Journal of Chromatography A, 1164 (2007) 139–144
Hydrophilic interaction liquid chromatography for separation and quantification of selected room-temperature ionic liquids Guillaume Le Rouzo a,∗ , Christine Lamouroux b , Carole Bresson a , Aline Guichard a , Philippe Moisy c , Gilles Moutiers d a
CEA Saclay, DEN/DANS/DPC/SECR/LSRM, 91191 Gif-sur-Yvette, France CEA Saclay DEN/DANS/DPC/SECR/LANIE, 91191 Gif-sur-Yvette, France c CEA Valrhˆ o DEN/VRH/DRCP/SCPS/LCA, 30207 Bagnols-sur-C`eze cedex, France d CEA Saclay DEN/DANS/DPC/SCP/DIR, 91191 Gif-sur-Yvette, France b
Received 16 April 2007; received in revised form 26 June 2007; accepted 28 June 2007 Available online 30 June 2007
Abstract Hydrophilic interaction liquid chromatography (HILIC) is an alternative technique to ion pairing-reversed-phase liquid chromatography (IPRPLC) and classical RPLC for separation of alkylimidazolium room-temperature ionic liquids (RTILs). Particularly, HILIC offers better retention and selectivity for short-chains RTILs imidazolium compounds. HILIC mechanisms were investigated by studying the influence of organic modifier content and salt concentration in the mobile phase. HILIC method was validated by quantifying 1-butyl-3-methylimidazolium cation (BMIM) degradation under gamma radiation at 2.5 MGy. Development of separative reproducible analytical methods, including for low concentration, applicable to RTILs are today mandatory to improve RTILs chemistry. © 2007 Elsevier B.V. All rights reserved. Keywords: Room-temperature ionic liquids; Hydrophilic interaction liquid chromatography; Ionic liquid cations; Ion-pairing reversed-phase chromatography
1. Introduction Room-temperature ionic liquids (RTILs), resulting from the combination of organic cations and various anions, are salts exhibiting melting points nearby room temperature. Among these, alkylmethylimidazolium RMIM have recently shown potential as unique solvents with a wide range of solubility, miscibility, and other interesting properties such as negligible vapour pressure, high thermal stability, good conductivity and wide electrochemical window (>4 V). These unique properties make RTILs an efficient alternative to volatile organic solvents as environmentally benign media in many industrially important chemical processes. Hence, they have been recently proposed as solvents in various chemical reactions in synthesis [1,2], catalysis [3,4], bioprocesses [5–7] and as electrolytes for batteries and fuel cells [8]. Their uses have been demonstrated in analytical chemistry as components for gas chromatography sta-
∗
Corresponding author. Tel.: +33 1 690 883 48; fax: +33 1 690 854 11. E-mail address:
[email protected] (G. Le Rouzo).
0021-9673/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.chroma.2007.06.053
tionary phases, running electrolytes for capillary electrophoresis or additives for high pressure liquid chromatography (HPLC) mobile phases [9–11]. In the nuclear fuel cycle, RTILs appear potentially attractive for future retreatment processes (for new Generation IV reactors). Firstly, as imidazolium RTILs have recently shown unique solvation properties for actinide and lanthanide cations [12–16], their electrochemical properties could make them potential media for lanthanides or actinides electrorefining [17]. Secondly, and for the same reasons, RTILs could replace conventional organic diluent for separation processes by solvent extraction [18]. However, to use RTILs in such nuclear applications, studies of their behaviour under irradiation are an essential prerequisite. Studies concerning radiochemical stability of ethylmethylimidazolium (EMIM), and hexymethylimidazolium (HMIM) under gamma irradiations have been reported for 400 kGy radiation doses [19] and for BMIM for 1200 kGy radiation doses [20]. Spectroscopic techniques like NMR show a lack of sensitivity for assessing the concentration of RTILs cations and furthermore, it does not allow the separation of RTILs from their impurities. Therefore, development of separative, sensitive and reproducible analytical
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methods that must be applicable to very low concentrations are required. Several attempts were reported to separate and determine alkylimidazolium RTILs cations [21]. Capillary zone electrophoresis (CZE) has been used for separation and detection of various imidazolium entities [22] and for resolving selected imidazolium RTILs cations in standard mixtures [23]. Reversedphase liquid chromatography (RPLC) has been widely used for separations of various imidazolium cations on alkyl-bonded stationary phases [24–29], phenyl-bonded stationary phases [30] or on silica packings functionalized with mixed polar and apolar functions [25–27]. Ion chromatography (IC) was also investigated with a strong exchange cation adsorbent [27]. However, these studies show clear limitations for the separation of short alkyl chains compounds like EMIM and BMIM in terms of retention and selectivity. In order to increase retention for these less retained cations, two different techniques can be considered. On the one hand, IP-RPLC can be used with hydrophobic surfactants like alkylsulfonate salts [31,32]. On the other hand, HILIC has been usually described for separation of natural charged compounds such as proteins, oligonucleotides and carbohydrates [33–35]. This separation mode implies polar stationary phases and less polar mobile phases, commonly a mixture of an organic solvent and water. The present study deals with the advantage of HILIC over IP-RPLC for separation of RMIM cations. HILIC was used to study degradation of BMIM cation under 2400 kGy gamma irradiation, showing applicability for quantification of RMIM cations. 2. Experimental 2.1. Chemicals Ionic liquids were purchased from Aldrich and Fluka, or were synthesized according to classical metathesis protocol previously described [12]. They were purified with activated carbon
(12 h) and separated on a column with small amounts of acidic alumina as previously described [12,36]. Solutions obtained were dried in vacuo (5 mBar) at 80 ◦ C for 6 h. [BMIM][Tf2 N] was irradiated at room temperature using MARCEL facility equipped with an IBL 137 Cis-Bio Irradiator 137 Cs was used as the gamma source with a dose rate of 32–34 kGy min−1 . Dosimetry was performed using a conventional ferioxalate dosimeter. Radiolysis was performed under argon atmosphere to prevent the absorption by RTILs of water and oxygen from the atmosphere during radiolysis. HPLC grade acetonitrile were purchased from Aldrich, sodium 1-decanesulfonate (>99% purity) from Fluka and ammonium acetate (analytical grade) from Labosi. Deionized water filtered through a Millipore (Milli-Q) water purification system to 18 M quality was used. 2.2. Samples preparation Ionic liquids used in these studies are listed by order of decreasing polarity in Table 1. They were prepared at 5 × 10−2 mol L−1 solutions by weighting the suitable quantity of RTILs and followed by the convenient dilution and vigorous stirring to obtain homogenous RTILs solutions. Solutions were prepared in the acetonitrile/water solutions with the same compositions than the eluting mobile phase from 10−5 mol L−1 to 10−3 mol L−1 . 2.3. Chromatographic conditions Experiments were performed on a Shimadzu HPLC System equipped with a binary pump (LC-10ADVP), an autosampler (SIL-10AF) and a diode array detector (SPD-M10AVP). As the ionic liquids studied exhibit their absorption maximum around 210 nm, the wavelength range of the detector was chosen from 190 to 300 nm. Samples volumes injected were from 3 L to 10 L and elutions were carried out under isocratic conditions. Following columns were used in the experiments: C8 Metasil
Table 1 Structure of selected RTILs for HILIC method development No.
Name
Melting point
Molecular weight (g mol−1 )
Cation formula
1
1-Ethyl-3-methyl imidazolium chloride
85 ◦ C
146.7
EMIM
2
1-Butyl-3-methyl imidazolium bis(trifluoromethylsulfonyl)imide
−2 ◦ C
419.4
BMIM
3
1-Butyl-2-methyl-3-methyl imidazolium bis(trifluoromethylsulfonyl)imide
Non available
433.4
BMMIM
4
1-Hexyl-3-methyl imidazolium chloride
Non available
202.7
HMIM
5
1-Methyl-3-octyl imidazolium chloride
Non available
230.8
OMIM
Formula
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Basic 150 × 4.6 I.D. (Varian), Uptisphere OH 250 × 2.0 I.D. (Interchrom), YMC-Pack OH 250 × 2.0 I.D. (Interchrom) and Nucleosil OH 250 × 2.0 I.D. (Machery Nagel). The particle size for each column was 5 m. 3. Results and discussion 3.1. Development of HILIC method for separation of RMIM RTILs cations 3.1.1. IP-RPLC separation of RMIM cations Separation of imidazolium RTILs cations mixtures by RPLC has been described for apolar stationary phases with butyl, octyl, octadecyl bonded functions [24–29]. Dispersive interactions between alkyl side chains of imidazolium cations and apolar bonded moieties of stationary phases are the main interaction of the retention mechanisms. Furthermore, electrostatic interactions between cations and residual charged silanols also play a determinant role in retention [24,25]. In these applications, mobile phases often contain additive anionic species, like ammonium acetate/acetic acid [29], potassium phosphate/phosphoric acid mixtures [26,27,30] or trifluoracetic acid [27], resulting in better reproducibility and improvements in peak symmetry. However, for the less hydrophobic cations with alkyl side chains of less than four carbon atoms, retention and selectivity on classical RP stationary phases is slightly weaker [29]. In order to improve retention of the less hydrophobic cations, IPRPLC seems to be a method of choice as ion-pairs formation has been stated to play a determinant role in retention properties of imidazolium RTILs cations [24,28]. By IP-RPLC, cations separation may involve the formation of neutral ion-pairs involving hydrophobic interactions with stationary phase or a dynamic ion exchange between adsorbed counteranions of the mobile phase and eluted species [37–39]. According to the nature of analytes that must be separated, the choice of ion-pairing reagent appears to be decisive as various organic phase modifier have been reported [40–43]. In the present study, alkylsulfonate salts with long hydrophobic chains have been chosen as ion-pairing agents. Fig. 1A shows the separation of five RMIM RTILs cations (listed in Table 1), varying by the length of alkyl chains on imidazolium ring. Separation was performed on a C8 Metasil Basic stationary phase in a buffered acetonitrile/water mobile phase under isocratic conditions. Retention of polar compounds like EMIM or BMIM is improved compared to classical RPLC [29]. RTILs cations are eluted by order of increasing hydrophobicity. Capacity factor Ln k are linearly dependent of the hydrophobic character of each RTILs, i.e. the nc number of methylene group of the alkyl chain (Ln k = −0.022nc + 0.239, r2 = 0.970). Additional investigations have been carried out on the influence of organic modifier content and ion-pairing reagent concentration. As it was predicted by previous studies in IP-RPLC [37–39], an increase of retention is observed with a decrease of organic modifier content and an increase of ion-pairing reagent concentration in the mobile phase (data not shown). IP-RPLC with alkylsulfonate additives leads to a good separation for RMIM cations in terms of retention, selectivity and
Fig. 1. Isocratic separation of 10 L of a five RTILs imidazolium mixture (10−4 M). Compounds numbered as in Table 1. UV detection 210 nm (A) stationary phase: C8 Metasil Basic; mobile phase: AcN/H2 O 40/60 (v/v) with 10 mM of sodium 1-decanesulfonate; 0.5 mL min−1 , (B) stationary phase: Uptisphere OH; mobile phase: AcN/H2 O 90/10 (v/v) with 10 mM ammonium acetate; 0.2 mL min−1 .
resolution regarding to other RPLC and IC techniques. However, lack of volatility of sulfonate salts involves a significant decrease of ESI-MS signal as well as other additive salts commonly used [44]. This represents a major drawback for further characterization by LC/ESI-MS. 3.1.2. HILIC separation of RMIM cations Recent efforts were made by Stepnowski et al. in order to improve retention properties of short chains polar imidazolium RTILs cations. In particular, IC was used with a strong cation exchange stationary phase. Retention mechanisms include both electrostatic interactions and reversed-phase mechanisms according to the organic modifier content [27]. RPLC phenyl-bonded stationary phase was also recently described [30]. In this case, retention mechanisms involve – interactions between imidazolium ring and stationary phase. In order to develop more polar interactions between imidazolium RTILs cations and stationary phase, modified silica packings groups have been investigated [26]. These types of packing include mixed polar and apolar groups. Therefore, both hydrophilic and hydrophobic interactions between analytes and stationary phase are involved in retention process. Nevertheless, hydrophobic interactions are predominant as imidazolium cations are eluted in order of increasing hydrophobicity. Therefore, HILIC provides an alternative approach for enhancement of retention of polar RMIM RTILs by involving only hydrophilic interactions. Historically, HILIC is considered as a “normal phase separa-
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tion” in a “reversed-phase fashion” wherein separation occurs on polar stationary phases with aqueous-organic mobile phases, where water is the strongly eluting solvent [34]. Generally, when charged molecules are analyzed by HILIC, salt (or buffer) is an essential component in the mobile phase [45]. Indeed, the presence of a salt in the mobile phase leads to the formation of neutral ion-pairs between RMIM cation and acetate counter anion developing hydrophilic interactions with the stationary phase. Different stationary phases have been reported for HILIC separations of a wide range of charged or polar solutes: naked silica or bounded silica with various aminopropyl, amide, poly(succinimide), diol silica, cyanopropyl or sulfoalkylbetaine functionalities which have been recently reviewed [35]. Fig. 1B shows the chromatographic separation of the same mixture of RMIM RTILs cations on an Uptisphere diol stationary phase in buffered acetonitrile/water isocratic conditions. Compounds are eluted by order of increasing hydrophilicity. It has been stated that separations of polar compounds are based on the partition of analytes into and out of an adsorbed water-enriched layer of stagnant eluent on the stationary phase by hydrogen bonding and dipole–dipole interactions [34]. However, the role and the structure of water layer at the surface of stationary phase has not yet been elucidated [35]. For RMIM RTILs separation, there is also a linear correlation between capacity factor Ln k and number of methylene groups of alkyl chain (Ln k = −0.007nc + 2.635, r2 = 0.940), but instead of IP-RPLC, for nc number lower than 4, relation is no longer linear. This is also consistent with various interactions including both hydrogen bonding and hydrophilic interactions between solutes and stationary phase [46]. Retention and selectivity are also improved for polar EMIM and BMIM cations, which suffer from low retention or poor peak shapes on reversed stationary phase. Furthermore, HILIC method provides shorter analysis times compared to IP-RPLC, thus leading to the interest of the development of HILIC for quantitative analyses of imidazolium RTILs cations samples. In addition, use of high aqueous volatile mobile phase only buffered with ammonium acetate is a major advantage for further structural investigations by HILIC/ESI-MS of hydrophilic degradation products produced during imidazolium RTILs radiolysis. Investigations have been carried out on mobile and stationary phases in order to optimize separation. 3.1.3. Effect of acetonitrile content The effect of acetonitrile content in the mobile phase was studied for a range from 30 to 95%. Fig. 2 shows the variation of capacity factors as a function of the acetonitrile content in the mobile phase for the same mixture of five RTILs imidazolium cations. For acetonitrile compositions varying from 60 to 95%, RTILs retention shows a typical HILIC behaviour [45,47,48] with an enhancement of hydrophilic interactions between solutes and adsorbed water layer of the stationary phase as organic modifier content increases. For acetonitrile compositions varying from 40 to 60%, capacity factors for each compound tend to a constant value but the resolution of separation dramatically decreases as the acetonitrile content decreases. These modifications possibly involve secondary interactions between solutes and mobile phase at low acetonitrile content as it was stated in
Fig. 2. Plots of Ln k vs. acetonitrile content in the mobile phase for five RTILs (ref. in Table 1). Stationary phase: Uptisphere OH; mobile phase: AcN/H2 O with 10 mM ammonium acetate; 0.2 mL min−1 .
previous report [48]. For acetonitrile mobile phase composition below 30%, RTILs are co-eluted. As retention of RMIM RTILs cations increases with a decrease of the acetonitrile content of the mobile phase, stationary phase shows in these conditions typical RPLC behaviour. 90/10 (v/v) acetonitrile/water mobile phases have been chosen for further studies, as this mobile phase composition represents the better compromise between times of analysis and selectivity of separation. 3.1.4. Effect of ammonium acetate concentration No elution was obtained for RTILs imidazolium cations on diol stationary phase with a salt-free mobile phase. HILIC retention mechanism states that compounds are not eluted due to their strong charge-charge interactions with the residual silanols of the stationary phase. As it was explained previously, additive salts play a determinant role in HILIC retention. Particularly, ammonium acetate is often used as a convenient buffer salt in the mobile phase due to its good solubility at high organic content [49]. Therefore, effect of ammonium acetate on separation was investigated by varying acetate concentration in the acetonitrile/water mobile phase from 5 to 20 mmol L−1 . A decrease of capacity factors is observed when ammonium acetate concentration was raised from 5 to 20 mmol L−1 as shown on Fig. 3. At low salt concentrations, compounds are highly retained. They have a much greater tendency to reside in the stagnant water layer. As the salt concentration increases, excess amount of acetate counter anions promotes the formation of neutral ion-pairs with the charged solutes having better solubility in the mobile phase, thus resulting in shorter retention times. These behaviour is in agreement with previous works on diol packing [45] but retention behaviour with buffer concentration strongly depends of the polar stationary phase as it was observed by Guo et al. [47,48]. A 10 mM concentration of ammonium acetate was chosen for further studies. 3.1.5. Loading studies on stationary phase OMIM was selected for loading studies. Solutions of OMIM varying from 10−5 mol L−1 to 10−3 mol L−1 were injected in
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Fig. 3. Plots of Ln k vs. ammonium acetate concentration in the mobile phase for three RTILs (ref. in Table 1). Stationary phase: Uptisphere OH; mobile phase: AcN/H2 O with 10 mM ammonium acetate; 0.2 mL min−1 .
triplicate with a 10 L volume and eluted with a 90/10 acetonitrile mobile phase. Fig. 4 shows the overlaid chromatograms with injected amounts varying from 0.5 to 10 nmol. Injecting higher amounts of OMIM produces a characteristic fronting shape and overload of diol bounded functions occurs in this condition at 1 nmol injected, as a characteristic shift on retention time is observed. This profile corresponds to a concave isotherm of distribution for the analytes between stationary and mobile phase. Limit of detection (LOD) was evaluated at 2.57 × 10−5 mol L−1 and limit of quantification (LOQ) at 5.82 × 10−5 mol L−1 , which underlines a good sensitivity in regard to previous NMR analysis [19] or previous HPLC methods [21]. The area of the OMIM peak is proportional to the injected amount (y = 1010 x + 124643, r2 = 0.997). Retention times RSDs ranged from 0.79 to 1.16% and peak area RSDs ranged from 0.22 to 3.00%, which underlines good reproducibility of the method. As Uptisphere diol stationary phase provides both resolution for RTILs separation and sensitivity for quantitative measurements, it will be used for further studies on BMIM cation degradation.
Fig. 4. Overlaid chromatograms of OMIM RTIL. Injected amounts from 0.5 to 10 nmol. Stationary phase: Uptisphere OH; mobile phase: AcN/H2 O 90/10 with 10 mM of ammonium acetate; UV detection 210 nm.
Fig. 5. (A) Overlaid chromatograms of standard additions of pure BMIM solution (5 × 10−2 mol L−1 ) to irradiated BMIM solution (10−4 mol L−1 ) solution with OMIM as internal standard (10−4 mol L−1 ). (a) No addition; (b) addition of 40 L of pure BMIM at 5 × 10−2 mol L−1 in 10 mL of irradiated BMIM at 10−4 mol L−1 ; (c) addition of 60 L in 10 mL; (d) addition of 80 L in 10 mL; (e) addition of 100 L in 10 mL. (B) Standard addition curves for pure and gamma-irradiated BMIM solutions. Stationary phase: Uptisphere OH; mobile phase: AcN/H2 O 80/20 (v/v) with 10 mM of ammonium acetate; 0.2 mL min−1 .
3.2. Application of HILIC method for assessing BMIM gamma degradation Standard addition method is adapted for quantification of imidazolium RTILs cations in irradiated media. OMIM cation was used as internal standard. A more polar 80/20 (v/v) acetonitrile/water mobile phase was used in order to decrease times of analyses. Four additions of pure standard BMIM solution were done for both pure and gamma irradiated BMIM solution. Each sample was injected in triplicate. Overlaid chromatograms of the four standard additions for irradiated BMIM solution are presented on Fig. 5A. Standard calibration curve for pure and irradiated solution are presented on Fig. 5B. Concentration of pure BMIM solution was evaluated at (9.02 ± 0.01) × 10−5 mol L−1 and irradiated BMIM solution was evaluated at (7.19 ± 0.01) × 10−5 mol L−1 . The peaks heights ratios RSDs, corrected with the appropriate student factor (N = 95%), ranged from 0.66 to 2.68% for pure BMIM solution and from 0.45 to 2.62% for irradiated BMIM solution. By this method, results have shown that approximately 20% of BMIM cation is degraded under 2400 kGy gamma irradiation. This result confirms radioresistance of RTILs compared to conventional diluents used in reprocessing nuclear fuels [19]. It is thought that RTILs radiolysis products result of anion–cation
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recombination during radiolysis steps [20]. No extra peaks appear on HILIC chromatograms, probably due to the formation of unretained apolar species that elute with the injection peaks and radiolysis gases were also supposed in previous studies [19]. Formation of various degradation products at concentrations below the LOD could also explain the absence of extra peak on the chromatogram. Linearity and reproducibility combined with compatibility of HILIC mobile phases with ESI-MS make a convenient analytical method for quantitative analysis of imidazolium RTILs cations in irradiated samples and qualitative analysis of polar degradation products. 4. Conclusion Imidazolium RTILs cations with various hydrophobic characteristics have been separated by IP-RPLC on C8 stationary phase and by HILIC on diol stationary phases. Satisfactory separations have been obtained in terms of retention, selectivity and resolution in both cases. Although IP-RPLC with highly hydrophobic ion-pair reagents like alkylsulfonate provides a satisfactory separation with a sufficient retention for the most polar imidazolium cations, the use of mobile phase with high amounts of surfactants is clearly not compatible with ESI-MS detection for further investigations to determine RTILs radiolysis products. In this context, HILIC provides an efficient separation for both polar and apolar cations in terms of retention and selectivity. First results have confirmed the potential of HILIC method for quantification of imidazolium RTILs cations in irradiated samples. The uses of HILIC method for the determination of polar degradation compounds are under progress. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12]
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