Fullerene-impregnated ionic liquid stationary phases for gas chromatography

A new method has been developed to facilitate the use of fullerenes as stationary phases (SPs) in gas chromatography (GC). In this method, ionic liquids (ILs) are used as solvents to coat fullerenes (C(60), amino-C(60) and hydroxy-C(60)) onto GC columns. However, the ILs serve not just as coating solvents but also act synergistically with fullerenes to provide unique properties as stationary phases, namely dual modal characteristics. They act as non-polar SPs when separating non-polar analytes (aromatic hydrocarbon mixtures and alkane mixtures), and as polar SPs for polar analytes (e.g., alcohol mixtures). The polarity of the stationary phase can be adjusted by changing either the type of the IL and/or by adding either C(60) (or its amino or hydroxy derivatives) to the IL. It was found that C(60) and its derivatives produce not only a change in the polarity of the SP but also substantial enhancement in separation efficiencies for both non-polar and polar analytes. More importantly, when added to the IL SP, C(60) improves separation efficiencies not just for non-polar analytes (aromatic hydrocarbon mixtures and alkane mixtures) but also for polar analytes (mixtures of ortho-, meta- and para-xylene and alcohol mixtures) as well. Moreover, C(60) SP provides higher efficiencies than amino-C(60) and hydroxy-C(60) for separation of polar analytes. This is rather surprising considering that not only are amino-C(60) and hydroxy-C(60) more polar than C(60), but that the IL used to coat the amino- and hydroxy-C(60) (i.e., N-ethylpyridinium trifluoroacetate, [EtPy(+) CF(3)COO(-)]) is more polar than the IL used to coat the C(60) (i.e., octylmethylimidazolium bis(trifluoromethyl)sulfonyl)amide], [OMIm(+) (CF(3)SO(2))(2)N(-)]). Moreover, compared to its amino and hydroxy derivatives, the concentration of C(60) in the column was 10 times lower.     

Wrong gas/liquid partition data by gas chromatography

In a paper published in 1992 [K.S. Reddy, J.-Cl. Dutoit, E.sz. Kováts, Pair-wise interactions by gas chromatography. I. Interaction free enthalpies of solutes with non-associated primary alcohol groups, J. Chromatogr. 609 (1992) 229] retention indices and standard chemical potential differences (SPOT-s) are given for about 160 solutes on a liquid branched paraffin hydrocarbon of the carbon number, z = 78. For the temperature dependence of the SPOT-s the proposal of Kirchhoff was accepted i.e. that at higher temperatures the molar heat capacity difference of the solute between the gas phase and the solution is nearly constant in a broad temperature range. The data were determined under conditions where the effect of adsorption could be neglected. By comparing the published data with those determined on another branched paraffin of the carbon number, z = 87 [F. Riedo, D. Fritz, G. Tarján, E.sz. Kováts, A tailor-made C87 hydrocarbon as a possible non-polar standard stationary phase for gas chromatography, J. Chromatogr. 126 (1976) 63] it is shown that the reported thermodynamic constants of solutes determined experimentally at higher temperatures (the group "H") are wrong on C78. In the present paper, the method of correction and the resulting corrected data are given. In later papers retention data are given on eight polar derivatives of the hydrocarbon C78 relative to those measured on the nonpolar standard [R. Cloux, G. Défayes, K. Fóti, J.-Cl. Dutoit, E.sz. Kováts, Pair-wise interactions by gas chromatography. III. Synthesis of isosteric stationary phases for gas chromatography, Synthesis (1993) 909; G. Défayes, K.S. Reddy, A. Dallos, E.sz. Kováts, Pair-wise interactions by gas chromatography. V. Interaction free enthalpies of solutes with primary chloro- and bromo-alkanes, J. Chromatogr. A 699 (1995) 131; K.S. Reddy, E.sz. Kováts, Pair-wise interactions by gas chromatography. II. Sociation free enthalpies of some hydrogen bonding solutes with non-associated primary alcohol groups, Chromatographia 34 (1992) 249; K.S. Reddy, R. Cloux, E.sz. Kováts, Pair-wise interactions by gas chromatography. IV. Interaction free enthalpies of solutes with trifluoromethyl-substituted alkanes, J. Chromatogr. A 673 (1994) 181; K.S. Reddy, R. Cloux, E.sz. Kováts, Pair-wise interactions by gas chromatography. VI. Interaction free enthalpies of solutes with primary methoxyalkane, cyanoalkane and alkanethiol groups, J. Chromatogr. A 704 (1995) 387; A. Dallos, A. Sisak, Z. Kulcsár, E.sz. Kováts, Pair-wise interactions by gas chromatography. VII. Interaction free enthalpies of solutes with secondary alcohol groups, J. Chromatogr. A 904 (2000) 211]. Obviously, the same data which are wrong in the nonpolar standard are also wrong in these solvents. The corrected data in the polar solvents are given in supplementary tables.     

Retention of opioids and their glucuronides on a combined zwitterion and hydrophilic interaction stationary phase

A stationary phase combining zwitterionic ion chromatography and hydrophilic interaction chromatography (ZIC-HILIC) from SeQuant was evaluated for the chromatography of some opiates and their polar metabolites. The effects of mobile phase constitution on retention and resolution were extensively evaluated. Different aspects of mobile phase constitution such as ion strength and type of buffer, type and amount of organic modifier and pH were examined. The selectivity and retention of the opiates compared to their glucuronides could be substantially altered with small changes of the mobile phase, especially when the type of buffer, i.e., formate or acetate and organic modifier, i.e., acetonitrile or methanol were changed. The retention on the ZIC-HILIC was dominated by hydrophilic interaction chromatography (HILIC) but considerable effects on the selectivity was observed, possibly caused by an ion exchange mechanism due to interactions with the charges on the stationary phase.     

The Choice of Polar Stationary Phases for Gas–Liquid Chromatography by Statistical Analysis of Retention Data

We have studied the retention indices of 127 volatile substances on a C₇₈-paraffin and on its seven nearly isochor and isomorphous polar derivatives over a temperature range of 90–210 °C. The retention index of a substance on the C₇₈ paraffin has been considered as the standard. The additional retention on the polar derivative was given by the difference of its retention index on the polar solvent and on the C₇₈-paraffin. Statistical analyses of the additional retention have shown that with respect to retention, the seven polar solvents can be classified into three groups: Type I: TTF (tetrakistrifluoromethyl), MTF (monotrifluoromethyl), Type II: PCN (primary cyano), PSH (primary thiol) and Type III: TMO (tetramethoxy), SOH (secondary alcohol) and POH (primary alcohol). It is shown that these three types are best represented by the solvents TTF, PCN and TMO. PSH (primary thiol) is aligned with PCN at temperatures up to about 150 °C, but is similar to TMO at 210 °C.     

Comparison of common mobile‐phase volume markers with polar‐group‐containing reversed‐phase stationary phases

A systematic study of the behavior of several common mobile‐phase volume markers using traditional and polar‐group‐containing reversed‐phase stationary phases is presented. Examined mobile‐phase volume markers include two neutral molecules, uracil and thiourea, concentrated (0.10 M) and dilute (0.0001 M) KNO₃, and D₂O. Mobile‐phase volumes are examined over the entire reversed‐phase mobile‐phase range of 100% water to 100% methanol or acetonitrile. The behavior of these mobile‐phase volume markers is compared with a maximum theoretical value (i.e. the void volume), as determined by pycnometry. The data suggest that: (i) uracil begins to fail as a mobile‐phase volume marker in mobile phases below about 40% strong solvent for polar group containing phases; (ii) in nearly all cases, the mobile‐phase volume measured dynamically is smaller than the pycnometric void volume; (iii) a significant dependence of measured mobile‐phase volume on salt concentration is seen on the polar endcapped phase, which is not observed on the traditional and embedded polar group phase; and (iv) D₂O does not work well as a mobile‐phase volume marker with polar‐group‐containing phases, possibly due to interaction with the stationary phase polar group.     

Analysis of Solvent–Solvent Interactions in Mixed Isosteric Solvents by Inverse Gas Chromatography

The Flory–Huggins interaction parameter, χ ₂₃, characterizing the interaction between solvents in a mixed stationary phase has been calculated using inverse gas chromatography (IGC). The χ ₂₃ parameters for four mixed solvent systems formed by mixing a non-polar branched alkane, 19, 24-dioctadecyldotetracontane (C78), with four different polar solvents are analysed as a function of temperature. Two of the polar solvents are formed by replacing one of the –CH₃ groups of C78 by –OH (POH) group and –CN (PCN) group and the other two polar solvents are formed by replacing all the four –CH₃ groups of C78 by four –CF₃ (TTF) groups and four –OCH₃ (TMO) groups. The five solvents have similar structures and nearly the same molar volume and in mixtures, they form regular solutions. For the mixture, C78 + POH, χ ₂₃ was calculated at two compositions and for the remaining three mixtures χ ₂₃ was calculated at equimolar composition. Eight solute probes representing all possible chemical structures and polarity were used under conditions approaching infinite dilution. The effects of temperature, concentration of the mixed solvent and probe solute on χ ₂₃ are presented and discussed. The probe-independent χ ₂₃ values were also calculated by linear regression analysis using the retention data of all the solute probes. In all the mixed solvents considered here the χ ₂₃ values are probe-dependent and decrease with increase of temperature.     

Screening approach for chiral separation of pharmaceuticals IV. Polar organic solvent chromatography

The aim of this work is to determine generic screening conditions and an initial simple separation strategy allowing the rapid separation of drug enantiomers in polar organic solvent chromatography (POSC). Four cellulose/amylose-based stationary phases were investigated in detail using two mobile phase basis solvents commonly applied in this mode, i.e. acetonitrile and methanol. Polar mode is interesting for use in purification of enantiomers. In a first step, the parameters potentially influencing the separation, such as addition of an alcohol to the polar organic solvent or the type of mobile phase additive(s), were examined by means of experimental designs. Afterwards, the factors found most important are investigated in more detail. Results showed that the cellulose- and amylose-based stationary phases have very broad and complementary enantiorecognition abilities in the POSC mode. The type of organic solvent for the mobile phase appeared to have a dramatic influence on the quality of the separation. Based on the results, a screening strategy was proposed. Enantioseparation was observed in more than 85% of the tested compounds and analysis times of last eluted peak were usually below 10 min.     

羧基甜菜碱型亲水作用色谱柱的制备及性能研究

以甲基丙烯酰氧乙基二甲基乙酸铵(CBMA)为功能单体,利用表面引发原子转移自由基聚合(Surface-initiated atom transfer radical polymerization, SI-ATRP)技术,将CBMA接枝到硅胶表面,得到接枝聚合物CBMA的亲水作用色谱固定相(Silica-CBMA).通过改变SI-ATRP反应体系中单体的浓度,制备了3种不同接枝量的亲水作用色谱固定相.考察了Silica-CBMA固定相对有机酸类化合物的分离性能以及流动相中pH值、盐浓度、水含量等因素对溶质保留行为的影响.结果表明,在亲水作用色谱模式下,Silica-CBMA固定相对有机酸类化合物的分离是离子交换作用与亲水作用的混合色谱模式.流动相中盐浓度增大,溶质保留减弱,符合离子交换作用特征;固定相和溶质的离子化程度受流动相pH值影响较大,pH值增大,溶质保留增强;而溶质的保留时间随流动相水含量增加而降低则是典型的亲水作用色谱特征.使用自制Silica-CBMA柱,建立了芦丁片中维生素C、芦丁含量的亲水作用色谱测定方法,操作方法简单,为强极性样品的分离测定提供了新方法.     

Simple and Rapid Determination of Denaturants in Alcohol Formulations by Hydrophilic Interaction Chromatography

A hydrophilic interaction chromatography (HILIC) technique has been developed and validated for determination of common denaturants (denatonium benzoate, crystal violet and methylene blue) in denaturated alcohol formulations. Among the three different polar stationary phases (i.e., aminopropyl, cyanoethyl and silica) studied the cyanoethyl phase provided much stronger retention for the organic cations. It was shown that high efficiencies were reached only with anionic ion-pairing reagent that reduces the interactions with the silanol groups. The anion ion-pairing strength under HILIC conditions was: acetate < formate < trifluoroacetate < perchlorate. This study also investigated the effect of various experimental factors on the retention of the cyanoethyl stationary phase, such as acetonitrile content, pH and ion-pairing anion concentration in the mobile phase. The separation of three denaturants was achieved in about 8 min with a mobile phase containing 60% (v/v) acetonitrile and 10 mmol L⁻¹ HClO4. The proposed method was validated and applied to the determination of danaturating agents in various Lithuanian denaturated alcohol formulations.     

Comparison of the Retention of Aliphatic Hydrocarbons with Polar Groups in RP-HPLC Systems with Different Modifiers of the Binary Eluent

The retention of aliphatic hydrocarbons with polar groups has been compared in respect to the separation selectivity changes in reversed-phase high-performance liquid chromatography with C18 stationary phase type and binary water eluent composed of methanol, acetonitrile, or tetrahydrofuran as modifiers. The changes in separation selectivity when one modifier is replaced by another in the eluent is explained, taking into consideration molecular interactions of the solutes with components of the stationary phase region, i.e., extracted modifier, and ordering of the stationary phase by the modifier.     

双柱气相色谱法测定有机物的正辛醇/水分配系数

通过对烷烃，醇类等有机物在五种不同极性固定相上的气相色谱保留值的测定，提出了从有机物气相色谱保留值测定其正辛醇/分配系数的公式，研究了从不同极性固定相上测得的保留值来测定其分配系数的准确性，得出烷烃类在所有柱上测得的保留值与其分配系数间均具有很好的关联，烷烃醇类在极性相异柱上的保留值之比值与其分配系数间也有很好的相关性，从而为烷烃及醇类等有机物分配系数的测定提供了一种简单、可靠，易行的新方法。     

Comparison of eluents in supercritical fluid chromatography as based on density and free volume

A comparative study of chromatographic properties of different supercritical eluents has been performed by considering the retention at equal density or at equal free volume for different mobile phases. At equal density, the temperature dependence of the capacity ratios of aromatic hydrocarbons is the same for different alkane mobile phases, whereas significant deviations are observed when carbon dioxide is employed as the mobile phase. At equal free volume, a comparison of the capacity ratios measured in carbon dioxide and in alkane eluents showed a pronounced similarity for the different mobile phases when the free volume was low, i.e., at high densities. With increasing free volume, the chemical differences of the alkanes on the one hand and carbon dioxide on the other become more apparent in their elution behavior. Furthermore, it is demonstrated that the free volume can also be used as a reference basis for the data of solvatochromic shift experiments.     

Multi-modal applicability of a reversed-phase/weak-anion exchange material in reversed-phase,anion-exchange,ion-exclusion,hydrophilic interaction and hydrophobic interaction chromatography modes

We recently introduced a mixed-mode reversed-phase/weak anion-exchange type separation material based on silica particles which consisted of a hydrophobic alkyl strand with polar embedded groups (thioether and amide functionalities) and a terminal weak anion-exchange-type quinuclidine moiety. This stationary phase was designed to separate molecules by lipophilicity and charge differences and was mainly devised for peptide separations with hydroorganic reversed-phase type elution conditions. Herein, we demonstrate the extraordinary flexibility of this RP/WAX phase, in particular for peptide separations, by illustrating its applicability in various chromatographic modes. The column packed with this material can, depending on the solute character and employed elution conditions, exploit attractive or repulsive electrostatic interactions, and/or hydrophobic or hydrophilic interactions as retention and selectivity increments. As a consequence, the column can be operated in a reversed-phase mode (neutral compounds), anion-exchange mode (acidic compounds), ion-exclusion chromatography mode (cationic solutes), hydrophilic interaction chromatography mode (polar compounds), and hydrophobic interaction chromatography mode (e.g., hydrophobic peptides). Mixed-modes of these chromatographic retention principles may be materialized as well. This allows an exceptionally flexible adjustment of retention and selectivity by tuning experimental conditions. The distinct separation mechanisms will be outlined by selected examples of peptide separations in the different modes.     

Elution parameters in constant-pressure, single-ramp temperature-programmed gas chromatography

The dependence of the degree of interaction of a solute with the stationary phase at the time of its elution from the column in temperature-programmed GC is best described by interaction level of the solute. The latter represents the fraction of a solute residing in the stationary phase relative to the total amount of the solute. A simple approach to the evaluation of interaction levels of eluting solutes in a single-ramp temperature program is proposed. In a single-ramp temperature program having no preceding temperature plateau, all solutes that elute at temperatures that are about 60 degrees C higher than the initial temperature of the heating ramp elute with nearly the same interaction levels that can be found as exp(-r), where r is dimensionless heating rate. A specially designed temperature plateau preceding the ramp causes all solutes eluting during the entire time of the ramp to elute with nearly the same interaction levels equal to exp(-r). A transformation of the interaction level of a solute into its retention factor or mobility factor (a fraction of a solute in a mobile phase in relation to the total amount of the solute) and vice versa is also described.     

Effect of ionic additives on the elution of sodium aryl sulfonates in supercritical fluid chromatography

Addition of a small amount of polar solvent (i.e., modifier) to CO2 in packed column supercritical fluid chromatography (SFC) has shown major improvements in both polar analyte solubility and interaction of the polar analyte with the stationary phase. Recently, the addition of an ionic component (i.e., additive) to the primary modifier by one of us has been shown to extend even further the application of SFC to polar analytes. In this work, the effect of various ionic additives on the elution of ionic compounds, such as sodium 4-dodecylbenzene sulfonate and sodium 4-octylbenene sulfonate, has been studied. The additives were lithium acetate, ammonium acetate, tetramethylammonium acetate, tetrabutylammonium acetate, and ammonium chloride dissolved in methanol. Three stationary phases with different degrees of deactivation were considered: conventional cyanopropyl, deltabond cyanopropyl, and bare silica. The effect of additive concentration and additive functionality on analyte retention was investigated. Sodium 4-dodecylbenzene sulfonate was successfully eluted using all the additives with good peak shape under isocratic/isobaric/isothermal conditions. Different additives, however, yielded different retention times and in some cases different peak shapes.     

Comparison of the Retention of Aliphatic Hydrocarbons with Polar Groups in RP-HPLC Systems with Different Modifiers of the Binary Eluent

The retention of aliphatic hydrocarbons with polar groups has been compared in respect to the separation selectivity changes in reversed-phase high-performance liquid chromatography with C18 stationary phase type and binary water eluent composed of methanol, acetonitrile, or tetrahydrofuran as modifiers. The changes in separation selectivity when one modifier is replaced by another in the eluent is explained, taking into consideration molecular interactions of the solutes with components of the stationary phase region, i.e., extracted modifier, and ordering of the stationary phase by the modifier.

     

Capillary electrochromatography with monolithic silica columns. IV. Electrochromatographic characterization of polar bonded monolithic stationary phases having surface-bound cyano functionalities

Two polar ligands, namely 3-hydroxypropionitrile and 1H-imidazole-4,5-dicarbonitrile (IDCN) were covalently attached to epoxy-activated silica-based monolithic capillary columns via an epoxide ring-opening reaction to yield CN-OH-Monolith and 2CN-OH-Monolith, respectively. The silica monolith was prepared by a sol-gel process, and the resulting "rod-like" stationary phase was subjected to pore tailoring with an alkaline solution to convert small pore domains to mesopore domains, thus yielding a monolith with bimodal pore structure consisting of flow through pores (i.e., flow channels for mobile-phase flow) and mesopores that provide most of the adsorption capacity of the monolith toward the separated solutes. The two polar monoliths, CN-OH-Monolith and 2CN-OH-Monolith, were evaluated in normal-phase CEC with organic-rich mobile phases less polar than the stationary phase. The 2CN-OH-Monolith bearing more polar functions than the CN-OH-Monolith exhibited more retention and improved selectivity toward model polar solutes.     

Mobile phase effects in reversed-phase liquid chromatography: a comparison of acetonitrile/water and methanol/water solvents as studied by molecular simulation

Molecular simulations of water/acetonitrile and water/methanol mobile phases in contact with a C(18) stationary phase were carried out to examine the molecular-level effects of mobile phase composition on structure and retention in reversed-phase liquid chromatography. The simulations indicate that increases in the fraction of organic modifier increase the amount of solvent penetration into the stationary phase and that this intercalated solvent increases chain alignment. This effect is slightly more apparent for acetonitrile containing solvents. The retention mechanism of alkane solutes showed contributions from both partitioning and adsorption. Despite changes in chain structure and solvation, the molecular mechanism of retention for alkane solutes was not affected by solvent composition. The mechanism of retention for alcohol solutes was primarily adsorption at the interface between the mobile and stationary phase, but there were also contributions from interactions with surface silanols. The interaction between the solute and surface silanols become very important at high concentrations of acetonitrile.     

Study of enantioselectivity on an immobilized amylose carbamate stationary phase under subcritical fluid chromatography

In this paper, we describe the enantiomeric separation of a chiral alcohol under subcritical fluid chromatography conditions using a 3 μm particle size bonded amylose carbamate stationary phase. Linear and branched alcohols were used as polar modifiers in CO₂. The studies with linear alcohols showed a decrease in selectivity factor as the number of carbons in the linear chain increased. For branched alcohols, as the bulk of substituents at the α carbon atom increases the separation factor decreases. Thermodynamic studies showed that in the presence of the alcohols studies, except methanol and ethanol, a positive ΔΔS was observed. Molecular mechanics simulation brought more insights into the mechanism of enantiomeric separation on this stationary phase under subcritical fluid chromatography.     

HPLC Characterization of the Association Mechanism of Organic Compounds with β Cyclodextrin Using a Novel Boron Nitride Nanotube Particulate Stationary Phase

In this paper, a simple homogeneous coating of silica spherical particles with pristine boron nitride nanotubes (BNNTs) was described. BNNTs dissolved in dimethylacetamide (DMAc) were mixed with amino-functionalized silica particles having a 5 μm diameter. Favorable interaction between the amino group and the BNNT surfaces induces the absorption of the BNNTs on the silica. The BNNT-coated silica particles were used as stationary phase for HPLC. For the first time, it was demonstrated that this new particulate BNNT stationary phase can be used for the study of the complexation of solute molecules (terpene molecules used as test drugs in this work) with β cyclodextrin (βCD). The apparent formation constants Kf of terpene derivative/βCD were in the same magnitude as those reported in the literature. The plot of Kf versus the water fraction in the methanol/water mobile phase showed that the BNNT surface played an active role in the complex formation due to terpene/BNNT-specific polar interactions. This work demonstrated that our novel particulate BNNT HPLC stationary phase was an efficient tool to study molecular recognition mechanism and more specifically the association between a drug substance and a target molecule with the aim of reaching biopharmaceutic and clinical applications.