Stationary Phases In Terms Research Articles

2D metal-organic framework nanosheets-assembled core-shell composite material as stationary phase for hydrophilic interaction liquid chromatography

The core-shell composites were first prepared by surface modification of silica spheres with two-dimensional MOF-FDM-23 nanosheets via a simple approach. The successful immobilization of 2D MOF-FDM-23 on the silica (2D MOF-FDM-23@silica) was confirmed by series of method including elemental analysis, X-ray photoelectron spectroscopy (XPS), Fourier-transform infrared (FT-IR) transmission electron microscopy (TEM), scanning electron microscopy (SEM), powder X-ray diffraction (XRD) and the Brunauer–Emmett–Teller (BET). The obtained material showed excellent separation performance for multiple polar compounds such as sulfonamides, alkaloids, antibiotic and amino acid compounds etc. In addition, it was found to be superior to most reported MOF-based stationary phases in terms of separation performance, preparative reproducibility and chromatographic stability. The reproducibility of material preparation was demonstrated by the difference in retention time of the same mixture on three batches of the material at different times. The relative standard deviation (RSD) of the retention time for preparation repeatability was found to range from 0.5% to 1.4%. In short, the core-shell composite material prepared by this method broadened the application of MOF-based composites as chromatography stationary phase, and significantly developed the field of 2D MOF nanosheets in chromatography.

An alternative approach for preparation of amide-embedded stationary phase for reversed-phase liquid chromatography

Polar-embedded stationary phases are superior to conventional alkyl stationary phases in terms of water-wettability, silanol activity and polar selectivity. A novel method was proposed for efficient preparation of amide-embedded stationary phases bearing extended aliphatic chain and large aromatic ring. The present approach, which simply required isocyanate and desired carboxylic acid, rather than the normal combination of amine/acyl chloride/proton scavenger, produced aromatic pyrenyl amide-embedded (PYE-Amide) and aliphatic docosanyl amide-embedded (C22-Amide) stationary phases. The as-synthesized packing materials were characterized and evaluated in comparison with conventional octadecyl stationary phases in terms of bonding density, aromatic and hydrophobic natures, charge-transfer capability, and selectivity towards isomers of various unsaturated compounds. A high affinity of the unsaturated ligand for the unsaturated solutes, especially the electron-deficient ones, was observed. The PYE-Amide stationary phase exhibited unique aromaticity-dependent shape selectivity, which was beneficial to the resolution of certain geometric isomers of polycyclic arenes. The applicability of such stationary phases was demonstrated by effective separations of estrogens and sulfonamide drugs.

An insight into hydrogen isotopic separation on iron-alumina and chromium-alumina as stationary phase in gas chromatographic method

In gas chromatographic separation of hydrogen isotopes iron-alumina as a stationary phase is conventionally used. Further, Chromium-alumina as an alternative stationary phase has also been mentioned in the literature. In this present study, a detail comparative study of both the stationary phases in terms of their hydrogen isotope separation ability has been carried out. This study shows that chromium-alumina is better alternative than iron-alumina which might be attributed to its improved resolution, higher column efficiency, easy mode of preparation and shorter retention time along with equally simple regeneration procedure for regaining the performance of column material. To understand the basis behind their difference in ability of hydrogen isotope separation, hydrogen adsorption/desorption phenomenon at experimentally condition along with different surface related properties of both stationary phases have been evaluated. These experimental findings have been correlated with hydrogen isotope separation ability in terms of its retention time and resolution obtained during hydrogen isotopic separation and it illustrates the fact that hydrogen adsorption/desorption phenomenon is one of the governing factor in deciding the performance of the column materials.

Separation of flavonoids on different phenyl-bonded stationary phases-the influence of polar groups in stationary phase structure

Four phenyl-bonded stationary phases, differing in polar embedded group between spacer and phenyl ring, were used for the separation of flavonoids in reversed-phase conditions. In addition, the work was focused on the comparison of these stationary phases in terms of retention and nature of interactions between flavonoid solutes and both, mobile and stationary phases. The differences and similarities between the columns and between individual flavonoids were evaluated by a statistical analysis. The retention over the wider range of mobile phase composition was described using well known model suggested for partition chromatographic systems. Due to differences in polarity of flavonoids, gradient elution had to be applied to achieve appropriate conditions for the successful separation. A chromatographic optimization software was employed for establish the appropriate profiles of gradient separations using UV detection at 275nm. The most appropriate conditions for the separation of flavonoids were apparent on the phenyl and phenoxy columns.

Alkali Metal Modification of Silica Gel-Based Stationary Phase in Gas Chromatography

Modification of the precipitated silica gel was done by treatment with alkali metal (NaCl) before and after calcination. The silica surfaces before and after modification were confirmed by infrared spectroscopy in order to observe the strength and abundance of the acidic surface OH group bands which play an important role in the adsorption properties of polar and nonpolar solutes. The surface-modified silica gels were tested as GC solid stationary phases in terms of the separation efficiency for various groups of non-polar and polar solutes. Also, thermodynamic parameters (<svg style="vertical-align:-0.0pt;width:25.924999px;" id="M1" height="11.425" version="1.1" viewBox="0 0 25.924999 11.425" width="25.924999" xmlns:xlink="http://www.w3.org/1999/xlink" xmlns="http://www.w3.org/2000/svg"> <g transform="matrix(.017,-0,0,-.017,.062,11.363)"><path id="x394" d="M600 0h-557v24l268 633l28 8l261 -641v-24zM497 50l-194 489l-196 -489h390z" /></g><g transform="matrix(.017,-0,0,-.017,10.959,11.363)"><path id="x1D43B" d="M865 650q-1 -4 -4 -14t-4 -14q-62 -5 -77 -19.5t-29 -82.5l-74 -394q-12 -61 -0.5 -77t75.5 -21l-6 -28h-273l8 28q64 5 82 21t29 76l36 198h-380l-37 -197q-11 -64 0.5 -78.5t79.5 -19.5l-6 -28h-268l6 28q60 6 75.5 21.5t26.5 76.5l75 394q13 66 2 81.5t-77 20.5l8 28&#xA;h263l-6 -28q-58 -5 -75.5 -21t-30.5 -81l-26 -153h377l29 153q12 67 2 81t-74 21l5 28h268z" /></g> </svg>, <svg style="vertical-align:-0.23206pt;width:23.325001px;" id="M2" height="11.75" version="1.1" viewBox="0 0 23.325001 11.75" width="23.325001" xmlns:xlink="http://www.w3.org/1999/xlink" xmlns="http://www.w3.org/2000/svg"> <g transform="matrix(.017,-0,0,-.017,.062,11.4)"><use xlink:href="#x394"/></g><g transform="matrix(.017,-0,0,-.017,10.959,11.4)"><path id="x1D43A" d="M713 296l-5 -25q-47 -7 -59 -20t-23 -72l-15 -79q-9 -48 -3 -74q-15 -3 -55 -13t-63 -15t-59.5 -10t-70.5 -5q-149 0 -243 80t-94 220q0 169 127.5 276.5t336.5 107.5q91 0 206 -36l-10 -165l-29 -1q1 85 -47.5 126t-146.5 41q-153 0 -243.5 -97t-90.5 -242&#xA;q0 -122 68.5 -198.5t188.5 -76.5q121 0 139 75l20 86q13 58 -1.5 70.5t-99.5 20.5l5 26h267z" /></g> </svg>, and <svg style="vertical-align:-0.23206pt;width:19.174999px;" id="M3" height="11.75" version="1.1" viewBox="0 0 19.174999 11.75" width="19.174999" xmlns:xlink="http://www.w3.org/1999/xlink" xmlns="http://www.w3.org/2000/svg"> <g transform="matrix(.017,-0,0,-.017,.062,11.4)"><use xlink:href="#x394"/></g><g transform="matrix(.017,-0,0,-.017,10.959,11.4)"><path id="x1D446" d="M457 488l-30 -3q-17 148 -131 148q-53 0 -84.5 -34.5t-31.5 -82.5q0 -42 25.5 -72t74.5 -62l33 -22q63 -42 95 -85t32 -102q0 -84 -67 -137t-163 -53q-58 0 -113 22t-70 43l-4 152l27 4q4 -32 15 -62.5t31 -59.5t53.5 -47t76.5 -18q56 0 92 35t36 96q0 39 -25 70t-78 68&#xA;l-31 22q-32 23 -53.5 41.5t-45 57t-23.5 77.5q0 82 58 132.5t156 50.5q46 0 101 -17l18.5 -6t17 -6t8.5 -3q-4 -55 0 -147z" /></g> </svg>) were determined using <i >n</i>-hexane as a probe in order to show the adsorbate-adsorbent interaction. It was observed that the non-polar solutes could be separated Independent on the reactivity and porosity of the silica surfaces. The efficiency of the surface-modified silica gels to separate the aromatic hydrocarbons seemed to be strongly influenced by the density of the surface hydroxyls.

Improvement of chromatographic performances of in-situ synthesized hybrid C 8 silica monoliths by reduction of structural radial heterogeneities

Several modifications of a previously described protocol are proposed to improve the performances of in-situ synthesized C 8 hybrid silica monoliths. Our attention was focused on reducing the sources of radial heterogeneity that may be responsible for the poor efficiencies observed in the hydrodynamic elution mode. It was demonstrated that a decrease in the temperature of the capillary during the filling step equally to that of the polymerization mixture (0 °C), associated with a decrease of the gelation temperature to 20 °C along with a new pre-treatment of the capillary's internal walls [with a mixture of tetraethoxysilane (TEOS)/EtOH (1/3, v/v)] allows (i) increasing the radial homogeneity of the monolith, thus further enhancing the performances in the nano-liquid chromatography (nano-LC) mode, (ii) improving the capillary to capillary reproducibility in terms of permeability and efficiencies. In fact, the average minimum plate height H min was lowered from 24 to 14 μm and the capillary-to-capillary reproducibility of the synthesis was widely improved by factors two and three of reduction on the calculated standard deviation, respectively for both the efficiency in the nano-LC mode and the permeability. At last, the improved radial homogeneity and anchoring of the synthesized monoliths allowed increasing the inner diameter of the capillary (up to 150 μm) without any significant loss in efficiency. Finally, long term stability of the as-obtained monolithic stationary phases in terms of retention and efficiency was studied. In addition, the evaluation of their chromatographic behaviour was also achieved with the Tanaka test and the results were compared to those already published for commercial monoliths (Chromolith) as well as for particulate stationary phases.

Stationary phases for chromatography prepared by ring opening metathesis polymerization

The chemistry of metathesis polymerization-derived stationary phases is summarized. Since both ring-opening metathesis and 1-alkyne polymerization triggered by well-defined transition metal alkylidenes are living polymerization methods, they allow for the controlled and highly reproducible synthesis of stationary phases in terms of both the nature and total content of the functional group(s) of interest. In addition, the high functionality tolerance of these polymerization techniques allows for creating chromatographic supports with an unprecedented diversity in terms of functional groups that may be introduced. Their applications in various areas of separation science such as SPE, ion-chromatography, RP chromatography, chiral chromatography, and the high-performance liquid chromatographic separation of biomolecules are summarized. Within the context of the latter topic, special attention will be given to metathesis polymerization-derived monolithic supports. Consideration will also be given to those aspects of polymer chemistry that are relevant to the separation performance of these supports.

Chromatographic properties of open tubular columns with an adsorption layer of aerosil

The chromatographic properties of a porous layer open tubular (PLOT) capillary column with an adsorption layer of highly dispersed silicon dioxide (Aerosil) were studied. It was found that the high efficiency and capacity of the column were retained at high rates of a gas-chromatographic process. The test adsorbent is similar to polydimethyl and polyphenylmethyl siloxane stationary phases in terms of selectivity. Complex multicomponent hydrocarbon mixtures can be separated on the PLOT/SiO2 column because of low peak asymmetry.

Solute-solvent interactions between a range of solutes and trifluoropropyl siloxane stationary phases in terms of gas-liquid chromatography activity coefficients

Several poly(3,3,3-methyltrifluoropropyl siloxane) stationary phases with a low percentage of trifluoropropyl have been recharacterised by means of activity coefficients at temperatures in the range 60–140°C. The temperature effect of activity coefficients was studied. Thermodynamic magnitudes: excess Gibbs energy, excess enthalpy and excess entropy for 44 solutes on these polymers were calculated, and their relationships with solutes’ molecular connectivity indexes were tested. Solute-polymer interactions were calculated at 120°C according to the solvation parameter model, and several correlations for selected solutes and polymers were investigated, mainly the effect of solutes’ structure on the non-polar interactions and the effect of the solute dipole moment on the polar interactions. In addition, the influence of polymer polarity on the different polar and non-polar interactions was investigated.

Laterally attached liquid crystalline polymers as stationary phases in reversed-phase high-performance liquid chromatography: II. Optimization of the molecular parameter of the polymer

Stationary phases obtained by coating side-chain liquid crystalline polymers (LCPs) with the mesogenic rod like units laterally attached to a polysiloxane backbone via a flexible spacer have been already reported. These phases show excellent planarity and shape recognition for polynuclear aromatic hydrocarbon (PAH) solutes in reversed-phase liquid chromatography. Optimization of these stationary phases in terms of molecular parameters of the polymer is here described. Fifteen stationary phases have been prepared varying different parameters such as the spacer length, the aliphatic tail length, and the proportion of laterally attached mesogenic units along the polymer chain. The results show that the combination of a long spacer and long terminal chains, which generates a smectic phase in the polymer bulk, leads to the best chromatographic performances towards planarity and shape recognition for PAH solutes.

Structure-driven retention model for optimization of ternary solvent systems in reversed-phase liquid chromatography

A new approach is presented fro the prediction of retention in reversed-phase liquid chromatography using ternary mobile phase compositions. The solvation parameter model is used to create the system constants that characterize the properties of the mobile and stationary phases in terms of the relative ease of cavity formation, and differences in dispersion, lone pair electron, dipole-type, and hydrogen-bond association interactions for all solvent compasitions employed. A statistical mixture-design approach provides system surfaces and models for the variation of individual system constants as a function of ternary solvent composition. The models for the system surfaces allow the retention factor to be predicted as a function of mobile phase composition for any compound whose descriptor values are known, or can be obtained from estimation rules or determined by experiment. The method is validated using a large literature data base for the retention factor of up to 46 varied aromatic solutes in 72 methonolacetonitrile-water and methonol-tetrahydrofuran-water mobile phase compositions.

Surface-modified silica gels as solid stationary phases in gas chromatography

Modifications of the precipitated silica gel were done by different treatments including calcination, hydrothermal treatment, silylation and treatment with alkali salt (NaCl). In addition, the hydrothermally treated silica gel was subjected to further treatments, namely silylation or modification by alkali metal. Density of surface hydroxyls ( N OH/100 Å 2) was determined thermogravimetrically and confirmed by infrared spectroscopy. Surface textural characteristics were investigated from nitrogen adsorption-desorption isotherms at −196 °C. It was found that the hydrothermal treatment has a pronounced influence in removing hydroxyls from the silica surface. Different pore systems were found to exist depending on the adopted treatments. The surface-modified silica gels were tested as GC solid stationary phases in terms of the separation efficiency for various groups of non-polar and polar solutes. Also, thermodynamic parameters (Δ H, Δ G and Δ S) were determined using n-hexane as a probe in order to show the sdsorbate-adsorbent interaction. It was observed that the non-polar solutes could be separated independent of the reactivity and porosity of the silica surfaces. The efficiency of the surface-modified silica gels to separate the aromatic hydrocarbons seemed to be strongly influenced by the density of the surface hydroxyls. It was suggested that the separation of the polar solutes ( n-alcohols) is enhanced by the inertness of the silica surface and a fraction of wider pores located mostly at 80 Å.

Mobile phase effects on enantiomer resolution using molecularly imprinted polymers

The aim of this study was to rationalise retention behaviour of a chiral solute on molecularly imprinted polymer (MIP) HPLC stationary phases in terms of variation of the mobile phase. It is generally held that the most important interaction governing the separation of enantiomers on such materials is H-bonding, and that retention times increase with decreasing H-bonding potential of the mobile phase. Previous studies have largely concerned mobile phases containing chloroform with acetic acid as a polar modifier. Boc-L-Phenylalanine (Boc-L-Phe-OH) MIPs were prepared, processed, and packed into HPLC columns, which were then used to investigate the retention characteristics of Boc-L-Phe-OH and Boc-D-Phe-OH with a range of mobile phases. The main observations were as follows: (1) in chloroform-based mobile phases there was generally a linear relationship between the H-bond donator factor of the polar modifier and capacity (K′). Results also indicated a hydrogen bond donor parameter value for a polar modifier at which retention became concentration independent; (2) For given values of K′L, K′D varied depending on the polar modifier, indicating that enantiomer resolution was solvent dependent; (3) Using mobile phases based on solvents of lower polarity/H-bonding potential than chloroform, substantial increases in K′ were observed, although enantioselectivity was greatly reduced. Chirality 9:238–242, 1997. © 1997 Wiley-Liss, Inc.

Mobile phase effects on enantiomer resolution using molecularly imprinted polymers

The aim of this study was to rationalise retention behaviour of a chiral solute on molecularly imprinted polymer (MIP) HPLC stationary phases in terms of variation of the mobile phase. It is generally held that the most important interaction governing the separation of enantiomers on such materials is H-bonding, and that retention times increase with decreasing H-bonding potential of the mobile phase. Previous studies have largely concerned mobile phases containing chloroform with acetic acid as a polar modifier. Boc-L-Phenylalanine (Boc-L-Phe-OH) MIPs were prepared, processed, and packed into HPLC columns, which were then used to investigate the retention characteristics of Boc-L-Phe-OH and Boc-D-Phe-OH with a range of mobile phases. The main observations were as follows: (1) in chloroform-based mobile phases there was generally a linear relationship between the H-bond donator factor of the polar modifier and capacity (K′). Results also indicated a hydrogen bond donor parameter value for a polar modifier at which retention became concentration independent; (2) For given values of K′L, K′D varied depending on the polar modifier, indicating that enantiomer resolution was solvent dependent; (3) Using mobile phases based on solvents of lower polarity/H-bonding potential than chloroform, substantial increases in K′ were observed, although enantioselectivity was greatly reduced. Chirality 9:238–242, 1997. © 1997 Wiley-Liss, Inc.

Chemometric classification of the solvent properties (selectivity) of commonly used gas chromatographic stationary phases

Abraham's solvation parameter model was used to characterize the solvent properties of 33 stationary phases in terms of their capacity for specific intermolecular interactions at 121.4°C. The capacity of the phases for dispersion, orientation, induction, and hydrogen-bond base interactions, combined with solvent cohesion (ease of cavity formation) were important in defining their behavior. The capacity for π- and n-electron pair acceptor interactions was less important and none of the phases possessed a significant capacity for solvent hydrogen-bond acid interactions. Principal component analysis and hierarchical clustering techniques were used to classify the phases by their similarity for specific intermolecular interactions. These methods provide a visual means of quantitatively comparing the properties of the stationary phases and a mechanism to develop a short list of phases able to represent the range of properties available in the larger data set.

A general method of classifying stationary phases and estimating their polarity and selectivity on the basis of thermodynamic characteristics

1. Empirical equations used for classifying stationary phases in terms of differences in retention indices have been developed. It has been shown that the energy equivalent of the index unit varies from one phase to the other. 2. A system is described by means of which one can estimate the polarities of stationary phases and adsorbents from the differential molar free energies of dissolution of n-alkane methyl groups and five test compounds. 3. A thermodynamic criterion for the selection of a comparison phase in selectivity determinations has been set up. 4. A quantitative method for estimating stationary-phase selectivities from differences in the excess free energy of mixing has been described.

Characterization of the selectivity of stationary phases from the partial molar free energies of solution of standards

A thermodynamic description is proposed to express the polarity of stationary phases in terms of the partial molar free energies of solution obtained for standards. This expression is an absolute value for any stationary phase and does not need squalane as the phase with “zero polarity”. Equations are given to express the contributions of dispersive interaction and of the sum of inductive, orientative and specific interactions to the partial molar free energies of solution for any substance in any phase.