Reversed-phase high-performance liquid chromatography behavior of chaotropic counteranions

Area of Science:

• Analytical Chemistry
• Chromatography

Background:

• Inorganic liophilic anions typically influence protonated basic analyte retention in reversed-phase HPLC.
• The exact mechanism of this influence, similar to ion-pairing agents, requires detailed investigation.

Purpose of the Study:

• To investigate the retention behavior of inorganic liophilic anions (perchlorate, tetrafluoroborate, hexafluorophosphate) in reversed-phase HPLC.
• To elucidate the role of mobile phase composition and column chemistry in the retention of these ions.
• To understand how these ions affect the retention of protonated basic analytes.

Main Methods:

• Studied HPLC retention of perchlorate (ClO4-), tetrafluoroborate (BF4-), and hexafluorophosphate (PF6-) ions.
• Utilized six reversed-phase columns with diverse bonded phases (alkyl, phenyl, perfluorophenyl).
• Investigated effects of mobile phase ionic strength and organic modifiers (acetonitrile, methanol).

Main Results:

• Significant retention of liophilic ions was observed in acetonitrile-water eluents.
• Multilayer adsorption of acetonitrile on the reversed-phase surface and its interactions with liophilic ions were identified as key factors.
• A semi-empirical expression was proposed to describe the retention profile versus eluent composition.

Conclusions:

• Inorganic liophilic anions exhibit notable retention in reversed-phase HPLC, particularly with acetonitrile-rich mobile phases.
• Acetonitrile adsorption and dispersive/pi-pi interactions play a crucial role in retaining these ions.
• The accumulation of these ions on the stationary phase introduces an electrostatic component influencing protonated basic analyte retention.