The effect of particle and ligand types on retention and peak shape in liquid chromatography

Purification of testosterone and its metabolites in urine using two-dimensional high-performance liquid chromatography for 13C/12C ratios analysis by gas chromatography/combustion/isotope ratio mass spectrometry

Two-dimensional high-performance liquid chromatography (2D-HPLC) is employed as a sample purification method for the isolation and enrichment of testosterone and its main metabolites in urine samples, thereby facilitating subsequent determination of 13C/12C ratios (δ13C) using Gas Chromatography/Combustion/Isotope Ratio Mass Spectrometry (GC/C/IRMS). The orthogonality of the HPLC columns was optimized by leveraging the distinct differences in polarity and steric hindrance effects between the stationary phases, enabling effective separation of endogenous interferents from testosterone in urine. Urine samples purified with 2D-HPLC exhibit significantly reduced matrix interferences in the fractions of testosterone (T), 5α-androstane-3α,17β-diol (5α-diol), and 5β-androstane-3α,17β-diol (5β-diol), thereby enhancing the accuracy and reliability of GC/C/IRMS analysis. The method achieves limit of quantification as low as 2 ng/mL for testosterone, and 3 ng/mL for both 5β-diol and 5α-diol. Furthermore, the simple instrument configuration enables direct transfer of 1D-purified fractions to 2D-HPLC for further isolation and enrichment, providing a flexible and efficient workflow for method development, particularly in handling complex biological matrices.

Mechanistic studies to understand peak tailing due to sulfinic acid- and carboxylic acid-silanophilic interactions in reversed-phase liquid chromatography

Silanophilic interactions are a primary contributor to peak tailing of acidic pharmaceutical compounds, thus a thorough understanding is especially important for reversed-phase liquid chromatography (RPLC) method development. Herein, a sulfinic acid compound that exhibited severe peak tailing in RPLC with acidic mobile phases was carefully studied. Results indicated that the neutral protonated form of the sulfinic acid is involved in the strong interaction that leads to peak tailing, but that tailing can be mitigated with a blocking effect achieved through use of acetic acid modifier in the mobile phase. Peak tailing was also observed with other structurally-similar sulfinic acids and carboxylic acids but was, in general, less severe with the latter. The Hydrophobic Subtraction Model (HSM) was applied to six commercial C18 columns that exhibited different tailing behaviors for the sulfinic acid compound in attempts to identify key sites of interaction within the stationary phase. A combination of heated acid column wash experiments and density functional theory (DFT) calculations indicate that the differential interactions of the acids with vicinal silanol pairs in the stationary phase play a major role in peak tailing.

Characterization and comparison of mixed-mode and reversed-phase columns; interaction abilities and applicability for peptide separation

In this work, two mixed-mode columns from a different manufacturers and one marketed as a reversed-phase column were characterized and compared in the terms of their interaction abilities, retentivity, peak symmetry, and applicability for peptide separation. All the tested columns contain octadecyl ligand and positively charged modifier, i.e. pyridyl group for the reversed-phase column XSelect CSH C18, quaternary alkylamine for mixed-mode column Atlantis PREMIER BEH C18 AX, and permanently charged moiety (details not available from the manufacturer) for mixed-mode column Luna Omega PS C18. For detailed characterization and comparison of their interaction potential, several approaches were used. First, a simple Walters test was performed to estimate hydrophobic and silanophilic interactions of the tested columns. The highest values of both parameters were observed for column Atlantis PREMIER BEH C18 AX. To investigate the effect of pH and buffer concentration on retention, mobile phases composed of acetonitrile and buffer (ammonium formate, pH 3.0; ammonium acetate pH 4.7 and pH 6.9) in various concentrations (5mM; 10mM; 15mM and 20mM) were used. The analysis of permanently charged compounds was used to describe the electrostatic interaction abilities of the stationary phases. The most significant contribution of electrostatic interactions to the retention was observed for Atlantis PREMIER BEH C18 AX column in the mobile phase with buffer of pH 3.0. A set of ten dipeptides, three pentapeptides and one octapeptide was used to investigate the effects of pH and buffer concentration on retention and peak symmetry. Each of the tested columns provides the optimal peak shape under different buffer pH and concentration. The gradient separation of the 14 tested peptides was used to verify the application potential of the tested columns for peptide separation. The best separation was achieved within 4 minutes on column Atlantis PREMIER BEH C18 AX.