Research Article Stability constants determination of successive metal complexes by hyphenated CE-ICPMS The study of radionuclides speciation requires accurate evaluation of stability constants, which can be achieved by CE-ICPMS. We have previously described a method for 1:1 metal complexes stability constants determination. In this paper, we present its extension to the case of successive complexations and its application to uranyl-oxalate and lanthanum- oxalate systems. Several significant steps are discussed: analytical conditions choice, mathematical treatment by non-linear regression, ligand concentration and ionic strength corrections. The following values were obtained: at infinite dilution, log(b 1 1 (UO 2 Oxa)) 5 6.9370.05, log(b 2 1(UO 2 (Oxa) 2 2 )) 5 11.9270.43 and log(b 3 1(UO 2 (Oxa) 3 4 )) 5 15.1170.12; log(b 1 1(LaOxa 1 )) 5 5.9070.07, log(b 2 1(La(Oxa) 2  )) 5 9.1870.19 and log(b 3 1(La(Oxa) 3 3 )) 5 9.8170.33. These values are in good agreement with the literature data, even though we suggest the existence of a new lanthanum-oxalate complex: La(Oxa) 3 3 . This study confirms the suitability of CE-ICPMS for complexation studies. Keywords: CE / Hyphenation / Lanthanum-oxalate / Stability constants / Uranyl-oxalate DOI 10.1002/elps.200900295 1 Introduction Many analytical techniques are suitable for direct speciation analysis: UV/vis. spectrophotometry, conductimetry, poten- tiometry or species-selective techniques such as Mo ¨ssbauer spectroscopy, X-ray photoelectron spectroscopy, electron spin resonance spectroscopy or electrochemical techniques [1], but these analytical techniques usually fail at trace levels in a real- sample matrix. The speciation study resorts thus to hyphenated techniques [2, 3]: the species-selectivity is achieved by chromatography or electrophoresis, whereas element-selectivity and sensitivity are obtained by atomic or MS. Among the available hyphenated techniques, recent improvements in CE-ICPMS hyphenation and developments of commercial interfaces allow CE-ICPMS to be regarded as an attractive technique for speciation studies [4–8], gathering many advantages: low sample and electrolyte consumption, better respect of metal speciation, high sensitivity, possibility of implementation in a radiological controlled area, etc. [9]. For all these reasons, CE-ICPMS was chosen in this study. The nuclear energy development has raised new research topics in the past years, such as the radionuclides speciation in the environment. This issue is crucial in the context of nuclear waste repository, because the physico- chemical forms of an element rule its biochemical proper- ties like toxicity, bioavailability and mobility. This is why this study is dedicated to radionuclide behaviour in solution. Understanding the radionuclides behaviour requires an accurate determination of stability and kinetic rate constants between radionuclides and naturally occurring ligands. This work is concentrated on the determination of uranium(VI)- oxalate and lanthanum(III)-oxalate. Indeed, uranium(VI) can be regarded as a representative element for actinides at 16 oxidation state, whereas lanthanum(III) belongs to the lanthanides group. Ligand oxalate is a small organic compound with good complexation abilities, ubiquitous in natural waters [10, 11] and crucial in some nuclear contexts. The analytical method for stability constants determina- tion depends on the stability or lability of the studied complex. In order to predict its behaviour, Sonke and Salters [12] have built a diagram representing the complex stability areas, according to their stability and kinetic rate constants. The lability of the uranyl-oxalate and the lanthanum-oxalate complexes has already been established [5]. In CE, the most common method used to estimate labile complexes stability constants is named ACE [2, 13] and involves measuring the electrophoretic mobility variation of a metal ion with the concentration of ligand in the buffered electrolyte. This method, developed to calculate the 1:1 metal-to-ligand ratio stability constants, has been described previously [5]. In a real sample matrix, it appears that metal speciation is made up of 1:1 metal–ligand complexes but also various superior order complexes. The aim of the present study is to extend ACE method to superior order complexes of labile systems, which has little been published in the literature [8]. This paper will Jeremy Petit 1 Jean Aupiais 2 Sylvain Topin 2 Vale ´ rie Geertsen 1 Catherine Beaucaire 3 Moncef Stambouli 4 1 CEA/DEN/DPC/SECR/LANIE, Ba ˆ timent, Gif-sur-Yvette. France 2 CEA/DAM Ile de France, Bruye ` res-le-Cha ˆ tel, Arpajon, France 3 CEA/DEN/DPC/SECR/L3MR, Ba ˆ timent, Gif-sur-Yvette, France 4 Ecole Centrale Paris/Laboratoire de Ge ´ nie des Proce ´ de ´s et Mate ´ riaux, Cha ˆ tenay-Malabry, France Received May 6, 2009 Revised July 16, 2009 Accepted August 19, 2009 Abbreviation: NEA, OECD nuclear energy agency Correspondence: Dr. Jeremy Petit, CEA Saclay, Ba ˆ t. 391 Pie ` ce 10B, 91191 Gif-sur-Yvette Ce ´ dex, France E-mail: jeremy.petit@centraliens.net Fax: 133169085411 & 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim www.electrophoresis-journal.com Electrophoresis 2010, 31, 355–363 355