A retention index for micellar electrokinetic chromatography

Area of Science:

• Analytical Chemistry
• Separation Science

Background:

• Developing reliable retention index systems is crucial for chromatographic analysis.
• Micellar electrokinetic chromatography (MEKC) offers unique separation capabilities but requires robust standardization.

Purpose of the Study:

• To develop and evaluate a retention index system for micellar electrokinetic chromatography (MEKC).
• To identify suitable homologous series for use as retention index standards in MEKC.
• To assess the impact of different micellar pseudostationary phases and organic modifiers on retention indices.

Main Methods:

• Investigated three homologous series (alkyl aryl ketones, 1-nitroalkanes, alkylbenzenes) as potential retention index standards.
• Employed various micellar systems including sodium dodecyl sulfate (SDS) alone and with Brij 35 or SB-12.
• Evaluated the effect of organic modifiers (acetonitrile, methanol, 1-propanol) on retention indices of neutral compounds in SDS-based MEKC.

Main Results:

• Retention indices for neutral compounds showed minimal variation (<2.0% RSD) across surfactant concentrations in micellar systems without organic modifiers.
• In systems with organic modifiers, retention indices exhibited slight variations (<3.0% RSD) with changing SDS concentrations.
• 1-Nitroalkanes demonstrated the highest suitability as retention index standards.
• Alkyl aryl ketones were effective for hydrophobic solutes but not hydrophilic ones.
• Alkylbenzenes were generally too hydrophobic for use as standards in this MEKC system.

Conclusions:

• 1-Nitroalkanes are recommended as reliable retention index standards for neutral compounds in MEKC.
• The choice of retention index standard is critical and depends on the analyte's hydrophobicity and the MEKC mobile phase composition.
• Further optimization of MEKC conditions may be needed for analyzing very hydrophilic or hydrophobic analytes using specific retention index standards.