The silanol group and its role in liquid chromatography

Enantioseparation of amfepramone (rac-diethylpropion): preparative separation of the enantiomers and enantioselective analysis

The enantiomers of the anorectic drug amfepramone [rac-diethylpropion, rac-2-(diethylamino)-1-phenyl-1-propanone; rac-DEP] were separated in the preparative scale by crystallization. With enantiopure di-O-benzoyltartaric acid as salt-forming chiral selector, diastereoisomeric salts of DEP enantiomers with a final purity of more than 97.5% were obtained. Analytical liquid chromatographic and electrophoretic methods for the control of the enantiomeric purity and the stoichiometry of the salts were developed. The enantioseparation of rac-DEP by capillary electrophoresis (CE) using hydroxypropyl-beta-cyclodextrin (HP-beta-CD) as chiral discriminator and phosphate buffer (pH 3.3) as run buffer led to good separations. HPLC methods were developed using polysaccharide chiral stationary phases (CSP). The separation of the two enantiomers and the two main degradation products (1-phenyl-1,2-propanedione and propiophenone), known from solid and liquid pharmaceutical preparations, was attained in one run on the silica-based CSP cellulose tris(3,5-dimethylphenylcarbamate) (Chiralcel OD). The conditions which might affect the enantioselectivity and the quality of the enantiomeric separation were investigated for Chiralcel OD and the related CSP amylose tris(3,5-dimethylphenylcarbamate) (Chiralpak AD). Both CSPs showed very similar chromatographic properties. The separation factors could be influenced significantly by varying the polar organic modifier added to the mobile phase.

Chromatographic properties of reversed-phase stationary phases under pressure- and electro-driven conditions

Seven different reversed-phase (RP) stationary phases were examined under high-performance liquid chromatographic (pressure-driven, HPLC), and capillary electrochromatographic (electro-driven, CEC) conditions. Characterization of the stationary phases was performed following well-established test procedures providing a number of distinct column descriptors: hydrophobicity, hydrophobic selectivity and silanol activity. These parameters were used to describe the behavior of the RP-columns under both HPLC and CEC conditions. It is shown that chromatographic characteristics of porous RP-phases greatly depend on the mode of operation. By contrast, column descriptors of a non-porous viz. solid RP-phase material hardly differed for HPLC and CEC conditions.

Solid-phase extraction: method development, sorbents, and coupling with liquid chromatography

The objective of this review is to provide updated information about the most important features of the new solid-phase extraction (SPE) materials, their interaction mode and their potential for modern SPE. First, the recent developments are given in formats, phases, automation, high throughput purpose and set-up of new types of procedures. Emphasis is then placed on the large choice of sorbents for trapping analytes over a wide range of polarities, such as highly cross-linked copolymers, functionalized copolymers, graphitized carbons or some specific n-alkylsilicas. The method development is given which is based on prediction from liquid chromatographic retention data or solvation parameters in order to determine the main parameters of any sequence (type and amount of sorbent, sample volume which can be applied without loss of recovery, composition and volume of the clean-up solution, composition and volume of the desorption solution). Obtaining extracts free from matrix interferences in a few steps--one step when possible--is now included in the development of SPE procedure. New selective phases such as mixed-mode and restricted access matrix sorbents or emerging phases such as immunosorbents or molecularly imprinted polymers are reviewed. Selectivity obtained by combining two sorbents is described with the use of ion-exchange or ion-pair sorbents. Special attention is given to complete automation of the SPE sequence with its on-line coupling with liquid chromatography followed by various detection modes. This represents a fast, modern and reliable approach to trace analysis. Many examples illustrate the various features of modern SPE which are discussed in this review. They have been selected in both biological and environmental areas.

Repeatability and reproducibility of retention data and band profiles on reversed-phase liquid chromatography columns. III. Results obtained with Kromasil C18 columns

The reproducibility of the retention data and the band profiles was investigated with Kromasil C18 columns (silica-based monomeric type reversed-phase packing material). High precision data were obtained and statistically compared among five columns from the same batch (column-to-column reproducibility) and six columns, one from each of six different batches (batch-to-batch reproducibility). These data were acquired under five different sets of chromatographic conditions, for a group of 30 neutral, acidic and basic compounds selected as probes following an experimental protocol previously described. Data characterizing the retention time, the retention factor, the separation factor, the column efficiency and the peak asymmetry for the different probe compounds are reported. Factors describing the silica surface interaction with the selected probe compounds, such as the hydrophobic interaction selectivity, the steric selectivity, and the separation factors of basic compounds at different pH values were also determined. The influence of the underlying silica on these data and correlations between the chromatographic and physico-chemical properties of the different batches are discussed.

Molecular mechanism of retention in reversed-phase high-performance liquid chromatography and classification of modern stationary phases by using quantitative structure-retention relationships

Quantitative structure-retention relationships (QSRRs) were derived for logarithms of retention factors normalised to a hypothetical zero percent organic modifier eluent, log kw, determined on 18 reversed-phase high-performance liquid chromatography (RP-HPLC) columns for 25 carefully designed, structurally diverse test analytes. The study was aimed at elucidating molecular mechanism of retention and at finding an objective manner of quantitative comparison of retention properties and classification of modern stationary phases for RP-HPLC. Three QSRR approaches were employed: (i) relating log kw to logarithms of octanol-water partition coefficient (log P); (ii) describing log kw in terms of linear solvation-energy relationship-based parameters of Abraham; (iii) regressing log kw against simple structural descriptors acquired by calculation chemistry. All the approaches produced statistically significant and physically interpretable QSRRs. By means of QSRRs the stationary phase materials were classified according to the prevailing intermolecular interactions in the separation process. Hydrophobic properties of the columns tested were parametrized. Abilities of individual phases to provide contributions to the overall retention due to non-polar London-type intermolecular interactions were quantified. Measures of hydrogen-bond donor activity and dipolarity of stationary phases are proposed along with two other phase polarity parameters. The parameters proposed quantitatively characterize the RP-HPLC stationary phases and provide a rational explanation for the differences in retention patterns of individual columns observed when applying the conventional empirical testing methods.

Comparative study of divalent metals and amines as silanol-blocking agents in reversed-phase liquid chromatography

In this work we compare the silanol-blocking ability of different alkaline earth metal cations, including calcium, magnesium and barium, and strong amine silanol blockers, such as triethylamine and octylamine, using six basic probe solutes at pH 7 on a conventional octadecylsilane phase. Some amines are better blocking agents than the metal cations but this varies with the amine and analyte structure. Among the metals, barium is the best blocker. For certain solutes barium is as effective at blocking silanols as some of the amine blockers. It produces short retention times and good peak shapes with satisfactory peak symmetry factors. However, amines with long alkyl chains, such as octylamine, are better blocking agents than barium. Peak symmetry is still poor for some solutes even in the presence of the strongest blocking agent in the eluent.

Reproducibility of the separation of astaxanthin stereoisomers on Pirkle covalent L-leucine and D-phenylglycine columns

The reproducibility of the separation of astaxanthin stereoisomers on columns packed with Pirkle covalent L-leucine chiral stationary phase (CSP) was examined by comparing six columns purchased from the same manufacturer. Differences were found even for columns packed with CSP from the same lot. The reproducibility of columns packed with Pirkle covalent D-phenylglycine CSP was also examined by comparing columns purchased from the same manufacturer as well as from different manufacturers. Significant differences were found for columns packed by different manufacturers. Chiral column-to-column reproducibility for complex stereoisomeric separations should therefore not be taken for granted.

Peptide separation in normal-phase liquid chromatography. Study of selectivity and mobile phase effects on various columns

An experimental procedure for peptide separation by normal-phase liquid chromatography (NPLC) was proposed in previous papers. In the present study, the chromatographic behavior of amino, cyano, amide, diol and silica columns, which have been used in non-aqueous NPLC, is investigated anew. The amino column was not appropriate for peptide separation because of poor recovery. The cyano column could not be used due to lack of retention. The amide, diol and silica columns were useful for peptide separation. The chromatograms on amide, diol and silica columns were a little different when the mobile phase composition was changed. The recovery of peptides was good: diol > amide > silica. Repeatability and reproducibility using amide, diol and silica columns was satisfactory.