Reversed-phase high-performance liquid chromatography of basic compounds at pH 11 with silica-based column packings

A biomimetic hybrid material consisting of CaCO3 mesoporous microspheres and an alternating copolymer for reversed-phase HPLC

We developed a biomineral-inspired hybrid material composed of CaCO3 and an organic polymer as a column packing material for HPLC. This material combines a hierarchical mesoporous structure and the functionality of the polymer. The surface of monodispersed mesoporous CaCO3 microspheres was modified with poly(maleic acid-alt-1-octadecene) (PMAcO) comprising hydrophobic alkyl chains and anionic carboxylate groups. PMAcO adsorbed onto the surface of CaCO3 through electrostatic interaction between Ca2+ sites and carboxylate groups, resulting in an octadecene coated microsphere interface. These microspheres were applied as a HPLC column and exhibited reversed-phase retention behavior in the separation of alkylbenzenes. This column showed high alkaline mobile phase resistance compared with the conventionally applied ODS column packing material. Quantitative analysis of the basic antidepressants clomipramine and imipramine spiked into whole blood was achieved with an alkaline mobile phase, demonstrating the potential of the biomineral-inspired material as a HPLC stationary phase for practical applications in routine analyses of basic drugs requiring alkaline mobile phases.

Optimization of normal-phase chromatographic separation of compounds with primary, secondary and tertiary amino groups

The retention behavior of primary, secondary and tertiary amines was studied using normal-phase-HPLC on silica, diol, and cyano stationary phases. Several classes of amines, including benzylamines, anilines, ephedrines, tryptamines, and azatryptamines were chromatographed using mixtures of hexane and ethoxynonafluorobutane with methylene chloride and methanol. Peak tailing, diminished selectivity and low plate count were minimized by the addition of volatile amines to the mobile phase. The optimal additive was n-propylamine at 0.1% concentration. On diol columns, the elution order of free primary, N-N-methyl, and N,N-dimethylamines was predictable, while the elution order of primary and secondary amines on cyano columns varied depending on the alcohol modifier concentration. The feasibility of preparative normal-phase chromatography was demonstrated by the separation of a mixture of primary, secondary and tertiary amines obtained by direct methylation of norephedrine. The procedures described may provide a practical alternative to traditional methods of analysis and purification of potential drug candidates.

Applications of silica supports in affinity chromatography

The combined use of silica-based chromatographic supports with immobilized affinity ligands can be used in many preparative and analytical applications. One example is the use of silica-based affinity columns in HPLC, giving rise to a method known as high-performance affinity chromatography (HPAC). This review discusses the role that silica has played in the development of affinity chromatography and HPAC and the applications of silica in these methods. This includes a discussion of the types of ligands that have been employed with silica and the methods by which these ligands have been immobilized. Various formats have also been presented for the use of silica in affinity chromatographic methods, including assays involving direct or indirect analyte detection, on-line or off-line affinity extraction, and chiral separations. The use of silica-based affinity columns in studies of biological systems based on zonal elution and frontal analysis methods will also be considered.

HPLC analysis and purification of peptides

High-performance liquid chromatography (HPLC) has proved extremely versatile over the past 25 yr for the isolation and purification of peptides varying widely in their sources, quantity and complexity. This article covers the major modes of HPLC utilized for peptides (size-exclusion, ion-exchange, and reversed-phase), as well as demonstrating the potential of a novel mixed-mode hydrophilic interaction/cation-exchange approach developed in this laboratory. In addition to the value of these HPLC modes for peptide separations, the value of various HPLC techniques for structural characterization of peptides and proteins will be addressed, e.g., assessment of oligomerization state of peptides/proteins by size-exclusion chromatography and monitoring the hydrophilicity/hydrophobicity of amphipathic alpha-helical peptides, a vital precursor for the development of novel antimicrobial peptides. The value of capillary electrophoresis for peptide separations is also demonstrated. Preparative reversed-phase chromatography purification protocols for sample loads of up to 200 mg on analytical columns and instrumentation are introduced for both peptides and recombinant proteins.

Unusual retention behaviour of 4-substituted piperidines on polybutadiene and polystyrene coated zirconia by comparison to reverse phase silica

The retention properties of a range of N-methylated and N-des-methyl 4-substituted piperidines on polybutadiene (PBD) and polystyrene (PS) coated zirconia have been studied and compared to those of Xterra RP(18) and Genesis C(18) silica. The effect of buffer type and pH are investigated with regard to the elution order on all three stationary phases. The change in the elution order is linked to the degree of substitution of the piperidine nitrogen and appears to be independent of the rest of the structure, indicating that this moiety is most heavily involved in the separation mechanism on the zirconia phases.

Fast and high-resolution ion chromatography at high pH on short columns packed with 1.8μm surfactant coated silica reverse-phase particles

Rapid ion chromatographic separations of small inorganic anions are performed on columns packed with high-pH resistant Zorbax Extend-C18 1.8 microm silica particles. Seven anions (iodate, chloride, nitrite, bromide, nitrate, phosphate, sulphate) are separated with 1.3 and 2 cm long x 0.46 cm I.D. C18 columns coated with the surfactant didodecyldimethylammonium bromide (DDAB). A 40 s separation is achieved at 2 mL/min with a 2.5 mM 4-hydroxybenzoic acid eluent at pH 10. Finally, the DDAB removal procedure is improved to eliminate the pressure build-up caused by precipitation of the surfactant in the column upon uncoating.

Preparation and evaluation of a hydrolytically stable amide-embedded stationary phase

We have developed a new hydrolytically stable amide-embedded stationary phase via a simple and effective synthetic method. The preparation of the new phase involves the synthesis of multifunctional silane ligands and the surface modification of porous silica particles via multiple attachments of these ligands to the silica surface. A hydrolytically stable coating was produced as a result of multiple covalent linkages formed between silane ligands and the silica surface, and cross-linking between adjacent ligands. The resulting amide-embedded stationary phase showed excellent hydrolytic stability over a wide pH range. Like other existing amide-embedded columns, this new stationary phase exhibits higher retention for polar compounds and different selectivity as compared to conventional C18 columns. The new phase is compatible with 100% aqueous mobile phases, and also provides high column efficiency and good peak shapes for both acidic and basic compounds.

Immobilized polymeric stationary phases using metalized silica supports

Immobilized presynthesized polymers on porous metalized (zirconized or titanized) silica particles as new stationary phases with improved chemical stability for RP-HPLC are reviewed. The preparations using different polymers, such as poly(methyloctylsiloxane), poly(methyltetradecylsiloxane), and poly(butadiene), different immobilization steps (gamma radiation, thermal treatment, and microwave radiation), and the chromatographic performances of these phases for polar, apolar, acidic, and basic compounds are discussed. The stability of some of these stationary phases using alkaline mobile phases is also presented.

Cellulose Dimethylphenylcarbamate-bonded Carbon-clad Zirconia for Chiral Separation in High Performance Liquid Chromatography

Porous zirconia particles are very robust material and have received considerable attention as a stationary phase support for HPLC. We prepared cellulose dimethylphenylcarbamate-bonded carbon-clad zirconia (CDMPCCZ) as a chiral stationary phase (CSP) for separation of enantiomers of a set of 14 racemic compounds in normal phase (NP) and reversed-phase (RP) liquid chromatography. Retention and enantioselectivity on CDMPCCZ were compared to those on CDMPC-coated zirconia (CDMPCZ) to see how the change in immobilization method of the chiral selector affects the retention and chiral selectivity. In NPLC, retention was longer and the number of resolved racemates was smaller on CDMPCCZ than on CDMPCZ. However, chiral selectivity factors for some resolved racemates were better on CDMPCCZ than on CDMPCZ. The longer retention on CDMPCCZ is likely due to strong, non-chiral discriminating interactions with the carbon layer on CDMPCZ. In RPLC only two racemates were resolved on CDMPCCZ, but retention times were shorter than, and resolutions were comparable to, those in NPLC, indicating a potential for improving chromatographic performance of the CDMPCCZ column in RPLC with optimized column preparation and separation conditions.

Determination of intrinsic hydrophilicity/hydrophobicity of amino acid side chains in peptides in the absence of nearest-neighbor or conformational effects

Understanding the hydrophilicity/hydrophobicity of amino acid side chains in peptides/proteins is one the most important aspects of biology. Though many hydrophilicity/hydrophobicity scales have been generated, an "intrinsic" scale has yet to be achieved. "Intrinsic" implies the maximum possible hydrophilicity/hydrophobicity of side chains in the absence of nearest-neighbor or conformational effects that would decrease the full expression of the side-chain hydrophilicity/hydrophobicity when the side chain is in a polypeptide chain. Such a scale is the fundamental starting point for determining the parameters that affect side-chain hydrophobicity and for quantifying such effects in peptides and proteins. A 10-residue peptide sequence, Ac-X-G-A-K-G-A-G-V-G-L-amide, was designed to enable the determination of the intrinsic values, where position X was substituted by all 20 naturally occurring amino acids and norvaline, norleucine, and ornithine. The coefficients were determined by reversed-phase high-performance liquid chromatography using six different mobile phase conditions involving different pH values (2, 5, and 7), ion-pairing reagents, and the presence and absence of different salts. The results show that the intrinsic hydrophilicity/hydrophobicity of amino acid side chains in peptides (proteins) is independent of pH, buffer conditions, or whether C(8) or C(18) reversed-phase columns were used for 17 side chains (Gly, Ala, Cys, Pro, Val, nVal, Leu, nLeu, Ile, Met, Tyr, Phe, Trp, Ser, Thr, Asn, and Gln) and dependent on pH and buffer conditions, including the type of salt or ion-pairing reagent for potentially charged side chains (Orn, Lys, His, Arg, Asp, and Glu).

Synthesis and characterization of chemically bonded stationary phases on hydride surfaces by hydrosilation of alkynes and dienes

A hydrosilation reaction was used to bind four compounds with one or more alkyne groups or two alkene functionalities. The diffuse reflectance infrared Fourier transform, the 29Si cross-polarization magic-angle spinning (CP-MAS) NMR, and the 13C CP-MAS NMR spectra were used to characterize the various bonded materials. The bonded phase density was determined from carbon elemental analysis. The two ten-carbon hydrophobic stationary phases were characterized chromatographically and static stability tests were run in acidic and basic solutions.

The analysis of thiamin and its derivatives in whole blood samples under high pH conditions of the mobile phase

In this study a protocol for the analysis of thiamin and thiamin coenzymes in whole blood was developed. Thiamin and its coenzymes are analyzed by reversed phase liquid chromatography (RPLC), precolumn derivatisation with alkaline potassium ferricyanide and fluorescence detection, all at pH 10. Under these relatively high pH conditions the detectability of the analytes and the robustness of the method were substantially improved. The use of a high pH resistant RPLC column was a crucial step in developing this analysis method. Reproducibility, linearity, recovery, detection limit and column robustness were investigated. The within-batch CV was <2.5%, the between-batch CV <4.5%. The method was linear far above the physiological relevant concentration level. Recovery was almost 100% on an average. The limit of quantification was 1 nmol/l. The robustness of the RPLC column proved to be very high. Up to 1500 injections hardly any substantial changes in retention times and efficiency were observed. In summary: Using a high pH resistant RPLC column resulted in a robust, sensitive and precise method for the analysis of total Vitamin B1 and especially of TDP.

Titanized silicas, modified by C18, as promising stationary phases for high pH separations

To enhance the high pH stability of silica based reversed phases, chemically bonded octadecyl phases were prepared through silanization of titanized silica particles containing approximately 14% titanium oxide on the surface. The present work describes some spectroscopic characterizations using infrared, solid-state 13C and 29Si nuclear magnetic resonance (NMR) and X-ray absorption spectroscopy (XAS). Chromatographic characterizations for the titanized phase as well as for a conventional C18 phase, based on the same silica support without titanization, are also described using three different test mixtures containing neutral, polar and basic compounds. After an artificial stability test at pH 10, the titanized phase was again characterized by elemental and X-ray fluorescence analyses to determine the remaining carbon and titanium contents. As an application to real world samples, the separation of some herbicides and highly basic drugs using buffered mobile phases are also shown.

Separation of racemic 2,4-dinitrophenyl amino acids on zirconia-immobilized quinine carbamate in reversed-phase liquid chromatography

Zirconia is known to be one of the best chromatographic support materials due to its excellent chemical, thermal, and mechanical stability. A quinine carbamate-coated zirconia was prepared as a chiral stationary phase for separation of enantiomers of DNP-amino acids in reversed-phase liquid chromatography. Retention and enantioselectivity of this phase were compared to those for quinine carbamate bonded onto silica. Most amino acids studied were separated on the quinine carbamate-zirconia CSP although retention was longer and chiral selectivity was somewhat lower than on the corresponding silica CSP. Increased retention and decreased selectivity are probably due to strong non-enantioselective Lewis acid-base interactions between the amino acid molecule and the residual Lewis acid sites on the zirconia surface.

Selectivity differences in the separation of amphipathic α-helical peptides during reversed-phase liquid chromatography at pHs 2.0 and 7.0

In an ongoing effort to understand the effect of varying reversed-phase high-performance liquid chromatography (RP-HPLC) parameters on the retention behaviour of peptides, necessary for the rational development of separation/optimization protocols, we believe it is important to delineate the contribution of alpha-helical structure to the selectivity of peptide separations. The present study reports the effects of varying column packing, mobile phase conditions and temperature on RP-HPLC retention behaviour at pHs 2.0 and 7.0 of peptides based on the amphipathic peptide sequence Ac-EAEKAAKEXEKAAKEAEK-amide (with position X in the centre of the hydrophobic face of the alpha-helix), where position X is substituted by L- or D-amino acids. At pH 2.0, an increase in trifluoroacetic acid concentration or the addition of sodium perchlorate to a phosphoric acid-based mobile phase had the similar effect of improving peak shape as well as increasing peptide retention time due to ion-pairing effects with the positively-charged peptides; in contrast, at pH 7.0, the addition of salt had little effect save an improvement in peak shape. Temperature was shown to have a complex influence on peptide selectivity due to varying effects on peptide conformation. In addition, subtle effects on peptide selectivity were also noted based on the column packings employed at pHs 2.0 and 7.0.

Chromatographic and column stability at pH 7 of a C18 dimethylurea polar stationary phase

Chromatographic evaluations of a C18 dimethylurea phase in 150 mm x 3.9 mm HPLC columns were performed using the Tanaka and Engelhardt test mixtures. The applicability of the new C18 dimethylurea phase was also evaluated with a mixture of some herbicides and their metabolites. An artificial aging procedure was also performed by passing a potassium phosphate mobile phase buffered at pH 7.0 through C18 50 mm x 3.9 mm dimethylurea columns. The column stability was evaluated by means of the chromatographic parameters obtained for the separation of some compounds from the Neue test mixture, using apolar, polar and highly basic analytes.

Rapid method for evaluating reversed-phase high-performance liquid chromatography column stability

A procedure is presented for the rapid evaluation of HPLC stationary phase stability at pH 8.4 or 10.1 using a temperature of 60 degrees C. Mobile phase (MeOH-0.1 mol l(-1) aqueous NaHCO3, 50:50, v/v) is continuously passed through the column with periodic injections of a test solution until the several chromatographic parameters of the resulting chromatograms are degraded. The tests were applied to several commercial and laboratory-made stationary phases. After degradation two of these phases, one commercial and one laboratory-made, were examined by elemental analysis and scanning electron microscopy to elucidate the degradation process.

Analysis of basic compounds at high pH values by reversed-phase liquid chromatography

Reversed phase high performance liquid chromatography (RPLC) is currently the method of choice for the analysis of basic compounds. However, with traditional silica materials, secondary interactions between the analyte and residual silanols produce peak tailing which can negatively affect resolution, sensitivity, and reproducibility. In order to reduce these secondary interactions, which comprise ion exchange, hydrogen bonding, and London forces interactions, chromatographic analyses can be carried out at low or high pH values where silanol groups and basic compounds are mostly uncharged. The chromatographic behaviour of a particular bidentate stationary phase, Zorbax Extend C18, was studied with a set of basic and neutral compounds. Thanks to a higher chemical stability than traditional silica based supports, analyses were carried out with a high pH mobile phase, which represents a good alternative to the acidic mobile phases generally used to reduce ion exchange interactions. The performance of this bidentate stationary phase was also compared with that of other supports and it was proved that it is advantageous to work with high pH mobile phases when analyzing basic compounds.

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.

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

The first part of the review contrasts the main drawbacks of silica-based packings such as their relative thermal and chemical instability with excellent stability of metal oxides. The paper concerns mainly ZrO2, TiO2 and Al2O3. Methods of preparation of spherical particles for HPLC are described. Surface chemistry of the oxides is, however, very different from that of silica. Ability of the oxides to ion- and ligand exchange is discussed from a chromatographic point of view.

High-performance liquid chromatographic stationary phases based on poly(methyloctylsiloxane) immobilized on silica. III. Stability evaluations

Several reversed-phase materials for high-performance liquid chromatography were obtained by deposition of poly(methyloctylsiloxane) (PMOS) on HPLC silica particles, followed by immobilization using different procedures. Each phase had characteristic physicochemical and chromatographic properties. The present work evaluates the stability of these phases with both neutral and basic mobile phases. All of the stationary phases were quite stable to neutral mobile phase, with less stability at higher pH. However, one thermally immobilized phase presented high stability even at an elevated temperature with a pH 10.0 mobile phase.

Mass spectrometry-compatible ICH (International Conference on Harmonization) impurity analysis with a high-pH mobile phase advantages and pitfalls

Recent advances in bonding chemistry and novel silica synthesis have significantly extended the pH range of silica-based HPLC columns. This extended range now enables the analysis of water-soluble basic drugs at high pH without ion-pairing reagents, thus offering an alternative approach to assay or impurity analyses. This paper describes the many advantages and potential pitfalls of using high-pH mobile phases in the development of MS-friendly LC gradient impurity analytical methods for water-soluble basic drugs under International Conference on Harmonization (ICH) guidelines. Operating at high-pH provides excellent peak shapes and retention, and accentuates selectivity differences of structurally similar impurities and degradants. However, several problems unique to the use of high-pH mobile phases, such as column lifetime, robustness of pH adjustments, peak fronting, and on-column dimerization, were encountered. Each of these problems is discussed with its respective remedy.

Evaluation of the applicability and the stability of a C18 stationary phase containing embedded urea groups

Chromatographic evaluations of a new C18 urea phase in 150x3.9 mm HPLC columns, involving the separation of different test mixtures, indicate good performance for both polar and basic compounds when compared with a commercial C18 reversed phase and also show promising results for the separation of some herbicides. An aging study was performed by passing a potassium phosphate mobile phase buffered at pH 7 through 50x3.9 mm HPLC columns. The column stability was evaluated by means of the chromatographic parameters obtained for the separation of some compounds of the Neue test mixture, containing apolar, polar and highly basic analytes. The applicability of the new C18 urea phase was evaluated with a herbicide mixture.