- Email: [email protected]
Determination of piperidinium ionic liquid cations in environmental water samples by solid phase extraction and hydrophilic interaction liquid chromatography
Accepted Manuscript Title: Determination of piperidinium ionic liquid cations in environmental water samples by solid phase extraction and hydrophilic interaction liquid chromatography Authors: Zi-qiang Fan, Hong Yu PII: DOI: Reference:
S0021-9673(17)30704-5 http://dx.doi.org/doi:10.1016/j.chroma.2017.05.015 CHROMA 358512
To appear in:
Journal of Chromatography A
Received date: Revised date: Accepted date:
25-1-2017 4-5-2017 5-5-2017
Please cite this article as: Zi-qiang Fan, Hong Yu, Determination of piperidinium ionic liquid cations in environmental water samples by solid phase extraction and hydrophilic interaction liquid chromatography, Journal of Chromatography Ahttp://dx.doi.org/10.1016/j.chroma.2017.05.015 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Determination of piperidinium ionic liquid cations in environmental water samples by solid phase extraction and hydrophilic interaction liquid chromatography Zi-qiang Fan, Hong Yu*
College of Chemistry and Chemical Engineering, Harbin Normal University, Harbin 150025, China
E-mail: [email protected] Highlights
Determination of piperidinium ionic liquid cations in environmental water
Effective enrichment and purification of the sample by solid phase extraction
Hydrophilic interaction liquid chromatography of indirect ultraviolet detection
Imidazolium ionic liquids as background ultraviolet absorbents and eluting agents
Abstract: This paper presents a novel analytical method for the determination of piperidinium ionic liquid cations in environmental water by hydrophilic interaction liquid chromatography and solid-phase extraction technology. The left standing, centrifuged and filtered river water samples were first purified and concentrated through the C18 solid phase extraction column, and eluted with 0.02 mol/L hydrochloric acid prepared in methanol and deionized water (80/20, v/v). Then the eluents were analyzed by a hydrophilic column combined with 0.8 mmol/L 1-propyl-3-methyl imidazolium tetrafluoroborate aqueous solution/acetonitrile (40/60, v/v) as the mobile phase and indirect ultraviolet detection. The detection limits of piperidinium cations were less than 0.4 mg/L. The relative standard deviations were less than 0.6%. The method has been successfully applied to the determination of piperidinium cations in Songhua River water 1
samples. Recoveries were 80.0%–98.3%. This research may provide a reference for studying the environmental effect of ionic liquids.
Keywords: solid phase extraction; hydrophilic interaction liquid chromatography; ionic liquids; piperidinium cations; indirect ultraviolet detection
1. Introduction
Ionic liquids (ILs) have characteristics of non-volatility and strong dissolving ability etc, thus they become the research hotspots in many fields [1-4]. However, some literatures report the biological effect of ILs and indicate that the toxicity increases as increasing the length of the alkyl chain in the ionic liquid cations [5]. Therefore, the detection of ILs in the environment is important. In recent years, the determination of ionic liquid cations [6-10] has been more reported but there are very few reports [11, 12] on the analysis of ionic liquid cations in the environment. Due to the complexity of environmental samples, sample pretreatment techniques are important in the analysis of environmental samples. Solid phase extraction (SPE) is mainly used for sample separation, purification and concentration and it is widely used in environment and other fields [13]. Therefore, SPE was used in this study.
The piperidinium ionic liquids are important ionic liquids and have wide ranges of applications. Some previous works mainly investigate the separation of piperidinium cations in the ion pair (IP) separation mode and they do not analyze piperidinium cations in environmental water [14, 15]. In the work, we use a new hydrophilic interaction liquid chromatography (HILIC) and indirect ultraviolet (IUV) detection [16] for the determination of
2
piperidinium cations in environmental water. Compared with the IP chromatography, HILIC is simpler for the analysis because of the avoidance of IP reagents. Moreover, the SPE can make the water sample be fully purified and enriched. This research may provide an analytical method of piperidinium cations in environmental water and provide the appropriate support for studying the environmental effect of ILs.
2. Experimental
2.1. Instrumentations
Sample pretreatment was carried out on an ASE-24 SPE apparatus (Tianjin Automatic Science Instrument Ltd.). The chromatographic analysis was carried out on a LC-20A liquid chromatograph (Shimadzu, Japan). UF-C18 SPE columns (250 mg/3mL, Dalian Zhongpu Technology Ltd.) were used.
2.2. Reagents
The ILs (99% purity) were N-methyl-N-ethyl piperidinium bromide ([MEPi][Br]), Nmethyl-N-propyl piperidinium bromide ([MPPi][Br]), N-methyl-N-butyl piperidinium bromide ([MBPi][Br]), 1-ethyl-3-methyl imidazolium tetrafluoroborate ([EMIm][BF4]), 1-propyl-3methyl imidazolium tetrafluoroborate ([PMIm][BF4]), 1-butyl-3-methyl imidazolium tetrafluoroborate ([BMIm][BF4]), 1-amyl-3-methyl imidazolium tetrafluoroborate ([AMIm][BF4]) and 1-hexyl-3-methyl imidazolium tetrafluoroborate ([HMIm][BF4]) purchased from Shanghai Chengjie Chemical Ltd. (Shanghai, China). Methanol, acetonitrile and sodium heptanesulfonate (Chromatographic purity) were obtained from Dikma Technologies (Shanghai, China). 3
2.3. Pretreatment of environmental water samples
The water samples were from the Songhua River of Harbin and the pH was 7.38. They were left standing, centrifuged and filtered through a 0.22 μm filter, and then sodium heptanesulfonate as the IP reagent was added to the sample at a concentration of 0.5 mmol/L. Prior to using, the UF-C18 SPE column was activated with 5 mL of methanol, equilibrated by 5 mL of deionized water and dried under vacuum. Then 30 mL of the pretreated water sample was passed through the column at a flow rate of 2 mL/min and the column was washed with 3 mL of 0.5 mmol/L sodium heptanesulfonate. Finally, the analyte was eluted with 1 mL of 0.02 mol/L hydrochloric acid prepared in methanol and deionized water (80/20, v/v) and the chromatographic analysis was conducted on the 1 mL of the collected eluent. The solid-phase extraction process is shown in Fig. 1.
2.4. Chromatographic conditions
The suitable chromatographic conditions were 0.8 mmol/L [PMIm][BF4] aqueous solution/acetonitrile (40/60, v/v) as the mobile phase, flow rate of 1.0 mL/min and a column temperature of 35°C. The separations were performed on a 250 mm×4.6 mm I.D., 5 μm TSKGEL Amide-80 HR column (TOSOH, Japan). The injection volume was 20μL. The IUV (210 nm) was employed.
3. Results and discussion
3.1. Optimization of the SPE step
3.1.1. Addition of the IP reagent and selection of the loading volume
4
In this study, the C18 SPE column was used. A small amount of two piperidinium cations were detected in the effluent after 30 mL of the standard mixture of three piperidinium cations ([MEPi]+, [MPPi]+ and [MBPi]+) was loaded. But no piperidinium cations were detected in the effluent after 30 mL of the standard mixture with 0.5 mmol/L sodium heptanesulfonate was loaded. This reason is that the retention of solutes in the SPE column depends primarily on the strength of the hydrophobic force. The retention of piperidinium ionic liquids is poor in the SPE column owing to their strongly hydrophilic character. However, the addition of IP reagents can improve the retention of the analyte on the reversed-phase SPE column [17]. The phenomenon is mainly explained by two models, namely IP model and dynamic ion exchange model [18]. In the ion pair model, the measured ions and the oppositely charged ions in the IP reagents form ion pairs. The formed ion pairs are reserved on the SPE column. In the dynamic ion exchange model, the IP reagents are adsorbed on the non-polar stationary phase, forming a stationary phase layer with ionexchange ability. Thus the ion-pair SPE method was used in this study. According to the literature [19], sodium heptanesulfonate at a concentration of 0.5 mmol/L was selected as the IP reagent.
Loading volume is a function of sorbent capacity and analytes concentrations. In order to select the best loading volume, loading volume in the range of 10-60 mL was investigated. This result indicated that breakthrough volume was 60 mL. In order to obtain a high enrichment factor, large loading volume is required but smaller loading volume may save time. Considering the factors of enrichment factors and analytical time, 30 mL was selected as the suitable loading volume. 5
3.1.2. Selection of eluents and the eluting volume
Solubility of the target compound is one of important factors in the selection of eluents. Due to the strong polarity of piperidinium cations and the polarity of methanol over the polarity of acetonitrile, methanol was chosen as the organic phase of the eluent. The effect of methanol concentrations was investigated using 0.02 mol/L hydrochloric acid prepared in 70%, 80% and 90% methanol and deionized water (v/v) as the eluent. The result showed that when the methanol concentration was 80%, the recovery was the highest. The reason may be that as the water content of the eluent increases, the solubility of the hydrophilic portion of the formed ion pairs increases. As the methanol content of the eluent increases, the solubility of the hydrophobic portion of the formed ion pairs increases. When the methanol content of the eluent was 80%, the recovery was the highest, which might be the common result. Therefore, 80% was selected as the optimum methanol concentration in the eluent.
The effect of HCl concentrations was investigated using 0.01, 0.02, 0.03 and 0.04 mol/L hydrochloric acid and the results of the study were shown in Table 1. The results showed that when the HCl concentration was 0.02 mol/L, the recovery was the highest. Therefore, 0.02 mol/L was selected as the optimum HCl concentration in the eluent.
In order to ensure a better recovery, larger eluting volume can be used. However, it is important to note, that the eluting volume cannot be excessively large because excessively large eluting volume increases the chance of eluting impurities and reduces the concentration of the analyte. The effect of eluting volume was investigated using 1 mL and 2
6
mL as the eluting volume. The result showed that when the eluting volume was 1 mL, the recoveries of piperidinium cations were almost the same as the recoveries of piperidinium cations using 2 mL as the eluting volume. In order to reduce the elution of impurities and increase the enrichment factor, 1 mL was selected as the eluting volume.
3.1.3. Selection of washing solutions
In order to reduce the interference of impurities in the analysis, impurities should be removed as much as possible without affecting the target compound recovery rate. This is often achieved by washing the SPE column, thus the effect of washing solutions was investigated as follows. When the washing solution was 10% acetonitrile, the retention of piperidinium cations on the SPE column was destroyed. When 10 mmol/L phosphoric acidsodium dihydrogen phosphate and 0.4% hydrochloric acid were used as washing solutions, the baseline was unstable and the chromatographic peaks of washing solutions themselves interfered with the detection of piperidinium cations. Therefore, 0.5 mmol/L sodium heptanesulfonate was selected as the washing solution to wash the impurities.
3.2. Optimization of the chromatographic conditions
3.2.1. Effect of imidazolium ILs
Imidazolium ILs are suitable UV absorption reagents because of their strong UV absorption, thus imidazolium ILs were selected as background UV absorption reagents. The effect of different imidazolium ILs ([EMIm][BF4], [PMIm][BF4], [BMIm][BF4], [AMIm][BF4] and [HMIm][BF4]) was investigated using 0.8 mmol/L imidazolium ILs aqueous solution/acetonitrile (40/60, v/v) as the mobile phase. The results showed three 7
piperidinium cations could be well separated and the elution order was that the less polar substance was eluted first and the more polar substance was eluted later. This followed HILIC characteristic [20]. The results also showed the retention time gradually increased with the increase of the length of the alkyl substituent of imidazolium cations. When [PMIm][BF4] was used as the mobile phase component, the resolution was the largest. So [PMIm][BF4] was selected as the suitable mobile phase component. The effect of [PMIm][BF4] concentrations was investigated at 0.3, 0.5, 0.8, 1.0 and 1.2 mmol/L, respectively. The results showed that the retention time of piperidinium cations was shortened with the increase of [PMIm][BF4] concentrations. When the concentration was 0.8 mmol/L, the detection limit was the lowest. So 0.8 mmol/L was selected as the suitable concentration.
3.2.2. Effect of organic solvents
The effect of acetonitrile volume fraction was investigated from 50% to 70%. The results showed that with the increase of the acetonitrile volume fraction, the retention time of piperidinium cations increased and the resolution became larger. So 60% was selected as the suitable acetonitrile volume fraction due to the factors of the retention time and the resolution.
3.2.3. Selection of UV detection wavelengths and column temperatures
The effect of the detection wavelength was investigated from 200 to 225 nm. When the detection wavelength was 210 nm, the detection limits were lower and the peak height was the highest. Thus, 210 nm was selected as the detection wavelength.
8
The effect of column temperatures was investigated from 30 to 40°C. The results showed that the column temperature had little effect on the determination of piperidinium cations. Thus, 35°C near room temperature was selected as an appropriate temperature.
3.3. Chromatographic quantitative parameters
According to the above investigation, the suitable chromatographic conditions are described as Section 2.4. The chromatogram of piperidinium cations with the chromatographic conditions was shown in Fig. 2.
The external calibration, namely standard curve method, was used for quantitative analysis. The linearity of the analytical method developed was determined for [MEPi]+, [MPPi]+ and [MBPi]+, spiked in seven concentrations, namely 5, 10, 20, 30, 50, 80, 100 mg/L. Linear regression equations were obtained from the relationship between the chromatographic peak area (integral value) and ion concentrations (mg/L). Detection limits were calculated with a tripled signal-to-noise ratio and the investigated signal of the noise value and measured substance is the peak height for the calculation of detection limits. Quantitation limits were calculated with a ten times signal-to-noise ratio. Relative standard deviations (RSD) were obtained from five repeated measurements. The data are listed in Table 2. The detection limits were less than 0.4 mg/L, RSD values were less than 0.6% and the correlation coefficients were more than 0.9994. Thus, the sensitivity, precision and linearity of this method are good.
3.4. Analysis of samples
9
The Songhua River water samples were determined by the descriptions in Section 2.3 and Section 2.4. In the determination of recovery, a certain amount of the standard substance was added to the sample before filtration and then the spiked sample was analyzed after the sample pretreatment. Analytical results and recoveries were listed in Table 3. The data in Table 3 are average values (n = 5) and RSD values are less than 2.3%. Recoveries were 80.0%-98.3%. The higher recoveries prove that the measured content of piperidinium cations dissolved in the environment water is not significantly different from the content of piperidinium cations actually flowing into the environment water. Thus this method is applicable to the determination of piperidinium cations flowing into the environment water.
4. Conclusion
A new analytical method for the determination of piperidinium ionic liquid cations in environmental water by using SPE combined with HILIC was developed. The SPE can make the water sample be fully purified and enriched. The retention behavior of piperidinium cations on the HILIC column has typical HILIC characteristic. The method can be applied to the analysis of environmental water samples because of its good extraction effects, good recoveries and simple operations.
Acknowledgments
This work was supported by the Natural Science Foundation of Heilongjiang Province (Grant No. B201307).
References 10
[1] H. Nan, C. Zhang, R.A. O’Brien, A. Benchea, J.H. Davis Jr., J.L. Anderson, Lipidic ionic liquid stationary phases for the separation of aliphatic hydrocarbons by comprehensive twodimensional gas chromatography, J. Chromatogr. A 1481 (2017) 127–136. [2] Y.Q. Liu, H. Yu, Ion chromatography with the indirect ultraviolet detection of alkali metal ions and ammonium using imidazolium ionic liquid as ultraviolet absorption reagent and eluent, J. Sep. Sci. 39 (2016) 3156–3162. [3] H.L. Yu, J. Merib, J.L. Anderson, Crosslinked polymeric ionic liquids as solid-phase microextraction sorbent coatings for high performance liquid chromatography, J. Chromatogr. A 1438 (2016) 10–21. [4] J.M. Delgado, D. Nunes, E. Fortunato, C.A.T. Laia, L.C. Branco, M. Vilarigues, The effect of three luminescent ionic liquids on corroded glass surfaces–a first step into stained-glass cleaning, Corros. Sci. 118 (2017) 109–117. [5] M.C. Bubalo, K. Radošević, I.R. Redovniković, J. Halambek, V.G. Srček, A brief overview of the potential environmental hazards of ionic liquids, Ecotoxicol. Environ. Saf. 99 (2014) 1–12. [6] F. Onink, W. Meindersma, B. Burghoff, W. Weggemans, G. Aerts, A. Haan, Ion chromatography as a novel method to quantify the solubility of pyridinium ionic liquids in organic solvents, J. Chromatogr. Sci. 53 (2015) 8–15. [7] C.A. Hawkins, A. Rud, M.L. Guthrie, M.L. Dietz, Rapid quantification of imidazolium-based ionic liquids by hydrophilic interaction liquid chromatography: methodology and an investigation of the retention mechanisms, J. Chromatogr. A 1400 (2015) 54–64. [8] Y. Zhang, H. Yu, M.Y. Wang, Determination of pyrrolidinium ionic liquid cations by ion
11
chromatography-indirect ultraviolet detection with imidazolium ionic liquids as both an ultraviolet absorption reagent and an eluting agent, Anal. Methods 7 (2015) 5654–5660. [9] M. Pyschik, M. Winter, S. Nowak, Determination and quantification of cations in ionic liquids by capillary electrophoresis-mass spectrometry, J. Chromatogr. A 1485 (2017) 131–141. [10] K.L. Zhuo, X.L. Ma, Y.J. Chen, C.Y. Wang, A.Q. Li, C.L. Yan, Molecularly imprinted polymer based potentiometric sensor for the determination of 1-hexyl-3-methylimidazolium cation in aqueous solution, Ionics 22 (2016) 1947−1955. [11] C.X. Lu, Z.G. Tang, C.B. Liu, X. Chen, Y. Wang, F.M. Dang, Determination of ionic liquid cations in soil samples by ultrasound-assisted solid-phase extraction coupled with liquid chromatography-tandem mass spectrometry, Anal. Methods 7 (2015) 5924–5933. [12] J. Nichthauser, W. Mrozik, A. Markowska, P. Stepnowski, Analysis of residual ionic liquids in environmental samples: development of extraction methods, Chemosphere 74 (2009) 515–521. [13] J. Płotka-Wasylka, N. Szczepańska, M. Guardia, J. Namieśnik, Modern trends in solid phase extraction: new sorbent media, Trends Anal. Chem. 77 (2016) 23–43. [14] X.X. Zhang, Y.Q. Liu, H. Yu, R.Q. Zhang, Rapid and simultaneous determination of piperidinium and pyrrolidinium ionic liquid cations by ion pair chromatography coupled with direct conductivity detection, Chin. Chem. Lett. 28 (2017) 126–130. [15] M.Y. Wang, H. Yu, P. Li, J. Li, Y.F. Gao, Determination of piperidinium ionic liquid cations by ion-pair chromatography-indirect ultraviolet detection, Chin. J. Chromatogr. 32 (2014) 773–778.
12
[16] S.L. Da, Indirect photometric high-performance liquid chromatography, Chin. J. Anal. Chem. 17 (1989) 372–381. [17] F. Han, Y.Z. He, L. Li, G.N. Fu, H.Y. Xie, W.E. Gan, Determination of benzoic acid and sorbic acid in food products using electrokinetic flow analysis-ion pair solid phase extractioncapillary zone electrophoresis, Anal. Chim. Acta 618 (2008) 79–85. [18] H.F. Zou, Y.K. Zhang, P.Z. Lu, Reversed-phase ion-pair chromatography and its application, Chin. J. Chromatogr. 10 (1992) 329–333. [19] P. Stepnowski, J. Nichthauser, Ion-pair solid-phase extraction of trace amounts of ionic liquid cations in fresh and seawater samples, Anal. Sci. 24 (2008) 1255–1259. [20] A.J. Alpert, Hydrophilic-interaction chromatography for the separation of peptides, nucleic acids and other polar compounds, J. Chromatogr. 499 (1990) 177–196.
13
Figure Captions
Fig. 1 The solid-phase extraction process of piperidinium ionic liquid cations in environmental water samples
Fig. 2 Chromatogram of a standard mixture solution of piperidinium ionic liquid cations
Chromatographic conditions: mobile phase, 0.8 mmol/L [PMIm][BF4] aqueous solution/acetonitrile (40/60, v/v); column, TSK-GEL Amide-80 HR (4.6 mm i.d. × 250 mm, 5 μm); flow rate, 1.0 mL/min; column temperature, 35°C; indirect UV detection, 210 nm; inject volumn, 20 μL. Peaks: 1, [MBPi]+; 2, [MPPi]+; 3, [MEPi]+ (The concentration of each cation is 50 mg/L).
14
Figr-1 Fig. 1
The solid-phase extraction process of piperidinium ionic liquid cations in
environmental water samples
15
Figr-2
Response /(uV×104)
5.0
2.5
0.0
1
-2.5
2
3
-5.0
-7.5 0.0
5.0
10.0
15.0
20.0
25.0
30.0
Time/min Fig. 2 Chromatogram of a standard mixture solution of piperidinium ionic liquid cations Chromatographic conditions: mobile phase, 0.8 mmol/L [PMIm][BF4] aqueous solution/acetonitrile (40/60, v/v); column, TSK-GEL Amide-80 HR (4.6 mm i.d. × 250 mm, 5 μm); flow rate, 1.0 mL/min; column temperature, 35°C; indirect UV detection, 210 nm; inject volumn, 20 μL. Peaks: 1, [MBPi]+; 2, [MPPi]+; 3, [MEPi]+ (The concentration of each cation is 50 mg/L).
16
Table 1 Recoveries obtained with eluents containing different concentrations of HCl The concentration of Recoveries (%) HCl in the eluent (mol/L)
[MBPi]+
[MPPi]+
[MEPi]+
0.01
74.9
77.1
44.0
0.02
95.5
95.6
83.2
0.03
59.2
61.0
53.9
0.04
71.7
63.9
45.3
17
Table 2 Parameters of the method
Parameters
[MBPi]+
[MPPi]+
[MEPi]+
Slope a
27047
31082
34877
Intercept b
–10934
–36203
–81403
Slope standard error
320
436
609
Intercept standard error
5962
8647
9219
Correlation coefficient (r)
0.9997
0.9996
0.9994
Linear range (mg/L)
5.0–100
5.0–100
5.0–100
Limit of detection (mg/L, S/N = 3)
0.22
0.27
0.36
Quantitation limit (mg/L, S/N = 10)
0.73
0.90
1.20
RSDt /RSDs (%, n = 5)
0.408/0.512
0.418/0.457
0.467/0.361
y = ax + b (y = area; x = concentration, mg/L)
18
19
Table 3 Contents and spiked recoveries of piperidinium cations in river water samples
Sample
Ion
Original
Added
Found
Recovery
ρo (mg/L)
ρA (mg/L)
ρF (mg/L)
R (%)
0.49
98.0
0.50 [MBPi]+
River water
[MPPi]+
[MEPi]+
n.d.
n.d.
n.d.
n.d. stands for “no detected”
20
0.80
0.77
96.3
1.20
1.17
97.5
0.50
0.48
96.0
0.80
0.75
93.8
1.20
1.18
98.3
0.50
0.40
80.0
0.80
0.67
83.8
1.20
1.01
84.2
S0021-9673(17)30704-5 http://dx.doi.org/doi:10.1016/j.chroma.2017.05.015 CHROMA 358512
To appear in:
Journal of Chromatography A
Received date: Revised date: Accepted date:
25-1-2017 4-5-2017 5-5-2017
Please cite this article as: Zi-qiang Fan, Hong Yu, Determination of piperidinium ionic liquid cations in environmental water samples by solid phase extraction and hydrophilic interaction liquid chromatography, Journal of Chromatography Ahttp://dx.doi.org/10.1016/j.chroma.2017.05.015 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Determination of piperidinium ionic liquid cations in environmental water samples by solid phase extraction and hydrophilic interaction liquid chromatography Zi-qiang Fan, Hong Yu*
College of Chemistry and Chemical Engineering, Harbin Normal University, Harbin 150025, China
E-mail: [email protected] Highlights
Determination of piperidinium ionic liquid cations in environmental water
Effective enrichment and purification of the sample by solid phase extraction
Hydrophilic interaction liquid chromatography of indirect ultraviolet detection
Imidazolium ionic liquids as background ultraviolet absorbents and eluting agents
Abstract: This paper presents a novel analytical method for the determination of piperidinium ionic liquid cations in environmental water by hydrophilic interaction liquid chromatography and solid-phase extraction technology. The left standing, centrifuged and filtered river water samples were first purified and concentrated through the C18 solid phase extraction column, and eluted with 0.02 mol/L hydrochloric acid prepared in methanol and deionized water (80/20, v/v). Then the eluents were analyzed by a hydrophilic column combined with 0.8 mmol/L 1-propyl-3-methyl imidazolium tetrafluoroborate aqueous solution/acetonitrile (40/60, v/v) as the mobile phase and indirect ultraviolet detection. The detection limits of piperidinium cations were less than 0.4 mg/L. The relative standard deviations were less than 0.6%. The method has been successfully applied to the determination of piperidinium cations in Songhua River water 1
samples. Recoveries were 80.0%–98.3%. This research may provide a reference for studying the environmental effect of ionic liquids.
Keywords: solid phase extraction; hydrophilic interaction liquid chromatography; ionic liquids; piperidinium cations; indirect ultraviolet detection
1. Introduction
Ionic liquids (ILs) have characteristics of non-volatility and strong dissolving ability etc, thus they become the research hotspots in many fields [1-4]. However, some literatures report the biological effect of ILs and indicate that the toxicity increases as increasing the length of the alkyl chain in the ionic liquid cations [5]. Therefore, the detection of ILs in the environment is important. In recent years, the determination of ionic liquid cations [6-10] has been more reported but there are very few reports [11, 12] on the analysis of ionic liquid cations in the environment. Due to the complexity of environmental samples, sample pretreatment techniques are important in the analysis of environmental samples. Solid phase extraction (SPE) is mainly used for sample separation, purification and concentration and it is widely used in environment and other fields [13]. Therefore, SPE was used in this study.
The piperidinium ionic liquids are important ionic liquids and have wide ranges of applications. Some previous works mainly investigate the separation of piperidinium cations in the ion pair (IP) separation mode and they do not analyze piperidinium cations in environmental water [14, 15]. In the work, we use a new hydrophilic interaction liquid chromatography (HILIC) and indirect ultraviolet (IUV) detection [16] for the determination of
2
piperidinium cations in environmental water. Compared with the IP chromatography, HILIC is simpler for the analysis because of the avoidance of IP reagents. Moreover, the SPE can make the water sample be fully purified and enriched. This research may provide an analytical method of piperidinium cations in environmental water and provide the appropriate support for studying the environmental effect of ILs.
2. Experimental
2.1. Instrumentations
Sample pretreatment was carried out on an ASE-24 SPE apparatus (Tianjin Automatic Science Instrument Ltd.). The chromatographic analysis was carried out on a LC-20A liquid chromatograph (Shimadzu, Japan). UF-C18 SPE columns (250 mg/3mL, Dalian Zhongpu Technology Ltd.) were used.
2.2. Reagents
The ILs (99% purity) were N-methyl-N-ethyl piperidinium bromide ([MEPi][Br]), Nmethyl-N-propyl piperidinium bromide ([MPPi][Br]), N-methyl-N-butyl piperidinium bromide ([MBPi][Br]), 1-ethyl-3-methyl imidazolium tetrafluoroborate ([EMIm][BF4]), 1-propyl-3methyl imidazolium tetrafluoroborate ([PMIm][BF4]), 1-butyl-3-methyl imidazolium tetrafluoroborate ([BMIm][BF4]), 1-amyl-3-methyl imidazolium tetrafluoroborate ([AMIm][BF4]) and 1-hexyl-3-methyl imidazolium tetrafluoroborate ([HMIm][BF4]) purchased from Shanghai Chengjie Chemical Ltd. (Shanghai, China). Methanol, acetonitrile and sodium heptanesulfonate (Chromatographic purity) were obtained from Dikma Technologies (Shanghai, China). 3
2.3. Pretreatment of environmental water samples
The water samples were from the Songhua River of Harbin and the pH was 7.38. They were left standing, centrifuged and filtered through a 0.22 μm filter, and then sodium heptanesulfonate as the IP reagent was added to the sample at a concentration of 0.5 mmol/L. Prior to using, the UF-C18 SPE column was activated with 5 mL of methanol, equilibrated by 5 mL of deionized water and dried under vacuum. Then 30 mL of the pretreated water sample was passed through the column at a flow rate of 2 mL/min and the column was washed with 3 mL of 0.5 mmol/L sodium heptanesulfonate. Finally, the analyte was eluted with 1 mL of 0.02 mol/L hydrochloric acid prepared in methanol and deionized water (80/20, v/v) and the chromatographic analysis was conducted on the 1 mL of the collected eluent. The solid-phase extraction process is shown in Fig. 1.
2.4. Chromatographic conditions
The suitable chromatographic conditions were 0.8 mmol/L [PMIm][BF4] aqueous solution/acetonitrile (40/60, v/v) as the mobile phase, flow rate of 1.0 mL/min and a column temperature of 35°C. The separations were performed on a 250 mm×4.6 mm I.D., 5 μm TSKGEL Amide-80 HR column (TOSOH, Japan). The injection volume was 20μL. The IUV (210 nm) was employed.
3. Results and discussion
3.1. Optimization of the SPE step
3.1.1. Addition of the IP reagent and selection of the loading volume
4
In this study, the C18 SPE column was used. A small amount of two piperidinium cations were detected in the effluent after 30 mL of the standard mixture of three piperidinium cations ([MEPi]+, [MPPi]+ and [MBPi]+) was loaded. But no piperidinium cations were detected in the effluent after 30 mL of the standard mixture with 0.5 mmol/L sodium heptanesulfonate was loaded. This reason is that the retention of solutes in the SPE column depends primarily on the strength of the hydrophobic force. The retention of piperidinium ionic liquids is poor in the SPE column owing to their strongly hydrophilic character. However, the addition of IP reagents can improve the retention of the analyte on the reversed-phase SPE column [17]. The phenomenon is mainly explained by two models, namely IP model and dynamic ion exchange model [18]. In the ion pair model, the measured ions and the oppositely charged ions in the IP reagents form ion pairs. The formed ion pairs are reserved on the SPE column. In the dynamic ion exchange model, the IP reagents are adsorbed on the non-polar stationary phase, forming a stationary phase layer with ionexchange ability. Thus the ion-pair SPE method was used in this study. According to the literature [19], sodium heptanesulfonate at a concentration of 0.5 mmol/L was selected as the IP reagent.
Loading volume is a function of sorbent capacity and analytes concentrations. In order to select the best loading volume, loading volume in the range of 10-60 mL was investigated. This result indicated that breakthrough volume was 60 mL. In order to obtain a high enrichment factor, large loading volume is required but smaller loading volume may save time. Considering the factors of enrichment factors and analytical time, 30 mL was selected as the suitable loading volume. 5
3.1.2. Selection of eluents and the eluting volume
Solubility of the target compound is one of important factors in the selection of eluents. Due to the strong polarity of piperidinium cations and the polarity of methanol over the polarity of acetonitrile, methanol was chosen as the organic phase of the eluent. The effect of methanol concentrations was investigated using 0.02 mol/L hydrochloric acid prepared in 70%, 80% and 90% methanol and deionized water (v/v) as the eluent. The result showed that when the methanol concentration was 80%, the recovery was the highest. The reason may be that as the water content of the eluent increases, the solubility of the hydrophilic portion of the formed ion pairs increases. As the methanol content of the eluent increases, the solubility of the hydrophobic portion of the formed ion pairs increases. When the methanol content of the eluent was 80%, the recovery was the highest, which might be the common result. Therefore, 80% was selected as the optimum methanol concentration in the eluent.
The effect of HCl concentrations was investigated using 0.01, 0.02, 0.03 and 0.04 mol/L hydrochloric acid and the results of the study were shown in Table 1. The results showed that when the HCl concentration was 0.02 mol/L, the recovery was the highest. Therefore, 0.02 mol/L was selected as the optimum HCl concentration in the eluent.
In order to ensure a better recovery, larger eluting volume can be used. However, it is important to note, that the eluting volume cannot be excessively large because excessively large eluting volume increases the chance of eluting impurities and reduces the concentration of the analyte. The effect of eluting volume was investigated using 1 mL and 2
6
mL as the eluting volume. The result showed that when the eluting volume was 1 mL, the recoveries of piperidinium cations were almost the same as the recoveries of piperidinium cations using 2 mL as the eluting volume. In order to reduce the elution of impurities and increase the enrichment factor, 1 mL was selected as the eluting volume.
3.1.3. Selection of washing solutions
In order to reduce the interference of impurities in the analysis, impurities should be removed as much as possible without affecting the target compound recovery rate. This is often achieved by washing the SPE column, thus the effect of washing solutions was investigated as follows. When the washing solution was 10% acetonitrile, the retention of piperidinium cations on the SPE column was destroyed. When 10 mmol/L phosphoric acidsodium dihydrogen phosphate and 0.4% hydrochloric acid were used as washing solutions, the baseline was unstable and the chromatographic peaks of washing solutions themselves interfered with the detection of piperidinium cations. Therefore, 0.5 mmol/L sodium heptanesulfonate was selected as the washing solution to wash the impurities.
3.2. Optimization of the chromatographic conditions
3.2.1. Effect of imidazolium ILs
Imidazolium ILs are suitable UV absorption reagents because of their strong UV absorption, thus imidazolium ILs were selected as background UV absorption reagents. The effect of different imidazolium ILs ([EMIm][BF4], [PMIm][BF4], [BMIm][BF4], [AMIm][BF4] and [HMIm][BF4]) was investigated using 0.8 mmol/L imidazolium ILs aqueous solution/acetonitrile (40/60, v/v) as the mobile phase. The results showed three 7
piperidinium cations could be well separated and the elution order was that the less polar substance was eluted first and the more polar substance was eluted later. This followed HILIC characteristic [20]. The results also showed the retention time gradually increased with the increase of the length of the alkyl substituent of imidazolium cations. When [PMIm][BF4] was used as the mobile phase component, the resolution was the largest. So [PMIm][BF4] was selected as the suitable mobile phase component. The effect of [PMIm][BF4] concentrations was investigated at 0.3, 0.5, 0.8, 1.0 and 1.2 mmol/L, respectively. The results showed that the retention time of piperidinium cations was shortened with the increase of [PMIm][BF4] concentrations. When the concentration was 0.8 mmol/L, the detection limit was the lowest. So 0.8 mmol/L was selected as the suitable concentration.
3.2.2. Effect of organic solvents
The effect of acetonitrile volume fraction was investigated from 50% to 70%. The results showed that with the increase of the acetonitrile volume fraction, the retention time of piperidinium cations increased and the resolution became larger. So 60% was selected as the suitable acetonitrile volume fraction due to the factors of the retention time and the resolution.
3.2.3. Selection of UV detection wavelengths and column temperatures
The effect of the detection wavelength was investigated from 200 to 225 nm. When the detection wavelength was 210 nm, the detection limits were lower and the peak height was the highest. Thus, 210 nm was selected as the detection wavelength.
8
The effect of column temperatures was investigated from 30 to 40°C. The results showed that the column temperature had little effect on the determination of piperidinium cations. Thus, 35°C near room temperature was selected as an appropriate temperature.
3.3. Chromatographic quantitative parameters
According to the above investigation, the suitable chromatographic conditions are described as Section 2.4. The chromatogram of piperidinium cations with the chromatographic conditions was shown in Fig. 2.
The external calibration, namely standard curve method, was used for quantitative analysis. The linearity of the analytical method developed was determined for [MEPi]+, [MPPi]+ and [MBPi]+, spiked in seven concentrations, namely 5, 10, 20, 30, 50, 80, 100 mg/L. Linear regression equations were obtained from the relationship between the chromatographic peak area (integral value) and ion concentrations (mg/L). Detection limits were calculated with a tripled signal-to-noise ratio and the investigated signal of the noise value and measured substance is the peak height for the calculation of detection limits. Quantitation limits were calculated with a ten times signal-to-noise ratio. Relative standard deviations (RSD) were obtained from five repeated measurements. The data are listed in Table 2. The detection limits were less than 0.4 mg/L, RSD values were less than 0.6% and the correlation coefficients were more than 0.9994. Thus, the sensitivity, precision and linearity of this method are good.
3.4. Analysis of samples
9
The Songhua River water samples were determined by the descriptions in Section 2.3 and Section 2.4. In the determination of recovery, a certain amount of the standard substance was added to the sample before filtration and then the spiked sample was analyzed after the sample pretreatment. Analytical results and recoveries were listed in Table 3. The data in Table 3 are average values (n = 5) and RSD values are less than 2.3%. Recoveries were 80.0%-98.3%. The higher recoveries prove that the measured content of piperidinium cations dissolved in the environment water is not significantly different from the content of piperidinium cations actually flowing into the environment water. Thus this method is applicable to the determination of piperidinium cations flowing into the environment water.
4. Conclusion
A new analytical method for the determination of piperidinium ionic liquid cations in environmental water by using SPE combined with HILIC was developed. The SPE can make the water sample be fully purified and enriched. The retention behavior of piperidinium cations on the HILIC column has typical HILIC characteristic. The method can be applied to the analysis of environmental water samples because of its good extraction effects, good recoveries and simple operations.
Acknowledgments
This work was supported by the Natural Science Foundation of Heilongjiang Province (Grant No. B201307).
References 10
[1] H. Nan, C. Zhang, R.A. O’Brien, A. Benchea, J.H. Davis Jr., J.L. Anderson, Lipidic ionic liquid stationary phases for the separation of aliphatic hydrocarbons by comprehensive twodimensional gas chromatography, J. Chromatogr. A 1481 (2017) 127–136. [2] Y.Q. Liu, H. Yu, Ion chromatography with the indirect ultraviolet detection of alkali metal ions and ammonium using imidazolium ionic liquid as ultraviolet absorption reagent and eluent, J. Sep. Sci. 39 (2016) 3156–3162. [3] H.L. Yu, J. Merib, J.L. Anderson, Crosslinked polymeric ionic liquids as solid-phase microextraction sorbent coatings for high performance liquid chromatography, J. Chromatogr. A 1438 (2016) 10–21. [4] J.M. Delgado, D. Nunes, E. Fortunato, C.A.T. Laia, L.C. Branco, M. Vilarigues, The effect of three luminescent ionic liquids on corroded glass surfaces–a first step into stained-glass cleaning, Corros. Sci. 118 (2017) 109–117. [5] M.C. Bubalo, K. Radošević, I.R. Redovniković, J. Halambek, V.G. Srček, A brief overview of the potential environmental hazards of ionic liquids, Ecotoxicol. Environ. Saf. 99 (2014) 1–12. [6] F. Onink, W. Meindersma, B. Burghoff, W. Weggemans, G. Aerts, A. Haan, Ion chromatography as a novel method to quantify the solubility of pyridinium ionic liquids in organic solvents, J. Chromatogr. Sci. 53 (2015) 8–15. [7] C.A. Hawkins, A. Rud, M.L. Guthrie, M.L. Dietz, Rapid quantification of imidazolium-based ionic liquids by hydrophilic interaction liquid chromatography: methodology and an investigation of the retention mechanisms, J. Chromatogr. A 1400 (2015) 54–64. [8] Y. Zhang, H. Yu, M.Y. Wang, Determination of pyrrolidinium ionic liquid cations by ion
11
chromatography-indirect ultraviolet detection with imidazolium ionic liquids as both an ultraviolet absorption reagent and an eluting agent, Anal. Methods 7 (2015) 5654–5660. [9] M. Pyschik, M. Winter, S. Nowak, Determination and quantification of cations in ionic liquids by capillary electrophoresis-mass spectrometry, J. Chromatogr. A 1485 (2017) 131–141. [10] K.L. Zhuo, X.L. Ma, Y.J. Chen, C.Y. Wang, A.Q. Li, C.L. Yan, Molecularly imprinted polymer based potentiometric sensor for the determination of 1-hexyl-3-methylimidazolium cation in aqueous solution, Ionics 22 (2016) 1947−1955. [11] C.X. Lu, Z.G. Tang, C.B. Liu, X. Chen, Y. Wang, F.M. Dang, Determination of ionic liquid cations in soil samples by ultrasound-assisted solid-phase extraction coupled with liquid chromatography-tandem mass spectrometry, Anal. Methods 7 (2015) 5924–5933. [12] J. Nichthauser, W. Mrozik, A. Markowska, P. Stepnowski, Analysis of residual ionic liquids in environmental samples: development of extraction methods, Chemosphere 74 (2009) 515–521. [13] J. Płotka-Wasylka, N. Szczepańska, M. Guardia, J. Namieśnik, Modern trends in solid phase extraction: new sorbent media, Trends Anal. Chem. 77 (2016) 23–43. [14] X.X. Zhang, Y.Q. Liu, H. Yu, R.Q. Zhang, Rapid and simultaneous determination of piperidinium and pyrrolidinium ionic liquid cations by ion pair chromatography coupled with direct conductivity detection, Chin. Chem. Lett. 28 (2017) 126–130. [15] M.Y. Wang, H. Yu, P. Li, J. Li, Y.F. Gao, Determination of piperidinium ionic liquid cations by ion-pair chromatography-indirect ultraviolet detection, Chin. J. Chromatogr. 32 (2014) 773–778.
12
[16] S.L. Da, Indirect photometric high-performance liquid chromatography, Chin. J. Anal. Chem. 17 (1989) 372–381. [17] F. Han, Y.Z. He, L. Li, G.N. Fu, H.Y. Xie, W.E. Gan, Determination of benzoic acid and sorbic acid in food products using electrokinetic flow analysis-ion pair solid phase extractioncapillary zone electrophoresis, Anal. Chim. Acta 618 (2008) 79–85. [18] H.F. Zou, Y.K. Zhang, P.Z. Lu, Reversed-phase ion-pair chromatography and its application, Chin. J. Chromatogr. 10 (1992) 329–333. [19] P. Stepnowski, J. Nichthauser, Ion-pair solid-phase extraction of trace amounts of ionic liquid cations in fresh and seawater samples, Anal. Sci. 24 (2008) 1255–1259. [20] A.J. Alpert, Hydrophilic-interaction chromatography for the separation of peptides, nucleic acids and other polar compounds, J. Chromatogr. 499 (1990) 177–196.
13
Figure Captions
Fig. 1 The solid-phase extraction process of piperidinium ionic liquid cations in environmental water samples
Fig. 2 Chromatogram of a standard mixture solution of piperidinium ionic liquid cations
Chromatographic conditions: mobile phase, 0.8 mmol/L [PMIm][BF4] aqueous solution/acetonitrile (40/60, v/v); column, TSK-GEL Amide-80 HR (4.6 mm i.d. × 250 mm, 5 μm); flow rate, 1.0 mL/min; column temperature, 35°C; indirect UV detection, 210 nm; inject volumn, 20 μL. Peaks: 1, [MBPi]+; 2, [MPPi]+; 3, [MEPi]+ (The concentration of each cation is 50 mg/L).
14
Figr-1 Fig. 1
The solid-phase extraction process of piperidinium ionic liquid cations in
environmental water samples
15
Figr-2
Response /(uV×104)
5.0
2.5
0.0
1
-2.5
2
3
-5.0
-7.5 0.0
5.0
10.0
15.0
20.0
25.0
30.0
Time/min Fig. 2 Chromatogram of a standard mixture solution of piperidinium ionic liquid cations Chromatographic conditions: mobile phase, 0.8 mmol/L [PMIm][BF4] aqueous solution/acetonitrile (40/60, v/v); column, TSK-GEL Amide-80 HR (4.6 mm i.d. × 250 mm, 5 μm); flow rate, 1.0 mL/min; column temperature, 35°C; indirect UV detection, 210 nm; inject volumn, 20 μL. Peaks: 1, [MBPi]+; 2, [MPPi]+; 3, [MEPi]+ (The concentration of each cation is 50 mg/L).
16
Table 1 Recoveries obtained with eluents containing different concentrations of HCl The concentration of Recoveries (%) HCl in the eluent (mol/L)
[MBPi]+
[MPPi]+
[MEPi]+
0.01
74.9
77.1
44.0
0.02
95.5
95.6
83.2
0.03
59.2
61.0
53.9
0.04
71.7
63.9
45.3
17
Table 2 Parameters of the method
Parameters
[MBPi]+
[MPPi]+
[MEPi]+
Slope a
27047
31082
34877
Intercept b
–10934
–36203
–81403
Slope standard error
320
436
609
Intercept standard error
5962
8647
9219
Correlation coefficient (r)
0.9997
0.9996
0.9994
Linear range (mg/L)
5.0–100
5.0–100
5.0–100
Limit of detection (mg/L, S/N = 3)
0.22
0.27
0.36
Quantitation limit (mg/L, S/N = 10)
0.73
0.90
1.20
RSDt /RSDs (%, n = 5)
0.408/0.512
0.418/0.457
0.467/0.361
y = ax + b (y = area; x = concentration, mg/L)
18
19
Table 3 Contents and spiked recoveries of piperidinium cations in river water samples
Sample
Ion
Original
Added
Found
Recovery
ρo (mg/L)
ρA (mg/L)
ρF (mg/L)
R (%)
0.49
98.0
0.50 [MBPi]+
River water
[MPPi]+
[MEPi]+
n.d.
n.d.
n.d.
n.d. stands for “no detected”
20
0.80
0.77
96.3
1.20
1.17
97.5
0.50
0.48
96.0
0.80
0.75
93.8
1.20
1.18
98.3
0.50
0.40
80.0
0.80
0.67
83.8
1.20
1.01
84.2