Class Of Stationary Phases Research Articles

Preparation of carboxyl-functionalized silica core-shell microspheres and their applications in weak cation exchange chromatography, heavy metal removal, and lysozyme enrichment.

Chromatography is a technique of separation based on adsorption and/or interaction of target molecules with stationary phases. Herein, we report the design and fabrication of BTDA@SiO2 core-shell microspheres as a new class of stationary phase and demonstrate its impressive performance for chromatographic separations. The silica microspheres of BTDA@SiO2 were synthesized by in situ method with 1,3,5-benzenetricarboxaldehyde and 3,5-diaminobenzoic to separate peptides and proteins on high-performance liquid chromatography. The BTDA@SiO2 core-shell structure has a high specific surface area and retention factor of 4.27 and 8.31 for anionic and cationic peptides, respectively. The separation factor and resolution were high as well. A typical chromatogram illustrated nearly baseline resolution of the two peptides in less than 3 min. The BTDA@SiO2 was also highly stable in the pH range of 1 to 14. Furthermore, the prepared BTDA@SiO2 core-shell material not only be used for chromatographic separation but also as heavy metal removal from water. Using a BTDA@SiO2, we also achieved a lysozyme enrichment with a maximum saturated adsorption capacity reaching 714mg/g. In summary, BTDA@SiO2 has great application prospects and significance in separation and purification systems.

New Stationary Ionic Liquid Phases with Quinolinium Cations for Capillary Gas Chromatography

The work provides the study of quinolinium ionic liquids (ILs)—a new class of stationary phases for capillary gas chromatography (GC). Quinolinium ILs were shown to have new attractive properties. They are not only highly polar, but also very selective for aromatic compounds and oxygenates. Depending on the substituents in the quinolinium ring, the studied ILs possess polarity numbers from 64 to 99, so they are polar and strongly polar. By the variation of the substituent and its position, changes in the prevailing types of interactions with analytes are observed. It is worth noting that Abraham’s constants for the quinolinium phases are significantly different from the previously known imidazolium and pyridinium ionic liquids (ILs) that cause significant variations in their selectivity. The most significant differences are observed in the π–π interactions and hydrogen bond acidity parameters. The investigation of background mass spectra for quinolinium ILs showed the most intense signals related to peaks at m/z 69 and 143 belonging to [CF3]+ and quinolinium ring fragments. The most thermostable ILs are N-propyl-6-methylqionoinium and bis(6-methylquinolinium)hexane that are stable up to 300 °C. With regard to the possible use of quinolinium ILs, apart from the separation of polar substances, it is expected that they will find applications in two-dimensional chromatography for the separation of complex mixtures compose of compounds that differ substantially in their properties. The applications of the columns for the determination of aromatic hydrocarbons in gasoline and phenolic compounds in pyrolysis biodiesel are demonstrated.

The Study of Chromatography Used In Separation Science

A procedure to determine effective diffusion coefficients of proteins in chromatographic gels, required as model input parameters, is presented. An experimental methodology based on frontal liquid chromatography was combined with a numerical methodology based on a detailed mathematical model describing the chromatographic process including the extra-column dispersion, the dispersion due to the packed bed, the external mass transfer from the bulk phase to the stationary phase, and the diffusive transport within the stationary phase. The procedure has several advantages compared to previously reported methods to determine diffusion coefficients in that no other equipment than an HPLC is required, any class of stationary phases can be investigated as long as the experiments are performed under non-binding conditions, and no modification, e.g., molding of slabs or membranes, to the stationary phase is required. To show the applicability of the procedure, the effective diffusion coefficients of lysozyme, bovine serum albumin, and immunoglobulin γ in Sepharose™ CL-4B were determined and shown to be comparable to those determined by other methods. Molecularly imprinted polymers (MIPs) are man-made polymeric materials with molecular recognition abilities. They mimic the molecular recognition of naturally occurring molecular recognition elements such as receptors and antibodies by binding target molecules by either non-covalent, covalent, or metal-coordinating interactions. Traditionally, MIPs are synthesized in the form of monolithic polymers which are subsequently crushed, ground, and sieved to an appropriate size range. In this thesis, a suspension polymerization method to prepare MIPs in the shape of spherical beads is presented. The method involves suspending a prepolymerization solution in mineral oil, used as the continuous phase. The droplets are transformed into solid spherical beads by free-radical polymerization. The beads have been shown to compare well to the traditional irregularly shaped particles prepared from monoliths. The advantages of the method compared to previously reported methods are the low cost and commercial availability of the continuous phase and the absence of the need for stabilizers for the formation of droplets of pre-polymerization solution in the mineral oil. When compared to the method to prepare particles from monolithic polymers, this method is advantageous due to the spherical shape of the resulting beads and the reduction in time needed to prepare a MIP. When a new MIP is designed, the traditional approach is to use eithera previously reported protocol or rules of thumb based on previous knowledge. This results in non optimized MIPs. The number of possible combinations of monomers, cross-linkers, solvents, and initiators are huge. A full optimization of a MIP formulation therefore requires a large number of experiments. To facilitate the efforts, chemometrics was applied to the work described in this thesis. Three factors (i.e., the amount of monomer, the amount of crosslinker, and the amount of porogen) were chosen as the factors in the model. Multivariate data analysis of the binding to the MIPs indicated how the factors influenced the binding and an optimized MIP composition was identified. The combined use of the suspension polymerization method to produce spherical beads with the application of chemometrics was shown in this thesis to drastically reduce the number of experiments and the time needed to design and optimize a new MIP.

Preparation and Application of Ionic Liquid-Modified Stationary Phases in High Performance Liquid Chromatography

Ionic liquid-modified stationary phases are an important class of stationary phase. They have been shown to have a capability of separating various classes of compounds in HPLC. In this review, we focus on the preparation and application of ionic liquid-modified stationary phases in HPLC. First, two common theories of retention mechanism in the ionic liquid-modified stationary phases in HPLC were discussed. Then, the preparations of two important types of stationary phase, ionic liquid-modified and poly(ionic liquids)-grafted silicas, were illustrated. Finally, applications of the ionic liquid-modified stationary phases in HPLC were reviewed, giving regard to the varieties of compounds separated or analyzed.

Comparison of stationary phases for packed column supercritical fluid chromatography based upon ionic liquid motifs: a study of cation and anion effects

A class of stationary phases for packed column supercritical fluid chromatography (SFC), referred to as immobilized ionic liquids (IILs), is evaluated with a two-part study: (1) a cation effect study and (2) an anion effect study. The former study compares six different IILs (tripropylphosphonium, tributylphosphonium, methyl-imidazolium, benzyl-imidazolium, triphenylphosphonium, and 4,4'-bipyridyl) on silica gel, evaluating their performance in SFC with all the counter anions as trifluoroacetate (TFA(-)). In the latter study, the stationary phase consisted of a bonded tributylphosphonium cation and varying counter anions (acetate, TFA(-), chloride, NTf (2)(-), and perchlorate). An order of retentivity was established for the cation effect study, and the favorable behavior of phosphonium-based stationary phases is reported for the first time in SFC. It was not possible to always assign a retentivity order for the anion effect study, but wide variations in selectivity are noted for different anions showing the tunable nature of this class of stationary phases.

Development of New HPLC Chiral Stationary Phases Based on Native and Derivatized Cyclofructans

An unusual class of chiral selectors, cyclofructans, is introduced for the first time as bonded chiral stationary phases. Compared to native cyclofructans (CFs), which have rather limited capabilities as chiral selectors, aliphatic- and aromatic-functionalized CF6s possess unique and very different enantiomeric selectivities. Indeed, they are shown to separate a very broad range of racemic compounds. In particular, aliphatic-derivatized CF6s with a low substitution degree baseline separate all tested chiral primary amines. It appears that partial derivatization on the CF6 molecule disrupts the molecular internal hydrogen bonding, thereby making the core of the molecule more accessible. In contrast, highly aromatic-functionalized CF6 stationary phases lose most of the enantioselective capabilities toward primary amines, however they gain broad selectivity for most other types of analytes. This class of stationary phases also demonstrates high "loadability" and therefore has great potential for preparative separations. The variations in enantiomeric selectivity often can be correlated with distinct structural features of the selector. The separations occur predominantly in the presence of organic solvents.

Considerations on HILIC and polar organic solvent-based separations: use of cyclodextrin and macrocyclic glycopetide stationary phases.

There is a natural tendency in science to prefer straightforward, logical classification systems. The use of mobile phase-stationary phase combinations that do not fit neatly into the standard "normal phase" or "reversed-phase" categories has been going on for over 50 years. The term "hydrophilic interaction chromatography" (HILIC) is sometimes being used as a general category for these "other" separations. In some cases, it may be appropriate and in others, not. Indeed the mechanistic constrains used to define the method seem to be varying with time. Given the name HILIC, it is assumed that water is not only present in the mobile phase, but also plays an essential role in the retention mechanism. However, there is residual water present in all organic solvents. Regardless, the number of reported separations in this alternative mode has increased tremendously in the last two decades. This is due to the advent of new stationary phases and an emphasis on polar, biologically important molecules. We discuss the relationships between HILIC and other chromatographic modes. We then examine two classes of stationary phases that have played a major role in these separations. These particular stationary phases can be used to provide appreciable mechanistic information as well.

Determination of diffusion coefficients of proteins in stationary phases by frontal chromatography

A strategy to determine effective diffusion coefficients of proteins in chromatographic gels is presented in this article. An experimental methodology based on frontal liquid chromatography was combined with a numerical methodology based on a mathematical model describing the chromatographic process including the extra-column dispersion, the dispersion due to the packed bed, the external mass transfer from the bulk phase to the stationary phase, and the diffusive transport within the stationary phase. The methodology has several advantages compared to previously reported methods to determine diffusion coefficients in that no other equipment than an HPLC is required, any class of stationary phases can be investigated as long as the experiments are performed under non-binding conditions, and no modification, e.g., moulding of slabs or membranes, to the stationary phase is required. To show the applicability of the methodology, the effective diffusion coefficients of lysozyme, bovine serum albumin, and immunoglobulin gamma in Sepharose CL-4B were determined and shown to be comparable with those determined with other methods.

Organic polymer monoliths as stationary phases for capillary HPLC

Modern rigid porous polymer monoliths were conceived as a new class of stationary phases in classical columns in the early 1990s and later extended to the capillary format. These monolithic materials are typically prepared using a simple molding process carried out within the confines of the capillary. Polymerization of a mixture comprising monomers, initiator, and porogenic solvent affords macroporous materials with large through-pores that enable applications in a rapid flow-through mode. Since all the mobile phase must flow through the monolith, convection considerably accelerates mass transport within the monolithic separation medium and improves the separations. As a result, monolithic columns perform well even at very high flow rates. Various mechanisms including thermally and UV initiated free radical polymerization as well as ring opening metathesis copolymerizations were demonstrated for the preparation of monolithic capillary columns. The versatility of these preparation techniques was demonstrated by their use with hydrophobic (styrene, divinylbenzene, butyl methacrylate, ethylene dimethacrylate), hydrophilic (2-hydroxyethyl methacrylate, methacrylamide, methylenebisacrylamide), ionizable (vinylsulfonic acid, 2-acrylamido-2-methyl-propanesulfonic acid), and tailor-made (norborn-2-ene, 1,4,4a,5,8,8a-hexahydro-1,4,5,8-exo,endo-dimethanonaphthalene) monomers. Variation of polymerization conditions enables control of the porous properties of the monolith over a broad range and mediates the hydrodynamic properties of the monolithic columns. The applications of polymer-based monolithic capillary columns are demonstrated for numerous separations in the microHPLC mode.

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.

High-performance liquid chromatographic separation of oligonucleotides and other nucleic acid constituents on multifunctional stationary phases

A new class of stationary phases has been created for high-performance liquid chromatography bonding various proportions of ionic and hydrophobic ligands to the surface of silica supports. The ratio of hydrophobic to ionic character can be reproducibly controlled by varying the volume ratio of monochlorosilane reagents in the bonding mixture. In this study, both octyl- and 3-chloropropyldimethylmono-chlorosilane ligands were bonded, after which the chloro groups were converted into quaternary amines by nucleophilic substitution with benzyldimethylamine. The resulting hydrophobic ion exchangers proved to be well suited for the separation of the oligonucleotides up to about 20 in chain length, improved separations being observed in comparison to existing ion-exchange or reversed-phase separations alone. Selectivities of these mixed-mode packings towards polynueleotides were found to be comparable with those observed for RPC-5 and similar resins.

The analysis of mixtures of the n-octenes, octalins, t-butylcyclohexenes, octanols, decalols, and t-butylcyclohexanols

Analytical methods, based on gas–liquid chromatography on one or two stationary phases, are described for the following mechanistically important sets of isomers and related compounds: the seven n-octenes (+n-octane); the three secondary octan-1-ols, their acetates and butyrates, and the corresponding octanones; the six octalins (+the two decalins); the five hydroxy-trans-decalins and 9-hydroxy-cis-decalin; the three t-butylcyclohexenes; and the seven t-butylcyclohexyl acetates, and the four 3- and 4-t-butylcyclohexyl butyrates. General recommendations for the analysis of such systems are made and a useful new class of stationary phase, ternary eutectics of aromatic polynitro-compounds, is described.