HPLC of basic drugs and quaternary ammonium compounds on microparticulate strong cation-exchange materials using methanolic or aqueous methanol eluents containing an ionic modifier

doi:10.1016/0021-9673(94)01116-V

Journal of Chromatography A

Volume 693, Issue 2, 24 February 1995, Pages 289-306

https://doi.org/10.1016/0021-9673(94)01116-V Get rights and content

Abstract

Sulphopropyl (SCX)-method silica HPLC columns used with methanolic or aqueous methanol eluents of appropriate pH and ionic strength can give good retention and peak shape for quaternary ammonium compounds and basic drugs. In the system studied, eluent pH influenced retention via protonation of basic analytes, the pKa indicating the pH where retention began to decrease at constant ionic strength. At constant eluent pH retention was inversely proportional to ionic strength for protonated bases and quaternary ammonium compounds. However, this effect was less marked at pH 8.3 as compared to results obtained under acidic conditions. Except for codeine, morphine and lignocaine, the addition of water had no major effects on retention or selectivity at concentrations up to 30% (v/v) at pH 6.7. However, and in contrast to behaviour on unmodified silica, the addition of up to 5% (v/v) water under strongly acidic conditions caused a doubling of retention for most analytes studied.

SCX-modified silica columns can be used in the HPLC of a range of basic drugs, including many compounds which are poorly retained on unmodified silica using methanol-rich eluents. The underlying retention mechanism appears to be ion exchange with the SCX moieties, although ionized surface silanols may also contribute to retention at higher eluent pH values. Applications of SCX columns in the HPLC of basic drugs include the analysis of antimalarials such as chloroquine, desethylchloroquine, hydroxychloroquine and quinine, benzodiazepines such as clonazepam, bronchodilators such as salbutamol and terbutaline, cardioactive drugs such as flecainide and lignocaine, and tricyclic antidepressants such as amitriptyline, dothiepin, and imipramine, and their N-demethyl, hydroxyl and sulphoxide metabolites.

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Strong cation exchange (SCX) is widely applied to separate various analytes including ions [1], proteins [2–4], peptides [5–7] and basic compounds [8–14].
This study investigated the influence of organic sample solvents on separation efficiency of basic compounds under strong cation exchange (SCX) mode. The mixtures of acidic aqueous solution and organic solvent such as acetonitrile, ethanol, methanol and dimethyl sulfoxide (DMSO) were tested as sample solvents. For later-eluting analytes, the increase of sample solvent elution strength was responsible for the decrease of separation efficiency. Thus, sample solvents with weak elution strength could provide high separation efficiencies. For earlier-eluting analytes, the retention of organic sample solvents was the main factor affecting separation efficiency. Weakly retained solvents could provide high separation efficiency. In addition, an optimized approach was proposed to reduce the effect of organic sample solvent, in which low ionic solvent was employed as initial mobile phase in the gradient. At last, the analysis of impurities in hydrophobic drug berberine was performed. The results showed that using acidic aqueous methanol as sample solvents could provide high separation efficiency and good resolution (R > 1.5).
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A novel type of silica-based sulfonate-modified reversed phase (⁻SO₃-HC-C₈) has been synthesized; it is based on a newly developed acid stable “hyper-crosslinked” C₈ derivatized reversed phase, denoted HC-C₈. The ⁻SO₃-HC-C₈ phases containing controlled amounts of sulfonyl groups were made by sulfonating the aromatic hyper-crosslinked network of the HC-C₈ phase at different temperatures. The ⁻SO₃-HC-C₈ phases are only slightly less hydrophobic than the parent HC-C₈ phase. The added sulfonyl groups provide a unique strong cation-exchange selectivity to the hydrophobic hyper-crosslinked substrate as indicated by the very large C coefficient as shown through Snyder's hydrophobic subtraction reversed-phase characterization method. This cation-exchange activity clearly distinguishes the sulfonated phase from all other reversed phases as confirmed by the very high values of Snyder's column comparison function F_s. In addition, as was found in previous studies of silica-based and zirconia-based reversed phases, a strong correlation between the cation-exchange interaction and hydrophobic interaction was observed for these sulfonated phases in studies of the retention of cationic solutes. The overall chromatographic selectivity of these ⁻SO₃-HC-C₈ phases is greatly enhanced by its high hydrophobicity through a “hydrophobically assisted” ion-exchange retention process.

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¹: Present address: Gasthuisberg, Onderwijs en Navorsing, Laboratory of Pharmacology, Herestraat 49, 3000 Leuven, Belgium.

²: Present address: Coagulation/Vitamin K Research Unit, Haemophilia Centre, 1st Floor, North Wing, St Thomas' Hospital, London SE1 7EW, UK.

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Regular paperHPLC of basic drugs and quaternary ammonium compounds on microparticulate strong cation-exchange materials using methanolic or aqueous methanol eluents containing an ionic modifier

Abstract

J. Chromatogr.

J. Pharmaceut. Biomed. Anal.

J. Pharmaceut. Biomed. Anal.

J. Pharmaceut. Biomed. Anal.

J. Chromatogr.

Trends Anal. Chem.

J. Chromatogr.

Trends Anal. Chem.

J. Pharmaceut. Biomed. Anal.

J. Chromatogr.

J. Chromatogr.

J. Chromatogr.

J. Chromatogr.

Biomed. Chromatogr.

Regular paper
HPLC of basic drugs and quaternary ammonium compounds on microparticulate strong cation-exchange materials using methanolic or aqueous methanol eluents containing an ionic modifier