Non-porous polybutadiene-coated silicas as stationary phases in reversed-phase chromatography

A detailed discussion of the influence of the amount of deposited polymer on the retention properties of polymer-coated silicas

Problems of the modification of the surface of micro-spherical silica gel with layers of polymer and the influence of the quantity of the coated polymer on the chromatographic properties of the resulting sorbents have been considered. The polymer modification of the surface of wide-pore micro-spherical silica gel obtained by means of hydrothermal treatment of the meso-porous silica gel under autoclave conditions is described. The polymer layer itself is formed by an octadecylmethacrylate-methylmethacrylate co-polymer. As a result, packings for RP-HPLC were obtained. An increase of the capacity factor (k) has been observed with an increase of the quantity of the polymer deposited during the polymer modification process. The indicated phenomenon depends on the increase of the number of C18 groups in the packing.

Characterization of Capillary-Channeled Polymer Fiber Stationary Phases for High-Performance Liquid Chromatography Protein Separations: Comparative Analysis with a Packed-Bed Column

Capillary-channeled polymer (C-CP) fibers are investigated as reversed-phase (RP) stationary phases for high-performance liquid chromatography of proteins. A comparative analysis of column characteristics for polypropylene and poly(ethylene terephthalate) C-CP fiber columns and a conventional packed-bed (C4-derivatized silica) column has been undertaken. Five proteins (ribonuclease A, cytochrome c, lysozyme, myoglobin, bovine serum albumin) were used to investigate the separation characteristics under typical RP gradient conditions. Column performance was compared under standard (identical) and optimized RP chromatographic conditions. The gradient compositions utilized with the C-CP fiber columns are similar to those used with conventional columns, employing flow rates in the 1-6 mL/min range and gradient rates of approximately 1%/min. The packed-bed column was operated as prescribed by the column manufacturer. The retention factor (k'), separation factor (alpha), resolution (Rs), asymmetry factor (As), elution order, and peak capacity values of a four protein separations performed on the C-CP fiber columns are compared to the same separation on the C4 column. One unique feature observed here is the lessening of the percentage of organic modifier necessary to elute the proteins from the fiber phases with increased linear velocity. The potential contribution of the different stationary phases to protein denaturation was evaluated through a spectrophotometric enzymatic activity assay. The repeatability of retention times under both sets of conditions for six consecutive injections of lysozyme on each C-CP fiber column is < or =1.5% RSD. The column-to-column reproducibility of retention times for three columns of each fiber type is also < or =1.5% RSD. The overall performance of the C-CP fiber columns was comparable to the conventional column used in these studies. Basic characteristics demonstrated here suggested further developments in the areas of ultrafast protein separations and preparative-scale protein chromatography.

Changes in the pore character of silicas caused by surface bonding and polymer coating

The pore character of packings for liquid chromatography, especially reversed-phase (RPLC) packings, has been studied by means of the nitrogen adsorption method (BET method). Micro-spherical silica gels with 9, 12, 30, 40 and 50 nm average pore diameter have been used as carriers. These silica gels have been modified with monochlorodimethyloctadecylsilane and several polymers (polyoctadecylmethacrylate-methylmethacrylate co-polymer, polyacrylamide gel, polyvinyl alcohol, poly-2-hydroxyethyl-methacrylate). A larger decrease in the specific surface area values was observed in the case of the polymer coating with the polyoctadecylmethacrylate-methylmethacrylate co-polymer compared with the derivatization by silanes or the modification with polymers without C18-groups. A new approach has been suggested to explain some questions concerning the interpretation of the data obtained during the measurements of the pore characteristics of the derivatized packings. An attempt has been made to reveal peculiarities connecting values of the measured surface of RP-packings with the specific surface area values of the initial silica, as well as with the chromatographically accessible surface.

High-performance liquid chromatographic stationary phases based on poly(dimethylsiloxane) immobilized on silica

This work describes the preparation and characterization of six stationary phases for high-performance liquid chromatography (HPLC) obtained by deposition of poly(dimethylsiloxane) (PDMS) in HPLC silica particles, followed by immobilization using different processes (thermal treatments, thermal treatment + microwave irradiation, self-immobilization + gamma irradiation and self-immobilization + microwave irradiation). The chromatographic parameters of all the phases were evaluated with a mixture of test compounds having varied natures (acid, basic and neutral). The stability of one of these phases was evaluated in both a neutral mobile phase and a higher pH mobile phase used at an elevated temperature, with promising results.

Influence of air on polybutadiene used in the preparation of stationary phases for high-performance liquid chromatography

A 100 ml bottle of polybutadiene, PBD, was repeatedly exposed to air over a period of 6 months. Samples were taken at time zero (PBD-0), after 3 months (PBD-3) and 6 months (PBD-6). These samples were sorbed onto HPLC silica by an open-air solution-evaporation procedure, which involved exposure to the atmosphere for 6 days. Portions of the three sets of samples were used to compare self-immobilization and the effects of 100 degrees C thermal treatments in air or nitrogen on HPLC performance of the resulting phases. It is concluded that self-immobilization is enhanced by prior exposure of sorbed PBD to air and subsequent heating at 100 degrees C further enhances column performance. The best performance (10(5) plates m(-1)) resulted from 4 h heating of PBD-6 material in nitrogen.

Microwave-immobilized polybutadiene stationary phase for reversed-phase high-performance liquid chromatography

Polybutadiene (PBD) has been immobilized on high-performance liquid chromatography (HPLC) silica by microwave radiation at various power levels (52-663 W) and actuation times (3-60 min). Columns prepared from these reversed-phase HPLC materials, as well as from similar non-irradiated materials, were tested with standard sample mixtures and characterized by elemental analysis (%C) and infrared spectroscopy. A microwave irradiation of 20 min at 663 W gives a layer of immobilized PBD that presented good performance. Longer irradiation times give thicker immobilized layers having less favorable chromatographic properties.

Part II. Chromatography using ultra-stable metal oxide-based stationary phases for HPLC

In this part of the review authors discuss methods used for modification of metal oxide surfaces. On the basis of literature data it is shown, that silanization of the surfaces do not form stable supports for chromatography. On the other hand, the success of polymer modified surfaces such as polybutadiene (PBD) and polystyrene (PS) is emphasized. Permanent modification of metal oxide surfaces with Lewis bases is also widely discussed. Chromatographic properties of polymer modified surfaces of zirconia are discussed in details. The perspectives of carbon-coated metal oxide surfaces in HPLC and high temperature separations are described.

Preparation of a polybutadiene stationary phase immobilized by gamma radiation for reversed-phase high-performance liquid chromatography

Polybutadiene (PBD) has been immobilized on HPLC silica by gamma radiation doses in the range from 5 to 180 kGy. Columns prepared from these reversed-phase materials, as well as from similar non-irradiated materials, were tested with standard sample mixtures and characterized by elemental analysis (% C) and infrared spectroscopy. A low dose of 5 kGy is sufficient to produce a layer of immobilized PBD which functions as an efficient and stable stationary phase. Higher doses give thicker immobilized layers having less favorable chromatographic properties.

High-performance liquid chromatographic stationary phases based on poly(methyloctylsiloxane) immobilized on silica. II. Chromatographic evaluation

This work describes the chromatographic characterization of stationary phases prepared by deposition of poly(methyloctylsiloxane) (PMOS) on silica followed by immobilization using one of several different processes: thermal treatments (120 or 220 degrees C for 4 h), microwave irradiation (495 W for 15 min), gamma radiation (dose of 80 kGy) or self-immobilization. This evaluation was based on the chromatographic parameters of several test solutes. The stationary phases immobilized at 220 degrees C and which underwent self-immobilization were not appropriate for chromatographic use but the other immobilized phases presented chromatographic performances similar in most respects to a commercial phase (Rainin C8) while the peak characteristics of the basic probe were significantly better with these phases.

New synthetic ways for the preparation of high-performance liquid chromatography supports

The latest developments and in particular important synthetic aspects for the preparation of modern HPLC supports are reviewed. In this context, the chemistry of inorganic supports based on silica, zirconia, titania or aluminum oxide as well as of organic supports based on poly(styrene-divinylbenzene), acrylates, methacrylates and other, more specialized polymers is covered. Special consideration is given to modern approaches such as sol-gel technology, molecular imprinting, perfusion chromatography, the preparation of monolithic separation media as well as to organic HPLC supports prepared by new polymer technologies such as ring-opening metathesis polymerization. Synthetic particularities relevant for the corresponding applications are outlined.

Influences of surface chemistry on the separation behavior of stationary phases for reversed-phase and ion-exchange chromatography: a comparison of coated and grafted supports prepared by ring-opening metathesis polymerization

A series of poly(norborn-2-ene) (poly-NBE), poly(7-oxanorborne-2-ene-5,6-dicarboxylic acid) (poly-ONDCA), as well as poly(norborn-2-ene-co-7-oxanorborne-2-ene-5,6-dicarboxylic acid) (poly-NBE-co-ONDCA) based silica supports were prepared via ring-opening metathesis polymerization (ROMP) using both coating and grafting techniques. Poly-NBE-grafted and poly-NBE-coated supports were used for the reversed-phase separation of phenols; poly-NBE, poly-ONDCA as well as poly-NBE-co-ONDCA-grafted supports were used for comparative studies on the separation of a series of anilines and lutidines. As expected, grafted supports possess superior separation capabilities compared to their coated analogues. Compared to pure poly-NBE- and poly-ONDCA-grafted stationary phases, supports consisting of poly-NBE-co-ONDCA block-copolymers possess both hydrophobic and ion-exchange sites and represent optimum stationary phases for the separation of isomeric basic analytes.

A comparative study of the chromatographic selectivity of polystyrene-coated zirconia and related reversed-phase materials

The chromatographic selectivities of polystyrene-coated zirconia (PS-ZrO2) have been investigated in detail by means of homologue series retention studies and studies of a variety of nonpolar aromatic positional isomers. This material has been compared to polybutadiene-coated zirconia, octadecyl- and phenyl-bonded silica, and polymer-based reversed phases. In addition, the separations of a wide variety of compound classes on PS-ZrO2 were compared with those on other aromatic and aliphatic RPLC phases. The PS-ZrO2 material exhibits substantial differences in selectivity toward nonpolar and polar analytes as compared to other phases, especially with respect to polybutadiene-coated zirconia (PBD-ZrO2). On PS-ZrO2, polynuclear aromatic hydrocarbons (PAHs) are retained much more than are alkylbenzenes, whereas on PBD-ZrO2 the opposite is observed. In addition, PS-ZrO2 has much greater selectivity than does PBD-ZrO2 for the separation of structural isomers which differ in the position of phenyl groups. In general, even though the carbon content of PS-ZrO2 is much lower than that of conventional reversed-phase materials, such as octadecyl-bonded silica (C18-SiO2), polar analytes exhibit sufficient retention and differential selectivity on PS-ZrO2 that resolution comparable to that on C18-SiO2 can be obtained but with substantially improved analysis times.

Reversed-phase liquid chromatography of proteins and peptides using multimodal copolymer-encapsulated silica

Multimodal copolymer-encapsulated particles for liquid chromatography were prepared by bonding 1-octadecene and unsaturated carboxylic acids on silica particles (5 microm diameter, 300 A pores) for liquid chromatography of proteins. These multimodal copolymer-encapsulated particles can provide both hydrophobic and hydrogen bonding interactions with polar compounds. The chromatographic performance of these multimodal copolymer-encapsulated particles for peptide and protein separations was evaluated under reversed-phase conditions. Compared with typical C8-bonded silica, polymer-encapsulated particles were more stable in acidic mobile phases and provided better recoveries, especially for large proteins (Mr>0.5 x 10(6)). Totally hydrophobic polymer-encapsulated particles were found to produce broad peaks for proteins, and significant improvements were observed by introducing hydrophilic groups (-COOH) onto the polymer-encapsulated surface to form a multimodal phase. For the reversed-phase liquid chromatography of peptides and proteins, improved selectivity and increased solute retention were found using the multimodal polymer-encapsulated particles. More peaks were resolved for the separation of complex peptide mixtures such as protein digests using the multimodal polymer-encapsulated particles as compared to totally hydrophobic polymer-encapsulated particles.

Factors influencing polybutadiene deposition within porous chromatographic zirconia

We have studied the effect of the conditions for the deposition of polymers in the preparation of polybutadiene-coated porous zirconia particles for reversed-phase chromatography. Chromatographic performance improves when the particle surface is pre-coated with elemental carbon by a chemical vapor deposition (CVD) process. Conversely, performance is significantly degraded when the solvent is removed very slowly during the deposition process. No improvement results when the particles are coated using small sequential loads of polymer. We hypothesize that the polymer deposition process is controlled by the rate at which the solvent meniscus recedes during solvent evaporation, by the affinity of the polymer for the zirconia surface, and by polymer-solvent and polymer-polymer interactions.

A study of the efficiency of polybutadiene-coated zirconia as a reversed-phase chromatographic support

This work describes the dynamic characteristics of columns packed with polybutadiene (PBD)-coated zirconia. Reduced plate height (h) vs reduced velocity (v) data for alkylbenzenes were acquired for three phases coated with different amounts of PBD. Additional data for benzene (k' = 0) on uncoated and four coated phases were also collected. These h vs v plots have a minimum between 4 and 5, indicating good but not excellent column efficiency. By fitting the Knox equation to these data, the A (packing quality), B (axial diffusion), and C (mass transfer resistance) coefficients were determined. The A coefficients were about 2 in all cases, and the B coefficients had values between 2 and 3. The C coefficients varied from 0.05 to 0.2, depending on the polymer load. The data suggest that the quality of packing and the efficiency of mass transfer resistance in the "film" of polymeric stationary phase, not the stagnant mobile phase, are key to further improvements in column performance. The amount of polymer loaded significantly affects the efficiency through the mass transfer resistance in the stationary phase.

Retention characteristics of polybutadiene-coated zirconia and comparison to conventional bonded phases

This paper presents a detailed study of retention on a reversed-phase material made by coating polybutadiene (PBD) on porous zirconia. PBD-coated zirconia particles with six different carbon loads (0.25-5.6% carbon by weight) were prepared by evaporatively depositing and cross-linking PBD on microparticulate porous zirconia. Retention data of a homologous series of alkylbenzenes were obtained on the six PBD phases as a function of mobile phase composition in methanol-water and acetonitrile-water mixtures from 20 to 50% (v/v). The results obtained for the phase were compared to those for conventional octadecylsilane (ODS) bonded phases, and the effect of the amount of PBD on retention was studied in detail. We find that, per amount of bonded phase, the PBD phase is less retentive than is the ODS phase, but it has comparable hydrophobic selectivity. Furthermore, the PBD phase has about the same sensitivity toward changes in mobile phase composition as does the ODS phase, and its solute shape selectivity is similar to that of a monomeric ODS phase. Finally, we conclude that retention arises primarily from a partition-like process.

Interactions of lysozyme with hydrophilic and hydrophobic polymethacrylate stationary phases in reversed phase chromatography (RPC)

Two silicas, one with a mean pore diameter of 30 nm and the other non-porous, were coated with polymethacrylates of increasing hydrophobicity in the sequence: poly-2-hydroxyethylmethacrylate (P2HEMA)1 polyethylmethacrylate (PEMA) and poly-n-octylmethacrylate (POMA). Association constants, Kass, between lysozyme and the coated silicas were determined by means of frontal analysis, and the apparent heats of adsorption, delta Happ, by means of microcalorimetry. Using Kass and delta Happ the changes in the apparent free energy, delta Gapp, and in the apparent entropy, delta Sapp, were calculated at a concentration of lysozyme < 10 mumol/l. The association between the lysozyme and the coated silica gave negative delta Happ and delta Sapp values. The delta Sapp values calculated from the experimental data in the absence of an added electrolyte and with 0.1 M sodium perchlorate present to be -12 (-96) J/mol deg on the P2HEMA silica, -27 (-51) J/mol deg on the PEMA silica, -98 (-186) J/mol deg on the POMA silica, respectively. The high delta Sapp value of the lysozyme on the POMA silica reflects a kind of stabilisation effect due to the conformational changes of lysozyme on the most hydrophobic POMA silica. As evidenced by RPC, lysozyme elutes on the POMA column in its totally unfolded form enabling a more disordered conformation with respect to entropy than in the native form.

Rapid analysis of somatostatin in pharmaceutical preparations by HPLC with a micropellicular reversed-phase column

A rapid high-performance liquid chromatography method for the analysis of somatostatin in pharmaceutical preparations is described. A commercially available column packed with 2 microns spherical non-porous silica-based reversed-phase sorbent is used, along with a mobile phase consisting of acetonitrile and aqueous phosphoric acid, adjusted to pH 2.8 with sodium hydroxide. The effect of the organic modifier content and column temperature on the retention behaviour of somatostatin is reported. The method is found to be highly selective and specific, as indicated by the baseline separation of a mixture containing somatostatin and two analogue peptides, which differ from the analyte for one and two amino acids, respectively. Down to 10 ng of somatostatin can be detected and the detector response is linear over the concentration range from 4.14 to 20.75 micrograms ml-1. The application of this method to two commercial pharmaceutical formulations of somatostatin is found to give a mean percentage recovery from each of the two commercial samples, subjected to multiple injection analysis (n = 5), of 100.9% with a RSD of 0.92%, and 102.6% with a RSD of 1.56%, respectively.

Capillary electrophoresis monitoring of the competitive adsorption of albumin onto the orosomucoid-coated polyisobutylcyanoacrylate nanoparticles

The simultaneous and rapid quantitation of a glycoprotein, human orosomucoid, and a protein, human serum albumin (HSA), by micellar electrokinetic capillary electrophoresis was developed and optimized (pH and sodium dodecyl sulfate concentration) in order to study the competitive adsorption of HSA onto orosomucoid-coated nanoparticles of polyisobutylcyanoacrylate. This method provided satisfactory repeatability and the response was linear for concentrations of orosomucoid and HSA ranging from 0.025 to 0.500 mg/mL (r > 0.994 for both proteins). To study the competitive adsorption of HSA, the amounts of free orosomucoid and free HSA were determined in the supernatant obtained after centrifugation of the orosomucoid-coated nanoparticle suspension incubated with HSA. Furthermore, the quantities of both orosomucoid and HSA absorbed onto nanoparticles were also determined after desorption of these molecules from the nanoparticle surface, using sodium dodecyl sulfate. No analytical interference was observed with this surfactant. The results showed that HSA did not at all absorb onto the orosomucoid-coated nanoparticles and, thus, that this protein did not displace the layer of absorbed orosomucoid from the nanoparticle surface.

Study of the irreversible adsorption of proteins on polybutadiene-coated zirconia

The cause of irreversible adsorption of proteins on polybutadiene-coated zirconia is investigated by comparing the chromatographic properties of polybutadiene-coated zirconia with that of other reversed-phase packing materials such as bonded phase silica, polybutadiene-coated alumina and polybutadiene-coated silica. We find that the polybutadiene-coated zirconia has a micropore size distribution similar to that of the polybutadiene-coated alumina, from which some proteins can be eluted. Thus, the irreversible adsorption of proteins on polybutadiene-coated zirconia is not caused by entrapment of proteins in the micropores of the packing. The high hydrophobicity of the polybutadiene coating and the strong Lewis acid sites on the zirconia surface cause strong interactions between proteins and the stationary phase, the combination of which lead to irreversible adsorption of proteins on polybutadiene-coated zirconia.

High-performance liquid chromatography of amino acids, peptides and proteins. CXXVI. Modelling of protein adsorption with non-porous and porous particles in a finite bath

Analytical solutions for a mathematical model describing dynamic adsorption processes of proteins onto non-porous adsorbent particles in a finite bath are presented. The model, based on the Langmuir adsorption isotherm, has been applied to experimental data obtained with affinity and ion-exchange adsorbents. The external film mass transfer resistance, as well as the rate of surface interaction between proteins and adsorbents, have been taken into account. The model has been extended to the case of adsorption onto porous particles by employing a linear driving force approximation for describing mass transfer in the pore fluid. This approach enables the derivation of an effective overall liquid phase mass transfer coefficient, permitting subsequent adaptation of the analytical solutions developed for non-porous particles. The evaluation of the effective liquid phase mass transfer coefficients is also described. Examples of a comparison between predicted and experimental dynamic adsorption curves for both dye-affinity and ion-exchange systems are presented. The application of the model for predicting the optimum operating conditions is discussed.