Binary ionic liquid mixtures as gas chromatography stationary phases for improving the separation selectivity of alcohols and aromatic compounds

Binary mixtures of two ionic liquids (ILs), 1-butyl-3-methylimidazolium chloride (BMIM-Cl) and 1-butyl-3-methylimidazolium bis[(trifluoromethyl)sulfonyl]imide (BMIM-NTf2), have been studied for the first time as gas chromatographic stationary phases. The two ILs differ only in the nature of the associated anion. The solvation parameter model was used to examine the change of solvation interactions with the IL stationary phase composition. The hydrogen bond basicity increased linearly as the stationary phase was enriched with the BMIM-Cl IL. The retention factor of short-chained alcohols increased by as much as 1100% when performing the separation on a column containing an IL mixture of 25% BMIM-NTf2/75% BMIM-Cl compared to that of the neat BMIM-NTf2 IL column. By tuning the composition of the IL-stationary phase, the separation selectivity and resolution factors of alcohols and aromatic compounds were improved. A reversal of elution order was observed for specific classes of analytes with enhancements in the stationary phase hydrogen bond basicity.     

Determination of solute partition behavior with room-temperature ionic liquid based micellar gas-liquid chromatography stationary phases using the pseudophase model

The use of micelles in ionic liquid based gas-chromatography stationary phases was evaluated using equations derived for a "three-phase" model. This model allows the determination of all three partition coefficients involved in the system, and elucidates the micellar contribution to retention and selectivity. Four types of micellar-ionic liquid columns were examined in this study: 1-butyl-3-methylimidazolium chloride with sodium dodecylsulfate or dioctyl sulfosuccinate, and 1-butyl-3-methylimidazolium hexafluorophosphate with polyoxyethylene-100-stearyl ether or polyoxyethylene-23-lauryl ether. The partition coefficients were measured for a wide range of probe molecules capable of a variety of types and magnitudes of interactions. In general, most probe molecules preferentially partitioned to the micellar pseudophase over the bulk ionic liquid component of the stationary phase. Therefore, addition of surfactant to the stationary phase usually resulted in greater solute retention. It is also shown that the selectivity of the stationary phase is significantly altered by the presence of micelles, either by enhancing or lessening the separation. The effects of surfactant on the interaction parameters of the stationary phase are determined using the Abraham solvation parameter model. The addition of sodium dodecylsulfate and dioctyl sulfosuccinate to 1-butyl-3-methylimidazolium chloride stationary phases generally increased the phase's hydrogen bond basicity and increased the level of dispersion interaction. Polyoxyethylene-100-stearyl ether and polyoxyethylene-23-lauryl ether surfactants, however, enhanced the pi-pi/n-pi, polarizability/dipolarity, and hydrogen bond basicity interactions of 1-butyl-3-methylimidazolium hexafluorophosphate to a greater degree than the ionic surfactants with 1-butyl-3-methylimidazolium chloride. However, these nonionic surfactants appeared to hinder the ability of the stationary phase to interact with solutes via dispersion forces. Therefore, it is possible to effectively predict which analytes will be most highly retained by these micellar-ionic liquid stationary phases.     

Evaluating the solvation properties of functionalized ionic liquids with varied cation/anion composition using the solvation parameter model

Ionic liquids (ILs) are promising gas chromatography (GC) stationary phases due to their high thermal stability, negligible vapor pressure, and ability to solvate a broad range of analytes. The tunability of ILs allows for structure modification in pursuit of enhanced separation selectivity and control of analyte elution order. In this study, the solvation parameter model is used to characterize the solvation interactions of fifteen ILs containing various cationic functional groups (i.e., dimethylamino, hydroxyl, and ether) and cation types paired with various counter anions, namely, tris(pentafluoroethyl)trifluorophosphate (FAP(-)), bis[(trifluoromethyl)sulfonyl]imide (NTf(2)(-)), thiocyanate (SCN(-)), tricyanomethide (C(CN)(3)(-)), tetracyanoborate (B(CN)(4)(-)), and bis[oxalate(2-)]borate (BOB(-)). The presence of functional groups affected the hydrogen bond basicity, hydrogen bond acidity, as well as dispersion interactions of the resulting ILs, while the change of cation type yielded modest influence on the dipolarity. The switch of counter anions in unfunctionalized ILs produced compounds with higher dipolarity and hydrogen bond basicity. The dipolarity and hydrogen bond basicity of ILs possessing cyano-containing anions appeared to be inversely proportional to the cyano content of the anion. The modification of IL structure resulted in a significant effect on the retention behavior as well as separation selectivity for many solutes, including reversed elution orders of some analytes. This study provides one of the most comprehensive examinations up-to-date on the relation between IL structure and the resulting solvation characteristics and gives tremendous insight into choosing suitable ILs as GC stationary phases for solute specific separations.     

Adjusting the chromatographic properties of poly(ionic liquid)‐modified stationary phases by substitution on the imidazolium cation

Poly(ionic liquid)‐modified stationary phases can have multiple interactions with solutes. However, in most stationary phases, separation selectivity is adjusted by changing the poly(ionic liquid) anions. In this work, two poly(ionic liquid)‐modified silica stationary phases were prepared by introducing the cyano or tetrazolyl group on the pendant imidazolium cation on the polymer chains. Various analytes were selected to investigate their mechanism of retention in the stationary phases using different mobile phases. Two poly(ionic liquid)‐modified stationary phases can provide various interactions toward solutes. Compared to the cyano‐functionalized poly(ionic liquid) stationary phase, the tetrazolyl‐functionalized poly(ionic liquid) stationary phase provides additional cation‐exchange and π‐π interactions, resulting in different separation selectivity toward analytes. Finally, applicability of the developed stationary phases was demonstrated by the efficient separation of nonsteroidal anti‐inflammatory drugs.     

Characterization of hexacationic imidazolium ionic liquids as effective and highly stable gas chromatography stationary phases

Polycationic ionic liquids (ILs) are an attractive class of ILs with great potential applicability as gas chromatography stationary phases. A family of hexacationic imidazolium ILs derived from the cycloalkanol family was chemically first prepared in a straightforward manner and then applied for analytical separation purposes. Four tuneable engineering vectors, namely cation ring size structure, anion nature, spatial disposition of cycloalkanol substituents and O‐substitution, were considered as experimental parameters for the design of the desired ionic liquids. A total number of five new phases based on a common benzene core respectively exhibited column efficiencies around to 2500 plates/m, broad operating temperature ranges and also, even more importantly, good thermal stabilities (bleeding temperature between 260 and 365°C), finding variations in the selectivity and analytes elution orders depending on the IL structures. Their solvation characteristics were evaluated using the Abraham solvation parameter model, establishing clear correlations between their cation structure and retention capability with respect to certain analytes. The study of relationships between the ILs structure and solvation parameters gives us an idea of the IL stationary phase to be used for specific separations.     

Lipophilicity measurement of drugs by reversed phase HPLC over Wide pH range using an alkaline-resistant silica-based stationary phase, XBridge Shield RP(18)

We propose a reversed phase HPLC (RP-HPLC) with an alkaline-resistant silica-based stationary phase, XBridge Shield RP(18), for the determination of the lipophilicity of drugs with diverse chemical nature ranging from acidic to basic. A set of 40 model compounds with well-defined solvatochromic parameters was selected to allow a broad distribution of structural properties. The chromatographic results showed that the lipophilicity index log k(w) obtained with XBridge Shield RP(18) was well correlated with experimental log P(oct) values (r(2)=0.96). Linear solvation free-energy relationship (LSER) analyses revealed that the retention mechanism of the stationary phase and 1-octanol/water partitioning were controlled by almost the same balance of intermolecular forces (hydrophobicity as expressed by the van der Waals volume V(w), H-bond acceptor basicity beta, and dipolarity/polarizability pi*). The results showed that XBridge Shield RP(18) phase overcomes the shortcomings of the silica-based stationary phases, the application of which to lipophilicity measurements had been limited to neutral and acidic compounds.     

Preparation and evaluation of a novel bonded imidazolium ionic liquid as stationary phase for gas chromatography

1‐Butyl‐3‐[(3‐trimethoxysilyl)propyl]imidazolium chloride ionic liquid was synthesized and chemically modified onto the inner wall of a fused capillary column as a stationary phase for gas chromatography. The 1‐butyl‐3‐[(3‐trimethoxysilyl)propyl]imidazolium chloride ionic liquid bonded capillary column was evaluated in detail. The results revealed that the ionic liquid bonded capillary column exhibited high column efficiency of 1.08 × 10⁴ plates/m, and good chromatographic separation selectivity (α ) for polar and non‐polar substances, and a good thermal stability between room temperature and 400°C. Moreover, the determination of thermodynamic parameters and the linear solvation energy relationship were further carried out. The results indicated that the chromatographic retention of each probe molecule on the ionic liquid bonded stationary phase was an enthalpy‐driven process, and the system constants of the linear solvation energy relationship signified that the dispersion interaction, the hydrogen bonding acidity and hydrogen bonding basicity were dominant interactions between probes and stationary phase among five interactions during the chromatographic separation. However, the contribution of each specific interaction for the stationary phase is ranked as the dispersion interaction > the hydrogen bonding basicity > the hydrogen bonding acidity.     

Characterization of stationary phases in subcritical fluid chromatography by the solvation parameter model. I. Alkylsiloxane-bonded stationary phases

Varied types of alkylsiloxane-bonded and fluoroalkylsiloxane-bonded stationary phases, all commercially available, were investigated with subcritical fluid mobile phase. The effect of the alkyl chain length (from C4 to C18) and of the nature of the bonding (fluorodecylsiloxane, phenyl-C18 and polar-embedded-C18) on the chromatographic behaviour was investigated by the use of a linear solvation energy relationship (LSER), the solvation parameter model. A large set of test compounds provides precise and reliable information on the intermolecular interactions responsible for retention on these stationary phases used with a subcritical mobile phase. First of all, the results underline the close properties between subcritical fluid and organic liquid. The use of non aqueous mobile phases reduces the cavity energy and the mobile phase acidity generally encountered with aqueous liquid phases, allowing other interactions to take a part in retention. As expected, an increase in the alkyl chain length favours the dispersive interactions between the solutes and the stationary phases. Changes in basicity and acidity of the stationary phases are also related to the chain length, but, in this case, mobile phase adsorption onto the stationary phase is supposed to explain these behaviours. The addition of a phenyl group at the bottom of the C18 chain, near the silica, does not induce great modifications in the retentive properties. The fluorodecylsiloxane and the polar-embedded alkylsiloxane phases display very different properties, and can be complementary to the classical alkylsiloxane-bonded phases. In particular, the fluorinated phase does not favour the dispersive interactions, in comparison to hydrogenated stationary phases, when the basicity of the polar-embedded phase is obviously greater than the one of classical alkylsiloxane-bonded phases, due to the amide function. Finally, logk-logk curves plotted between the different phases illustrate the effect of the interaction properties on the retention of different classes of compounds.     

硅烷基咪唑双三氟甲烷磺酰亚胺离子液体气相色谱固定相的性能评价

以双三氟甲烷磺酰亚胺离子([NTf₂]^-)为阴离子,合成阳离子烷基取代不同(C1、C2和C4)的硅烷基咪唑离子液体,以其为固定相制备气相色谱填充柱。 硅烷基咪唑离子液体为强极性固定相;阳离子结构影响固定相的热稳定性、极性和分离性能。 在这些离子液体固定相中,1-丁基-3-[(3-三甲氧基硅基)-丙基]咪唑双三氟甲烷磺酰亚胺([PBIM]NTf₂)对Grob试剂分离性能较好。 利用溶剂化作用参数模型,评价[PBIM]NTf₂固定相特性,研究固定相-组分分子之间相互作用机制;同时考察[PBIM]NTf₂色谱柱对不同类型化合物的分离性能。 结果表明,[PBIM]NTf₂固定相主要作用力是氢键碱性和偶极作用,对烷烃、醇、酯和胺等不同类型的样品组分表现出良好的分离能力。     

Chiral Separation of Organic Phosphonate Compounds on Polysaccharide-Based Chiral Stationary Phases

Four polysaccharide-based chiral stationary phases have been used to separate the enantiomers of fourteen O,O-dialkyl-1-benzyloxycarbonyl-aminoarylmethyl phosphonates. These polysaccharide-based chiral stationary phases are Chiralpak AD, Chiralpak AS, Chiralcel OG and Chiralcel OJ. The data obtained indicate that the chiral separation ability for these organophosphonate compounds are in the order Chiralpak AD > Chiralcel OG > Chiralcel OJ > Chiralpak AS. With Chiralpak AD, all of the studied compounds could be easily baseline separated. Those two polysaccharides possess different chiral discrimination mechanism due to of the difference of the conformational structures of amylose and cellulose. The chiral discrimination of derivatized amylose chiral stationary phases were based on the stereogenic fit of the analytes in the helical structures of amylose and the transient diastereomeric complex formation between the analyte and the amylose CSP through π–π interaction H-bond interactions and induced dipole interactions exerted by the substituents on the analyte molecules. The chiral discrimination, in case of derivatized cellulose chiral stationary phase is based on the stereogenic fit of the analytes in the grooves of cellulose followed by interactions mentioned above between the analytes and the cellulose CSP.     

Separation Characteristics of Ionic Liquids Grafted Polymethylsiloxanes Stationary Phases for Capillary GC

Ionic liquids (ILs) grafted polymethylsiloxane (PMS) stationary phases (IL-PMS) for capillary gas chromatography (CGC) are described. The stationary phases were synthesized by grafting 1-vinyl-3-hexylimidazolium (VHIm) with either NTf ₂ ^? or PF₆ ^? anion to poly(methylhydrosiloxane) (PMS-VHIm-NTf₂, PMS-VHIm-PF₆) and coated statically onto fused-silica capillary columns. Separation characteristics of the stationary phases involving Abraham solvation parameters, separation ability and thermal stability were investigated. The obtained solvation parameters reveal that both IL-PMS stationary phases exhibited unique intermolecular interactions compared with either ILs or PMS due to the synergistic effect of ILs and PMS chemically combining together. The separation performance of the IL-PMS stationary phases was investigated by a Grob mixture and a complex mixture of 26 compounds of different types. The results show that the present stationary phases exhibit excellent resolution and selectivity for the analytes of interest with narrow and symmetric peak shapes. Thermal stability was also investigated by column bleed profiles with satisfactory results. The satisfactory chromatographic performance and thermal stability of the IL-PMS stationary phases suggest their great potential as a new type of CGC stationary phases.     

Highly selective GC stationary phases consisting of binary mixtures of polymeric ionic liquids

GC stationary phases composed of binary mixtures of two polymeric ionic liquids (PILs), namely, poly(1‐vinyl‐3‐hexylimidazolium) bis[(trifluoromethyl)sulfonyl]imide (poly(ViHIm‐NTf₂))/poly(1‐vinyl‐3‐hexylimidazolium) chloride (poly(ViHIm‐Cl)) and poly(1‐vinyl‐3‐hexadecylimidazolium) bis[(trifluoromethyl)sulfonyl]imide (poly(ViHDIm‐NTf₂))/poly(1‐vinyl‐3‐hexadecylimidazolium) chloride (poly(ViHDIm‐Cl)), were evaluated in terms of their on‐set bleed temperature and separation selectivity. A total of six neat or binary PIL stationary phases were characterized using the solvation parameter model to investigate the effects of the polymeric cation and anion and PIL composition on the system constants of the resulting stationary phases. The hydrogen bond basicity of the mixed poly(ViHIm‐NTf₂)/poly(ViHIm‐Cl) stationary phases was enriched linearly with the increase in the poly(ViHIm‐Cl) content. Results revealed that tuning the composition of the stationary phase allowed for fine control of the retention factors and separation selectivity for alcohols and carboxylic acids as well as selected ketones, aldehydes, and aromatic compounds. A reversal of elution order was observed for particular classes of analytes when the weight percentage of the chloride‐based PIL was increased.     

Linear Solvation Energy Relationships in the Determination of Specificity and Selectivity of Stationary Phases

The retention of fifty structurally different compounds has been studied using linear solvation energy relationships. Investigations were performed with the use of six various stationary phases with two mobile phases (50/50?% v/v methanol/water and 50/50?% v/v acetonitrile/water). Packing materials were home-made and functionalized with octadecyl, alkylamide, cholesterol, alkyl-phosphate and phenyl molecules. This is the first attempt to compare all of these stationary phases synthesized on the same silica gel batch. Therefore, all of them may be compared in more complex and believable way, than it was performed earlier in former investigations. The phase properties (based on Abraham model) were used to the classification of stationary phases according to their interaction properties. The hydrophilic system properties s, a, b indicate stronger interactions between solute and mobile phase for most of the columns. Both e and v cause greater retention as a consequence of preferable interactions with stationary phase by electron pairs and cavity formation as well as hydrophobic bonds. However, alkyl-phosphate phase has different retention properties, as it was expressed by positive sign of s coefficient. It may be concluded that most important parameters influencing the retention of compounds are volume and hydrogen bond acceptor basicity. The LSER coefficients showed also the dependency on the type of organic modifier used as a mobile phase component.     

Linear Solvation Energy Relationships in the Determination of Specificity and Selectivity of Stationary Phases

The retention of fifty structurally different compounds has been studied using linear solvation energy relationships. Investigations were performed with the use of six various stationary phases with two mobile phases (50/50 % v/v methanol/water and 50/50 % v/v acetonitrile/water). Packing materials were home-made and functionalized with octadecyl, alkylamide, cholesterol, alkyl-phosphate and phenyl molecules. This is the first attempt to compare all of these stationary phases synthesized on the same silica gel batch. Therefore, all of them may be compared in more complex and believable way, than it was performed earlier in former investigations. The phase properties (based on Abraham model) were used to the classification of stationary phases according to their interaction properties. The hydrophilic system properties s, a, b indicate stronger interactions between solute and mobile phase for most of the columns. Both e and v cause greater retention as a consequence of preferable interactions with stationary phase by electron pairs and cavity formation as well as hydrophobic bonds. However, alkyl-phosphate phase has different retention properties, as it was expressed by positive sign of s coefficient. It may be concluded that most important parameters influencing the retention of compounds are volume and hydrogen bond acceptor basicity. The LSER coefficients showed also the dependency on the type of organic modifier used as a mobile phase component.

     

Characterisation of stationary phases in subcritical fluid chromatography with the solvation parameter model. III. Polar stationary phases

In this third paper, varied types of polar stationary phases, namely silica gel (SI), cyano (CN)- and amino-propyl (NH2)-bonded silica, propanediol-bonded silica (DIOL), poly(ethylene glycol) (PEG) and poly(vinyl alcohol) (PVA), were investigated in subcritical fluid mobile phase. This study was performed to provide a greater knowledge of the properties of these phases in SFC, and to allow a more rapid and efficient choice of polar stationary phase in regard of the chemical nature of the solutes to be separated. The effect of the nature of the stationary phase on interactions between solute and stationary phases and between solute and carbon dioxide-modifier mobile phases was studied by the use of a linear solvation energy relationship (LSER), the solvation parameter model. The retention behaviour observed with sub/supercritical fluid with carbon dioxide-methanol is close to the one reported in normal-phase liquid chromatography with hexane. The hydrogen bond acidity and basicity, and the polarity/polarizability favour the solute retention when the molar volume of the solute reduces it. As with non-polar phases, the absence of water in the subcritical fluid allows the solute/stationary phase interactions to play a greater part in the retention behaviour. As expected, the DIOL phase and the bare silica display a similar behaviour towards acidic and basic solutes, when interactions with basic compounds are lower with the NH2 phase. On the CN phase, all interactions (hydrogen bonding, dipole-dipole and charge transfer) have a nearly equivalent weight on the retention. The polymeric phases, PEG and PVA, provide the most accurate models, possibly due to their better surface homogeneity.     

Investigation of the ion-exchange properties of methacrylate-based mixed-mode monolithic stationary phases employed as stationary phases in capillary electrochromatography

The potential of methacrylate-based mixed-mode monolithic stationary phases bearing sulfonic acid groups for the separation of positively charged analytes (alkylanilines, amino acids, and peptides) by capillary electrochromatography (CEC) is investigated. The retention mechanism of protonated alkylanilines as positively charged model solutes on these negatively charged mixed-mode stationary phases is investigated by studying the influence of mobile phase and stationary phase parameters on the corrected retention factor which was calculated by taking the electrophoretic mobility of the solutes into consideration. It is shown that both solvophobic and ion-exchange interactions contribute to the retention of these analytes. The dependence of the corrected retention factor on (1) the concentration of the counter ion ammonium and (2) the number of methylene groups in the alkyl chain of the model analytes investigated shows clearly that a one-site model (solvophobic and ion-exchange interactions take place simultaneously at a single type of site) has to be taken to describe the retention behaviour observed. Comparison of the CEC separation of these charged analytes with electrophoretic mobilities determined by open-tubular capillary electrophoresis shows that mainly chromatographic interactions (solvophobic and ion-exchange interactions) are responsible for the selectivity observed in CEC, while the electrophoretic migration of these analytes plays only a minor role.     

Preparation and comparison of three zwitterionic stationary phases for hydrophilic interaction liquid chromatography

Surface‐bonded zwitterionic stationary phases have shown highlighted performances in separation of polar and hydrophilic compounds under hydrophilic interaction chromatography mode. So, it would be helpful to evaluate the characteristics of zwitterionic stationary phases with different arranged charged groups. The present work involved the preparation and comparison of three zwitterionic stationary phases. An imidazolium ionic liquid was designed and synthesized, and the cationic and anionic moieties respectively possessed positively charged imidazolium ring and negatively charged sulfonic groups. Then, the prepared ionic liquid, phosphorylcholine and an imidazolium‐based zwitterionic selector were bonded on the surface of silica to obtain three zwitterionic stationary phases. The selectivity properties were characterized and compared through the relative retention of selected solute pairs, and different kinds of hydrophilic solutes mixtures were used to evaluate the chromatographic performances. Moreover, the zwitterionic stationary phases were further characterized by the modified linear solvation energy relationship model to probe the multiple interactions. All the results indicated that the types and arrangement of charged groups in zwitterionic stationary phases mainly affect the retention and separation of ionic or ionizable compounds, and for interaction characteristics the contribution from n and π electrons and electrostatic interactions displayed certain differences.