Examination of ionic liquids and their interaction with molecules, when used as stationary phases in gas chromatography.

Stable room-temperature ionic liquids (RTILs) have been used as novel reaction solvents. They can solubilize complex polar molecules such as cyclodextrins and glycopeptides. Their wetting ability and viscosity allow them to be coated onto fused silica capillaries. Thus, 1-butyl-3-methylimidazolium hexafluorophosphate and the analogous chloride salt can be used as stationary phases for gas chromatography (GC). Using inverse GC, one can examine the nature of these ionic liquids via their interactions with a variety of compounds. The Rohrschneider-McReynolds constants were determined for both ionic liquids and a popular commercial polysiloxane stationary phase. Ionic liquid stationary phases seem to have a dual nature. They appear to act as a low-polarity stationary phase to nonpolar compounds. However, molecules with strong proton donor groups, in particular, are tenaciously retained. The nature of the anion can have a significant effect on both the solubilizing ability and the selectivity of ionic liquid stationary phases. It appears that the unusual properties of ionic liquids could make them beneficial in many areas of separation science.     

Immobilized ionic liquids as high-selectivity/high-temperature/high-stability gas chromatography stationary phases

Ionic liquids (ILs) are a class of nonmolecular solvents in which the cation/anion combination can be easily tuned to provide desired chemical and physical properties. When used as stationary phases in gas-liquid chromatography, ionic liquids exhibit dual nature retention selectivity. That is, they are able to separate polar molecules such as a polar stationary phase and nonpolar molecules such as a nonpolar stationary phase. However, issues such as optimization of the wetting ability of the ionic liquid on fused-silica capillaries, the maximum operating temperatures of the stationary phases, and nonuniform film thickness on the wall of the capillary at high temperatures have limited their use in gas chromatography. As described in this paper, these limitations are overcome by cross-linking a new class of ionic liquid monomers by free radical reactions to provide a more durable and robust stationary phase. By lightly cross-linking the ionic liquid stationary phase using a small amount of free radical initiator, high-efficiency capillary columns were produced that are able to endure high temperatures with little column bleed. Two types of cross-linked IL stationary phases are developed. A partially cross-linked stationary phase allows for high-efficiency separations up to temperatures of approximately 280 degrees C. However, by creating a more highly cross-linked stationary phase of geminal dicationic ILs, exclusively, an increase in efficiency is observed at high temperatures allowing for its use over 350 degrees C. In addition, through the use of solvation thermodynamics and interaction parameters, it was shown that the cross-linking/immobilization of the ionic liquid does not affect the selectivity of the stationary phase thereby preserving its dual nature retention behavior.     

Chiral ionic liquids as stationary phases in gas chromatography

Recently, it has been found that room-temperature ionic liquids can be used as stable, unusual selectivity stationary phases. They show "dual nature" properties, in that they separate nonpolar compounds as if they are nonpolar stationary phases and separate polar compounds as if they are polar stationary phases. Extending ionic liquids to the realm of chiral separations can be done in two ways: (1) a chiral selector can be dissolved in an achiral ionic liquid, or (2) the ionic liquid itself can be chiral. There is a single precedent for the first approach, but nothing has been reported for the second approach. In this work, we present the first enantiomeric separations using chiral ionic liquid stationary phases in gas chromatography. Compounds that have been separated using these ionic liquid chiral selectors include alcohols, diols, sulfoxides, epoxides, and acetylated amines. Because of the synthetic nature of these chiral selectors, the configuration of the stereogenic center can be controlled and altered for mechanistic studies and reversing enantiomeric retention.     

Solvent extraction of sr2+ and cs+ based on room-temperature ionic liquids containing monoaza-substituted crown ethers

A series of N-alkyl aza-18-crown-6 ethers were synthesized and characterized by NMR spectroscopy and mass spectrometry. These monoaza-substituted crown ethers in ionic liquids were investigated as recyclable extractants for separation of Sr(2+) and Cs(+) from aqueous solutions. The pH-sensitive complexation capability of these ligands allows for a facile stripping process to be developed so that both macrocyclic ligands and ionic liquids can be reused. The extraction efficiencies and selectivities of these monoaza-substituted crown ethers for Na(+), K(+), Cs(+), and Sr(2+) were studied in comparison to those of dicyclohexano-18-crown-6 under the same conditions. The extraction selectivity order for dicyclohexano-18-crown-6 in the ionic liquids investigated here was K(+) > Sr(2+) > Cs(+) > Na(+). The extraction selectivity order for N-alkyl aza-18-crown-6, in which the alkyl group is varied systematically from ethyl to n-dodecyl, was Sr(2+) > K(+) > Cs(+) > Na(+) in 1-ethyl-3-methylimidazolium bis[(trifluoromethyl)sulfonyl]amide and 1-butyl-3-methylimidazolium bis[(trifluoromethyl)sulfonyl]amide and K(+) > Sr(2+) > Cs(+) > Na(+) in 1-hexyl-3-methylimidazolium bis[(trifluoromethyl)sulfonyl] amide and 1-octyl-3-methylimidazolium bis[(trifluoromethyl)sulfonyl]amide. The strong dependence of selectivity on the type of ionic liquid indicates an important role played by solvation in solvent extraction processes based on ionic liquids. The optimization of macrocyclic ligands and ionic liquids led to an extraction system that is highly selective toward Sr(2+).     

Synthesis and evaluation of an aromatic polymer-coated zirconia for reversed-phase liquid chromatography

We synthesized a novel aromatic polymer-coated zirconia-based RPLC stationary phase by chemical adsorption of a copolymer of chloromethylstyrene and diethoxymethylvinylsilane onto zirconia (CMS/VMS-ZrO2). Characterization of the pore structure of the support by nitrogen porosimetry and inverse size-exclusion chromatography indicates that CMS/VMS-ZrO2 maintains the well-defined pore structure of the base material. Flow studies show that CMS/VMS-ZrO2 has good mass transfer characteristics. The reversed-phase retention characteristics of the new support are comparable to those of conventional silica-bonded phases. We have also evaluated the mechanical, thermal, and pH stability of CMS/VMS-ZrO2. The results show that CMS/VMS-ZrO2 is stable over a very wide range of pH (pH = 1-13) and at temperatures as high as 160 degrees C. Chromatographic separations of some low molecular weight aromatic analytes on CMS/VMS-ZrO2 and octadecyl-bonded silica phases indicate that there are some subtle but significant differences in the chromatographic selectivity of these two types of phases.     

Nonmolecular solvents in separation methods: dual nature of room temperature ionic liquids

Room temperature ionic liquids (RTIL) are molten salts starting to be used as nonmolecular solvents in separation methods mainly for their extremely low vapor pressure and thermal stability. RTILs are formed by an anion associated to a cation. This intrinsic structure gives them a dual nature. When used as additives in RPLC mobile phases to enhance basic compound separation, RTILs lose their particular physicochemical properties to become just salts. However, a given RTIL is not equivalent to another one made with the same cation. It is shown that both the anion and the cation contribute to solute retention and peak efficiency extending beyond simple "salting-out" or ion-pairing effects. Nine different alkyl-methyl-imidazolium ionic liquids with different alkyl chain length and chloride or BF(4-) or PF(6-) anions were used as additives (50 mM max. conc.) in the liquid chromatography separation of some cationic basic solutes on a Kromasil C18 column. It is shown with sodium salts and an acetonitrile-water 30/70 v/v mobile phase that anions can adsorb on the stationary phase surface according to their lyotropic character. They can also form ion pairs with the cationic basic solutes. Alkyl-imidazolium cations also adsorb on the C18 bonded stationary phase due to hydrophobic character depending on their alkyl chain length. Anion adsorption dramatically increases the cationic solute retention factors when cation adsorption decreases them. The cation adsorption is mainly responsible for peak shape and efficiency enhancements. RTILs are additives that enhance the basic cationic solute peak shape changing peak position. A wise choice of the appropriate combination of anion lyotropy with imidazolium cation hydrophobicity allows playing with solute selectivity and analysis duration.     

The effect of stationary-phase pore size on retention behavior in micellar liquid chromatography

One of the limitations that has restricted the applicability of micellar liquid chromatography (MLC) is the weak eluting power of micellar mobile phases compared to conventional hydro-organic mobile phases used in reversed-phase liquid chromatography. This may be the result of Donnan or steric exclusion of the micelles from the pores of the stationary phase, within which nearly all (> or = 99%) of the stationary phase resides and the analytes spend most of their time. To determine whether wide-pore stationary phases would overcome this limitation in MLC, several C8 and C18 stationary phases ranging from 100 to 4000 A were investigated using a diverse set of test solutes and micellar solutions of anionic, neutral, and cationic surfactants as mobile phases. With the larger pore size stationary phases, the eluting power of the MLC mobile phases was enhanced with all surfactant types, the greatest effect being with the neutral surfactant. Differences in retention behavior were observed between various solute types and between the C8 and C18 stationary phases. These differences appear to be related to the relative hydrophobicity of the solutes and to differences in the surfactant-modified stationary phases. Partitioning behavior of representative solutes on the large-pore C8 and C18 columns was shown to follow the three-phase partitioning model for MLC. Methylene group selectivity data showed only minor differences in the stationary-phase characteristics between the small- and large-pore size C18 columns. The true eluting power of micellar mobile phases was revealed with wide-pore stationary phases and was demonstrated by the separation and elution of an extended series of alkylphenones on C18 columns.     

Comparative study of hydrocarbon, fluorocarbon, and aromatic bonded RP-HPLC stationary phases by linear solvation energy relationships.

The retention properties of eight alkyl, aromatic, and fluorinated reversed-phase high-performance liquid chromatography bonded phases were characterized through the use of linear solvation energy relationships (LSERs). The stationary phases were investigated in a series of methanol/water mobile phases. LSER results show that solute molecular size and hydrogen bond acceptor basicity under all conditions are the two dominant retention controlling factors and that these two factors are linearly correlated when either different stationary phases at a fixed mobile-phase composition or different mobile-phase compositions at a fixed stationary phase are considered. The large variation in the dependence of retention on solute molecular volume as only the stationary phase is changed indicates that the dispersive interactions between nonpolar solutes and the stationary phase are quite significant relative to the energy of the mobile-phase cavity formation process. PCA results indicate that one PCA factor is required to explain the data when stationary phases of the same chemical nature (alkyl, aromatic, and fluoroalkyl phases) are individually considered. However, three PCA factors are not quite sufficient to explain the whole data set for the three classes of stationary phases. Despite this, the average standard deviation obtained by the use of these principal component factors are significantly smaller than the average standard deviation obtained by the LSER approach. In addition, selectivities predicted through the LSER equation are not in complete agreement with experimental results. These results show that the LSER model does not properly account for all molecular interactions involved in RP-HPLC. The failure could reside in the V2 solute parameter used to account for both dispersive and cohesive interactions since "shape selectivity" predictions for a pair of structural isomers are very bad.     

Matrix-assisted laser desorption/ionization mass spectrometric analysis of uncomplexed highly sulfated oligosaccharides using ionic liquid matrices

Direct UV matrix-assisted laser desorption/ionization (MALDI) mass spectrometric analysis of uncomplexed, underivatized, highly sulfated oligosaccharides has been carried out using ionic liquids as matrices. Under conventionally used MALDI time-of-flight experimental conditions, uncomplexed polysulfated oligosaccharides do not produce any signal. We report that 1-methylimidazolium alpha-cyano-4-hydroxycinnamate and butylammonium 2,5-dihydroxybenzoate ionic liquid matrices allow the detection of picomole amounts of the sodium salts of a disaccharide, sucrose octasulfate, and an octasulfated pentasaccharide, Arixtra. The experimental results indicate that both analytes undergo some degree of thermal fragmentation with a mass loss corresponding to cleavage of O-SO3Na bonds in the matrix upon laser irradiation, reflecting lability of sulfo groups.     

An approach to the concept of resolution optimization through changes in the effective chromatographic selectivity.

It is very common chromatographic practice to optimize resolution by making changes in selectivity by systematically varying key retention controlling factors. In many instances, a change in conditions merely results in monotonic, systematic variation in the relative retention of all pairs of peaks. Useful or "effective" changes in selectivity generally result when we see peak crossovers, changes in elution order or differential changes in band position of three or more peaks upon changing some operating condition. In this work, we demonstrate that changes in what we now call the effective selectivity can only take place when retention depends on a minimum of two solute molecular properties and further the dependencies must differ for the two sets of conditions. To verify our concept, real chromatographic data are examined from the viewpoint of linear solvation energy relationships (LSERs) and linear solvent strength theory. Five different RPLC stationary phases in different eluents are compared to elucidate the similarities and differences in their effective selectivities. Of major importance is our finding that the effective selectivity can only be understood when it is viewed in terms of the ratios of system-dependent interaction coefficients, such as the LSER coefficients, and not merely the absolute values of the coefficients. We confirm, both theoretically and experimentally, that a change in mobile-phase volume fraction and in column temperature is not as powerful a mechanism for tuning the effective selectivity as is a change in stationary-phase type.     

A new class of magnetic fluids: bmim[FeCl/sub 4/] and nbmim[FeCl/sub 4/] ionic liquids

The responses to a magnet of two room-temperature ionic liquids containing tetrachloroferrate(III) ions, 1-butyl-3-methylimidazolium tetrachloroferrate (bmim[FeCl/sub 4/]) and 1-butyronitrile-3-methylimidazolium tetrachloroferrate (nbmim[FeCl/sub 4/]) are compared. Although their magnetic susceptibilities are similar, the observed responses are distinct from each other, suggesting that the response is determined not only by the magnetic susceptibility but also by the other factors including density, viscosity, and surface tension. The two magnetic ionic liquids constitute a new class of magnetic fluids that hold many attractive physical properties for practical applications.     

Hydrophilic interaction chromatography using methacrylate-based monolithic capillary column for the separation of polar analytes

A porous zwitterionic monolith was prepared by thermal copolymerization of N,N-dimethyl-N-methacryloxyethyl-N-(3-sulfopropyl)ammonium betaine and ethylene dimethacrylate inside a 100-mum-i.d. capillary. The resulting monolith was evaluated as a hydrophilic liquid chromatography (HILIC) stationary phase. No evidence of swelling or shrinking of the monolith in different polarity solvents was observed. A typical HILIC mechanism was observed at higher organic solvent content (ACN% > 60%). The poly(SPE-co-EDMA) monolith showed very good selectivity for neutral, basic, and acidic polar analytes. For charged analytes, both hydrophilic interactions and electrostatic interactions contributed to their retention, which provide chromatographers more choice to optimize the separations.     

Capillary electrochromatography with fused-silica capillaries packed with copolymeric reversed-phase adsorbent and ion exchangers

Macroporous poly(styrene-divinylbenzene) (PSDVB), PRP-1, a reversed-phase adsorbent, and PSDVB-based strong acid cation exchangers and strong base and weak base anion exchangers were evaluated as stationary phases for capillary electrochromatography (CEC). Electroosmotic flow (EOF) for adsorbent and exchanger packed fused-silica capillaries for acetone as the marker increases with increasing ion exchange capacity, buffer organic solvent concentration, and applied voltage, is nearly independent of pH, and decreases with increased buffer ionic strength. For anion exchangers, EOF is reversed. Thiourea, acetone, acrylamide, nitromethane, propanal, and acetic acid were evaluated as EOF markers and undergo weak interaction with the PSDVB-based stationary phases. EOF in a basic buffer is greater than or equal to silica-based C-18 and cation exchanger packed capillaries. For an acidic buffer, EOF for a PRP-1 capillary is almost twice the C-18 packed capillary. As analyte hydrophobicity increases, retention and migration time increases for the PSDVB-based stationary phases. As exchange capacity increases, availability of the polymeric matrix for analyte partitioning decreases, causing analyte migration time to decrease. Increasing buffer organic solvent concentration decreases analyte retention. The PSDVB-based stationary phases provide good resolving power and reproducibility and are applicable to the CEC separation of neutral, weakly acidic, and basic analytes. Efficiency, however, is less than obtained with silica-based stationary phases. Because of stability in a strong acid buffer, the CEC separation of weak acids, where dissociation is suppressed, and weak bases as cations is possible. Separations of short-chain alkyl aldehydes, methyl ketones, aromatic hydrocarbons, substituted benzene derivatives, and short-chain carboxylic acids are described.     

Adjusting selectivity in liquid chromatography by use of the thermally tuned tandem column concept

In this study, we propose the novel "thermally tuned tandem column (T3C)" concept for the optimization of selectivity in LC by continuous adjustment of the stationary phase. Two columns with distinctly different chromatographic selectivities (e.g., polybutadiene- and carbon-coated zirconia) are serially coupled and independently temperature-controlled. Selectivity is "tuned" by adjusting the individual temperatures of the two columns. The effect of changing column temperature is quite analogous to changing the relative column lengths, thereby altering the relative and absolute contribution each column makes to the overall retention time in T3C. The distinct selectivity differences between polybutadiene- and carbon-coated zirconia as well as the extraordinary thermal stability of zirconia-based phases (thermally stable to 200 degrees C) allow us to tune the overall chromatographic selectivity over a very substantial range. We have developed a simplified useful model, which characterizes retention and selectivity for the T3C system as a function of the two column temperatures. The model is in good agreement with the experimental results. We also describe a simple computer-assisted optimization strategy based on the window diagram method, which facilitates the optimization of the T3C system with only four or five initial runs.     

Ionic liquids: a new class of sensing materials for detection of organic vapors based on the use of a quartz crystal microbalance

A QCM device employing ionic liquids as the sensing materials for organic vapors has been developed and evaluated. The sensing mechanism is based on the fact that the viscosity of the ionic liquid membrane decreases rapidly due to solubilization of analytes in the ionic liquids. This change in viscosity, which varies with the chemical species of the vapors and the types of ionic liquids, results in a frequency shift of the corresponding quartz crystal. The QCM sensor demonstrated a rapid response (average response time of less than 2 s) to organic vapors with an excellent reversibility because of the fast diffusion of analytes in ionic liquids. Furthermore, the ionic liquids, with zero vapor pressure and stable chemical properties, ensure a long-term shelf life for the sensor.     

Ionic liquids as advantageous solvents for headspace gas chromatography of compounds with low vapor pressure

The potential of ionic liquids as solvents for headspace gas chromatography was investigated. Three compounds with boiling points above 200 degrees C were selected to demonstrate the feasibility of the concept described. 2-Ethylhexanoic acid, formamide, and tri-n-butylamine as examples of acidic, neutral, and basic analytes were dissolved in acidic 1-n-butyl-3-methylimidazolium hydrogen sulfate (1), neutral 1-n-butyl-2,3-dimethylimidazolium dicyanamide (2), and 2 containing 1,8-diazabicyclo[5.4.0]undec-7-ene to adjust basic conditions. All analytes could be determined with limits of detection and limits of quantification in the low-ppm concentration range.     

Direct coupling of ionic liquid based single-drop microextraction and GC/MS

The use of ionic liquids as extracting media in single-drop liquid-phase microextraction (SDME) and its direct coupling to gas chromatography/mass spectrometry (GC/MS) is presented. For this purpose, a new removable interface that enables the introduction of the extracted analytes into the GC system, while preventing the ionic liquid from entering the column, has been developed. The determination of three representative pollutants in water samples has been used as a model analytical problem in order to demonstrate the feasibility of the proposed interface. The analytes (dichloromethane, p-xylene, and n-undecane) were coextracted from the aqueous sample in a 2-microL drop of 1-butyl-3-methylimidazolium hexaflourophosphate. Then, the syringe used to perform the SDME was directly introduced into the interface, which was held at 140 degrees C in order to achieve a complete volatilization of the target compounds. After the injection, the ionic liquid was retained in the interface, while a carrier gas transferred the volatilized analytes into the GC inlet. The optimization of the operational variables affecting the new interface (temperature, carrier flow rate, sample volume and injection technique) was accomplished. The analytes could be determined with detection limits in the low-nanogram per milliliter concentration range, and the relative standard deviations were between 3.3 and 4.4%.     

Extended thermodynamic approach to ion interaction chromatography

The chromatographic behavior of charged analytes in ion interaction chromatography (IIC) is theoretically investigated. The chemical modifications of the stationary and mobile phases in the presence of ion interaction reagent (IIR) are theoretically shown to change the partition coefficient for charged molecules. The most reliable literature experimental results concerning retention behavior of charged molecules in IIC were used to test the new theory. Retention equations are compared with those that can be obtained from the most important retention models in IIC. The present exhaustive retention model, which is well-founded in physical chemistry, goes further than the previous ones whose retention equations can be viewed as limiting cases of the present theory. The present extended thermodynamic approach reduces to stoichiometric or electrostatic retention models if the surface potential or pairing equilibria are respectively neglected. Moreover, it is able to quantitatively explain experimental evidences that cannot be rationalized by the existing retention models.     

Retention of ions on nonporous charged stationary phases

The interactions between ion-exchange resins and counterions consist of several mechanisms, such as ion-pair formation between active sites and counterions, specific adsorption, solvation changes, and double-layer accumulation. The double-layer accumulation of ions, which is a typical nonstoichiometric mechanism, is an important factor governing overall ion-exchange chromatographic retention when a major part of the stationary-phase surface is in contact with eluent flows. Nonporous stationary phases, where solutes are accessible to the surfaces by convection as well as by diffusion, possibly highlight this nonstoichiometric contribution through the coupling of a flow profile with an electrostatic potential function. The retention of ions on nonporous stationary phases has been interpreted by a model derived on the basis of the Poisson-Boltzmann equation including solvation change terms. Unusual retention behaviors have been confirmed only for anions, and can be explained by the model including the assumption that anions undergo solvation changes in a thin layer (approximately 5 nm thickness) at the vicinity of the stationary phase; the thickness should be a function of eluent flow rates. This strongly suggests that there is a difference in solvation nature between cations and anions. It can be inferred that water molecules interacting with polymer domains of the stationary phase behave like single molecules and cannot form a stable hydration shell around an anion as usually seen in bulk solution.