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Fast determination of tetrafluoroborate by high-performance liquid chromatography using a monolithic column
Journal of Chromatography A, 1206 (2008) 200–203
Contents lists available at ScienceDirect
Journal of Chromatography A journal homepage: www.elsevier.com/locate/chroma
Short communication
Fast determination of tetrafluoroborate by high-performance liquid chromatography using a monolithic column Shuang Zhou, Hong Yu ∗ , Ling Yang, Hongjing Ai College of Chemistry and Chemical Engineering, Harbin Normal University, Harbin 150025, China
a r t i c l e
i n f o
Article history: Received 23 May 2008 Received in revised form 12 August 2008 Accepted 14 August 2008 Available online 19 August 2008 Keywords: Monolithic High-performance liquid chromatography Tetrafluoroborate Conductivity detection Ionic liquids
a b s t r a c t Fast determination of tetrafluoroborate by high-performance liquid chromatography using a silica-based monolithic column and direct conductivity detection was carried out. Chromatographic separation was performed on a Chromolith Speed ROD RP-18e column (50 mm × 4.6 mm i.d.) with tetrabutylammonium hydroxide (TBA-OH) + phthalic acid as eluent. The effects of eluent concentration, eluent pH, column temperature and flow rate on retention time of tetrafluoroborate were investigated. The optimized chromatographic conditions for determination of tetrafluoroborate were using 0.5 mM TBA-OH + 0.31 mM phthalic acid (pH 5.5) as eluent, column temperature of 30 ◦ C and flow rate of 6.0 mL/min. Retention time of tetrafluoroborate was less than 1 min under the conditions. Common anions (Cl− , Br− , NO3 − and SO4 2− ) did not interfere with the determination of tetrafluoroborate. Detection limit (S/N = 2) for tetrafluoroborate was 1.4 mg/L. The linear range of calibration curve between peak area and the concentration of tetrafluoroborate was from 1.4 to 100.0 mg/L. The reproducibility was 0.09% and 1.8% (n = 5) relative standard deviation (RSD) for retention time and peak area, respectively. The method has been applied to the determination of tetrafluoroborate in ionic liquids. Recoveries of tetrafluoroborate after spiking were 98.2–101.5%. © 2008 Elsevier B.V. All rights reserved.
1. Introduction Tetrafluoroborate (BF4 − ) is one of the important anions which exist in ionic liquids. Recently, ionic liquids, the studies and applications of which are very extensive involving organic synthesis, catalysis, analytical chemistry and so on [1–3], have been considered to be environmentally friendly “green product”, and attract more and more attention in chemistry field. In the process of preparation and application of ionic liquids, the purity of the ionic liquids can be determined by the determination of BF4 − . With the development of industrialized ionic liquids in future, it is essential to develop relative methods used for monitoring and control. Tetrafluoroborate is also widely applied in electroplating; it has a great effect on the quality of the electroplating solution and plating layer. Moreover, fluoride pollution is caused by F− formed by slow hydrolysis of BF4 − when it was diluted at room temperature, so the determination of BF4 − in electroplating solution and industrial wastewater is necessary [4]. Ion-selective electrodes [5], flow injection ion-selective electrodes [4,6] and ion chromatography (IC) [7,8] are the main methods for the determination of BF4 − . Li et al. [7]
∗ Corresponding author. Tel.: +86 451 86321534. E-mail address: [email protected] (H. Yu). 0021-9673/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.chroma.2008.08.048
achieved the determination of BF4 − and other impurity anions (F− , Cl− and Br− ) in ionic liquids by ion-exchange chromatography with suppressed conductivity. Katagiri et al. [8] studied the total amount of boron, through converting boron to tetrafluoroborate which was determined by IC with suppressed conductivity detection. Villagran et al. [9] and Hao et al. [10] achieved the determination of halide anions in ionic liquids by IC, respectively. The retention time of BF4 − using conventional IC is usually more than 10 min. As a novel stationary phase, monolithic columns have been widely applied for high-performance liquid chromatography (HPLC), capillary electrochromatography (CEC) and so on [11,12], and they have attracted increasing concern in recent years. Due to its macroporous and mesoporous structure, good permeability and efficiency are obtained; it is very suitable for fast and high-throughput analysis. The main forms of monoliths are organic polymer monoliths and silica-based monoliths [11,13]. There are two methods to separate anions using silica-based monolithic column.
(1) Monoliths are modified with cationic surfactant to form ionexchange sites, and then ions are separated by ion-exchange high-performance liquid chromatography [14–17]. (2) Adding ion-pair reagent to mobile phase, ions are separated by ion-pair high-performance liquid chromatography [18–20].
S. Zhou et al. / J. Chromatogr. A 1206 (2008) 200–203
In this work, a silica-based monolithic column is employed to achieve fast determination of BF4 − by high-performance liquid chromatography with direct conductivity detection. Some factors influencing retention time of BF4 − are discussed. A method that employes a silica-based monolithic column to achieve fast determination of tetrafluoroborate by high-performance liquid chromatography is established and applied to achieve the analysis of BF4 − in ionic liquids. 2. Experimental 2.1. Equipments All experiments were carried out on a LC-20A ion chromatograph (Shimadzu, Japan), which consisted of a Model LP-20ADsp liquid delivery pump, a conductivity detector Model CDD-10Avp, an autosample injector Model SIL-20A, a Model CTO-20AC column oven and a system controller Model SCL-10Avp. The column and the conductivity detection cell were placed inside the CTO-20AC column oven for temperature control. The exact column temperature was controlled within 0.1 ◦ C using the column oven. The chromatographic system control, data acquisition and data analysis were performed using the LCsolution Ver 1.1 workstation (Shimadzu, Japan). A Model PHSF-3F pH meter (China) was used for pH measurement. A Millipore Milli-Q water purification system (Millipore, Bedford, MA, USA) was used to deionize distilled water, and the deionized water produced was prepared for eluents and sample solutions. A Model DOA-P504-BN pump (IDEX, USA) was used to degas eluents. 0.22 m membrane filter (Automatic science, China) was used to filter eluents and sample solutions. 2.2. Reagents For preparation of the eluent, tetrabutylammonium hydroxide (TBA-OH) was supplied by Tianjin Guangfu Fine Chemical Research Institute (China) as a 25% (w/w) solution in water, and phthalic acid was obtained from Shanghai Chemical Reagent Factory (China). Sodium tetrafluoroborate used to prepare standard solutions was supplied by J&K Chemical Ltd. (China). Chloride, bromide and sulfate used as their sodium salts were obtained from Shanghai Chemical Reagent Factory (China). Potassium nitrate was obtained from Shanghai Chemical Reagent Factory (China). All reagents were analytical grade or better. Acetonitrile (HPLC grade) was obtained from Tianjin Guangfu Fine Chemical Research Institute (China). Ionic liquids used as the samples were obtained from Shanghai Chengjie Chemical Ltd. (China). Eluents were prepared by titrating the desired amount of TBAOH with phthalic acid to a required pH. The eluents were filtered through a 0.22 m filter, and then degassed for 15 min before using. Standard solutions of inorganic anions were prepared in 18.2 M cm−1 water. Stock standard solutions of concentration 1000 mg/L were prepared monthly. Working standard solutions from each respective stock solution were prepared in the eluent on a daily basis as required. The solutions were filtered using 0.22 m membrane filter before injection.
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column was equilibrated for at least 30 min with water–acetonitrile (95:5, v/v) eluent, and then the column was flushed with the mobile phase to achieve stable base line before injection. In order to eliminate hydrophobic TBA-OH absorbed on the column, the column should be flushed using at least 30 mL water–acetonitrile (95:5, v/v) eluent after analysis everyday. In this way, the column could be recovered to the previous state. 3. Results and discussion 3.1. Effect of eluent concentration on the retention time of tetrafluoroborate In this experiment, flow rate was 1.0 mL/min, column temperature was 30 ◦ C, eluent used were different concentrations of TBA-OH and phthalic acid. The retention time of tetrafluoroborate varying with the eluent concentration was discussed. The TBA-OH eluents with the concentrations of 0.25, 0.5, 1.0, 1.5, 2.0, 3.0, 4.0 and 5.0 mM were prepared, and then the solutions of TBA-OH were titrated to pH 6.0 with phthalic acid, resulting in the corresponding concentrations of phthalic acid 0.13, 0.25, 0.52, 0.75, 0.99, 1.47, 1.98 and 2.47 mM, respectively. The relationship between the eluent concentration and the retention time of tetrafluoroborate is shown in Fig. 1. It shows that the retention time of tetrafluoroborate is prolonged at first and then shortened with the increase of the elution concentration. This phenomenon is the result of the combined effect of TBA-OH and phthalic acid. The retention mechanism of tetrafluoroborate is mainly anion-exchange. Firstly, TBA-OH in eluent is adsorbed on the surface of monolith to form a dynamic functional layer with anion-exchange groups, and then the analyte anions (e.g. BF4 − ) are retained onto stationary phase by anionexchange mechanism, and are eluted by phthalate anion. TBA-OH adsorbed on the column can be removed with water–acetonitrile (95:5, v/v) eluent after analysis, thus the column is regenerated. With increasing concentration of TBA-OH, the number of the sites of anion-exchange will increase, so the retention of anions will increase to some extent, as a result the retention time of anions is prolonged. However, since the concentration of phthalate anion is increased with the increment of the concentration of phthalic acid, it will reduce the retention time of anions. The experimental results showed that the background conductivity values of the eluent increased from 29 to 469 S/cm with TBA-OH concentration from
2.3. Chromatographic conditions All separations were performed on a Chromolith Speed ROD RP18e column which was bonded with C18 (50 mm × 4.6 mm i.d., Merk KGaA, Darmstadt, Germany). The optimized mobile phase for the rapid separation consisted of 0.5 mM TBA-OH + 0.31 mM phthalic acid (pH 5.5). The flow rate was set at 6.0 mL/min. Column temperature was 30 ◦ C. Inject volume was 20 L. Direct conductivity detection was employed. Prior to performing any separations, the
Fig. 1. Relationship curve between retention time of tetrafluoroborate and eluent concentration. Chromatographic conditions: Chromolith Speed ROD RP-18e column (50 mm × 4.6 mm i.d.); eluent of TBA-OH + phthalic acid (pH 6.0); 1.0 mL/min flow rate; column temperature of 30 ◦ C; 20 L inject volume; direct conductivity detection.
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0.25 to 5.0 mM, and the baseline noise values (0.03–0.04 S/cm) were basically unchanged. The low background conductivity of eluent is helpful to improve detection sensitivity of direct conductivity detection. The retention time of BF4 − was shorter and the background conductivity of eluent was lower with the TBA-OH concentration of 0.5 mM. Meanwhile, other common anions (Cl− , Br− , NO3 − and SO4 2− ) did not interfere with the determination of tetrafluoroborate. So the appropriate concentration of TBA-OH was 0.5 mM, corresponding to the phthalic acid concentration of 0.25 mM. 3.2. Effect of eluent pH on the retention time of tetrafluoroborate The suitable pH range is 2.0–7.5 for the silica-based monolithic column. Higher pHs will dissolve the silica, creating voids in the column. Lower pHs can eventually strip away some of the bonded phase. All these will cause changes in retention times and loss of resolution. The effect of eluent pH on the retention time of tetrafluoroborate was investigated using column temperature of 30 ◦ C and flow rate of 1.0 mL/min. Eluent pHs were changed through adding phthalic acid to the solution of 0.5 mM TBA-OH. When pH was lower than 4.5 or higher than 6.5, the baseline was unstable, and the system peak was formed, bring interference to the determination of analyte anions. When the pH varied from 4.5 to 6.5, the retention time of BF4 − was prolonged with the increase of pH, and both the background conductivity values of eluent (52–58 S/cm) and baseline noise values (0.03–0.04 S/cm) showed little change. Because the retention time of tetrafluoroborate was shorter, and both the other anions and system peak had no interference on the determination of tetrafluoroborate, the optimum pH was 5.5. 3.3. Effect of column temperature on the retention time of tetrafluoroborate Eluent was 0.50 mM TBA-OH + 0.31 mM phthalic acid (pH 5.5) and flow rate was 6.0 mL/min in this research. When column temperatures were 25, 30, 35 and 40 ◦ C, the change of retention time of tetrafluoroborate was investigated. The results showed that the retention time of tetrafluoroborate was shortened slightly with increasing column temperature, so column temperature had little effect on the retention of tetrafluoroborate. 30 ◦ C of column temperature was appropriate.
Fig. 2. Fast separation of a standard mixture of tetrafluoroborate and four common anions by high-performance liquid chromatography with a monolithic column. Peaks: (1) 5.0 mg/L Cl− ; (2) 10.0 mg/L Br− ; (3) 5.0 mg/L NO3 − ; (4) 10.0 mg/L BF4 − ; (5) 5.0 mg/L SO4 2− ; (sp) system peak. Chromatographic conditions: Chromolith Speed ROD RP-18e column (50 mm × 4.6 mm i.d.); eluent of 0.5 mM TBA-OH + 0.31 mM phthalic acid (pH 5.5); 6.0 mL/min flow rate; column temperature of 30 ◦ C; 20 L inject volume; direct conductivity detection.
est, and the baseline is stable. Column back-pressure is raised with the increase of flow rate, however, at the flow rate of 6.0 mL/min or even higher, the pressure is still lower than conventional HPLC operating pressure. Column efficiency shows little change with the increase of the flow rate. The results show that monolithic columns employed to achieve the determination of tetrafluoroborate by high-performance liquid chromatography have the advantages of rapid and high efficiency. By combining above factors, the optimum chromatographic conditions for the determination of tetrafluoroborate were 0.5 mM TBA-OH + 0.31 mM phthalic acid (pH 5.5) as eluent, column temperature of 30 ◦ C, flow rate of 6.0 mL/min. At the chromatographic condition, the chromatogram of a standard mixture of tetrafluoroborate and other common anions (Cl− , Br− , NO3 − and SO4 2− ) was obtained in Fig. 2. The retention time of tetrafluoroborate was less than 1 min, and the other common anions had no interference with the determination. Compared with the analysis of BF4 − by IC [7–10] (retention time of BF4 − was usually 10–20 min), a large amount of analysis time was saved by this method. 3.5. Quantitative parameters Detection limit, calibration curve and precision were obtained by determining a series of standard solutions of BF4 − under the optimized chromatographic conditions. Calculating as twice the signal-to-noise ratio (S/N = 2), detection limit of tetrafluoroborate
3.4. Effect of flow rate on the retention time of tetrafluoroborate, column back-pressure and column efficiency In investigating the effect of flow rate, eluent was 0.5 mM TBAOH + 0.31 mM phthalic acid (pH 5.5), column temperature was 30 ◦ C, flow rate was changed from 1.0 to 6.0 mL/min. Table 1 lists the change of retention time, column back-pressure and column efficiency with change of flow rate. It shows that the retention time of tetrafluoroborate is obviously shortened with the increase of flow rate. At the flow rate of 6.0 mL/min, separation time is the short-
Table 1 Effect of flow rate on retention time of tetrafluoroborate, column back-pressure and column efficiency Parameter
Retention time (min) Column back-pressure (MPa) Plate height (m)
Flow rate (mL/min) 1
2
3
4
5
6
4.76 3.1 30.1
2.20 4.3 27.9
1.47 5.4 28.4
1.11 6.5 28.7
0.88 7.5 28.9
0.76 8.7 29.2
Fig. 3. Chromatograms of ionic liquids and standard solution obtained using a monolithic column. (a) Standard solution. (b) 1-Propyl-3-methylimidazolium tetrafluoroborate ionic liquid. (c) 1-Hexyl-3-methylimidazolium tetrafluoroborate ionic liquid. Peaks: (1) Cl− ; (2) Br− ; (3) NO3 − ; (4) BF4 − ; (5) SO4 2− ; (sp) system peak. Chromatographic conditions as in Fig. 2.
S. Zhou et al. / J. Chromatogr. A 1206 (2008) 200–203
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Table 2 Analytical results and recoveries of tetrafluoroborate found in ionic liquids Ionic liquid
Original (o /mg L−1 )
Added (A /mg L−1 )
Found (F /mg L−1 )
Recovery (R/%)
Content in ionic liquids (w/%)
RSD (%, n = 5)
PMIM BF4 BMIM BF4 AMIM BF4 HMIM BF4
17.84 25.06 22.60 20.92
10.0 10.0 10.0 10.0
27.99 35.19 32.42 30.8
101.5 101.3 98.2 98.8
26.06 38.2 35.71 34.09
2.2 2.4 2.0 2.5
PMIM BF4 : 1-propyl-3-methylimidazolium tetrafluoroborate; BMIM BF4 : 1-butyl-3-methylimidazolium tetrafluoroborate; AMIM BF4 : 1-amyl-3-methylimidazolium tetrafluoroborate; HMIM BF4 : 1-hexyl-3-methylimidazolium tetrafluoroborate.
was 1.4 mg/L. Relative standard deviation (RSD) of chromatographic peak area and retention time obtained by repeated measurement of a standard sample of 10.0 mg/L BF4 − was 1.8% and 0.09% (n = 5), respectively. The resolution (R) of BF4 − and SO4 2− was 4.8. The linear range of calibration curve between peak area and the concentration of tetrafluoroborate was from 1.4 to 100.0 mg/L. Linear regression equation as follows: y = 120.6x − 113.3 where y is integral value of peak area, x is the concentration of BF4 − expressed in mg/L. Correlation coefficient (r) is 0.9998 (n = 5). 3.6. Analysis of sample The proposed method was applied to the determination of BF4 − in four kinds of ionic liquids, namely 1-propyl-3methylimidazolium tetrafluoroborate, 1-butyl-3-methylimidazolium tetrafluoroborate, 1-amyl-3-methylimidazolium tetrafluoroborate and 1-hexyl-3-methylimidazolium tetrafluoroborate. The ionic liquids of exactly quantified weights (0.1–0.2 g) were diluted to 100 mL with the eluent as stock solutions. Then the stock solutions of 1 mL were taken out and diluted to 25 mL. The diluents filtered through 0.22 m membrane filter were used for the determination of tetrafluoroborate with the selected chromatographic conditions. The chromatograms of ionic liquids are shown in Fig. 3. There existed Br− in these ionic liquids besides BF4 − . Especially, there were lots of Br− in the ionic liquid of 1-propyl-3-methylimidazolium tetrafluoroborate. Recoveries
were tested by standard addition method. Analytical results and recoveries of tetrafluoroborate in ionic liquids are listed in Table 2. Data in Table 2 are the average values of five mensurations. Relative standard deviation (RSD) of analytical results is less than 2.5%. Recoveries of tetrafluoroborate after spiking are 98.2–101.5%. The results indicate that this method has the advantages of high accuracy and good precision, and it has been successfully applied to achieve quantitative analysis of tetrafluoroborate in ionic liquids. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20]
T.L. Greaves, C.J. Drummond, Chem. Rev. 108 (2008) 206. S.A. Shamsi, N.D. Danielson, J. Sep. Sci. 30 (2007) 1729. A. Berthod, M.J. Ruiz-Ángel, S. Carda-Broch, J. Chromatogr. A 1184 (2008) 6. A.N. Araújo, M.B. Etxebarria, J.L.F.C. Lima, M.C.B.S.M. Montenegro, R. Pérez Olmos, Anal. Chim. Acta 293 (1994) 35. F. Kluger, C. Koeberl, Anal. Chim. Acta 175 (1985) 127. T. Imato, T. Yoshizuka, N. Ishibashi, Anal. Chim. Acta 233 (1990) 139. X.H. Li, H.L. Duan, J.T. Pan, L.F. Wang, Chin. J. Anal. Chem. 34 (2006) S192. J. Katagiri, T. Yoshioka, T. Mizoguchi, Anal. Chim. Acta 570 (2006) 65. C. Villagran, M. Deetlefs, W.R. Pitner, C. Hardacre, Anal. Chem. 76 (2004) 2118. F. Hao, P.R. Haddad, T. Ruther, Chromatographia 67 (2008) 495. G. Guiochon, J. Chromatogr. A 1168 (2007) 101. R.A. Wu, L.H. Hu, F.J. Wang, M.L. Ye, H.F. Zou, J. Chromatogr. A 1184 (2008) 369. O. Nunej, K. Nakanishi, N. Tanaka, J. Chromatogr. A 1191 (2008) 231. P. Hatsis, C.A. Lucy, Anal. Chem. 75 (2003) 995. B. Paull, P.N. Nesterenko, Trends Anal. Chem. 24 (2005) 295. D. Schaller, E.F. Hilder, P.R. Haddad, J. Sep. Sci. 29 (2006) 1705. S.D. Chambers, K.M. Glenn, C.A. Lucy, J. Sep. Sci. 30 (2007) 1628. P. Hatsis, C.A. Lucy, Analyst 127 (2002) 451. R.S. Li, H. Yu, H.J. Ai, J. Anal. Sci. 24 (2008) 6. H. Yu, R.S. Li, Chin. J. Anal. Chem. 36 (2008) 835.
Contents lists available at ScienceDirect
Journal of Chromatography A journal homepage: www.elsevier.com/locate/chroma
Short communication
Fast determination of tetrafluoroborate by high-performance liquid chromatography using a monolithic column Shuang Zhou, Hong Yu ∗ , Ling Yang, Hongjing Ai College of Chemistry and Chemical Engineering, Harbin Normal University, Harbin 150025, China
a r t i c l e
i n f o
Article history: Received 23 May 2008 Received in revised form 12 August 2008 Accepted 14 August 2008 Available online 19 August 2008 Keywords: Monolithic High-performance liquid chromatography Tetrafluoroborate Conductivity detection Ionic liquids
a b s t r a c t Fast determination of tetrafluoroborate by high-performance liquid chromatography using a silica-based monolithic column and direct conductivity detection was carried out. Chromatographic separation was performed on a Chromolith Speed ROD RP-18e column (50 mm × 4.6 mm i.d.) with tetrabutylammonium hydroxide (TBA-OH) + phthalic acid as eluent. The effects of eluent concentration, eluent pH, column temperature and flow rate on retention time of tetrafluoroborate were investigated. The optimized chromatographic conditions for determination of tetrafluoroborate were using 0.5 mM TBA-OH + 0.31 mM phthalic acid (pH 5.5) as eluent, column temperature of 30 ◦ C and flow rate of 6.0 mL/min. Retention time of tetrafluoroborate was less than 1 min under the conditions. Common anions (Cl− , Br− , NO3 − and SO4 2− ) did not interfere with the determination of tetrafluoroborate. Detection limit (S/N = 2) for tetrafluoroborate was 1.4 mg/L. The linear range of calibration curve between peak area and the concentration of tetrafluoroborate was from 1.4 to 100.0 mg/L. The reproducibility was 0.09% and 1.8% (n = 5) relative standard deviation (RSD) for retention time and peak area, respectively. The method has been applied to the determination of tetrafluoroborate in ionic liquids. Recoveries of tetrafluoroborate after spiking were 98.2–101.5%. © 2008 Elsevier B.V. All rights reserved.
1. Introduction Tetrafluoroborate (BF4 − ) is one of the important anions which exist in ionic liquids. Recently, ionic liquids, the studies and applications of which are very extensive involving organic synthesis, catalysis, analytical chemistry and so on [1–3], have been considered to be environmentally friendly “green product”, and attract more and more attention in chemistry field. In the process of preparation and application of ionic liquids, the purity of the ionic liquids can be determined by the determination of BF4 − . With the development of industrialized ionic liquids in future, it is essential to develop relative methods used for monitoring and control. Tetrafluoroborate is also widely applied in electroplating; it has a great effect on the quality of the electroplating solution and plating layer. Moreover, fluoride pollution is caused by F− formed by slow hydrolysis of BF4 − when it was diluted at room temperature, so the determination of BF4 − in electroplating solution and industrial wastewater is necessary [4]. Ion-selective electrodes [5], flow injection ion-selective electrodes [4,6] and ion chromatography (IC) [7,8] are the main methods for the determination of BF4 − . Li et al. [7]
∗ Corresponding author. Tel.: +86 451 86321534. E-mail address: [email protected] (H. Yu). 0021-9673/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.chroma.2008.08.048
achieved the determination of BF4 − and other impurity anions (F− , Cl− and Br− ) in ionic liquids by ion-exchange chromatography with suppressed conductivity. Katagiri et al. [8] studied the total amount of boron, through converting boron to tetrafluoroborate which was determined by IC with suppressed conductivity detection. Villagran et al. [9] and Hao et al. [10] achieved the determination of halide anions in ionic liquids by IC, respectively. The retention time of BF4 − using conventional IC is usually more than 10 min. As a novel stationary phase, monolithic columns have been widely applied for high-performance liquid chromatography (HPLC), capillary electrochromatography (CEC) and so on [11,12], and they have attracted increasing concern in recent years. Due to its macroporous and mesoporous structure, good permeability and efficiency are obtained; it is very suitable for fast and high-throughput analysis. The main forms of monoliths are organic polymer monoliths and silica-based monoliths [11,13]. There are two methods to separate anions using silica-based monolithic column.
(1) Monoliths are modified with cationic surfactant to form ionexchange sites, and then ions are separated by ion-exchange high-performance liquid chromatography [14–17]. (2) Adding ion-pair reagent to mobile phase, ions are separated by ion-pair high-performance liquid chromatography [18–20].
S. Zhou et al. / J. Chromatogr. A 1206 (2008) 200–203
In this work, a silica-based monolithic column is employed to achieve fast determination of BF4 − by high-performance liquid chromatography with direct conductivity detection. Some factors influencing retention time of BF4 − are discussed. A method that employes a silica-based monolithic column to achieve fast determination of tetrafluoroborate by high-performance liquid chromatography is established and applied to achieve the analysis of BF4 − in ionic liquids. 2. Experimental 2.1. Equipments All experiments were carried out on a LC-20A ion chromatograph (Shimadzu, Japan), which consisted of a Model LP-20ADsp liquid delivery pump, a conductivity detector Model CDD-10Avp, an autosample injector Model SIL-20A, a Model CTO-20AC column oven and a system controller Model SCL-10Avp. The column and the conductivity detection cell were placed inside the CTO-20AC column oven for temperature control. The exact column temperature was controlled within 0.1 ◦ C using the column oven. The chromatographic system control, data acquisition and data analysis were performed using the LCsolution Ver 1.1 workstation (Shimadzu, Japan). A Model PHSF-3F pH meter (China) was used for pH measurement. A Millipore Milli-Q water purification system (Millipore, Bedford, MA, USA) was used to deionize distilled water, and the deionized water produced was prepared for eluents and sample solutions. A Model DOA-P504-BN pump (IDEX, USA) was used to degas eluents. 0.22 m membrane filter (Automatic science, China) was used to filter eluents and sample solutions. 2.2. Reagents For preparation of the eluent, tetrabutylammonium hydroxide (TBA-OH) was supplied by Tianjin Guangfu Fine Chemical Research Institute (China) as a 25% (w/w) solution in water, and phthalic acid was obtained from Shanghai Chemical Reagent Factory (China). Sodium tetrafluoroborate used to prepare standard solutions was supplied by J&K Chemical Ltd. (China). Chloride, bromide and sulfate used as their sodium salts were obtained from Shanghai Chemical Reagent Factory (China). Potassium nitrate was obtained from Shanghai Chemical Reagent Factory (China). All reagents were analytical grade or better. Acetonitrile (HPLC grade) was obtained from Tianjin Guangfu Fine Chemical Research Institute (China). Ionic liquids used as the samples were obtained from Shanghai Chengjie Chemical Ltd. (China). Eluents were prepared by titrating the desired amount of TBAOH with phthalic acid to a required pH. The eluents were filtered through a 0.22 m filter, and then degassed for 15 min before using. Standard solutions of inorganic anions were prepared in 18.2 M cm−1 water. Stock standard solutions of concentration 1000 mg/L were prepared monthly. Working standard solutions from each respective stock solution were prepared in the eluent on a daily basis as required. The solutions were filtered using 0.22 m membrane filter before injection.
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column was equilibrated for at least 30 min with water–acetonitrile (95:5, v/v) eluent, and then the column was flushed with the mobile phase to achieve stable base line before injection. In order to eliminate hydrophobic TBA-OH absorbed on the column, the column should be flushed using at least 30 mL water–acetonitrile (95:5, v/v) eluent after analysis everyday. In this way, the column could be recovered to the previous state. 3. Results and discussion 3.1. Effect of eluent concentration on the retention time of tetrafluoroborate In this experiment, flow rate was 1.0 mL/min, column temperature was 30 ◦ C, eluent used were different concentrations of TBA-OH and phthalic acid. The retention time of tetrafluoroborate varying with the eluent concentration was discussed. The TBA-OH eluents with the concentrations of 0.25, 0.5, 1.0, 1.5, 2.0, 3.0, 4.0 and 5.0 mM were prepared, and then the solutions of TBA-OH were titrated to pH 6.0 with phthalic acid, resulting in the corresponding concentrations of phthalic acid 0.13, 0.25, 0.52, 0.75, 0.99, 1.47, 1.98 and 2.47 mM, respectively. The relationship between the eluent concentration and the retention time of tetrafluoroborate is shown in Fig. 1. It shows that the retention time of tetrafluoroborate is prolonged at first and then shortened with the increase of the elution concentration. This phenomenon is the result of the combined effect of TBA-OH and phthalic acid. The retention mechanism of tetrafluoroborate is mainly anion-exchange. Firstly, TBA-OH in eluent is adsorbed on the surface of monolith to form a dynamic functional layer with anion-exchange groups, and then the analyte anions (e.g. BF4 − ) are retained onto stationary phase by anionexchange mechanism, and are eluted by phthalate anion. TBA-OH adsorbed on the column can be removed with water–acetonitrile (95:5, v/v) eluent after analysis, thus the column is regenerated. With increasing concentration of TBA-OH, the number of the sites of anion-exchange will increase, so the retention of anions will increase to some extent, as a result the retention time of anions is prolonged. However, since the concentration of phthalate anion is increased with the increment of the concentration of phthalic acid, it will reduce the retention time of anions. The experimental results showed that the background conductivity values of the eluent increased from 29 to 469 S/cm with TBA-OH concentration from
2.3. Chromatographic conditions All separations were performed on a Chromolith Speed ROD RP18e column which was bonded with C18 (50 mm × 4.6 mm i.d., Merk KGaA, Darmstadt, Germany). The optimized mobile phase for the rapid separation consisted of 0.5 mM TBA-OH + 0.31 mM phthalic acid (pH 5.5). The flow rate was set at 6.0 mL/min. Column temperature was 30 ◦ C. Inject volume was 20 L. Direct conductivity detection was employed. Prior to performing any separations, the
Fig. 1. Relationship curve between retention time of tetrafluoroborate and eluent concentration. Chromatographic conditions: Chromolith Speed ROD RP-18e column (50 mm × 4.6 mm i.d.); eluent of TBA-OH + phthalic acid (pH 6.0); 1.0 mL/min flow rate; column temperature of 30 ◦ C; 20 L inject volume; direct conductivity detection.
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0.25 to 5.0 mM, and the baseline noise values (0.03–0.04 S/cm) were basically unchanged. The low background conductivity of eluent is helpful to improve detection sensitivity of direct conductivity detection. The retention time of BF4 − was shorter and the background conductivity of eluent was lower with the TBA-OH concentration of 0.5 mM. Meanwhile, other common anions (Cl− , Br− , NO3 − and SO4 2− ) did not interfere with the determination of tetrafluoroborate. So the appropriate concentration of TBA-OH was 0.5 mM, corresponding to the phthalic acid concentration of 0.25 mM. 3.2. Effect of eluent pH on the retention time of tetrafluoroborate The suitable pH range is 2.0–7.5 for the silica-based monolithic column. Higher pHs will dissolve the silica, creating voids in the column. Lower pHs can eventually strip away some of the bonded phase. All these will cause changes in retention times and loss of resolution. The effect of eluent pH on the retention time of tetrafluoroborate was investigated using column temperature of 30 ◦ C and flow rate of 1.0 mL/min. Eluent pHs were changed through adding phthalic acid to the solution of 0.5 mM TBA-OH. When pH was lower than 4.5 or higher than 6.5, the baseline was unstable, and the system peak was formed, bring interference to the determination of analyte anions. When the pH varied from 4.5 to 6.5, the retention time of BF4 − was prolonged with the increase of pH, and both the background conductivity values of eluent (52–58 S/cm) and baseline noise values (0.03–0.04 S/cm) showed little change. Because the retention time of tetrafluoroborate was shorter, and both the other anions and system peak had no interference on the determination of tetrafluoroborate, the optimum pH was 5.5. 3.3. Effect of column temperature on the retention time of tetrafluoroborate Eluent was 0.50 mM TBA-OH + 0.31 mM phthalic acid (pH 5.5) and flow rate was 6.0 mL/min in this research. When column temperatures were 25, 30, 35 and 40 ◦ C, the change of retention time of tetrafluoroborate was investigated. The results showed that the retention time of tetrafluoroborate was shortened slightly with increasing column temperature, so column temperature had little effect on the retention of tetrafluoroborate. 30 ◦ C of column temperature was appropriate.
Fig. 2. Fast separation of a standard mixture of tetrafluoroborate and four common anions by high-performance liquid chromatography with a monolithic column. Peaks: (1) 5.0 mg/L Cl− ; (2) 10.0 mg/L Br− ; (3) 5.0 mg/L NO3 − ; (4) 10.0 mg/L BF4 − ; (5) 5.0 mg/L SO4 2− ; (sp) system peak. Chromatographic conditions: Chromolith Speed ROD RP-18e column (50 mm × 4.6 mm i.d.); eluent of 0.5 mM TBA-OH + 0.31 mM phthalic acid (pH 5.5); 6.0 mL/min flow rate; column temperature of 30 ◦ C; 20 L inject volume; direct conductivity detection.
est, and the baseline is stable. Column back-pressure is raised with the increase of flow rate, however, at the flow rate of 6.0 mL/min or even higher, the pressure is still lower than conventional HPLC operating pressure. Column efficiency shows little change with the increase of the flow rate. The results show that monolithic columns employed to achieve the determination of tetrafluoroborate by high-performance liquid chromatography have the advantages of rapid and high efficiency. By combining above factors, the optimum chromatographic conditions for the determination of tetrafluoroborate were 0.5 mM TBA-OH + 0.31 mM phthalic acid (pH 5.5) as eluent, column temperature of 30 ◦ C, flow rate of 6.0 mL/min. At the chromatographic condition, the chromatogram of a standard mixture of tetrafluoroborate and other common anions (Cl− , Br− , NO3 − and SO4 2− ) was obtained in Fig. 2. The retention time of tetrafluoroborate was less than 1 min, and the other common anions had no interference with the determination. Compared with the analysis of BF4 − by IC [7–10] (retention time of BF4 − was usually 10–20 min), a large amount of analysis time was saved by this method. 3.5. Quantitative parameters Detection limit, calibration curve and precision were obtained by determining a series of standard solutions of BF4 − under the optimized chromatographic conditions. Calculating as twice the signal-to-noise ratio (S/N = 2), detection limit of tetrafluoroborate
3.4. Effect of flow rate on the retention time of tetrafluoroborate, column back-pressure and column efficiency In investigating the effect of flow rate, eluent was 0.5 mM TBAOH + 0.31 mM phthalic acid (pH 5.5), column temperature was 30 ◦ C, flow rate was changed from 1.0 to 6.0 mL/min. Table 1 lists the change of retention time, column back-pressure and column efficiency with change of flow rate. It shows that the retention time of tetrafluoroborate is obviously shortened with the increase of flow rate. At the flow rate of 6.0 mL/min, separation time is the short-
Table 1 Effect of flow rate on retention time of tetrafluoroborate, column back-pressure and column efficiency Parameter
Retention time (min) Column back-pressure (MPa) Plate height (m)
Flow rate (mL/min) 1
2
3
4
5
6
4.76 3.1 30.1
2.20 4.3 27.9
1.47 5.4 28.4
1.11 6.5 28.7
0.88 7.5 28.9
0.76 8.7 29.2
Fig. 3. Chromatograms of ionic liquids and standard solution obtained using a monolithic column. (a) Standard solution. (b) 1-Propyl-3-methylimidazolium tetrafluoroborate ionic liquid. (c) 1-Hexyl-3-methylimidazolium tetrafluoroborate ionic liquid. Peaks: (1) Cl− ; (2) Br− ; (3) NO3 − ; (4) BF4 − ; (5) SO4 2− ; (sp) system peak. Chromatographic conditions as in Fig. 2.
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Table 2 Analytical results and recoveries of tetrafluoroborate found in ionic liquids Ionic liquid
Original (o /mg L−1 )
Added (A /mg L−1 )
Found (F /mg L−1 )
Recovery (R/%)
Content in ionic liquids (w/%)
RSD (%, n = 5)
PMIM BF4 BMIM BF4 AMIM BF4 HMIM BF4
17.84 25.06 22.60 20.92
10.0 10.0 10.0 10.0
27.99 35.19 32.42 30.8
101.5 101.3 98.2 98.8
26.06 38.2 35.71 34.09
2.2 2.4 2.0 2.5
PMIM BF4 : 1-propyl-3-methylimidazolium tetrafluoroborate; BMIM BF4 : 1-butyl-3-methylimidazolium tetrafluoroborate; AMIM BF4 : 1-amyl-3-methylimidazolium tetrafluoroborate; HMIM BF4 : 1-hexyl-3-methylimidazolium tetrafluoroborate.
was 1.4 mg/L. Relative standard deviation (RSD) of chromatographic peak area and retention time obtained by repeated measurement of a standard sample of 10.0 mg/L BF4 − was 1.8% and 0.09% (n = 5), respectively. The resolution (R) of BF4 − and SO4 2− was 4.8. The linear range of calibration curve between peak area and the concentration of tetrafluoroborate was from 1.4 to 100.0 mg/L. Linear regression equation as follows: y = 120.6x − 113.3 where y is integral value of peak area, x is the concentration of BF4 − expressed in mg/L. Correlation coefficient (r) is 0.9998 (n = 5). 3.6. Analysis of sample The proposed method was applied to the determination of BF4 − in four kinds of ionic liquids, namely 1-propyl-3methylimidazolium tetrafluoroborate, 1-butyl-3-methylimidazolium tetrafluoroborate, 1-amyl-3-methylimidazolium tetrafluoroborate and 1-hexyl-3-methylimidazolium tetrafluoroborate. The ionic liquids of exactly quantified weights (0.1–0.2 g) were diluted to 100 mL with the eluent as stock solutions. Then the stock solutions of 1 mL were taken out and diluted to 25 mL. The diluents filtered through 0.22 m membrane filter were used for the determination of tetrafluoroborate with the selected chromatographic conditions. The chromatograms of ionic liquids are shown in Fig. 3. There existed Br− in these ionic liquids besides BF4 − . Especially, there were lots of Br− in the ionic liquid of 1-propyl-3-methylimidazolium tetrafluoroborate. Recoveries
were tested by standard addition method. Analytical results and recoveries of tetrafluoroborate in ionic liquids are listed in Table 2. Data in Table 2 are the average values of five mensurations. Relative standard deviation (RSD) of analytical results is less than 2.5%. Recoveries of tetrafluoroborate after spiking are 98.2–101.5%. The results indicate that this method has the advantages of high accuracy and good precision, and it has been successfully applied to achieve quantitative analysis of tetrafluoroborate in ionic liquids. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20]
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