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Rapid Separation and Quantification of Iron in Uranium Nuclear Matrix by Capillary Zone Electrophoresis (CZE)

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Rapid Separation and Quantification of Iron in Uranium Nuclear Matrix by Capillary Zone Electrophoresis (CZE) American Journal of Analytical Chemistry, 2011, 2, 46-55 doi:10.4236/ajac.2011.21005 Published Online February 2011 (http://www.SciRP.org/journal/ajac) Rapid Separation and Quantification of Iron in Uranium Matrix by Capillary Zone Electrophoresis (CZE) Vivekchandra Mishra, Mrinal Kanti Das, Subbiah Jeyakumar, Ramesh Mahadeo Sawant, Karanam Lakshminarayana Ramakumar Radioanalytical Chemistry Division, Bhabha Atomic Research Centre, Mumbai, India E-mail: [email protected] Received August 12, 2010; revised November 30, 2010; accepted December 2, 2010 Abstract A method was developed for rapid separation and determination of iron by employing capillary zone elec- trophoresis (CZE) technique with direct UV detection. Iron could be separated from matrix uranium by di- rect injection of dissolved sample solution into capillary using a mixture of 10 mM HCl and 65 mM KCl (pH = 2) as background electrolyte (BGE) at an applied voltage of 15 kV. The developed method has a very high tolerance for the matrix element U (100 mg/mL) and as such may not need prior separation of iron from the matrix. Iron could be separated with better than 95% recovery. The method showed a linear calibration over a concentration range 1-50 ppm of Fe(III). The migration times for the iron peak were reproducible within 1% for both pure Fe(III) and in presence of matrix uranium (80 mg/mL). The precision (RSD, n = 22) of peak area obtained for 1 ppm of iron was 3.5%. The limit of detection (LOD) (3) was 0.1 ppm and the ab- solute LOD was 9 × 10-14 g considering the sample injection volume of 1.5 nL. The developed method has been validated by separating and determining iron in two certified reference materials of U3O8. The method was applied for the determination of iron in different uranium based nuclear materials. The CZE method is versatile for routine analysis as it is simple, rapid and has simple sample preparation procedure. Keywords: Capillary Zone Electrophoresis, Iron, Uranium, Nuclear Industry, Specification Analysis 1. Introduction dispensable to confirm the quantitative recovery of iron. This necessitates a simple, reliable, rapid and specific Separation and determination of trace elements in nu- method to minimize the labour involved and to achieve clear materials is important in nuclear industry as their maximum sample throughput. performance in the reactor critically depends on their In this context, the application of CZE for metal ion purity [1]. During the initial stages of nuclear fuel fabri- determination deserves increased attention due to its cation, the designers require the purity check analyses of speed, high separation efficiency, resolving power, mini- the unfinished fuel with respect to a few elements like Fe mal sample and reagent requirements. Ever since the first and Ca [2]. Depending upon the concentration of iron, paper concerning the indirect detection of inorganic ions the designers are required to take adequate measures to by using CE in 1967 [10], the application of CE to the achieve the desired purity. separation and determination of inorganic substances has Several methods are available for the determination of increased rapidly and many papers have been published iron in nuclear materials. The methods involving ICP- and some of critical reviews have also been summarized AES [3,4], ICP-MS [5], spectrophotometry [6], ion chro- [11-15]. Different approaches have been reported to real- matography [7,8] and HPLC [9] are well known. A dis- advantage of these methods is the associated laborious ize the full potential of CE for separation of metal ions by and time consuming sample preparation procedures and CE. These include employing non-aqueous media [16,17], the need for prior separation of matrix uranium using use of partial or complete complexation techniques [18, solvent extraction or ion exchange before carrying out 19], using CE in conjunction with ICP-MS [20-22] and the measurement. Also analysis of a matrix-matched use of quantitative microchip CE [23]. Despite the in- reference material every time along with samples is in- creased applications to inorganic materials, the applica- Copyright © 2011 SciRes. AJAC V. MISHRA ET AL. 47 tion of CE in nuclear industry has been somewhat limited higher pH electrolyte media (typically pH 6-10) are used [24,25]. for the effective coating of CTAB on the capillary sur- The separation principle of CE is based on the differ- face and subsequent formation of double layer necessary ential electrophoretic mobility of charged compounds. for reversing the EOF. Under such pH conditions, the The differences in the mobility of the analytes are related, bulk uranium will undergo hydrolysis significantly and in turn, to their charge densities, i.e., the charge-to-mass severely obstruct the movements of ions inside the capil- ratio. One of the problems in the analysis of metal ions lary path. This is because unlike the hydroxycarboxylic by CE is that most of transition metal cations have al- acids, the aminocarboxylic acids cannot prevent the hy- most the same mobility due to their similar size and drolysis of metals at higher pH. Moreover, the pH of the identical charge. Obviously, the enhancement of separa- sample cannot be kept low (pH < 3) as effective com- tion selectivity is the only alternative to achieve a satis- plexation does not occur between metal and aminocar- factory resolution. Generally, there are two main ap- boxylic acids. proaches in this direction that imply the addition of a The aim of the work was to develop a method for the complexing ligand to either the carrier electrolyte or a separation and quantification of iron from uranium. As sample solution before introduction into the capillary direct injection of the sample solution into capillary is [26]. In the first case, the mobility of sample cations to- envisaged, significant reduction in analysis time is possi- ward the cathode can be selectively moderated due to the ble which is desirable during process control operations. partial complexation within the capillary, followed by At the same time, interferences from other metal cations the formation of metal complexes of different stability can be eliminated due to the anionic complex formation and thereby effective charge [27,28]. The second ap- with chloride ligand. proach provides the complete pre-capillary or on- capil- lary conversion of metal ions into stable, charged com- 2. Experimental plexes, which can move with different mobilities de- pending on their charge, size and stability [29]. The latter 2.1. Instrumentation approach was exploited in the present investigation. Several CZE methods have been reported for the de- Separations were performed on a commercial capillary termination of iron in various matrices like water, elec- electrophoresis apparatus (Prince Technologies, CEC-770, troplating baths and cyanide complexes [30-34]. CZE Netherlands) equipped with a photodiode array detector. separation of Fe(II) and Fe(III) was demonstrated after Fused silica capillaries of 50 µm i.d. and 60 cm long complexing with o-phenanthroline and EDTA respec- were used. A capillary having 75 µm i.d. and 60 cm long tively and the same indicates separation feasibility of was also used. Sample injection was done in hydrody- Fe(III) as its EDTA complex [35]. Another study reports namic mode by the application of 50 mbar pressure for the separation of Fe(III) as its DTPA complex [36]. In 0.2 min on the sample vial which corresponds to a sam- these reported methods, for the determination of either ple volume of 1.5 nL. System DAX software was used total iron or speciation, Fe-aminocarboxylic acid anionic for data acquisition. Direct UV detection was performed complex was separated from other metal-aminocarbo- at 214 nm. All the experiments were conducted at room xylic acid anionic complexes. It would be difficult to temperature (25-27˚C). The pH measurements were done apply these methods directly for the nuclear materials with a pH meter (Eutech, Tutor-model, Malaysia). like uranium compounds because U(VI) also forms ani- onic complex with aminocarboxylic acids. Between the aminocarboxylic acid complexes of U(VI) and Fe(III), 2.2. Reagents and Solutions the net negative charge of the U(VI) complex is more than that of the Fe(III)-complex (eg. Fe(III) [33] and Standard stock solution of Fe(III) was prepared by dis- U(VI) [37] with EDTA form Fe-EDTA- and solving Fe(NO3)3•9H2O (99.99%, Aldrich Chemicals, 2- USA) in 0.01N HNO . Subsequently, the working stan- UO2-EDTA respectively). Due to the higher charge on 3 the uranyl complex, the migration order follows uranium dards were made from the stock by appropriate dilutions. and then iron and large amount of uranium in the sample High purity HCl and HNO3 acids (suprapure, MERCK, solution produces a bulk and long tailing peak, possibly Germany) were used for the sample and electrolyte masking the iron peak or affecting its recovery. It is preparations. Potassium chloride (GR grade, MERCK, worth mentioning that while separating the metal-ami- Germany) was used. Nuclear grade UO2 (NFC, India) nocarboxylic acid anionic complexes, it would be diffi- was used for the preparation of standard uranium solu- cult to achieve a large separation factor to overcome the tion and the concentration of U was obtained from biam- problem of peak masking. Moreover, in these methods perometric determination [38]. All solutions, electrolytes Copyright © 2011 SciRes. AJAC 48 V. MISHRA ET AL. and standard solutions were prepared with ultrapure wa- analyte should be faster than matrix element. Under these ter (18 MΩ) obtained from a MilliQ-Academic System conditions, the analyte reaches the detector much earlier (Millipore, India). than the matrix element and therefore, it will be free from matrix effects. In the present case, since both uranyl 2.3.
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