobutyric acid (HIBA) 23, 24, 25, 26, 27, 28, 29, 30, 31or lactic acid 29, 30and using indirect photometric detection. Direct absorption detection in the UV range has been utilised in the complete separation of 13 lanthanides as complexes with 1,2-diamino-cyclohexane-N,N,N′,N′-tetraacetic acid (CDTA) 32, 33in the total complexation mode. Somewhat surprisingly, no successful applications of metallochromic ligands to the separation of lanthanides have been reported. It should be noted that all the lanthanide separations have been achieved with relatively small auxiliary ligands.

Probably the most sensitive photometric reagents for a large number of di- and higher valent metals including uranium, thorium and lanthanides is arsenazo III (abbreviated AIII) 5, 34. It has been used in CE for separation of a model mixture of several transition metals, lanthanum, cerium(III) and uranium(VI) [9], but peaks for lanthanum and cerium(III) were poorly shaped and co-migrating. AIII has been also used for the determination of uranium(VI) in waters [15], but the peak obtained was broad. Arsenazo III is a relatively large molecule, so that differences in electrophoretic mobility caused by complexation with different metal ions are unlikely to be significant and it is therefore questionable whether a complete separation of a number of similar metal ions can be achieved using the total complexation mode.

The aim of the present study was to determine whether partial complexation with AIII using a competing auxiliary ligand added together with the AIII into the background electrolyte (BGE) is a workable way to separate lanthanides, uranium(VI) and thorium(IV) by CE and to study the role of the complexation equilibria in the BGE in the CE separation.

Section snippets

Reagents

Carbowax 20M (poly(ethylene glycol) compound with 2,2′-[1-methylethylidene)bis(4,1-phenyleneoxymethylene)]bisoxirane, CW20M) and arsenazo III (2,2′-(1,8-dihydroxy-3,6-disulfonaphthylene-2,7-bisazo)-bisbenzenearsonic acid) were purchased from Aldrich. The purity of AIII determined using the earlier reported CE method [35]was 79% (external standard method using 98% pure AIII, Aldrich). Standard solutions of metals were prepared from the chemicals listed in Table 1. All chemicals other than

General approach

The majority of the previously published separations of metals as complexes with metallochromic ligands have used pre-capillary complexation as the preferred method of complex formation [3], with on-capillary complexation being used in only a few cases, such as with PAR [10], arsenazo I 20, 21or sulfonazo III [22]. Pre-capillary formation is preferable when the metal complexes are stable and often also kinetically inert. In these cases the metal ions are usually complexed totally. For totally

Conclusions

The following conclusions can be drawn with regard to (i) separation selectivity, (ii) separation efficiency and (iii) sensitivity of direct detection:

(i) Even a large ligand such as AIII can provide good separation selectivity towards a group of similar metals, such as the lanthanides, when secondary complexation equilibria are employed as the source of separation selectivity.

(ii) This is possible only if sufficiently fast kinetics of the complexation equilibria can be achieved, which in turn

Acknowledgements

The authors would like to thank Professor G. Deacon of Monash University, Melbourne, Australia, for the donation of several lanthanides. Financial support from the Australian Research Council, Dionex Corporation, and the Department of Industry, Science and Tourism (for a travel grant to P.N.) are gratefully acknowledged.

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