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Analysis of aromatic acids by non-aqueous capillary electrophoresis with ionic-liquid electrolytes

Yuanqi Lu 1,2,3,4, Dunqing Wang1, Chunyan Kong1,

Hao Zhong3*, Michael C.

Breadmore 2* 1

Analysis and Testing Centre, Dezhou University, University West Road 566, Dezhou

253023, P. R. China 2

Austrialian Centre for Research on Separation Science, School of Chemistry,

University of Tasmania, Hobart, Tasmania, 7001, Australia 3

Institute of Materia Medica, Shandong Academy of Medical Sciences, Jinan 250062, P.

R. China 4

Key Laboratory of Coordination Chemistry and Functional Materials in Universities of

Shandong, Dezhou University, University West Road 566, Dezhou 253023, P. R. China

Tel.

86-534-8985552

Email:

[email protected] [email protected](additional)

Received: 11-May-2014; Revised: 27-Jul-2014; Accepted: 28-Jul-2014 This article has been accepted for publication and undergone full peer review but has not been through the copyediting, typesetting, pagination and proofreading process, which may lead to differences between this version and the Version of Record. Please cite this article as doi: 10.1002/elps.201400242. This article is protected by copyright. All rights reserved.

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ABSTRACT The separation of six kinds of aromatic acids by capillary zone electrophoresis with EMIMCl and EMIMHSO4, two kinds of ionic liquids (IL) as background electrolytes and acetonitrile as solvent were investigated. The six kinds of aromatic acids can be separated under positive voltage with low IL concentration with either of the two ILs and separation with EMIMHSO4 is better in consideration of peak shapes and separation efficiency. But the migration order is different when the IL is different. Under negative voltage with high IL concentration, the six analytes can be separated with EMIMCl as background electrolytes and the migration order of the analytes is opposites to those with low concentration of EMIMCl as background electrolyte. The separations are based on the combination effects of heteroconjungation between the anions and cations in the ILs and the analytes, of which the heteroconjungation between the anions in the ILs and the analytes plays a dominant role. The heteroconjungation between the anions of the ILs and the analytes is proton sensitive and only a very small amount of proticsovlents added into the electrolyte solution can harm the separation. When EMIMCl concentration is high, the heteroconjungation between the IL anions and the proton in the analytes make the effective mobility of the analytes much higher than the EOF and their migration direction reversed. Finally, the six aromatic acids in water samples was analyzed by nonaqueous capillary electrophoresis with low concentration of EMIMHSO4 as background electrolytes with satisfactory results.

Keywords Capillary electrophoresis; ionic liquids; aromatic acids

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1

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Introduction Non-aqueous capillary electrophoresis (NACE), which is based on the use of electrolyte solutions

prepared from pure organic solvents, offers a number of attractive features such as improved selectivity by changing the solvent or solvent mixture, extended application scope with a better solubility for hydrophobic compounds , reduced electrophoretic currents and Joule Heating and easiness to couple to other detectors such as mass spectrometer, has drawn much attention in recent years [1-11].

One of the

limitations of NACE is the limited solubility of many electrolytes typically used in aqueous CE, which influences the flexibility and ability to optimize a separation. The solution may be to use ionic liquids (ILs): substances with melting points considerably lower than typical for ionic salts [12]. They present a variety of desirable properties. They are environmentally benign, nonvolatile and nonflammable with a high thermal stability and are good solvent for a wide range of both inorganic and organic materials. In recent years, they have been widely utilized in CE. In aqueous CE, ILs have been reported to be used as electrolytes [12~14], additives in electrolytes [15,16] , covalent coating reagents of the capillary [17,18] and for on-line [19] or off-line [20] extraction. As some ionic liquids are also readily soluble in organic solvents, they have also began to be used in NACE [2, 5~11]. In non-aqueous CE, ILs was often used as background electrolytes in acetonitrile, propylene carbonate and other nonprotogenic solvents [2,5-11]. Previously, we examined the migration behavior of three flavonoids by NACE with three 1-ethyl-3-methylimidazolium ILs in ACN and noted significant difference in migration behavior and selectivity depending on the type and concentration of the IL [5]. These were attributed to heteroconjugation of the flavonoids with the ionic liquid anion, with the results somewhat different to that reported by Yanes et al. but consistent with that rereported by Vaher et al. on This article is protected by copyright. All rights reserved.

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the separation of phenols[6-9] and dyes [10]. The interaction between the anions in the back ground electrolytes and the cations of ionic liquids as analytes has also been reported to separate some cations of ionic liquids using contactless conductivity detection [11]. In this paper, we continue to examine the migration under NACE with ionic liquids, focusing on the separation of six kinds of aromatic acids, four of which are positional isomers. These acids are interesting because of the challenge in separating the different structures as well as the relatively low pKa values (2.90–4.30) which means they may be partially charged in the IL background electrolyte and making their migration more interesting than those of phenols and dyes previously studies. The effect of IL type and concentration , capillary temperature and solvent mixture were studied, and the resulting separations compared to aqueous separations as well as to results in the literature. 2. Materials and methods 2.1 Apparatus and conditions Separatsions were performed in a P/ACE MDQ HPCE system (Beckman Coulter, Fullerton, CA, USA) equipped with a diode array detector operated at 199 nm. A 60.2 cm  50 m id fused-silica capillary (Polymicro Technologies, Phoenix, AZ USA) was utilized with an effective length of 50.0 cm, and its temperature was maintained at 25°C. Data were collected and analyzed using the Karat 7.0 software from Beckman Coulter running on an IBM Pentium IV-1 GHz computer. Before use, the capillary was rinsed with 1 M NaOH, water, MeOH, and then with ACN and separation electrolyte, each for 10 min. Between analyses the capillary was washed with ACN for 3 min and then with separation electrolyte for 3 min. 2.2 Chemicals All chemicals, unless otherwise stated, were of analytical reagent grade. Phenyl acetic acid, benzoic This article is protected by copyright. All rights reserved.

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acid, 2,3-dihydroxylbenzoic acid, 2,4-dihydroxylbenzoic acid, 2,5-dihydroxylbenzoic acid and 3,5-dihydroxylbenzoic acid were purchased from Aldrich Chemical Co.(Milwawkee, USA). Standard solutions of each acid at a concentration of 1.0 mg·mL-1were prepared in ACN. All stock solutions were stored in refrigerator with temperature at 4°C. Calibration standard solutions were prepared by diluting the stock

solutions

with

ACN.

1-ethyl-3-methylimidazolium

chloride

(EMIMCl)

and

1-ethyl-3-methylimidazolium hydrogen sulfate (EMIMHSO4) were purchased from Sigma Chemical Co. (St. Louis, MO, USA). Acetonitrile and MeOH of HPLC grade were from Tedia Company (Fairfield, USA). Ethyl acetate of HPLC grade was from Sigma Chemical Co. All solutions were stored in a desiccator and filtered through a 0.45 m membrane filter and sonicated for 10 min prior to usage. Because of the significant influence of MeOH and H2O on the heteroconjugation, all the ACN solvents used in this work were HPLC grade and all the ACN solutions were newly prepared and stored in desiccators before analysis, and the air in the CE was dried with silica gel before use.

2.3 Sample Preparation Waste water was collected from Zhangwei Nan River (Dezhou, Shandong Province, China). Before analysis, the samples were filtered through a 0.45 m membrane syringe filter in order to eliminate particulate matter. The samples were stored in the refrigerator at 4°C. 50 mL of sample or spiked sample was acidified with 1.0 mL of 1 mol/L hydrochloric acid to pH 2.0. Then 10 mL of ethyl acetate was added. After shaking, the sample was then centrifuged at 3000 rpm for 5 min and the ethyl acetate layer was collected. The extraction process was repeated three times and the pooled ethyl acetate layers was combined and about 0.5 g anhydrous calcium chloride was added to eliminate the water in it. After filtration, the filtrate was evaporated to 1 mL under a N-EVAPTM 111 nitrogen evaporator (Organomation This article is protected by copyright. All rights reserved.

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Associates, USA).

3 Results and Discussion 3.1

The effects of ionic liquids on separation In our previous work, IL concentration and the type of anion can affect the NACE separation of

flavonoids in ACN media [5]. Based on these results, the effect of 1-ethyl-3-methylimidazolium (EMIM) ionic liquids with chloride and hydrogen sulfate counter ions on the separation of the six aromatic acids were selected for investigation. As shown in Fig. 1, over the addition of 0-50 mM, the EOF decreased with the increase of the IL concentration while the electrophoretic mobilities of the analytes increased as the IL concentraton increased. The decrease of EOF may be due to the decreased the zeta potential of the capillary surface with the increase of the IL concentration.

At concentrations of 20 mM EMIMCl and

under +20 kV, benzoic acid (2) and phenyl acetic acid (1) migrate after the EOF with migration time around 40 min (36.9 min and 45.7 min). While the peaks of the other four analytes only appear when -18 kV was applied indicating that their electrophoretic mobility is greater than the EOF and hence they migrate in the opposite direction to the EOF. When the EMIMCl concentration is higher than 20 mM, the migration direction of all the analytes are reversed and they all migrate with a velocity higher than the EOF with net migration towards the anode. This can be seen clearly in Fig 2b which shows the electropherograms in 5 mM EMIMCl (+20 kV) and 50 mM EMIMCl (-18 kV),. Thus, in 5 mM EMIMCl the migration order is 2, 1, 6, 3, 4, 5 while with 50 mM EMIMCl it is exactly reverse, with only slight differences observed in the relative position

of 2,3-dihydroxybenzoic acid (peak 3) and

2,5-dihydroxybenzoic acid (peak 4). This change in migration order is primarily due to a reduction in the magnitude of the EOF and not to changes in mobility of the acid-IL conjugate, as the mobility values for the acids are relatively constant (figure 1). This article is protected by copyright. All rights reserved.

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When using EMIMHSO4, the aromatic acids had a lower electrophoretic mobility than the EOF under all conditions examined and always migrated after the EOF, with the electrophoretic mobility values approximately half in HSO4- of what they are in Cl-.

These trends are similar to those we reported

previously for the flavonoids. More interesting is the subtle difference in selectivity with HSO4- - the migration order is 1, 2, 3, 4, 5, 6 (Fig. 2a). This contrasts very clearly with 5, 4, 3, 6, 1, 2 in 50 mM EMIMCl (Fig. 2b) and 1, 2, 4, 6, 3, 5 in aqueous sodium tetraborate (Fig. 2c), all of which are counter-EOF separations. This is in contrast to our previous work with flavanoids in which there was no change in selectivity between the Cl- and HSO4- ILs and sodium tetraborate. This different selectivity is likely a result of differences in relative acidity and the extent of ionization, and heteroconjugation of the acids with the IL anion. As EMIMHSO4, gave better peak shapes and the separation was much quicker this IL was selected for further optimisation.

3.2 Effects of solvents mixture The effects of MeOH on separations with the ILs as background electrolytes has been reported by Vaher et al. [6~7,10]. They found that the self-dissociation and proton donation ability of MeOH hinders the heteroconjungation, reducing electrophoretic mobilities and reducing the practical value of the method. We also previously observed this, but importantly a little MeOH added into the ACN can enhance the solubility of flavonoids, and the decrease in heteroconjuugation can be overcome by increasing the concentration of the IL to ensure a satisfactory separation [5]. Here, we examined the addition of three solvents, MeOH, H2O and ethyl acetate, the physical properties of which are shown in Table 1 [22,23], on the acid separation with 15 mM EMIMHSO4. Small amounts of H2O and MeOH were selected to examine the decrease in heteroconugation, and for water specifically, to This article is protected by copyright. All rights reserved.

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see whether further selectivity changes could be observed by influencing changes in ionization.

Ethyl

acetate was selected to investigate the solvent effects because it is a kind of nonprotogenic solvent often used for solvent extraction and its effects on the separation based on heteroconjungation has not been reported. Figure 3a, b, and c show the changes in electrophoretic mobility and EOF observed with MeOH, H2O and ethyl acetate added to 15 mM EMIMHSO4 with the separations shown in Fig. 4. As shown in Fig. 3, the EOF decreased as the concentration of solvents increased, decreasing rapidly with MeOH, while it decreased similarly with H2O and ethyl acetate although to lesser extent. The more pronounced change with MeOH is most likely due to changes in viscocity and is charaterstic of the use of this solvent in CE. In MeOH and H2O, the electrophoretic mobility of the aromatic acids decreased quickly, with the resolution becoming worse with the increase of solvent concentration due to the disruption of the heteroconjugation. When MeOH is higher than 20% or H2O is higher than 8%, the acids could not be resolved at all. However, with ethyl acetate, the electrophoretic mobility of the aromatic acids decreased slowly and the migration time and the resolution increased, with resolution still possible with 10% ethyl acetate added to the electrolyte. This is due to the fact that ethyl acetate is a non-protogenic solvent and it can’t donate proton and disrupt the heteroconjugation. The slight decrease in electrophoretic mobility is therefore due to changes in solvent properties: the permittivity is much lower and the viscosity is a little higher than that of ACN, the combination of these effects decreasing the electrophoretic mobility of the aromatic acids and the EOF slightly lower when compared to those in pure ACN. Also as shown in Fig. 5, when ethyl acetate was used as the sample sovent (trace b) the sensitivity of was about half of that when using ACN (trace a). Aside from work by Valkó et al. [23] who measured the EOF in a range of pure non-aqueous solvents with no ions present, the only other use of ethyl acetate in the BGE in CE is for microemulsion electrokinetic This article is protected by copyright. All rights reserved.

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chromatography (MEEKC). The combination of NACE with ILs presents an interesting opportunity to use more unconventional solvents that have yet to find widespread applicability in CE.

3.3 Analytical application From the IL electrolytes examined above, 15 mM EMIMHSO4 in pure ACN was selected as the most promising BGE for the separation of the six aromatic acids as they could be separated in 11 min with good resolution and peak shapes (Fig. 6a). This system was partially validated and the results are shown in table 2 and table 3. The reproducibility was estimated by making five replicate injection of a standard mixture with the results shown in Table 2 where it can be seen that the relative standard deviation of the six aromatic acids based on migration time and peak area were in the 0.42–0.74% , and 3.9–6.1%, respectively. The calibration was also investigated and the results was shown in Table 3 which is satisfactory. As also shown in Table 3, The detection limits based on three times noise ranged from 0.16–0.60 µg/mL, which is more than two times lower than those in aqueous CE (20 mmol/L sodium tetraborate as background electrolyte, other conditions the same as Fig. 2c.). To demonstrate the potential of the NACE method, aromatic acids were determined in waters from China. Aromatic acids are the main organic compounds in water [20] and the maximum permitted amounts of organic compounds are 3 mg/L in drinking water in China [24]. To determine the concentration of aromatic acids in waste water is also very important for pollution control [25]. In this work, the six aromatic acids in the water from Zhangwei Nan River (Dezhou, Shandong Province, China) were analysed with ACN solution of 15 mM EMIMHSO4 as background electrolytes. The water was treated with 1.0 mL of 1 mol/L hydrochloric acid to pH 2.0 and extracted with ethyl acetate. At first, the ethyl acetate extracts were directly injected into the CE system. But the results showed that the peaks of the analytes overlapped with the EOF, which may be due to the effects of the water solved in ethyl acetate. Anhydrous calcium chloride This article is protected by copyright. All rights reserved.

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was added to eliminate the water and the results were satisfactory. Finally, the ethyl acetate layer was evaporate to 1 mL. Fig. 6b shows results from the direct injection of the ethyl acetate extracts of water sample and those spiked with suitable amount of each of the analytes. No analytes peaks were observed in the sample and recoveries ranged from 53.4 to 90.0% .

4. Concluding remarks The application of ionic liquids to separate six kinds of aromatic acids were investigated. The effects of ionic liquids concentration and kinds as well as solvent mixture were studied. Amphiprotic solvent MeOH can hinder the heteroconjungation between the anion of ionic liquids and hurt the separation. Acknowledgement This work was supported by funding from the Key Technologies R&D Programme of Shandong Provine (2010GSF10615) and the Australian Research Council (DP0984745). References [1] Humam, M., Bieri, S., Geiser, L., Muñoz, O., Veuthey, J. L., Christen, P., Phytochem. Anal. 2005, 16: 349-356. [2] Porras, S. P., Palonen, R. K. S., Riekkola, M. L., J. Chromatogr. A 2003,990, 35-44. [3] Lu,Y., Breadmore, M. C., J. Chromatogr. A 2010, 1217, 7282-7287. [4] Lu,Y., Breadmore, M. C., J. Sep. Sci. 2010, 33, 2140-2144. [5] Lu , Y., Jia, C., Yao, Q., Zhong, H., Breadmore, M. C., J. Chromatogr. A 2013, 1319, 160-165. [6] Vaher, M., Koel, M., Kaljurand, M., Electrophoresis 2002, 23,426-430. [7] Vaher, M., Koel, M., Kaljurand, M., J. Chromatogr. A 2002, 979, 27-32 [8] Kuldvee, R., Vaher, M., Koel, M., Kaljurand, M., Electrophoresis 2003, 24, 1627-1634. [9] Vaher, M., Koel, M., J. Chromatogr. A 2005, 1068, 83-88.

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[10] Vaher, M., Koel, M., Kaljurand, M., Chromatographia 2001, 53, S302- S306. [11] Borissova, M., Gorbatšova, J., Ebber, A., Kaljurand, M., Koel, M., Vaher, M., Electrophoresis 2007, 28, 3600-3605. [12] Yanes, E. G., Gratz, S. R., Baldwin, M. J., Robison, S. E., Stalcup, A. M., Anal. Chem. 2001, 73, 3838-3844. [13] Yanes, E. G., Gratz, S. R., Stalcup, A. M., Analyst 2000, 125, 1919-1923. [14] Wu, X., Wei, W. , Su, Q. , Xu, L. , Chen, G., Electrophoresis 2008, 29, 2356-2362. [15] Laamanen, P. L., Busi, S., Lahtinen, M., Matilainen, R. , J. Chromatogr. A 2005, 1095, 164-171. [16] Zhang, H., Tian, K., Tang, J., Qi, S., Chen, H. , Chen, X., Hu, Z., J. Chromatogr. A 2006, 1129, 304–307. [17] Qin, W., Li, S. F. Y., J. Chromatogr. A 2004, 1048, 253-256. [18] Qin, W., Wei, H., Li, S. F. Y., J. Chromatogr. A 2003, 985, 447-454. [19] Breadmore, M. C., J. Chromatogr. A 2011, 1218, 1347-1352. [20] Qin, B., Zhang. L., Petrochemical Technology 2007,36, 1056-1059. [21] Liu, G., Ma, L., Liu, J, Physical Property Data Handbook in Chemistry and Chemical Engineering (Inorganic volume), Chemical Industry Press, Beijing 2002, p. 1. [22] Liu, G., Ma, L., Liu, J., Physical Property Data Handbook in Chemistry and Chemical Engineering (Inorganic volume) (Organic volume), Chemical Industry Press, Beijing 2002, p. 398, 463, 542.. [23] Valkó, I. E., Sirén, H., Riekkola, M. L., J. Microcol. Sep. 1999, 11, 199-208. [24] Ministry of Health of the People’s Republic of China& Standardization Administration of the People’s Republic of China .Standards for drinking water quality( GB5749-2006). [25] Wang, Q., Qiu, H., Li, J., Liu, X., Jiang, S., J. Chromatogr. A 2010, 1217, 5434-5439. This article is protected by copyright. All rights reserved.

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Table 1 Physical properties of the solvents in this work Compounds Acetonitrile Methanol Ethyl acetate Polarity 5.8 5.1 4.3 Viscosity (mPa∙S) 0.34 0.54 0.43 Permittivity 37.5 32.6 5.1 Boiling point (°C) 82 65 77 Saturated vapor 11.53 (24°C) 16.82 13.33 (27°C) pressure (kPa) Unless otherwise specified, the temperature and air pressure are 20°C respectively.

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Water 10.2 0.89 80.4 100 3.1684 and 101.3 kPa,

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Table 2 Reproducibility of the peak area and migration time of the analytes (n = 5). Analytes Concentration Migration time Peak area -1 (g٠mL ) Mean RSD (%) Mean RSD (%) Phenyl acetic acid 30.00 7.636 0.51 18892 5.2 Benzoic acid 30.00 8.125 0.48 27142 4.7 2,3-Dihydroxybenzoic acid 30.00 8.502 0.74 7500 6.1 2,5-Dihydroxybenzoic acid 30.00 9.103 0.63 9306 3.9 2,4-Dihydroxybenzoic acid 30.00 9.467 0.42 6481 4.8 3,5-Dihydroxybenzoic acid 30.00 10.370 0.59 12571 5.4 CE conditions are the same as in Fig. 6

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Table 3 The regression equations and detection limits Compound Regression equationa Correlation coefficient Phenyl acetic acid y = 629.73x – 18.63 0.9932 Benzoic acid y = 904.09x – 21.05 0.9941 2,3-Dihydroxybenzoic acid y = 242.14x + 16.33 0.9915 2,5-Dihydroxybenzoic acid y = 310.20x + 17.08 0.9926 2,4-Dihydroxybenzoic acid y = 216.03x + 11.09 0.9953 3,5-Dihydroxybenzoic acid y = 419.27x + 12.26 0.9962

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Linear range (g٠mL-1) 1.0~40.0 1.0~40.0 2.0~60.0 2.0~60.0 2.0~60.0 2.0~60.0

Detection limitb (g٠mL-1) 0.19 0.16 0.50 0.40 0.60 0.30

CE conditions are the same as in Fig. 5 a. In the regression equation, the x value is the concentration of analytes (g٠mL-1), the y value is the peak area b. The detection limit is evaluated on the basis of a signal-to-noise of 3.

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Table 4 Results of sample analysis and the recovery (n = 5) Compounds Original Added Found (µg/mL) (µg/mL) (µg/mL) Phenyl acetic acid 0 0.20 0.149  0.024 Benzoic acid 0 0.20 0.138  0.010 2,3-Dihydroxybenzoic acid 0 0.20 0.113  0.011 2,5-Dihydroxybenzoic acid 0 0.20 0.109  0.008 2,4-Dihydroxybenzoic acid 0 0.20 0.157  0.010 3,5-Dihydroxybenzoic acid 0 0.20 0.180  0.017 CE conditions are the same as in Fig. 5

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Recovery (%) 74.7  12.2 68.8  5.0 56.4  5.7 53.4  4.2 78.4  5.2 90.0  8.6

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Figure 1: The changes in EOF and mobility with the three different ILs. a. EMIMCl; b. EMIMHSO4. 1. phenyl acetic acid; 2. benzoic acid; 3. 2,3-dihydroxybenzoic acid; 4. 2,5-dihydroxybenzoic acid; 5. 2,4-dihydroxybenzoic acid; 6. 3,5-dihydroxybenzoic acid (1, 2, 3, 4, 5, 6 have the same meaning in the whole paper). Notes: Other conditions: applied voltage, 20 kV(-18 kV, only for EMIMCl when its concentration is higher than 10 mM); wavelength, 199 nm

Fig. 1 EMIMCl EMIMHSO4 10

10

8

(a)

6

6

2 -1 -1

2

4

-8

-8

2 -1 -1

EOF 1 2 3 4 5 6

8

ep(10 m v s )

4

ep(10 m v s )

(b)

EOF 1 2 3 4 5 6

0

2

0

-2 -4

-2 0

10

20

30

Concentration(mM)

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40

50

0

10

20

30

Concentration(mM)

40

50

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Figure 2: Electropherograms of the analytes in EMIMHSO4 (a), EMIMCl (b) and 20 mM sodium tetraborate (c) of both low and high concentrations.Conditions: applied voltage, 20 kV(-18 kV, only for EMIMCl when its concentration is 50 mM); wavelength, 199 nm

Fig. 2 EMIMHSO4

EMIMCl

Aqueous sodium tetraborate

0.015 (c)

0.015

0.012

0.02

5 mM EMIMCl 1 50 mM EMIMCl

(b) 2

0.01

(a) 5 mM EMIMHSO4 50 mM EMIMHSO4

4,5,6

1

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2 0.009

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AU

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AU

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time(min)

0.000 0

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24

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time(min)

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64

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time(min)

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Figure 3: Changes in mobility with MeOH, H2O and ethyl acetate in ACN for 15 mM EMIMHSO4. Conditions: applied voltage, 20 kV; wavelength, 199 nm.

Fig. 3 (a)

EOF 1 2 3 4 5 6

2 2 -1 -1

ep(10 m v s )

1

3

1

0

0

0

-1

-1

-1

-2

-2 0

5

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15

MeOH(%, V/V)

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20

EOF 1 2 3 4 5 6

2

-8

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-8

-8

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ep(10 m v s )

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EOF 1 2 3 4 5 6

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(c) (b)

-2 0

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04

H2O(%,V/V)

2

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6

ethyl acetate(%,V/V)

8

10

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Figure 4: Separations of the analytes in EMIMHSO4 with different solvent mixture. a. ACN; b. 20% (v/v) MeOH; c. 8% (v/v) H2O; d. 10% (v/v) ethyl acetate. Conditions: applied voltage, 20 kV; wavelength, 199 nm; Sample injection, 5 s (0.5 psi) .

Fig. 4

0.03

1,2,3,4,5

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0

3

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time(min)

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15

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Figure 5: Separations of the analytes in 15 mM EMIMHSO4 with different solvents for the analytes. Analytes Concentration, 30 g/mL A. ACN; b. Ethyl acetate. Conditions: applied voltage, 20 kV; wavelength, 199 nm; Sample injection, 5 s (0.5 psi) .

Fig. 5

-0.002

-0.002

(b)

(a)

2

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Figure 6: Electropherograms of the standards (a) and the acetonitrile extract of water sample (b). Analytes Concentration, 30 g/mL; Conditions: background electrolyte, 15 mM EMIMHSO4 solution of ACN; applied voltage, 20 kV; wavelength, 199 nm; Sample injection, 5 s (0.5 psi)

Fig. 6

-0.006

-0.006

2 (a) -0.007

(b)

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Analysis of aromatic acids by nonaqueous capillary electrophoresis with ionic-liquid electrolytes.

The separation of six kinds of aromatic acids by CZE with 1-ethyl-3-methylimidazolium chloride (EMIMCl) and 1-ethyl-3-methylimidazolium hydrogen sulfa...
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