Solute Attributes and Molecular Interactions Contributing to Retention on a Fluorinated High-Performance Liquid Chromatography Stationary Phase
Open Access
- Author:
- Bell, David Scott
- Graduate Program:
- Chemistry
- Degree:
- Doctor of Philosophy
- Document Type:
- Dissertation
- Date of Defense:
- April 29, 2005
- Committee Members:
- A Daniel Jones, Committee Chair/Co-Chair
Alan James Benesi, Committee Member
Mark Maroncelli, Committee Member
Christopher Albert Mullin, Committee Member - Keywords:
- liquid chromatography
fluorinated
retention mechanisms
mass spectrometry
pentafluorophenyl
stationary phase - Abstract:
- Structural attributes and molecular interactions contributing to “U-shape” retention on pentafluorophenylpropyl (PFPP) HPLC stationary phases are systematically investigated. Basic analytes exhibit retention that increases with the acetonitrile content in mixtures of acetonitrile and aqueous ammonium acetate. Retention of the quaternary ammonium salt bretylium showed minimal influence of ion-exchange mechanisms on the C18 phase, however a significant influence of ion-exchange mechanisms was observed for both PFPP and bare silica. These findings suggest ion-exchange mechanisms dominate retention of basic analytes in the high acetonitrile realm on PFPP. The PFPP stationary phase exhibits a substantial increase in effects of ionized surface silanol groups compared to the alkyl phase despite similar surface coverage. Retention of some basic analytes on a PFPP phase was enhanced relative to retention on silica alone, and implicates other dispersive interactions that might be exploited for selectivity different from either alkyl phases or silica alone. NMR spectroscopy is a rapid and useful technique for the determining pKa values of solutes in HPLC mobile phases. The variation of chemical shift data for protons in the vicinity of a basic nitrogen atom as a function of the medium pH can be related through the Henderson-Hasselbalch equation to estimate analyte pKa values. The present study demonstrates that one can not assume that bases are protonated in high acetonitrile content even if the aqueous pH is adjusted to less than two pKa units from the literature pKa value. The pKa values for the basic analytes were shown to decrease by approximately one pKa unit in approximately 90 v/v% acetonitrile from their aqueous value. Improved prediction and manipulation of HPLC selectivity results from more accurate knowledge of the analyte degree of dissociation values. Knowledge of analyte degree of dissociation allows one to further explore the fundamental mechanisms of retention in chromatographic processes.