Optimization of the indirect fluorimetric detection for the simultaneous analysis of inorganic anions and polycharged organic anions by capillary electrophoresis

Original articles Analusis, 1998, 26, 107-115 © EDP Sciences, Wiley-VCH Optimization of the indirect fluorimetric detection for the simultaneous analysis of inorganic anions and polycharged organic anions by capillary electrophoresis P.L. Desbène*, C. Morin and A. Desbène Laboratoire d’Analyse des Systèmes Organiques Complexes, I.R.C.O.F. et I.F.R. n° 23, Université de Rouen, 43 rue Saint-Germain, 27000 Evreux, France Abstract. Until now, fluorescein sodium salt has been used to generate the background signal for indirect fluorimetric detection of inorganic cations or mono-charged organic anions separated by Capillary Zone Electrophoresis (CZE). In this paper, we extend the use of fluorescein sodium salt to the simultaneous CZE analysis of inorganic anions (Cl–, NO–2 , NO–3, SO42–, F–) and polycharged organic anions (oxalate and citrate). The electroosmotic flow was reversed by using tetradecyltrimethylammonium bromide (TTAB) or hexadimethrine. The specific problems linked to the simultaneous utilization of fluorescein sodium salt and a positively charged capillary were studied and resolved by reducing the fluorescein/TTAB interactions and by optimizing the fluorophore concentration in the electrolyte. The study of reproducibility and the quantitative approach led to very good results. In such conditions the quantification of Cl–, NO–3 and SO42–, in different mineral and spring waters, has been successfully realized. In addition, the detection thresholds reached the range of one hundred to a few hundred ppb. Key words. Capillary zone electrophoresis – indirect fluorimetric detection – fluorescein sodium salt – inorganic anions – polycharged organic anions. Introduction • either to coat positively the capillary walls and so to reverse the electroosmotic flow allowing the visualization of all anions, whatever be their electrophoretic mobility [16-38], Liquid Phase Chromatography is still considered today as a predilection technique for the inorganic anions analysis in several domains such as: environment, clinical diagnosis or food industry. Ions exchangers columns are used for the separation of these species, the detection being performed essentially by conductometry [1] and sometimes by indirect photometric detection [2]. However these techniques, although being still commonly used, compete more and more with High Performance Capillary Electrophoresis (HPCE), a relatively recent analytical technique that matches up a small cost of utilization, a certain facility of implementation and very high performances. • or to constitute micelles and so to analyze these inorganic ions by Micellar ElectroKinetic Chromatography [39]. Many cationic surfactants, generally quaternary ammonium surfactants, can be used as an electroosmotic flow reverser. For instance OFM Anion - BT [16-18,32,35], cetyltrimethylammonium (CTAB) [21-25,36,37], hexamethonium [20,27,28,34] or trimethyltetradecylammonium (TTAB) [27, 29-31,33,36-38] can be noted. Concerning the detection of inorganic anions separated by HPCE, we can observe, according to literature data, almost no loss of sensitivity as compared to Liquid Phase Chromatography. Several detection methods have been used, including amperometric detection in non aqueous media [14] or suppressed conductivity detection [4]. Nevertheless the direct or indirect photometric detection methods are mainly employed. The direct UV detection applies unfortunately only to a few number of inorganic anions such as Br–, I–, NO–2 NO–3 and SCN– [19,24,25,33,36,39-44]. On the other hand, the indirect UV detection appears more attractive due to its universality. The chromophore commonly introduced into the electrolyte to generate the background signal is the chromate [12,16-18,21,27,29,31,37]. However the utilization of dichromate [11,30,34], phtalate [7,14,20], pyromellitic acid [20,26,27] or polysulfonated naphtalenes [8] can be also noted. The introduction of these chromophores into the electrolyte often being matched by the addition of a quaternary ammonium surfactant, used as electroosmotic flow reverser, it is important to insure that incompatibility does In the case of bare fused silica capillary, the electrophoretic mobility of inorganic anions is diametrically opposed to the electroosmotic flow. As a result, if a complex mixture containing inorganic anions with a wide range of electrophoretic mobilities is analyzed, some of these anions will never reach the detection window whatever the injection (anodic or cathodic). To prevent this analytical problem three strategies can be envisaged. The first one is based upon the use of a strong electroosmotic flow [3,4] in order to obtain a satisfactory separation rate. This strategy shows however some limitations, anions presenting a very high electrophoretic mobility not being able to be analyzed. The second one consists in the reduction or the suppression of the electroosmotic flow [5-15], detection and injection being respectively performed near the anode and at the cathode. Nevertheless for the inorganic anions and polycharged organic anions analysis, a third strategy is currently the most frequently used, the use of cationic surfactants: * Cor respondence and reprints. Received No vember 28, 1997; r evised F ebruary 10, 1998; accepted February 17, 1998. 107 Article available at http://analusis.edpsciences.org or http://dx.doi.org/10.1051/analusis:1998119 Original articles not exist between these two components of the electrophoretic system, as is the case for the vanadium salts, that precipitate in front of some quaternary ammonium salts [16]. Otherwise the indirect fluorimetric detection, although less used, offers interesting potentialities concerning the sensitivity, for example in the case of the analyses of eleven priority phenols [45], of isopropenylpyrophosphates [46], of explosives [47] or of triorganotin compounds [48]. It has already been used for the analysis of inorganic anions, without reversion of the electroosmotic flow, by using detectors specially conceived for these applications [3,6,9]. Results and discussion The first studies mentioned in this paper were carried out by introducing into the buffer solutions the fluorophore (i.e. the fluorescein sodium salt) at 10−5 M. Indeed this concentration appeared to be during our previous works, performed without any electroosmotic flow modifier [49,50], the optimal concentration in regard of the concentration limit of detection (LOD). Similarly, the pH of the running buffer was fixed to 8.5 during these first approaches since this pH is perfectly suited to the utilization range of the fluorophore [49]. The aim of this study is to evaluate the potentialities of the first commercially available fluorimetric detector for the analysis of inorganic and poly ch a rged organic anions, without any modification of the equipment. These analyses have been carried out with an eletroosmotic flow modifier (TTAB or hexadimethrine), the fluorescein sodium sa (...truncated)