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Rapid and simultaneous determination of imidazolium and pyridinium ionic liquid cations by ion-pair chromatography using a monolithic column
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Chinese Chemical Letters 23 (2012) 843–846 www.elsevier.com/locate/cclet
Rapid and simultaneous determination of imidazolium and pyridinium ionic liquid cations by ion-pair chromatography using a monolithic column Xu Huang, Hong Yu *, Ying Jie Dong College of Chemistry and Chemical Engineering, Harbin Normal University, Harbin 150025, China Received 20 March 2012 Available online 9 June 2012
Abstract A method for rapid and simultaneous determination of imidazolium and pyridinium ionic liquid cations by ion-pair chromatography with ultraviolet detection was developed. Chromatographic separations were performed on a reversed-phase silica-based monolithic column using 1-heptanesulfonic acid sodium–acetonitrile as mobile phase. The effects of ion-pair reagent and acetonitrile concentration on retention of the cations were investigated. The retention times of the cations accord with carbon number rule. The method has been successfully applied to the determination of four ionic liquids synthesized by organic chemistry laboratory. # 2012 Hong Yu. Published by Elsevier B.V. on behalf of Chinese Chemical Society. All rights reserved. Keywords: Ionic liquids; Ion-pair chromatography; Monolithic column; Ultraviolet detection
Ionic liquids (ILs), are organic molten salts that are composed of large organic cations and inorganic anions or organic anions. With the development of ILs applied to the fields of synthesis, electrochemistry and analytical chemistry [1], it has become increasingly necessary to develop analytical techniques for determination of ILs. The separation and determination of ionic liquids mainly involved analysis of cations and anions in ionic liquids. In the process of preparation and application of ionic liquids, the determination of cations or anions can determine the kind and the purity of ion liquids. Recently, the methods reported for determination of ionic liquid anions are mainly ion chromatography (IC) [2]. Some investigations have been reported on the determination of ionic liquid cations using primarily capillary electrophoresis [3], reversed-phase liquid chromatography [4], hydrophilic interaction liquid chromatography [5], IC [6] and ion-pair chromatography (IPC) [7]. IPC is a very versatile chromatographic mode that is used for the separation of analytes ranging from small ionic species to large biomacromolecules. In recent years, the monolithic columns have been intensively studied as the alternative solution for the high speed separation operated with relatively low back pressure [8]. At present, the simultaneous analysis of imidazolium and pyridinium ionic liquid cations by IPC using a monolithic column has scarcely been reported. The aim of this work was to develop a simple, rapid and practical IPC method on a monolithic column to separate imidazolium and pyridinium ionic liquid cations simultaneously.
* Corresponding author. E-mail address: [email protected] (H. Yu). 1001-8417/$ – see front matter # 2012 Hong Yu. Published by Elsevier B.V. on behalf of Chinese Chemical Society. All rights reserved. http://dx.doi.org/10.1016/j.cclet.2012.04.029
844
X. Huang et al. / Chinese Chemical Letters 23 (2012) 843–846
1. Experimental ILs (99%) used as the standard solution were obtained from Shanghai Chengjie Chemical Ltd. (Shanghai, China). ILs were: 1-ethyl-3-methylimidazolium tetrafluoroborate ([EMIm]BF4), 1-propyl-3-methylimidazolium tetrafluoroborate ([PMIm]BF4), 1-butyl-3-methylimidazolium tetrafluoroborate ([BMIm]BF4), 1-amyl-3-methyl-imidazolium tetrafluoroborate ([AMIm]BF4), 1-hexyl-3-methylimidazolium tetrafluoroborate ([HMIm]BF4), N-ethylpyridinium iodide ([EPy]I), N-butylpyridinium trifluoromethanesulfonate ([BPy]CF3SO3), N-butyl-4-methyl-pyridinium tetrafluoroborate ([BMPy]BF4), N-hexylpyridinium bromide ([HPy]Br). 1-Heptanesulfonic acid sodium and 1-pentanesulfonic acid sodium were obtained from Tianjin Guangfu Fine Chemical Research Institute (Tianjin, China). Acetonitrile (LC grade) was obtained from Dikma Technologies. Citric acid was supplied by Shanghai Chemical Reagent Factory (Shanghai, China). Standard solutions of all cations were prepared in 18.2 MV cm deionized distilled water from a Millipore Milli-Q water purification system (Millipore, Bedford, MA, USA). The solutions were filtered using a 0.22 mm membrane filter before injection. Milli-Q water was also used to prepare mobile phase. Mobile phases were prepared with the desired amount of 1heptanesulfonic acid sodium, then citric acid was used to adjust the required pH (Model PHSF-3F, China). The mobile phases were filtered through a 0.22 mm filter and then degassed for 15 min with a Model DOA-P504-BN pump (IDEX, USA). All chromatographic experiments were carried out on an Agilent 1200 HPLC system (Agilent, USA), which consisted of a quaternary pump, a detector, an autosample injector, a column oven and a degasser system. The chromatographic system control, data acquisition and data analysis were performed using the Agilent Rev.B.04.01 workstation. All separations were performed on a Chromolith Performance RP-18e column (4.6 mm i.d. 100 mm, Merck, Germany). The mobile phase was (A) 1.0 mmol/L 1-heptanesulfonic acid sodium (pH 4.0 adjusted by citric acid)–(B) acetonitrile. The flow rate was set at 2.0 mL/min. Column temperature was 30 8C. The injection volume was 20 mL. UV detection (210 nm) was employed. 2. Results and discussion The influences of two ion-pair reagents on the retention of nine cations were discussed. Column temperature was 30 8C, and flow rate was 1.0 mL/min. By adopting 80% (v/v) 1.0 mmol/L 1-pentanesulfonic acid sodium or 1heptanesulfonic acid sodium (pH 4.0)–20% acetonitrile as mobile phase, the separation of nine cations (EMIm, PMIm, BMIm, AMIm, HMIm, EPy, BPy, BMPy, HPy) was performed. The hydrophobicity of 1-pentanesulfonic acid sodium is weaker than that of 1-heptanesulfonic acid sodium, and therefore the retention of imidazolium and pyridinium cations was weaker when using 1-pentanesulfonic acid sodium in the mobile phase than with 1-heptanesulfonic acid sodium. In order to increase the retention of imidazolium and pyridinium cations and optimize the separation, 1heptanesulfonic acid sodium was chosen as ion-pair reagent for this study. The concentrations of 1-heptanesulfonic acid sodium varied from 0.5 to 1.5 mmol/L in order to choose the optimal concentration. It was found that the retention factors of imidazolium and pyridinium cations totally enlarged with the increase of the concentration of 1-heptanesulfonic acid sodium. While the concentration was 0.5 mmol/L, the retention times of imidazolium and pyridinium cations were so short that the system peak disturbed separation. However, when the concentration was more than 1.5 mmol/L, the column could be polluted to some degree since the ion-pair reagent concentration was excessive. Consequently, 1.0 mmol/L of 1-heptanesulfonic acid sodium was the appropriate concentration. The effect of the acetonitrile concentration on the retention of nine cations was investigated using 1.0 mmol/L 1heptanesulfonic acid sodium–15, 20, 25 and 30% acetonitrile as mobile phase. The column temperature was 30 8C and flow rate was 1.0 mL/min. The retention factors of imidazolium and pyridinium cations decrease with the increase of the acetonitrile concentration. Among the cations, the decline magnitude of the longer alkyl-chain cations retention factor was significant. In LC, using acetonitrile as mobile phase, the relationship between retention factor (k) and the percentage of acetonitrile in the mobile phase (%ACN) is: log k = c%ACN + d [4], in which c is the slope of the line and d is the theoretical retention factor with no ACN added. The relationship curves were listed in Table 1. Known from the results, the log k vs %ACN of cations shows a better linear relationship. This indicates that the retentions of the nine cations are in line with reversed-phase mode using the monolithic column.
X. Huang et al. / Chinese Chemical Letters 23 (2012) 843–846
845
Table 1 Linear regression equation for the log k vs %ACN relationships obtained with the retention time of imidazolium and pyridinium cations. Analyte
Linear regression equation
Analyte
Linear regression equation
EPy PMIm BMIm AMIm HMIm
log k = log k = log k = log k = log k =
EMIm BPy BMPy HPy
log k = log k = log k = log k =
0.073%ACN + 1.028 (r = 0.9726) 0.057%ACN + 0.9276 (r = 0.9937) 0.066%ACN + 1.359 (r = 0.9952) 0.076%ACN + 1.905 (r = 0.9927) 0.081%ACN + 2.346 (r = 0.9986)
0.071%ACN + 1.036 0.065%ACN + 1.239 0.068%ACN + 1.504 0.072%ACN + 1.998
(r = 0.9757) (r = 0.9866) (r = 0.9930) (r = 0.9934)
Absorbance (mAU)
When flow rates were set to 1, 2, 3, 4 mL/min, the effect of flow rate on retention time of imidazolium and pyridinium cations was investigated. In the experiment, the mobile phase of 1.0 mmol/L 1-heptanesulfonic acid sodium (pH 4.0)–20% acetonitrile was used, and column temperature was 30 8C. Totally, the retention times of imidazolium and pyridinium cations were shortened with the increase of flow rate, and the column pressure was raised with the increase of flow rate. While flow rate was higher than 3 mL/min, the baseline was not steady, and the retention times of EMIm and EPy were close. So 2 mL/min were selected as the optimum flow rate. Under this flow rate, the baseline was smooth, and a better separation of the cations was achieved. In order to further optimize the separation, elution gradient was designed using the mobile phase of (A) 1.0 mmol/L 1-heptanesulfonic acid sodium (pH 4.0) and (B) acetonitrile. By optimizing, the elution gradient: 0–3.0 min, 7% B; 3.0–3.5 min, 7–18% B; 3.5–5.0 min, 18% B; 5.0–5.5 min, 18–25% B; 5.5–7.0 min, 25% B; 7.0–7.5 min, 25–30% B was chosen as the optimum gradient condition. The retention factors for homologous series and the carbon number follow the carbon number rule by the following relationship: log k = a nC + b [4] (k is the retention factor; nC is the carbon number of the alkyl chain). The drawing of retention factors of imidazolium cations and the carbon numbers of the imidazolium alkyl chain was obtained under the optimum gradient condition. The linear regression equation for the five cations EMIm, PMIm, BMIm, AMIm and HMIm is log k = 1.668 nC 1.784, r = 0.9937. At the same time, the drawing of retention factors of pyridinium cations and the carbon numbers of the pyridinium alkyl chain was obtained under the optimum gradient condition. The linear regression equation for the three cations EPy, BPy and HPy is log k = 1.665 nC 0.5967, r = 0.9945. Known from the linear regression equations, log k and nC have a good linear regression, so the retention of imidazolium and pyridinium cations accords with the carbon number rule under the gradient condition and the monolithic column. The chromatogram of a standard solution of nine cations by using the best chromatographic condition is show in Fig. 1. Detection limits (S/N = 3) were 0.11–0.62 mg/L, and noise of baseline was 0.1941 mAU. Relative standard deviations (RSD) of peak areas obtained by determining a standard solution of nine cations were 0.06–0.49%. The linear regression equations of calibration curves for nine cations were as follows: EPy, y = 8.06x 8.63; EMIm, y = 9.27x 10.81; PMIm, y = 8.76x 6.81; BPy, y = 7.40x 9.60; BMIm, y = 7.71x 3.62; BMPy, y = 6.47x 6.57; AMIm, y = 7.82x + 8.48; HPy, y = 9.97x 3.01; HMIm, y = 7.42x + 6.30. The linear ranges of calibration curves between peak area and the concentration for nine cations were 0.20–200 mg/L. This method was applied to the determination of cations in four kinds of ionic liquids synthesized by organic chemistry lab, namely [EMIm]BF4, [BMIm]Br, [BPy]CF3SO3, [BMPy]BF4. The [EMIm]BF4 of exactly quantified 800 600 400 1 2
200 0
1
2
3 4 Time (min)
3
4 5 6
5
7
6
8 9
7
8
Fig. 1. Chromatogram of a mixture solution of nine ionic liquid cations. Elution gradient: 0–3.0 min, 7% B; 3.0–3.5 min, 7–18% B; 3.5–5.0 min, 18% B; 5.0–5.5 min, 18–25% B; 5.5–7.0 min, 25% B; 7.0–7.5 min, 25–30% B. Eluent A, 1.0 mmol/L 1-heptanesulfonic acid sodium (pH 4.0 adjusted by citric acid); eluent B, acetonitrile; flow rate, 2.0 mL/min; column temperature, 30 8C. Peaks (mg/L): 1, EPy (50.0); 2, EMIm (40.0); 3, PMIm (60.0); 4, BPy (40.0); 5, BMIm (30.0); 6, BMPy (50.0); 7, AMIm (60.0); 8, HPy (60.0); 9, HMIm (80.0).
X. Huang et al. / Chinese Chemical Letters 23 (2012) 843–846
846
Table 2 Analytical results and recoveries of imidazolium and pyridinium cations found in ionic liquid samples. Ionic liquid
Ionic liquid cations
Original, ro (mg/L)
Added, rA (mg/L)
Found, rF (mg/L)
Recovery, R (%)
Content of cations in ionic liquids synthesized, w (%)
EMImBF4 BMImBF4 BPyCF3SO3 BMPyBF4
EMIm BMIm BPy BMPy
31.18 39.02 28 27.6
10 10 10 10
41.01 48.58 38.2 37.8
98.3 95.6 102.0 102.0
50.00 61.86 55.41 65.90
weight 0.1559 g, the [BMIm]Br of exactly quantified weight 0.1577 g, the [BPy]CFSO3 of exactly quantified weight 0.1263 g and the [BMPy]BF4 of exactly quantified weight 0.1047 g were diluted to 100 mL as stock solutions. Then the stock solutions of 2 mL [EMIm]BF4, 2 mL [BMIm]Br, 2 mL [BPy]CFSO3 and 2 mL [BMPy]BF4 were taken out and diluted to 50 mL. The diluents filtered through a 0.22 mm memberance filter were used for the determination of imidazolium and pyridinium cations with the selected chromatographic condition. The chromatograms are show in Fig. S2 (Supporting information). Recoveries were tested by the standard addition method. Analytical results and recoveries of imidazolium and pyridinium cations in the ionic liquids are listed in Table 2. The data from Table 2 prove the method to be accurate and reproducible. 3. Conclusion A practical method for rapid and simultaneous determining nine imidazolium and pyridinium ionic liquid cations was developed. The application of monolithic column can reduce the analysis time of the cations. The nine cations were baseline separated by a gradient elution procedure using 1-heptanesulfonic acid sodium and acetonitrile as mobile phase. In addition, the retention times of cations accord with carbon number rule. This method has been successfully applied to determine imidazolium and pyridinium cations in ionic liquids synthesized by organic chemistry lab, and the results are accurate, the reproducibility is satisfactory. Acknowledgments This work was supported by the Natural Science Foundation of Heilongjiang Province (No. B200909) and the Program for Scientific and Technological Innovation Team Construction in Universities of Heilongjiang Province (No. 2011TD010). Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/ j.cclet.2012.04.029. References [1] [2] [3] [4] [5] [6] [7] [8]
L.Q. Yin, D.X. Yuan, M. Zhang, Chin. Chem. Lett. 21 (2010) 1457. Y.Y. Zhou, W.F. Zhou, X.L. Zeng, et al. Chin. Chem. Lett. 22 (2011) 461. M.J. Markuszewski, P. Stepnowski, M.P. Marszall, Electrophoresis 25 (2004) 3450. M.J. Ruiz-Angel, A. Berthod, J. Chromatogr. A 1113 (2006) 101. C. Lamouroux, G. Foglia, G. LeRouzo, J. Chromatogr. A 1218 (2011) 3022. H. Yu, S. Zhou, W. Gao, Chromatographia 72 (2010) 225. L.M. Meng, H. Yu, Y.Z. Liu, Chromatographia 73 (2011) 367. Q. Chen, H. Yu, J. Instrum. Anal. 30 (2011) 705.
Chinese Chemical Letters 23 (2012) 843–846 www.elsevier.com/locate/cclet
Rapid and simultaneous determination of imidazolium and pyridinium ionic liquid cations by ion-pair chromatography using a monolithic column Xu Huang, Hong Yu *, Ying Jie Dong College of Chemistry and Chemical Engineering, Harbin Normal University, Harbin 150025, China Received 20 March 2012 Available online 9 June 2012
Abstract A method for rapid and simultaneous determination of imidazolium and pyridinium ionic liquid cations by ion-pair chromatography with ultraviolet detection was developed. Chromatographic separations were performed on a reversed-phase silica-based monolithic column using 1-heptanesulfonic acid sodium–acetonitrile as mobile phase. The effects of ion-pair reagent and acetonitrile concentration on retention of the cations were investigated. The retention times of the cations accord with carbon number rule. The method has been successfully applied to the determination of four ionic liquids synthesized by organic chemistry laboratory. # 2012 Hong Yu. Published by Elsevier B.V. on behalf of Chinese Chemical Society. All rights reserved. Keywords: Ionic liquids; Ion-pair chromatography; Monolithic column; Ultraviolet detection
Ionic liquids (ILs), are organic molten salts that are composed of large organic cations and inorganic anions or organic anions. With the development of ILs applied to the fields of synthesis, electrochemistry and analytical chemistry [1], it has become increasingly necessary to develop analytical techniques for determination of ILs. The separation and determination of ionic liquids mainly involved analysis of cations and anions in ionic liquids. In the process of preparation and application of ionic liquids, the determination of cations or anions can determine the kind and the purity of ion liquids. Recently, the methods reported for determination of ionic liquid anions are mainly ion chromatography (IC) [2]. Some investigations have been reported on the determination of ionic liquid cations using primarily capillary electrophoresis [3], reversed-phase liquid chromatography [4], hydrophilic interaction liquid chromatography [5], IC [6] and ion-pair chromatography (IPC) [7]. IPC is a very versatile chromatographic mode that is used for the separation of analytes ranging from small ionic species to large biomacromolecules. In recent years, the monolithic columns have been intensively studied as the alternative solution for the high speed separation operated with relatively low back pressure [8]. At present, the simultaneous analysis of imidazolium and pyridinium ionic liquid cations by IPC using a monolithic column has scarcely been reported. The aim of this work was to develop a simple, rapid and practical IPC method on a monolithic column to separate imidazolium and pyridinium ionic liquid cations simultaneously.
* Corresponding author. E-mail address: [email protected] (H. Yu). 1001-8417/$ – see front matter # 2012 Hong Yu. Published by Elsevier B.V. on behalf of Chinese Chemical Society. All rights reserved. http://dx.doi.org/10.1016/j.cclet.2012.04.029
844
X. Huang et al. / Chinese Chemical Letters 23 (2012) 843–846
1. Experimental ILs (99%) used as the standard solution were obtained from Shanghai Chengjie Chemical Ltd. (Shanghai, China). ILs were: 1-ethyl-3-methylimidazolium tetrafluoroborate ([EMIm]BF4), 1-propyl-3-methylimidazolium tetrafluoroborate ([PMIm]BF4), 1-butyl-3-methylimidazolium tetrafluoroborate ([BMIm]BF4), 1-amyl-3-methyl-imidazolium tetrafluoroborate ([AMIm]BF4), 1-hexyl-3-methylimidazolium tetrafluoroborate ([HMIm]BF4), N-ethylpyridinium iodide ([EPy]I), N-butylpyridinium trifluoromethanesulfonate ([BPy]CF3SO3), N-butyl-4-methyl-pyridinium tetrafluoroborate ([BMPy]BF4), N-hexylpyridinium bromide ([HPy]Br). 1-Heptanesulfonic acid sodium and 1-pentanesulfonic acid sodium were obtained from Tianjin Guangfu Fine Chemical Research Institute (Tianjin, China). Acetonitrile (LC grade) was obtained from Dikma Technologies. Citric acid was supplied by Shanghai Chemical Reagent Factory (Shanghai, China). Standard solutions of all cations were prepared in 18.2 MV cm deionized distilled water from a Millipore Milli-Q water purification system (Millipore, Bedford, MA, USA). The solutions were filtered using a 0.22 mm membrane filter before injection. Milli-Q water was also used to prepare mobile phase. Mobile phases were prepared with the desired amount of 1heptanesulfonic acid sodium, then citric acid was used to adjust the required pH (Model PHSF-3F, China). The mobile phases were filtered through a 0.22 mm filter and then degassed for 15 min with a Model DOA-P504-BN pump (IDEX, USA). All chromatographic experiments were carried out on an Agilent 1200 HPLC system (Agilent, USA), which consisted of a quaternary pump, a detector, an autosample injector, a column oven and a degasser system. The chromatographic system control, data acquisition and data analysis were performed using the Agilent Rev.B.04.01 workstation. All separations were performed on a Chromolith Performance RP-18e column (4.6 mm i.d. 100 mm, Merck, Germany). The mobile phase was (A) 1.0 mmol/L 1-heptanesulfonic acid sodium (pH 4.0 adjusted by citric acid)–(B) acetonitrile. The flow rate was set at 2.0 mL/min. Column temperature was 30 8C. The injection volume was 20 mL. UV detection (210 nm) was employed. 2. Results and discussion The influences of two ion-pair reagents on the retention of nine cations were discussed. Column temperature was 30 8C, and flow rate was 1.0 mL/min. By adopting 80% (v/v) 1.0 mmol/L 1-pentanesulfonic acid sodium or 1heptanesulfonic acid sodium (pH 4.0)–20% acetonitrile as mobile phase, the separation of nine cations (EMIm, PMIm, BMIm, AMIm, HMIm, EPy, BPy, BMPy, HPy) was performed. The hydrophobicity of 1-pentanesulfonic acid sodium is weaker than that of 1-heptanesulfonic acid sodium, and therefore the retention of imidazolium and pyridinium cations was weaker when using 1-pentanesulfonic acid sodium in the mobile phase than with 1-heptanesulfonic acid sodium. In order to increase the retention of imidazolium and pyridinium cations and optimize the separation, 1heptanesulfonic acid sodium was chosen as ion-pair reagent for this study. The concentrations of 1-heptanesulfonic acid sodium varied from 0.5 to 1.5 mmol/L in order to choose the optimal concentration. It was found that the retention factors of imidazolium and pyridinium cations totally enlarged with the increase of the concentration of 1-heptanesulfonic acid sodium. While the concentration was 0.5 mmol/L, the retention times of imidazolium and pyridinium cations were so short that the system peak disturbed separation. However, when the concentration was more than 1.5 mmol/L, the column could be polluted to some degree since the ion-pair reagent concentration was excessive. Consequently, 1.0 mmol/L of 1-heptanesulfonic acid sodium was the appropriate concentration. The effect of the acetonitrile concentration on the retention of nine cations was investigated using 1.0 mmol/L 1heptanesulfonic acid sodium–15, 20, 25 and 30% acetonitrile as mobile phase. The column temperature was 30 8C and flow rate was 1.0 mL/min. The retention factors of imidazolium and pyridinium cations decrease with the increase of the acetonitrile concentration. Among the cations, the decline magnitude of the longer alkyl-chain cations retention factor was significant. In LC, using acetonitrile as mobile phase, the relationship between retention factor (k) and the percentage of acetonitrile in the mobile phase (%ACN) is: log k = c%ACN + d [4], in which c is the slope of the line and d is the theoretical retention factor with no ACN added. The relationship curves were listed in Table 1. Known from the results, the log k vs %ACN of cations shows a better linear relationship. This indicates that the retentions of the nine cations are in line with reversed-phase mode using the monolithic column.
X. Huang et al. / Chinese Chemical Letters 23 (2012) 843–846
845
Table 1 Linear regression equation for the log k vs %ACN relationships obtained with the retention time of imidazolium and pyridinium cations. Analyte
Linear regression equation
Analyte
Linear regression equation
EPy PMIm BMIm AMIm HMIm
log k = log k = log k = log k = log k =
EMIm BPy BMPy HPy
log k = log k = log k = log k =
0.073%ACN + 1.028 (r = 0.9726) 0.057%ACN + 0.9276 (r = 0.9937) 0.066%ACN + 1.359 (r = 0.9952) 0.076%ACN + 1.905 (r = 0.9927) 0.081%ACN + 2.346 (r = 0.9986)
0.071%ACN + 1.036 0.065%ACN + 1.239 0.068%ACN + 1.504 0.072%ACN + 1.998
(r = 0.9757) (r = 0.9866) (r = 0.9930) (r = 0.9934)
Absorbance (mAU)
When flow rates were set to 1, 2, 3, 4 mL/min, the effect of flow rate on retention time of imidazolium and pyridinium cations was investigated. In the experiment, the mobile phase of 1.0 mmol/L 1-heptanesulfonic acid sodium (pH 4.0)–20% acetonitrile was used, and column temperature was 30 8C. Totally, the retention times of imidazolium and pyridinium cations were shortened with the increase of flow rate, and the column pressure was raised with the increase of flow rate. While flow rate was higher than 3 mL/min, the baseline was not steady, and the retention times of EMIm and EPy were close. So 2 mL/min were selected as the optimum flow rate. Under this flow rate, the baseline was smooth, and a better separation of the cations was achieved. In order to further optimize the separation, elution gradient was designed using the mobile phase of (A) 1.0 mmol/L 1-heptanesulfonic acid sodium (pH 4.0) and (B) acetonitrile. By optimizing, the elution gradient: 0–3.0 min, 7% B; 3.0–3.5 min, 7–18% B; 3.5–5.0 min, 18% B; 5.0–5.5 min, 18–25% B; 5.5–7.0 min, 25% B; 7.0–7.5 min, 25–30% B was chosen as the optimum gradient condition. The retention factors for homologous series and the carbon number follow the carbon number rule by the following relationship: log k = a nC + b [4] (k is the retention factor; nC is the carbon number of the alkyl chain). The drawing of retention factors of imidazolium cations and the carbon numbers of the imidazolium alkyl chain was obtained under the optimum gradient condition. The linear regression equation for the five cations EMIm, PMIm, BMIm, AMIm and HMIm is log k = 1.668 nC 1.784, r = 0.9937. At the same time, the drawing of retention factors of pyridinium cations and the carbon numbers of the pyridinium alkyl chain was obtained under the optimum gradient condition. The linear regression equation for the three cations EPy, BPy and HPy is log k = 1.665 nC 0.5967, r = 0.9945. Known from the linear regression equations, log k and nC have a good linear regression, so the retention of imidazolium and pyridinium cations accords with the carbon number rule under the gradient condition and the monolithic column. The chromatogram of a standard solution of nine cations by using the best chromatographic condition is show in Fig. 1. Detection limits (S/N = 3) were 0.11–0.62 mg/L, and noise of baseline was 0.1941 mAU. Relative standard deviations (RSD) of peak areas obtained by determining a standard solution of nine cations were 0.06–0.49%. The linear regression equations of calibration curves for nine cations were as follows: EPy, y = 8.06x 8.63; EMIm, y = 9.27x 10.81; PMIm, y = 8.76x 6.81; BPy, y = 7.40x 9.60; BMIm, y = 7.71x 3.62; BMPy, y = 6.47x 6.57; AMIm, y = 7.82x + 8.48; HPy, y = 9.97x 3.01; HMIm, y = 7.42x + 6.30. The linear ranges of calibration curves between peak area and the concentration for nine cations were 0.20–200 mg/L. This method was applied to the determination of cations in four kinds of ionic liquids synthesized by organic chemistry lab, namely [EMIm]BF4, [BMIm]Br, [BPy]CF3SO3, [BMPy]BF4. The [EMIm]BF4 of exactly quantified 800 600 400 1 2
200 0
1
2
3 4 Time (min)
3
4 5 6
5
7
6
8 9
7
8
Fig. 1. Chromatogram of a mixture solution of nine ionic liquid cations. Elution gradient: 0–3.0 min, 7% B; 3.0–3.5 min, 7–18% B; 3.5–5.0 min, 18% B; 5.0–5.5 min, 18–25% B; 5.5–7.0 min, 25% B; 7.0–7.5 min, 25–30% B. Eluent A, 1.0 mmol/L 1-heptanesulfonic acid sodium (pH 4.0 adjusted by citric acid); eluent B, acetonitrile; flow rate, 2.0 mL/min; column temperature, 30 8C. Peaks (mg/L): 1, EPy (50.0); 2, EMIm (40.0); 3, PMIm (60.0); 4, BPy (40.0); 5, BMIm (30.0); 6, BMPy (50.0); 7, AMIm (60.0); 8, HPy (60.0); 9, HMIm (80.0).
X. Huang et al. / Chinese Chemical Letters 23 (2012) 843–846
846
Table 2 Analytical results and recoveries of imidazolium and pyridinium cations found in ionic liquid samples. Ionic liquid
Ionic liquid cations
Original, ro (mg/L)
Added, rA (mg/L)
Found, rF (mg/L)
Recovery, R (%)
Content of cations in ionic liquids synthesized, w (%)
EMImBF4 BMImBF4 BPyCF3SO3 BMPyBF4
EMIm BMIm BPy BMPy
31.18 39.02 28 27.6
10 10 10 10
41.01 48.58 38.2 37.8
98.3 95.6 102.0 102.0
50.00 61.86 55.41 65.90
weight 0.1559 g, the [BMIm]Br of exactly quantified weight 0.1577 g, the [BPy]CFSO3 of exactly quantified weight 0.1263 g and the [BMPy]BF4 of exactly quantified weight 0.1047 g were diluted to 100 mL as stock solutions. Then the stock solutions of 2 mL [EMIm]BF4, 2 mL [BMIm]Br, 2 mL [BPy]CFSO3 and 2 mL [BMPy]BF4 were taken out and diluted to 50 mL. The diluents filtered through a 0.22 mm memberance filter were used for the determination of imidazolium and pyridinium cations with the selected chromatographic condition. The chromatograms are show in Fig. S2 (Supporting information). Recoveries were tested by the standard addition method. Analytical results and recoveries of imidazolium and pyridinium cations in the ionic liquids are listed in Table 2. The data from Table 2 prove the method to be accurate and reproducible. 3. Conclusion A practical method for rapid and simultaneous determining nine imidazolium and pyridinium ionic liquid cations was developed. The application of monolithic column can reduce the analysis time of the cations. The nine cations were baseline separated by a gradient elution procedure using 1-heptanesulfonic acid sodium and acetonitrile as mobile phase. In addition, the retention times of cations accord with carbon number rule. This method has been successfully applied to determine imidazolium and pyridinium cations in ionic liquids synthesized by organic chemistry lab, and the results are accurate, the reproducibility is satisfactory. Acknowledgments This work was supported by the Natural Science Foundation of Heilongjiang Province (No. B200909) and the Program for Scientific and Technological Innovation Team Construction in Universities of Heilongjiang Province (No. 2011TD010). Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/ j.cclet.2012.04.029. References [1] [2] [3] [4] [5] [6] [7] [8]
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