TY - JOUR
AU - Öztekin,, Nevin
AB - Abstract A rapid and sensitive capillary zone electrophoresis method for the simultaneous separation and determination of lanthanide ions, combined with capacitively coupled contactless conductivity detection (C4D), is reported. The influence of experimental parameters on separation was investigated. The optimal separation conditions were obtained when 4.5 mM 2-hydroxyisobutyric acid and 1 mM acetic acid (HAc) at pH 4.5 were used as buffer solution. Under these conditions, complete separation of all 14 lanthanide ions was achieved in 6 min. With the use of the C4D detector, the sensitive detection of non-UV active lanthanide ions was achieved without the need of a UV active ligand or a visualization agent. The sensitivities were further enhanced with a sample stacking procedure. The limits of detection were to be found between 2.77 and 8.26 nmol/L and the limits of quantification were between 9.29 and 27.5 nmol/L for 14 lanthanide ions. Introduction Nowadays, rare earth elements (REEs) (specifically elements of lanthanide group, yttrium and scandium) are critical components for modern electronic devices, magnets, pigments, catalysts, specialty glasses and lenses. Analysis of the supply and demand for each of the REEs indicated that, in 2014, global demand is around 200,000 tons per year, which exceeds current production by over 75,000 tons per year (1). The significant increase in global demand has led to exploring and mining new sources. Contrary to their name, REEs are relatively abundant in the earth, but they are present at trace concentration levels in geological materials and the abundance of individual lanthanides varies from one mineral to another. Several separation methods have been introduced for the separation and quantification of individual lanthanides. However, due to very similar features of the group ions, the separation of 14 lanthanides is a challenging task for all analytical methods. High performance liquid chromatography and ion chromatographic (IC) applications, using cation or anion exchange resins as stationary phase and aqueous solutions of complexing agents as mobile phase, have been reported for lanthanide analysis (2–4). Commonly, after chromatographic separation, lanthanides are detected by post-column UV-visible spectrophotometric detection using a chromophore complexed with lanthanide ions. A limited number of detection systems coupled with IC have been introduced alternatively in scientific papers, like spectrofluorometry (5, 6), inductively coupled plasma atomic emission spectrometry (7) and fast Fourier transform continuous cyclic voltametry (8). Due to the high separation efficiency of capillary electrophoresis (CE), several successful simultaneous separation of lanthanides have been reported. Lanthanides have been separated in capillary column and online detected, either directly or indirectly. In the direct detection mode, some limited number of UV active complexing agents are used for the complexation with lanthanides and the separated complexes are spectrophotometrically detected directly (9–15). In the indirect detection mode, a complexing agent and a chromophore are added to the separation electrolyte (16–24). As the complexing agent provides differentiation in the electrophoretic mobilities of group members, the added chromophore enables indirect UV-visible detection of lanthanide metal ions. The commonly used complexing agent for lanthanide separation is 2-hydroxyisobutyric acid (HIBA). However, due to the equal charge, very similar ionic radius and complex forming abilities of group elements, in both direct and indirect modes, a second and sometimes third auxiliary ligand needs to be added along the main complexing agent. In that way, by the competitions of complexing agents toward lanthanide ions, resolutions are improved. As an alternative to the UV-visible detectors, very few alternative detection methods have been reported for lanthanide detection in CE separations, such as inductively coupled plasma mass spectrometry (25) and electrospray mass spectrometry (26). In addition, isotachophoresis (ITP) is also one of the electrophoretic methods used in the separation of lanthanide ions. The separation of yttrium and 14 lanthanide ions by ITP has been achieved by using a high frequency contactless conductivity detector (24, 27–30). Recently, capacitively coupled contactless conductivity detection (C4D) combined with CE has been widely used in the analysis of inorganic ions, organic ions and bio-molecules (31). The introduction of the C4D for CE was independently made by Zemann et al. and Da Silva and do Lago in 1998 (32, 33). With the use of C4D detectors in CE, fast and sensitive detection of non-UV active species can be achieved. In this process, derivatization of the compounds or the addition of a visualization agent for indirect optical detection is not needed. Furthermore, the C4D detectors have advantages like simplicity, speed, versatility, high sensitivity, minimal maintenance and low cost. No detection window being required, changing of the capillary is carried out quickly. Recently, the separation of lanthanide ions by CE-coupled C4D detector was first published by Nguyen et al. (34). In the present study, complete separation and detection of all lanthanides were achieved by CE coupled with C4D detector. Furthermore, by the application of stacking technique for the first time, the sensitivities of lanthanides were highly enhanced. Experimental Chemicals Tb2(CO3)3, Tm2(CO3)2, Gd(NO3)3, Pr6O11 and Lu2O3 were purchased from Sigma-Aldrich (St Louis, USA). La2O3, Sm2O3, Eu2O3, Dy2O3, Ho2O3, Er2O3, Yb2O3 and HIBA were from Fluka (Buchs, Steinheim, Switzerland); Tris, Ce(NO3)3·6H2O, Nd(NO3)3·5H2O, perchloric acid and glacial acetic acid were from Merck (Darmstadt, Germany). Stock solutions of lanthanides were prepared by dissolving their oxides or carbonates in an excess of ultrapure perchloric acid, and nitrates in water. Other chemicals used were of analytical reagent grade. Instrumentation Separations were performed with a commercial CE injection system (Prince, Emmen, Netherlands) in combination with a contactless conductivity detector (TraceDec, Innovative Sensor Technologies, Strasshof, Austria). The data processing was carried out with DAx8.0 Data Acquisition and Analysis software. All solutions were prepared with ultrapure water purified by an Elga Purelab Option-7-15 filtration system (Elga, UK). An Orion Dual Star pH-ISE meter (Thermo Fisher Scientific Beverly, MA, USA) with combined glass pH electrode was used for pH measurements. The fused silica capillaries used for separation were obtained from Polymicro Technologies (Phoenix, AZ, USA). Electrophoretic conditions The separation voltage was 28 kV. Sample injection was carried out with pressure (60 mbar, 0.1 min) for normal CE experiments and with electro kinetic injection (5 kV, 0.2 min) for stacking experiments. Injections were performed from the anodic side. A 61.5 cm x 50 μm I.D. fused silica capillary was used. The distance from the detection window was 50 cm. The capillary was flushed (at a pressure of 1,000 mbar) with 0.1 M HCl and water and running buffer each for 10 min at the beginning of each day. A washing step of 1 min with buffer between runs was applied. The C4D detector was operated at 0 Db of voltage and a gain of 100%. Results The composition and pH of the buffer were optimized to obtain complete separation and sensitive response of the C4D detector to the 14 lanthanide ions. Effects of concentration of HIBA and HAc were investigated using HIBA and acetic acid (HAc) as composition of the buffer. Effect of HIBA concentration Figure 1 shows the electropherogram of lanthanides with increasing HIBA concentrations. The concentration of HIBA was studied over the range of 1–6 mmol/L at pH 4.5. The pH of all solutions was adjusted by Tris. The pKa value of HIBA is 3.79. In addition to complexing agent, HIBA serves as buffer at this pH. The optimum HIBA concentration was selected as 4.5 mmol/L. Figure 1. Open in new tabDownload slide Separation of lanthanides with increasing HIBA concentrations: (a) 1.0 mmol/L, (b) 2.0 mmol/L, (c) 3.0 mmol/L, (d) 4.5 mmol/L and (e) 6.0 mmol/L. All buffers contain 1.0 mmol/L HAc at pH 4.5. Injection 60 mbar, 0.1 min. Run voltage 28 kV. Concentrations of lanthanide ions are 0.05 mmol/L. Peaks 1: La, 2: Ce, 3: Pr, 4: Nd, 5: Sm, 6: Eu, 7: Gd, 8: Tb, 9: Dy, 10: Ho, 11: Er, 12: Tm, 13: Yb, 14: Lu, *system peak. Figure 1. Open in new tabDownload slide Separation of lanthanides with increasing HIBA concentrations: (a) 1.0 mmol/L, (b) 2.0 mmol/L, (c) 3.0 mmol/L, (d) 4.5 mmol/L and (e) 6.0 mmol/L. All buffers contain 1.0 mmol/L HAc at pH 4.5. Injection 60 mbar, 0.1 min. Run voltage 28 kV. Concentrations of lanthanide ions are 0.05 mmol/L. Peaks 1: La, 2: Ce, 3: Pr, 4: Nd, 5: Sm, 6: Eu, 7: Gd, 8: Tb, 9: Dy, 10: Ho, 11: Er, 12: Tm, 13: Yb, 14: Lu, *system peak. Effect of acetic acid concentration Figure 2 shows the effect of HAc concentration as auxiliary ligand to the electrophoretic mobilities of lanthanide ions. HAc concentrations were studied in the range of 0–8 mmol/L in 4.5 mmol/L HIBA solutions. The pH of all solutions was adjusted to pH 4.5 with Tris. The optimum concentration was selected as 1 mmol/L HAc. Figure 2. Open in new tabDownload slide Plot of electrophoretic mobilities of lanthanides vs. acetic acid concentrations in a buffer of 4.5 mmol/L HIBA at pH 4.5. Sample concentrations are 0.05 mmol/L. Figure 2. Open in new tabDownload slide Plot of electrophoretic mobilities of lanthanides vs. acetic acid concentrations in a buffer of 4.5 mmol/L HIBA at pH 4.5. Sample concentrations are 0.05 mmol/L. Effect of pH of buffer The effect of pH on the separation and sensitivities was checked between pH 4 and 5. The optimal pH value was chosen as pH 4.5. Precisions and detection limits Figure 3 shows the complete separation of 14 lanthanides under the optimized conditions of the buffer system as 4.5 mmol/L HIBA and 1.0 mmol/L HAc at pH 4.5, but the pH of buffers was arranged with Tris (Figure 3a) or NaOH (Figure 3b). The results of repeatabilities of peak areas and the limits of detection (LODs) of lanthanide ions are as seen in Table I. Figure 3. Open in new tabDownload slide Electropherogram of 14 lanthanides. Buffer: 1.0 mmol/L HAc, 4.5 mmol/L HIBA at pH 4.5. Buffer pH was adjusted using (a) Tris and (b) NaOH. Concentrations of lanthanide ions are 0.05 mmol/L. Injection 60 mbar, 0.1 min. (c) Electropherogram of lanthanides with FASI method. Concentrations of lanthanide ions are 0.5 μmol/L containing 5% ACN aqueous solution. Injection 5 kV, 0.2 min. Other conditions are as in Figure 1. Figure 3. Open in new tabDownload slide Electropherogram of 14 lanthanides. Buffer: 1.0 mmol/L HAc, 4.5 mmol/L HIBA at pH 4.5. Buffer pH was adjusted using (a) Tris and (b) NaOH. Concentrations of lanthanide ions are 0.05 mmol/L. Injection 60 mbar, 0.1 min. (c) Electropherogram of lanthanides with FASI method. Concentrations of lanthanide ions are 0.5 μmol/L containing 5% ACN aqueous solution. Injection 5 kV, 0.2 min. Other conditions are as in Figure 1. Table I. The Results of Repeatabilities (RSD %) of Peak Areas (n = 5) and the LODs . RSDa % . LODa (µmol/L) . RSDb % . LODb (µmol/L) . La 3.22 0.315 4.41 0.655 Ce 3.77 0.428 2.92 0.570 Pr 3.51 0.623 4.48 0.544 Nd 4.43 1.234 3.06 0.665 Sm 4.45 1.870 1.13 0.413 Eu 3.29 0.921 1.71 0.374 Gd 6.98 0.809 4.32 0.428 Tb 4.73 0.460 3.47 0.363 Dy 2.08 0.374 4.49 0.299 Ho 3.19 0.352 4.48 0.291 Er 3.5 0.299 2.03 0.285 Tm 3.21 0.272 2.12 0.272 Yb 2.07 0.239 1.98 0.249 Lu 3.07 0.187 3.67 0.210 . RSDa % . LODa (µmol/L) . RSDb % . LODb (µmol/L) . La 3.22 0.315 4.41 0.655 Ce 3.77 0.428 2.92 0.570 Pr 3.51 0.623 4.48 0.544 Nd 4.43 1.234 3.06 0.665 Sm 4.45 1.870 1.13 0.413 Eu 3.29 0.921 1.71 0.374 Gd 6.98 0.809 4.32 0.428 Tb 4.73 0.460 3.47 0.363 Dy 2.08 0.374 4.49 0.299 Ho 3.19 0.352 4.48 0.291 Er 3.5 0.299 2.03 0.285 Tm 3.21 0.272 2.12 0.272 Yb 2.07 0.239 1.98 0.249 Lu 3.07 0.187 3.67 0.210 aBuffer pH was adjusted with Tris. bBuffer was adjusted with NaOH. Table I. The Results of Repeatabilities (RSD %) of Peak Areas (n = 5) and the LODs . RSDa % . LODa (µmol/L) . RSDb % . LODb (µmol/L) . La 3.22 0.315 4.41 0.655 Ce 3.77 0.428 2.92 0.570 Pr 3.51 0.623 4.48 0.544 Nd 4.43 1.234 3.06 0.665 Sm 4.45 1.870 1.13 0.413 Eu 3.29 0.921 1.71 0.374 Gd 6.98 0.809 4.32 0.428 Tb 4.73 0.460 3.47 0.363 Dy 2.08 0.374 4.49 0.299 Ho 3.19 0.352 4.48 0.291 Er 3.5 0.299 2.03 0.285 Tm 3.21 0.272 2.12 0.272 Yb 2.07 0.239 1.98 0.249 Lu 3.07 0.187 3.67 0.210 . RSDa % . LODa (µmol/L) . RSDb % . LODb (µmol/L) . La 3.22 0.315 4.41 0.655 Ce 3.77 0.428 2.92 0.570 Pr 3.51 0.623 4.48 0.544 Nd 4.43 1.234 3.06 0.665 Sm 4.45 1.870 1.13 0.413 Eu 3.29 0.921 1.71 0.374 Gd 6.98 0.809 4.32 0.428 Tb 4.73 0.460 3.47 0.363 Dy 2.08 0.374 4.49 0.299 Ho 3.19 0.352 4.48 0.291 Er 3.5 0.299 2.03 0.285 Tm 3.21 0.272 2.12 0.272 Yb 2.07 0.239 1.98 0.249 Lu 3.07 0.187 3.67 0.210 aBuffer pH was adjusted with Tris. bBuffer was adjusted with NaOH. Enhancement of detection sensitivity In order to improve the detection sensitivity, the field-amplified sample injection (FASI) technique was used. Figure 3c shows the electropherogram of lanthanide ions with FASI application. The result of FASI application is reaching nmol/L detection limits. The results of precision and LODs in FASI technique are shown in Table II. The obtained detection limits were enhanced between 71 and 91 times compared with LODs obtained without FASI application. The repeatability of the peak areas was obtained with lanthanides mixture in 0.5 µmol/L concentrations of each and for five successive injections. Table II. The Results of RSD% and LODs in FASI Application . RSD % (peak area) . LOD (nmol/L) . LOQ (nmol/L) . La 6.22 8.26 27.5 Ce 3.25 7.34 24.4 Pr 2.86 6.01 20.0 Nd 4.94 8.17 27.2 Eu 3.94 5.62 18.7 Eu 4.71 5.07 16.9 Gd 4.85 6.03 20.1 Tb 4.44 4.33 14.4 Dy 5.74 4.12 13.7 Ho 4.34 3.95 13.2 Er 3.35 3.68 12.3 Tm 3.78 3.51 11.7 Yb 4.38 2.96 9.85 Lu 3.20 2.77 9.29 . RSD % (peak area) . LOD (nmol/L) . LOQ (nmol/L) . La 6.22 8.26 27.5 Ce 3.25 7.34 24.4 Pr 2.86 6.01 20.0 Nd 4.94 8.17 27.2 Eu 3.94 5.62 18.7 Eu 4.71 5.07 16.9 Gd 4.85 6.03 20.1 Tb 4.44 4.33 14.4 Dy 5.74 4.12 13.7 Ho 4.34 3.95 13.2 Er 3.35 3.68 12.3 Tm 3.78 3.51 11.7 Yb 4.38 2.96 9.85 Lu 3.20 2.77 9.29 Table II. The Results of RSD% and LODs in FASI Application . RSD % (peak area) . LOD (nmol/L) . LOQ (nmol/L) . La 6.22 8.26 27.5 Ce 3.25 7.34 24.4 Pr 2.86 6.01 20.0 Nd 4.94 8.17 27.2 Eu 3.94 5.62 18.7 Eu 4.71 5.07 16.9 Gd 4.85 6.03 20.1 Tb 4.44 4.33 14.4 Dy 5.74 4.12 13.7 Ho 4.34 3.95 13.2 Er 3.35 3.68 12.3 Tm 3.78 3.51 11.7 Yb 4.38 2.96 9.85 Lu 3.20 2.77 9.29 . RSD % (peak area) . LOD (nmol/L) . LOQ (nmol/L) . La 6.22 8.26 27.5 Ce 3.25 7.34 24.4 Pr 2.86 6.01 20.0 Nd 4.94 8.17 27.2 Eu 3.94 5.62 18.7 Eu 4.71 5.07 16.9 Gd 4.85 6.03 20.1 Tb 4.44 4.33 14.4 Dy 5.74 4.12 13.7 Ho 4.34 3.95 13.2 Er 3.35 3.68 12.3 Tm 3.78 3.51 11.7 Yb 4.38 2.96 9.85 Lu 3.20 2.77 9.29 The calibration curves of 14 lanthanides gave linear calibration range between 0.05 and 1.5 µmol/L and correlation coefficients between 0.993 and 0.998. Discussion Buffer selection For C4D detection, the composition of the background electrolyte should be optimized to create as great as possible difference between the effective mobility of the analyte and effective mobility of the co-ion of the separation electrolyte, in order to obtain the highest signal-to-noise ratio. If the conductivity of the separation electrolyte is smaller than the conductivity of the analyte zone, positive peaks appear in electropherogram. If the conductivity of the separation electrolyte is greater than the conductivity of the analyte, zone negative peaks appear. Similar sensitivities can be obtained for positive and negative peaks. However, separation electrolytes having high conductivity cause joule heating in the capillary and C4D detection is more sensitive to excessive joule heat compared with UV detection. Joule heating causes an unstable baseline. Therefore, for C4D detections, narrow inner diameter capillary columns are used, which partly decrease the joule heat. Because of the same reason, low conductivity running buffers such as 2-(N-morpholino)ethanesulfonic acid (MES) and histidine are used frequently to minimize joule heating. The suitable pH for these buffers is equal to or higher than 6. However, the lanthanide ions hydrolyze readily in solutions above pH 6, so that MES is not a convenient separation electrolyte for lanthanide ions. On the other hand, lanthanides form weak complexes with organic acids like citric, lactic, acetic and HIBA. Due to very similar properties of the group elements, stability constants of formed complexes are very near each other. In order to create differences between electrophoretic mobilities, it is preferred to use more than one complexing agent competing with each other over lanthanide ions. Thus, in the present study, HIBA was selected as the complexing agent and acetic acid as the auxiliary ligand. In order to have a low conductivity buffer, pH adjustments were done with Tris instead of NaOH. Effect of HIBA concentration As seen in Figure 1, with the increase of HIBA concentration, a decrease is observed in the electrophoretic mobilities of lanthanides. This effect is seen more in the last (heavier) lanthanides. Accordingly, the resolution of peaks increases with increasing HIBA concentration. However, since the HIBA concentration affects the conductivities of lanthanide ions, the direction of the peaks changes with HIBA concentration. The increase of the HIBA concentration causes a decrease of the positive charges of ions due to increasing complexation degrees. Therefore, the conductivity of lanthanide ions becomes close to the conductivity of the buffer, so that the peak heights decrease. When the conductivities of the ions are equal to the conductivity of the buffer, these peaks become invisible in electropherogram. Obviously, the charge of lanthanide ions, i.e., the conductivities of the ions, decreases from La to Lu with the increase of HIBA concentration. When the conductivities of the lanthanide ions are lower than the conductivity of the buffer, ions appear at the electropherogram as negative peaks. The resolution of lanthanides was found compatible with the sensitivities of peaks at 4.5 mmol/L HIBA concentration. With further increase in HIBA concentration, the last peak (Lu) combines to the system peak marked with asterisk (*). Hence, the optimum HIBA concentration was chosen as 4.5 mmol/L. Effect of acetic acid concentration As seen from Figure 2, without HAc the resolution of heavier lanthanide ions did not happen. With the addition of 1 mmol/L HAc, the resolution of all lanthanides was observed. Further increase in HAc concentration does not cause a noticeable increase in resolutions. Thus, 1 mmol/L was selected as the optimal auxiliary ligand concentration. Effect of pH of buffer The best resolution, sensitivity and smooth baseline were obtained at pH 4.5. As mentioned before, this pH value is compatible with the pKa value of the complexing agent HIBA. In this way, HIBA provides a buffer effect at the same time. Moreover, the lanthanide ions do not hydrolyze at this pH. In basic medium, possible hydroxyl precipitation of lanthanides is expected. Precisions and detection limits In comparing the LOD values of ions, the LOD values of heavier lanthanide ions are lower compared with the LODs of medium lanthanides ions (Table I). Especially, the LOD values of Nd and Sm are comparatively higher from the LODs of the other ions. These two ions are located on the two sides of the border of changing peak locations from positive to negative. In order to improve the sensitivities of these two ions, the pH of buffer system was arranged with NaOH instead of Tris. As expected, the conductivities of the buffer ions increased and all of the lanthanide ions had less conductivity than the buffer. Therefore, all of the lanthanide ions gave negative peaks (Figure 3a and b). Analysis of metal ions via positive or negative peaks with similar high sensitivity for all species in CE-C4D systems was reported previously in the literature (35, 36). The increase in the conductivity of the buffer did not cause any baseline distortion and again repeatable peak areas were obtained as seen in the third column of Table I. Not only sensitivities of Nd and Sm, but the sensitivities of most of the lanthanides improved as given in the last column of Table I and Figure 4. The LOD values obtained are compatible or better than the reported values of CE-UV studies obtained with complexation of lanthanides with UV active ligands. With aromatic polyaminocarboxylate and nitrilotriacetic acid ligand, the LOD value of only Lu ion was reported as 0.42 µmol/L (15). With arsenazo III, LOD of La was given as 0.35 µmol/L (12). For cupferron, LODs of 14 analytes were reported as between 1.73 and 2.69 µmol/L (13), and for pyridine carboxylic acid were between 3.81 and 5.49 µmol/L (14). With CE-C4D, LODs of 14 lanthanides ions were reported as between 1.38 and 29.9 µmol/L (34). Figure 4. Open in new tabDownload slide Comparison of the sensitivities of lanthanides with different pH adjustments of buffer solution. Sample concentrations are 0.05 mmol/L. Figure 4. Open in new tabDownload slide Comparison of the sensitivities of lanthanides with different pH adjustments of buffer solution. Sample concentrations are 0.05 mmol/L. Enhancement of detection sensitivity In order to improve the detection sensitivity, the FASI technique is generally used as online preconcentration method in CE (37–39). In FASI, samples that have low conductivity are injected electrokinetically into a capillary containing a high conductivity running buffer. In order to increase detection sensitivities of separated lanthanides ions, FASI was applied. However, since it is not suitable to further increase the conductivity of the separation buffer due to the requirement of C4D detector, the conductivity of the sample zone was decreased with the addition of organic solvent to the sample zone. The same buffer composition as in the previous experiment was used. The buffer pH was arranged with NaOH. By adding 5% acetonitrile (ACN) to the sample, the conductivity and viscosity of the sample zone were decreased. Sample injection was made electrokinetically, applying 5 kV for 0.2 min. With the increase in the electrical field of the sample zone, the electrophoretic mobilities of the cations increase. When they reach the low electrical field in the buffer, they slow down and stack at the boundary of sample and buffer zone. The stacked ions migrate to the detector as a zone that is narrower than the injected sample plug. The electropherogram of lanthanide ions with FASI application is shown in Figure 3. By the application of stacking procedure, LOD values of lanthanides were decreased to between 2.77 and 8.26 nmol/L. As far as we know, these LOD values are the lowest of the reported values by conventional capillary electrophoretic systems. The only exception was reported by Xu et al. (40) in pmol/L levels for six lanthanide ions by the modification of CE systems, increasing sample vial volume to 17 mL, stirring sample solution and replacing the common wire electrode by a ring electrode. Conclusions In this study, all of the 14 lanthanide ions were simultaneously separated in ~6 min by using an on-capillary complexation method with HIBA/HAc ligands. Lanthanide ions were detected with C4D detector without the need of including a chromophore to the separation buffer. Detection sensitivities of lanthanides were further enhanced between 71- and 91-fold, with the application of a stacking procedure. The C4D detector offers an economic, rapid and sensitive alternative to the previously reported detection methods in CE for the determination of lanthanides. Acknowledgements We thank the Research Foundation of Istanbul Technical University for financial support. References 1 Rare Earth Elements: A Review of Production, Processing, Recycling, and Associated Environmental Issues, EPA 600/R-12/572. https://cluin.org/download/issues/mining/Hard_Rock/Wednesday_April_4/05_Rare_Earth_Elements/01_Weber_R.pdf or http://nepis.epa.gov (accessed May 25, 2016). 2 Kumar , P. , Jaison , P.G., Rao , D.R.M., Telmore , V.M., Sarkar , A., Aggarwal , S.K.; Determination of lanthanides and yttrium in high purity dysprosium by RP-HPLC using alpha-hydroxyisobutyric acid as an eluent ; Journal of Liquid Chromatography and Related Technologies , ( 2013 ); 36 : 1513 – 1527 . OpenURL Placeholder Text WorldCat 3 Dybczynski , R.S. , Kulisa , K., Pyszynska , M., Bojanowska-Czajka , A.; New reversed phase-high performance liquid chromatographic method for selective separation of yttrium from all rare earth elements employing nitrilotriacetate complexes in anion exchange mode ; Journal of Chromatography A , ( 2015 ); 1386 : 74 – 80 . Google Scholar Crossref Search ADS PubMed WorldCat 4 Jaison , P.G. , Kumar , P., Telmore , V.M., Aggarwal , S.K.; Comparative study of ion interaction reagents for the separation of lanthanides by reversed-phase high performance liquid chromatography (RP-HPLC) ; Journal of Liquid Chromatography and Related Technologies , ( 2009 ); 32 : 2146 – 2163 . Google Scholar Crossref Search ADS WorldCat 5 Kutun , S. , Akseli , A.; New elution agent, sodium trimetaphosphate, for the separation and determination of rare earths by anion-exchange chromatography ; Journal of Chromatography A , ( 1999 ); 847 : 261 – 269 . Google Scholar Crossref Search ADS WorldCat 6 Akseli , A. , Kutun , S.; Distribution coefficients and cation-exchange separation of rare earths in sodium trimetaphosphate media and application to monazite ; Separation Science Technology , ( 2000 ); 35 : 561 – 571 . Google Scholar Crossref Search ADS WorldCat 7 Borai , E. , Ekhlom , P., Harjula , R.; Group separation of heavy metals followed by subsequent and individual separation of lanthanides by chelation chromatography ; Journal of Liquid Chromatography and Related Technologies , ( 2014 ); 37 : 1614 – 1631 . Google Scholar Crossref Search ADS WorldCat 8 Pourjavid , M.R. , Norouzi , P., Rashedi , H., Ganjali , M.R.; Separation and direct detection of heavy lanthanides using new ion-exchange chromatography: fast fourier transform continuous cyclic voltammetry system ; Journal of Applied Electrochemistry , ( 2010 ); 40 : 1593 – 1603 . Google Scholar Crossref Search ADS WorldCat 9 Pourjavid , M.R. , Norouzi , P., Rashedi , H., Ganjali , M.R.; Metal-ion capillary electrophoresis with direct uv detection effect of a charged surfactant on the migration behavior of metal-chelates ; Journal of Chromatography A , ( 1994 ); 671 : 419 – 427 . Google Scholar Crossref Search ADS WorldCat 10 Timerbaev , A.R. , Semenova , O.P., Bonn , G.K., Fritz , J.S.; Determination of metal-ions complexed with 2,6-diacetylpyridine bis(n-methylenepyridiniohydrazone) by capillary electrophoresis ; Analytica Chimica Acta , ( 1994 ); 296 : 119 – 128 . Google Scholar Crossref Search ADS WorldCat 11 Timerbaev , A.R. , Semenova , O.P., Bonn , G.K.; Capillary zone electrophoresis of lanthanoid elements after complexation with aminopolycarboxylic acids ; Analyst , ( 1994 ); 119 : 2795 – 2799 . Google Scholar Crossref Search ADS WorldCat 12 Macka , M. , Nesterenko , P., Andersson , P., Haddad , P.R.; Separation of uranium(VI) and lanthanides by capillary electrophoresis using on-capillary complexation with arsenate III ; Journal of Chromatography A , ( 1998 ); 803 : 279 – 290 . Google Scholar Crossref Search ADS WorldCat 13 Öztekin , N. , Erim , F.B.; Separation and direct UV detection of lanthanides complexed with cupferron by capillary electrophoresis ; Journal of Chromatography A , ( 2000 ); 895 : 263 – 268 . Google Scholar Crossref Search ADS PubMed WorldCat 14 Öztekin , N. , Erim , F.B.; Separation and direct UV detection of lanthanides complexed with pyridine-2-carboxylic acid by capillary electrophoresis ; Journal of Chromatography A , ( 2001 ); 924 : 541 – 546 . Google Scholar Crossref Search ADS PubMed WorldCat 15 Saito , S. , Danzaka , N., Hoshi , S.; Highly sensitive determination of lanthanides by capillary electrophoresis with direct visible detection after precapillary complexation with aromatic polyaminocarboxylate and additionally applying dynamic ternary complexation with nitrilotriacetic acid ; Electrophoresis , ( 2006 ); 27 : 3093 – 3100 . Google Scholar Crossref Search ADS PubMed WorldCat 16 Foret , F. , Fanali , S., Nardi , A., Bocek , P.; Capillary zone electrophoresis of rare earth metals indirect UV absorbance detection ; Electrophoresis , ( 1990 ); 11 : 780 – 783 . Google Scholar Crossref Search ADS PubMed WorldCat 17 Weston , A. , Brown , P.R., Jandik , P., Jones , W.R., Heckenberg , A.L.; Factors affecting the separation of inorganic metal-cations by capillary electrophoresis ; Journal of Chromatography , ( 1992 ); 593 : 289 – 295 . Google Scholar Crossref Search ADS WorldCat 18 Shi , Y. , Fritz , J.S.; Separation of metal-ions by capillary electrophoresis with a complexing electrolyte ; Journal of Chromatography , ( 1993 ); 640 : 473 – 479 . Google Scholar Crossref Search ADS WorldCat 19 Chen , M. , Cassidy , R.M.; Separation of metal ions by capillary electrophoresis ; Journal of Chromatography , ( 1993 ); 640 : 425 – 431 . Google Scholar Crossref Search ADS WorldCat 20 Vogt , C. , Conradi , S.; Complex equilibria in capillary zone electrophoresis and their use for the separation of rare earth metal ions ; Analytica Chimica Acta , ( 1994 ); 294 : 145 – 153 . Google Scholar Crossref Search ADS WorldCat 21 Verma , S.P. , Garcia , R., Santoyo , E., Aparicio , A.; Improved capillary electrophoresis method for measuring rare-earth elements in synthetic geochemical standards ; Journal of Chromatography A , ( 2000 ); 884 : 317 – 328 . Google Scholar Crossref Search ADS PubMed WorldCat 22 Santoyo , E. , Garcia , R., Galicia-Alanis , K.A., Verma , S.P., Aparicio , A., Santoyo-Castelazo , A.; Separation and quantification of lanthanides in synthetic standards by capillary electrophoresis: a new experimental evidence of the systematic "odd-even" pattern observed in sensitivities and detection limits ; Journal of Chromatography A , ( 2007 ); 1149 : 12 – 19 . Google Scholar Crossref Search ADS PubMed WorldCat 23 Sun , Y. ; Detailed study on simultaneous separation of rare earth elements by capillary electrophoresis ; Journal of Chromatography A , ( 2004 ); 1048 : 245 – 251 . Google Scholar Crossref Search ADS PubMed WorldCat 24 Mao , Q. , Hashimoto , Y., Manabe , Y., Ikuta , N., Nishiyama , F., Hirokawa , T.; Separation of rare-earth ions by isotachophoresis and capillary zone electrophoresis ; Journal of Chromatography A , ( 1998 ); 802 : 203 – 210 . Google Scholar Crossref Search ADS WorldCat 25 Kautenburger , R. , Beck , H.P.; Complexation studies with lanthanides and humic acid analyzed by ultrafiltration and capillary electrophoresis-inductively coupled plasma mass spectrometry ; Journal of Chromatography A , ( 2007 ); 1159 : 75 – 80 . Google Scholar Crossref Search ADS PubMed WorldCat 26 Pitois , A. , de Las Heras , L.A., Betti , M.; Multi-component elemental and molecular analysis of lanthanides by capillary electrophoresis-electrospray mass spectrometry (CE-ESI-MS) ; International Journal of Mass Spectrometry , ( 2008 ); 273 : 95 – 104 . Google Scholar Crossref Search ADS WorldCat 27 Hirokawa , T. , Hashimoto , Y.; Simultaneous separation of yttrium and lanthanide ions by isotachophoresis ; Journal of Chromatography A , ( 1997 ); 772 : 357 – 367 . Google Scholar Crossref Search ADS WorldCat 28 Nukatsuka , I. , Taga , M., Yoshida , H.; Separation of lanthanides by capillary tube isotachophoresis using complex-forming equilibria ; Journal of Chromatography , ( 1981 ); 205 : 95 – 102 . Google Scholar Crossref Search ADS WorldCat 29 Hirokawa , T. , Xia , W., Kiso , Y.; Isotachophoretic separation of rare earth ions. I. Separation behaviour of yttrium and fourteen lanthanide ions forming complexes with tartaric acid and α-hydroxyisobutyric acid ; Journal of Chromatography A , ( 1995 ); 689 : 149 – 156 . Google Scholar Crossref Search ADS WorldCat 30 Hirokawa , T. , Aoki , N., Kiso , Y.; Complex-forming equilibria in isotachophoresis : VI. simulation of isotachophoretic equilibria of lathanoids and determination mobilities and stability constants of acetate and β-hydroxyisobutyrate complexes ; Journal of Chromatography , ( 1984 ); 312 : 11 – 29 . Google Scholar Crossref Search ADS WorldCat 31 Kuban , P. , Hauser , P.C.; Contactless conductivity detection for analytical techniques: developments from 2010 to 2012 ; Electrophoresis , ( 2013 ); 34 : 55 – 69 . Google Scholar Crossref Search ADS PubMed WorldCat 32 Zemann , A.J. , Schnell , E., Volgger , D., Bonn , G.K.; Contactless conductivity detection for capillary electrophoresis ; Analytical Chemistry , ( 1998 ); 70 : 563 – 567 . Google Scholar Crossref Search ADS PubMed WorldCat 33 da Silva , J.A.F. , do Lago , C.L.; An oscillometric detector for capillary electrophoresis ; Analytical Chemistry , ( 1998 ); 70 : 4339 – 4343 . Google Scholar Crossref Search ADS WorldCat 34 Nguyen , T.A.H. , Nguyen , V.R., Le , D.D., Nguyen , T.T.B., Van Hoang Cao , V.H., Nguyen , T.K.D., et al. .; Simultaneous determination of rare earth elements in ore and anti-corrosion coating samples using a portable capillary electrophoresis instrument with contactless conductivity detection ; Journal of Chromatography A , ( 2016 ); 1457 : 151 – 158 . Google Scholar Crossref Search ADS PubMed WorldCat 35 Haber , C. , Jones , W.R., Soglia , J., Surve , M.A., McGlynn , M., Caplan , A., et al. .; Conductivity detection in capillary electrophoresis - a powerful tool in ion analysis ; Journal of Capillary Electrophoresis , ( 1996 ); 3 : 1 – 11 . Google Scholar PubMed OpenURL Placeholder Text WorldCat 36 Tanyanyiwa , J. , Hauser , P.C.; High-voltage contactless conductivity detection of metal ions in capillary electrophoresis ; Electrophoresis , ( 2002 ); 23 : 3781 – 3786 . Google Scholar Crossref Search ADS PubMed WorldCat 37 Öztekin , N. , Aydın , H.M., Erim , F.B.; Stacking in CE for analysis of bromate flour additive ; Chromatographia , ( 2009 ); 70 : 987 – 990 . Google Scholar Crossref Search ADS WorldCat 38 Tan , F. , Yang , B., Guan , Y.; Determination of heavy metal ions by capillary electrophoresis with contactless conductivity detection after field-amplified sample injection ; Analytical Sciences , ( 2005 ); 21 : 955 – 958 . Google Scholar Crossref Search ADS PubMed WorldCat 39 Lau , H.F. , Quek , N.M., Law , W.S., Zhao , J.H., Hauser , P.C., Li , S.F.Y.; Optimization of separation of heavy metals by capillary electrophoresis with contactless conductivity detection ; Electrophoresis , ( 2011 ); 32 : 1190 – 1194 . Google Scholar Crossref Search ADS PubMed WorldCat 40 Xu , Z. , Nakamura , K., Timerbaev , A.R., Hirokawa , T.; Another approach toward over 100000-fold sensitivity increase in capillary electrophoresis: electrokinetic supercharging with optimized sample injection ; Analytical Chemistry , ( 2011 ); 83 : 398 – 401 . Google Scholar Crossref Search ADS PubMed WorldCat © The Author 2016. Published by Oxford University Press. All rights reserved. For Permissions, please email: journals.permissions@oup.com
TI - Separation and Sensitive Detection of Lanthanides by Capillary Electrophoresis and Contactless Conductivity Detection
JF - Journal of Chromatographic Science
DO - 10.1093/chromsci/bmw200
DA - 2017-04-01
UR - https://www.deepdyve.com/lp/oxford-university-press/separation-and-sensitive-detection-of-lanthanides-by-capillary-0Wbd0hsEEB
SP - 465
VL - 55
IS - 4
DP - DeepDyve
ER -