Review on the chemical and thermal stability of stationary phases for reversed-phase liquid chromatography.

At present, in high-performance liquid chromatography (HPLC) for the majority of analyses, reversed-phase liquid chromatography (RPLC) is the separation mode of choice. Faster method development procedures using aggressive eluents under elevated temperature conditions, the need for improved selectivities, efficiencies and resolution, the reduction of solvent consumption and also the decrease of analysis times require reversed-phase (RP) columns of high chemical and thermal stability. Until now, the majority of columns for RPLC separations are manufactured from silica substrates. Silica has many favorable properties making this material nearly ideal as a support for RP columns. However, its solubility, that increases considerably in eluents of pH above +/-7, is a drawback preventing its widespread use over the entire pH range. In addition, also the thermal stability of silica is limited. Recently, however, substantial progress has been made in the synthesis of RPLC silica-based stationary phases showing satisfactory thermal and chemical stability under many different experimental conditions. Also, new substrates mainly based on other inorganic substrates like, e.g. alumina and zirconia have been developed now as a starting material for the preparation of RPLC stationary phases of improved chemical and thermal stability. In addition, for the same reasons, many efforts have also been made to synthesize polymer and also polymer-coated phases. These latter phases, more particularly those based on zirconia, but also polymer phases show a high degree of chemical and thermal stability compared to silica counterparts. In this paper, an overview will be given of the state-of-the-art of the thermal and chemical stability of the different available stationary phases for RPLC.

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.

Stability of silica-based, endcapped columns with pH 7 and 11 mobile phases for reversed-phase high-performance liquid chromatography.

The goal of this study was to define practical conditions and limitations of using silica-based, endcapped bonded-phase columns in intermediate and higher pH environments for developing rugged HPLC methods. Bonded-phase degradation in this pH range is a result mainly of silica support dissolution; covalently-bound silane ligands are hydrolyzed very slowly if at all from silica supports at intermediate and higher pH. Based on rates of silica support dissolution determined by chemical measurements and comparable chromatographic studies, we now find that endcapping alkyl-bonded stationary phases increases column longevity at pH 7, compared to non-endcapped columns. As previously determined for non-endcapped packings, we also find that the type of silica support determines the stability of bonded-phase packings. Silicas made by the sol-gel process are more resistant to dissolution than supports made by a silicate-gel (xerogel) process. In addition, endcapping methods apparently affect column stability, with double-endcapping methods apparently superior to single-endcapping approaches. Degradation rates for several endcapped commercial bonded-phase C8 columns were found to be quite variable in highly aggressive pH 7 accelerated-lifetime tests. Column stability in the pH 7-11 range is enhanced by using buffers other than phosphate in the mobile phase, and by excluding higher column temperatures. Certain silica-based endcapped bonded-phase columns can be used for developing rugged methods to at least pH 11 when used with organic buffers at < or = 40 degrees C.