3014 Mikhail Kamencev1 Nina Yakimova1 Leonid Moskvin1 Irina Kuchumova1 Kirill Tkach1 Yulia Malinina1 Oleg Tungusov2 1 Institute
of Chemistry, St. Petersburg State University, Saint-Petersburg, Russian Federation 2 The Mayak Production Association, Ozersk, Russian Federation
Received August 26, 2015 Revised September 14, 2015 Accepted September 14, 2015
Electrophoresis 2015, 36, 3014–3017
Research Article
Isotopic separation of lithium ions by capillary zone electrophoresis Separation of 6 Li and 7 Li isotopes by CZE was demonstrated. The BGE contained 5 mM 4-aminopyridine, 0.9 mM oxalic acid, 0.25 mM CTAB, and 0.25% w/v Tween 20 (рМ = 9.2). The running conditions were +25 kV at 30°C with indirect photometric detection at 261 nm. Under optimal experimental conditions, the analysis time was less than 21 min. Separation of Li preparations with mole fraction of 6 Li ranging from 3.44 up to 90.38% was demonstrated. Keywords: Capillary electrophoresis / Isotope / Isotopic separation / Lithium DOI 10.1002/elps.201500399
1 Introduction Both 6 Li and 7 Li isotopes are widely used in industry. 7 LiOH as an additive to the steam water in nuclear power plants [1], 6 Li in weaponry [2]. Extensive 6 Li usage for military purposes caused depletion in 6 Li of nearly all commercially available preparations of lithium [2]. The most obvious choice for isotopic analysis is MS but in the case of lithium simple and affordable alternative is still of interest due to such extremely wide area of application. Preparative separation of lithium ions based on isotopic effect on electromigration was studied in early 1960s [1,3]. CE isotopic separations of the analytes that possess an ionizable group were first shown in late 1980s [4, 5]. The separation was based on isotopic effect on ionization constants of carboxyl acids and amines that was firstly demonstrated using HPLC [6, 7]. Isotopic separation of the analytes without ionizable groups was more challenging analytical task. In 1990s, CE was shown to be able to perform isotopic separation of different anionic species (namely chloride and bromide ions) using so-called mobility counterbalance mode [8–10]. No CE separation of cationic species like alkaline metal cations was demonstrated yet. The above mentioned approach is based on compensating the migration speed of the analytes by oppositely directed EOF, thus, decreasing apparent mobility of the analyte and increasing the selectivity. Nearly unlimited increase in separation time and selectivity is possible without the use of long capillaries (and consequent inevitable decrease in field strength). Apparent mobility difference of several percents was achieved compared to actual difference in intrinsic mobility of the isotopes (0.29% for 35 Cl and 37 Cl and 0.12%
for 79 Br and 81 Br) [10]. The main drawback of this approach is that small changes in EOF speed greatly affect the apparent mobility of the analytes so very stable EOF must be maintained. Adjustable and reproducible cathodic electroosmotic mobilities can be obtained through the adjustment of pH and ionic strength of BGE, but cathodic EOF is only suitable for mobility counterbalancing of anionic species [9, 10]. For the separation of cationic species using this technique reversed (anodic) EOF must be used, which is less dependent on the BGE pH and in many cases less reproducible. One of the approaches to adjust anodic EOF is to use the combination of cationic surfactant and zwitter-ionic one that can form less charged mixed micelles thus providing slower EOF. Yeung and Lucy [11–13] used the combination of CTAB and coco amidopropylhydroxyldimethylsulfobetaine (Rewoteric AM CAS U) in order to adjust EOF speed for isotopic separations of ammonia and aniline. This approach was successfully used in combination with isotopic effect on pKa for the separation of above mentioned ionizable species but never any of the metal ions. The aim of our work was to perform the separation of lithium isotopes using CE. Despite the high mass difference between the isotopes (about 15%) mobility difference between 6 Li and 7 Li is only 0.36% [1] that is close to other alkaline metals like39 K and 41 K (0.385%) [14] or other light isotopes. At the same time, the apparent mobility difference must be brought to whole percents using mobility counterbalance in order to achieve CE separation.
2 Materials and methods Correspondence: Mikhail Kamencev, Institute of Chemistry, St. Petersburg State University, Universitetskii pr., 26, St. Petersburg, 198504, Russia E-mail: [email protected] Fax: +7-812-428-69-39
C 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
2.1 Chemicals 4-Aminopyridine, oxalic acid dihydrate, Tween 20, and CTAB were purchased from Sigma (St. Luouis, MO, USA). All
www.electrophoresis-journal.com
CE and CEC
Electrophoresis 2015, 36, 3014–3017
3015
chemicals used were of analytical grade. Bidistilled water was used for all experiments. Lithium carbonate depleted and enriched in 6 Li was a gift from the manufacturer (The Mayak Production Association, Ozersk, Russian Federation).
2.2 Apparatus Electrophoretic experiments were performed using a Lumex (St. Petersburg, Russia) CE system (model Capel 105M). Separation was performed using uncoated fused silica capillary (Polymicro Technologies, Phoenix, USA) of 37 cm total length (27 cm effective length) and 75 m internal diameter. At the beginning of each working day capillary was flushed with water for 5 min, 1 M HCl for 5 min, water for 5 min, 0.5 M NaOH for 5 min, water for 5 min, and the BGE for 10 min. In between the runs, the capillary was rinsed with BGE for 5 min. The temperature was set at 30°C. The detection wavelength was set at 261 nm. Separation was performed at +25 kV unless otherwise stated. The resulting current was at about 10 A. Injection was performed under pressure (30 mbar for 5 s). At the end of the day the capillary was rinsed with water for 5 min, 1 M HCl for 5 min, water for 5 min, 0.5 M NaOH for 5 min, water for 5 min and left in water.
2.3 Buffer and sample preparation The BGE was prepared containing 5 mM 4-aminopyridine, 0.9 mM oxalic acid, 0.5 mM CTAB, and different concentrations of Tween 20 in the range of 0.05–0.7% w/v if not otherwise stated (pH = 9.2). Stock solutions of lithium was prepared using lithium carbonate depleted (3.44% mol of 6 Li) and enriched (90.38% mol of 6 Li) in 6 Li, that was supplied by the manufacturer. Sample solutions with total concentration of 0.1 mM of lithium ions (sum of isotopes) were prepared by dilution with water.
3 Results and discussion 3.1 Buffer optimization As very close effective migration velocities of the probe ion and lithium ions are needed in order to decrease electrodispersion a probe ions with slightly higher mobility compared to lithium was considered. Effective mobility of the probe ion might than be controllably decreased by BGE pH adjustment to match the mobility of lithium ions of 4.1 × 10−4 cm2 V−1 s−1 [15]. We used 4-aminopyridine with ionic mobility of about 4.9 × 10−4 cm2 V−1 s−1 [16]. 4-Aminopyridine was chosen for our experiments as it usually gives some increase in sensitivity compared to other fast migrating probe ions, such as creatinine or imidazole, due to higher absorption in UV region (ε = 18 500 at 261 nm) [17–20]. C 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
Figure 1. Effect of the concentration of Tween 20 on EOF mobility. The BGE contained 5 mM 4-aminopyridine, 0.9 mM oxalic acid, 0.25 mM CTAB, and various concentrations of Tween 20. The other conditions as in Section 2.2.
BGE pH was adjusted with oxalic acid to match the effective mobilities of the probe ion and lithium ions. Optimal lithium peak symmetry was obtained at the concentration of oxalic acid of 0.9 mM (pH = 9.2). We used oxalate as a counter-ion in order to prevent any of transition or alkalineearth metal ions interference. Strong complexation of oxalate prevents any peaks of transition or alkaline-earth metal ions from appearing in the electropherogram. As CTAB alone provided too fast EOF we examined the system containing CTAB in combination with nonionic surfactant to obtain lower and adjustable EOF speed. Tween 20 was considered as it is one of the most frequently used nonionic surfactants in CZE and is known to form mixed micelles with CTAB [21] and to decrease EOF speed [22]. Lithium ions mobility is about 4.1 × 10−4 cm2 V−1 s−1 [15] so EOF mobility should be adjusted to slightly lower value down from 5.7 × 10−4 cm2 V−1 s−1 when BGE without Tween 20 was used. Figure 1 shows the effect of the concentration of Tween 20 on EOF mobility. EOF velocity was measured using negative polarity (–25 kV) and 0.1% DMSO as EOF marker. As can be seen from Fig. 1, the EOF mobility could be adjusted in the range from 3.3 to 5.7 × 10−4 cm2 V−1 s−1 using this technique. Stable EOF velocities (RSD less than 1%) were obtained at any examined concentration of Tween 20.
3.2 Isotopic separation The separation of lithium isotopes was examined at different concentrations of Tween 20 and thus total analysis time. Partial separation of the isotopes was observed starting from 7 min with 6 Li being the faster one. Resolution of 1.0 was achieved within 17 min for isotopic ratio of 1:1 (Fig. 2). For baseline separation of the samples with high isotopic ratio about 20 min long analysis was needed that corresponds to www.electrophoresis-journal.com
3016
M. Kamencev et al.
Electrophoresis 2015, 36, 3014–3017
Figure 2. Effect of the concentration of Tween 20 on the separation of 6 Li and 7 Li. Tween 20 concentration (A) 0.5% w/v; (B) 0.4% w/v; (C) 0.3% w/v. The other conditions as in Fig. 1.
Figure 3. Electropherograms of lithium samples with mole fraction of 6 Li of (A) 3.44%; (B) 90.38%. Tween 20 concentration is 0.25% w/v. The other conditions as in Fig. 1.
Tween 20 concentration of 0.25% (Fig. 3) so these conditions was chosen for further experiments. Isotopic ratio was calculated directly from electropherograms using corrected peak areas as described previously [9]. Corrected peak area of the analyte was calculated as its peak area divided by its migration time. Mole fractions of the isotopes were then calculated using normalization procedure. Good linearity between corrected peak areas and mole fraction of lithium isotopes was observed (r = 0.9998) in the range from 3.44 to 90.38% (mol) of 6 Li. The results are given in Table 1. The LODs, defined as a S/N of 3, was 1 M for both 6 Li and 7 Li. The LOQs, defined as a S/N of 10, was 3 M for both isotopes that corresponds to mole fraction of 3% when overall lithium concentration in the sample is 0.1 mM. C 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
Table 1. Results of analysis of lithium preparations (n = 3)
Sample no.
6 Li, CE (% mol)
6 Li, MS (% mol) a)
1 2 3
3.5 50.2 90.5
3.44 50.04 90.38
a) The reference values were provided by the manufacturer (obtained by MS analysis).
4 Concluding remarks Relatively fast isotopic separation of lithium ions was demonstrated using CE in less than 21 min. Both depleted and enriched samples can be analyzed. The anodic EOF was www.electrophoresis-journal.com
Electrophoresis 2015, 36, 3014–3017
controlled by the combination of cationic surfactant (CTAB) and nonionic one (Tween 20).
CE and CEC
3017
[10] Aupiais, J., J. Solution Chem. 2011, 40, 1629–1644. [11] Yeung, K., Lucy, C., Anal. Chem. 1998, 70, 3286–3290. [12] Yeung, K., Lucy, C., Electrophoresis 1999, 20, 2554–2559.
The authors have declared no conflict of interest.
5 References [1] Arnikar, H., J. Inorg. Nucl. Chem. 1959, 11, 248–251. [2] Coplen, T., Pure Appl. Chem. 2002, 74, 1987–2017.
[13] Lucy, C., Yeung, K., Fu, C., Li, D., Hanselwood, T., Underhill, R., Can. J. Chem. 1999, 70, 281. [14] Brewer, A., Madorsky, S., Taylor, J., Dibeler, V., Bradt, P., Parham, O., Britten, R., Reid, J., J. Res. Natl. Bur. Stand. 1947, 38, 137–168. [15] Packakova, V., Coufal, P., Stulik, K., Gas, B., Electrophoresis 2003, 24, 1883–1891.
[3] Wagener, K., Thiemann, W., Z. Naturfosch. A 1963, 18, 228–235.
[16] Haddad, P., Foley, R., Anal. Chem. 1989, 61, 1435–1441.
[4] Terabe, S., Yashima, T., Anal. Chem. 1988, 60, 491–493.
[17] Wang, K., Wang, C., Electrophoresis 1997, 18, 241–246.
[5] Bushey, M., Jorgenson, J., Anal. Chem. 1989, 61, 491–493.
[18] Shakulashvili, N., Faller, T., Engelhardt, H., J. Chromatogr. A 2000, 895, 205–212.
[6] Tanaka, N., Araki, M., J. Am. Chem. Soc. 1985, 107, 7780–7781.
[19] Chiou, C., Shih, J., Analyst 1996, 121, 1107–1110.
[7] Tanaka, N., Yamaguchi, A., Araki, M., J. Am. Chem. Soc. 1985, 107, 7781–7783. [8] Avdalovic, N., Pohl, C., Rocklin, R., Stillian, J., Anal. Chem. 1993, 65, 1470–1475. [9] Lucy, C., McDonald, T., Anal. Chem. 1995, 67, 1074–1078.
C 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
[20] Johns, C., Macka, M., Haddad, P., Electrophoresis 2003, 24, 2150–2167. [21] Sidim, T., Akar, G., J. Surfactants Deterg. 2013, 16, 601–607. [22] Matchett, W., Brumley, W., J. Liq. Chromatogr. Relat. Technol. 1997, 20, 79–100.
www.electrophoresis-journal.com
of Chemistry, St. Petersburg State University, Saint-Petersburg, Russian Federation 2 The Mayak Production Association, Ozersk, Russian Federation
Received August 26, 2015 Revised September 14, 2015 Accepted September 14, 2015
Electrophoresis 2015, 36, 3014–3017
Research Article
Isotopic separation of lithium ions by capillary zone electrophoresis Separation of 6 Li and 7 Li isotopes by CZE was demonstrated. The BGE contained 5 mM 4-aminopyridine, 0.9 mM oxalic acid, 0.25 mM CTAB, and 0.25% w/v Tween 20 (рМ = 9.2). The running conditions were +25 kV at 30°C with indirect photometric detection at 261 nm. Under optimal experimental conditions, the analysis time was less than 21 min. Separation of Li preparations with mole fraction of 6 Li ranging from 3.44 up to 90.38% was demonstrated. Keywords: Capillary electrophoresis / Isotope / Isotopic separation / Lithium DOI 10.1002/elps.201500399
1 Introduction Both 6 Li and 7 Li isotopes are widely used in industry. 7 LiOH as an additive to the steam water in nuclear power plants [1], 6 Li in weaponry [2]. Extensive 6 Li usage for military purposes caused depletion in 6 Li of nearly all commercially available preparations of lithium [2]. The most obvious choice for isotopic analysis is MS but in the case of lithium simple and affordable alternative is still of interest due to such extremely wide area of application. Preparative separation of lithium ions based on isotopic effect on electromigration was studied in early 1960s [1,3]. CE isotopic separations of the analytes that possess an ionizable group were first shown in late 1980s [4, 5]. The separation was based on isotopic effect on ionization constants of carboxyl acids and amines that was firstly demonstrated using HPLC [6, 7]. Isotopic separation of the analytes without ionizable groups was more challenging analytical task. In 1990s, CE was shown to be able to perform isotopic separation of different anionic species (namely chloride and bromide ions) using so-called mobility counterbalance mode [8–10]. No CE separation of cationic species like alkaline metal cations was demonstrated yet. The above mentioned approach is based on compensating the migration speed of the analytes by oppositely directed EOF, thus, decreasing apparent mobility of the analyte and increasing the selectivity. Nearly unlimited increase in separation time and selectivity is possible without the use of long capillaries (and consequent inevitable decrease in field strength). Apparent mobility difference of several percents was achieved compared to actual difference in intrinsic mobility of the isotopes (0.29% for 35 Cl and 37 Cl and 0.12%
for 79 Br and 81 Br) [10]. The main drawback of this approach is that small changes in EOF speed greatly affect the apparent mobility of the analytes so very stable EOF must be maintained. Adjustable and reproducible cathodic electroosmotic mobilities can be obtained through the adjustment of pH and ionic strength of BGE, but cathodic EOF is only suitable for mobility counterbalancing of anionic species [9, 10]. For the separation of cationic species using this technique reversed (anodic) EOF must be used, which is less dependent on the BGE pH and in many cases less reproducible. One of the approaches to adjust anodic EOF is to use the combination of cationic surfactant and zwitter-ionic one that can form less charged mixed micelles thus providing slower EOF. Yeung and Lucy [11–13] used the combination of CTAB and coco amidopropylhydroxyldimethylsulfobetaine (Rewoteric AM CAS U) in order to adjust EOF speed for isotopic separations of ammonia and aniline. This approach was successfully used in combination with isotopic effect on pKa for the separation of above mentioned ionizable species but never any of the metal ions. The aim of our work was to perform the separation of lithium isotopes using CE. Despite the high mass difference between the isotopes (about 15%) mobility difference between 6 Li and 7 Li is only 0.36% [1] that is close to other alkaline metals like39 K and 41 K (0.385%) [14] or other light isotopes. At the same time, the apparent mobility difference must be brought to whole percents using mobility counterbalance in order to achieve CE separation.
2 Materials and methods Correspondence: Mikhail Kamencev, Institute of Chemistry, St. Petersburg State University, Universitetskii pr., 26, St. Petersburg, 198504, Russia E-mail: [email protected] Fax: +7-812-428-69-39
C 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
2.1 Chemicals 4-Aminopyridine, oxalic acid dihydrate, Tween 20, and CTAB were purchased from Sigma (St. Luouis, MO, USA). All
www.electrophoresis-journal.com
CE and CEC
Electrophoresis 2015, 36, 3014–3017
3015
chemicals used were of analytical grade. Bidistilled water was used for all experiments. Lithium carbonate depleted and enriched in 6 Li was a gift from the manufacturer (The Mayak Production Association, Ozersk, Russian Federation).
2.2 Apparatus Electrophoretic experiments were performed using a Lumex (St. Petersburg, Russia) CE system (model Capel 105M). Separation was performed using uncoated fused silica capillary (Polymicro Technologies, Phoenix, USA) of 37 cm total length (27 cm effective length) and 75 m internal diameter. At the beginning of each working day capillary was flushed with water for 5 min, 1 M HCl for 5 min, water for 5 min, 0.5 M NaOH for 5 min, water for 5 min, and the BGE for 10 min. In between the runs, the capillary was rinsed with BGE for 5 min. The temperature was set at 30°C. The detection wavelength was set at 261 nm. Separation was performed at +25 kV unless otherwise stated. The resulting current was at about 10 A. Injection was performed under pressure (30 mbar for 5 s). At the end of the day the capillary was rinsed with water for 5 min, 1 M HCl for 5 min, water for 5 min, 0.5 M NaOH for 5 min, water for 5 min and left in water.
2.3 Buffer and sample preparation The BGE was prepared containing 5 mM 4-aminopyridine, 0.9 mM oxalic acid, 0.5 mM CTAB, and different concentrations of Tween 20 in the range of 0.05–0.7% w/v if not otherwise stated (pH = 9.2). Stock solutions of lithium was prepared using lithium carbonate depleted (3.44% mol of 6 Li) and enriched (90.38% mol of 6 Li) in 6 Li, that was supplied by the manufacturer. Sample solutions with total concentration of 0.1 mM of lithium ions (sum of isotopes) were prepared by dilution with water.
3 Results and discussion 3.1 Buffer optimization As very close effective migration velocities of the probe ion and lithium ions are needed in order to decrease electrodispersion a probe ions with slightly higher mobility compared to lithium was considered. Effective mobility of the probe ion might than be controllably decreased by BGE pH adjustment to match the mobility of lithium ions of 4.1 × 10−4 cm2 V−1 s−1 [15]. We used 4-aminopyridine with ionic mobility of about 4.9 × 10−4 cm2 V−1 s−1 [16]. 4-Aminopyridine was chosen for our experiments as it usually gives some increase in sensitivity compared to other fast migrating probe ions, such as creatinine or imidazole, due to higher absorption in UV region (ε = 18 500 at 261 nm) [17–20]. C 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
Figure 1. Effect of the concentration of Tween 20 on EOF mobility. The BGE contained 5 mM 4-aminopyridine, 0.9 mM oxalic acid, 0.25 mM CTAB, and various concentrations of Tween 20. The other conditions as in Section 2.2.
BGE pH was adjusted with oxalic acid to match the effective mobilities of the probe ion and lithium ions. Optimal lithium peak symmetry was obtained at the concentration of oxalic acid of 0.9 mM (pH = 9.2). We used oxalate as a counter-ion in order to prevent any of transition or alkalineearth metal ions interference. Strong complexation of oxalate prevents any peaks of transition or alkaline-earth metal ions from appearing in the electropherogram. As CTAB alone provided too fast EOF we examined the system containing CTAB in combination with nonionic surfactant to obtain lower and adjustable EOF speed. Tween 20 was considered as it is one of the most frequently used nonionic surfactants in CZE and is known to form mixed micelles with CTAB [21] and to decrease EOF speed [22]. Lithium ions mobility is about 4.1 × 10−4 cm2 V−1 s−1 [15] so EOF mobility should be adjusted to slightly lower value down from 5.7 × 10−4 cm2 V−1 s−1 when BGE without Tween 20 was used. Figure 1 shows the effect of the concentration of Tween 20 on EOF mobility. EOF velocity was measured using negative polarity (–25 kV) and 0.1% DMSO as EOF marker. As can be seen from Fig. 1, the EOF mobility could be adjusted in the range from 3.3 to 5.7 × 10−4 cm2 V−1 s−1 using this technique. Stable EOF velocities (RSD less than 1%) were obtained at any examined concentration of Tween 20.
3.2 Isotopic separation The separation of lithium isotopes was examined at different concentrations of Tween 20 and thus total analysis time. Partial separation of the isotopes was observed starting from 7 min with 6 Li being the faster one. Resolution of 1.0 was achieved within 17 min for isotopic ratio of 1:1 (Fig. 2). For baseline separation of the samples with high isotopic ratio about 20 min long analysis was needed that corresponds to www.electrophoresis-journal.com
3016
M. Kamencev et al.
Electrophoresis 2015, 36, 3014–3017
Figure 2. Effect of the concentration of Tween 20 on the separation of 6 Li and 7 Li. Tween 20 concentration (A) 0.5% w/v; (B) 0.4% w/v; (C) 0.3% w/v. The other conditions as in Fig. 1.
Figure 3. Electropherograms of lithium samples with mole fraction of 6 Li of (A) 3.44%; (B) 90.38%. Tween 20 concentration is 0.25% w/v. The other conditions as in Fig. 1.
Tween 20 concentration of 0.25% (Fig. 3) so these conditions was chosen for further experiments. Isotopic ratio was calculated directly from electropherograms using corrected peak areas as described previously [9]. Corrected peak area of the analyte was calculated as its peak area divided by its migration time. Mole fractions of the isotopes were then calculated using normalization procedure. Good linearity between corrected peak areas and mole fraction of lithium isotopes was observed (r = 0.9998) in the range from 3.44 to 90.38% (mol) of 6 Li. The results are given in Table 1. The LODs, defined as a S/N of 3, was 1 M for both 6 Li and 7 Li. The LOQs, defined as a S/N of 10, was 3 M for both isotopes that corresponds to mole fraction of 3% when overall lithium concentration in the sample is 0.1 mM. C 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
Table 1. Results of analysis of lithium preparations (n = 3)
Sample no.
6 Li, CE (% mol)
6 Li, MS (% mol) a)
1 2 3
3.5 50.2 90.5
3.44 50.04 90.38
a) The reference values were provided by the manufacturer (obtained by MS analysis).
4 Concluding remarks Relatively fast isotopic separation of lithium ions was demonstrated using CE in less than 21 min. Both depleted and enriched samples can be analyzed. The anodic EOF was www.electrophoresis-journal.com
Electrophoresis 2015, 36, 3014–3017
controlled by the combination of cationic surfactant (CTAB) and nonionic one (Tween 20).
CE and CEC
3017
[10] Aupiais, J., J. Solution Chem. 2011, 40, 1629–1644. [11] Yeung, K., Lucy, C., Anal. Chem. 1998, 70, 3286–3290. [12] Yeung, K., Lucy, C., Electrophoresis 1999, 20, 2554–2559.
The authors have declared no conflict of interest.
5 References [1] Arnikar, H., J. Inorg. Nucl. Chem. 1959, 11, 248–251. [2] Coplen, T., Pure Appl. Chem. 2002, 74, 1987–2017.
[13] Lucy, C., Yeung, K., Fu, C., Li, D., Hanselwood, T., Underhill, R., Can. J. Chem. 1999, 70, 281. [14] Brewer, A., Madorsky, S., Taylor, J., Dibeler, V., Bradt, P., Parham, O., Britten, R., Reid, J., J. Res. Natl. Bur. Stand. 1947, 38, 137–168. [15] Packakova, V., Coufal, P., Stulik, K., Gas, B., Electrophoresis 2003, 24, 1883–1891.
[3] Wagener, K., Thiemann, W., Z. Naturfosch. A 1963, 18, 228–235.
[16] Haddad, P., Foley, R., Anal. Chem. 1989, 61, 1435–1441.
[4] Terabe, S., Yashima, T., Anal. Chem. 1988, 60, 491–493.
[17] Wang, K., Wang, C., Electrophoresis 1997, 18, 241–246.
[5] Bushey, M., Jorgenson, J., Anal. Chem. 1989, 61, 491–493.
[18] Shakulashvili, N., Faller, T., Engelhardt, H., J. Chromatogr. A 2000, 895, 205–212.
[6] Tanaka, N., Araki, M., J. Am. Chem. Soc. 1985, 107, 7780–7781.
[19] Chiou, C., Shih, J., Analyst 1996, 121, 1107–1110.
[7] Tanaka, N., Yamaguchi, A., Araki, M., J. Am. Chem. Soc. 1985, 107, 7781–7783. [8] Avdalovic, N., Pohl, C., Rocklin, R., Stillian, J., Anal. Chem. 1993, 65, 1470–1475. [9] Lucy, C., McDonald, T., Anal. Chem. 1995, 67, 1074–1078.
C 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
[20] Johns, C., Macka, M., Haddad, P., Electrophoresis 2003, 24, 2150–2167. [21] Sidim, T., Akar, G., J. Surfactants Deterg. 2013, 16, 601–607. [22] Matchett, W., Brumley, W., J. Liq. Chromatogr. Relat. Technol. 1997, 20, 79–100.
www.electrophoresis-journal.com
