Capillary electrophoresis in N,N-dimethylformamide

Area of Science:

• Analytical Chemistry
• Physical Chemistry

Background:

• N,N-Dimethylformamide (DMF) is a versatile solvent for capillary electrophoresis (CE).
• Accurate pH control of background electrolytes (BGE) is crucial for CE applications.
• Understanding solvent effects on analyte properties is essential for method development.

Purpose of the Study:

• To investigate the pH-dependent behavior of analytes in DMF-based BGEs for CE.
• To determine the electrophoretic mobilities and pK(a) values of substituted phenolates in DMF.
• To explore the influence of solvent composition on analyte-solvent interactions and separation mechanisms.

Main Methods:

• Preparation of benzoic acid-benzoate buffers in DMF with varying acid/salt ratios.
• Calculation of theoretical pH using the activity-corrected Henderson-Hasselbalch equation.
• Measurement of analyte mobilities across a range of pH values and determination of pK(a) values.
• Investigation of ion pairing, homoconjugation, and heteroconjugation effects.
• Analysis of the impact of water content on DMF solvent properties and separation parameters.

Main Results:

• Analyte mobilities in DMF exhibited a typical sigmoid dependence on pH.
• Determined pK(a) values for chloro- and nitro-substituted phenolates in DMF were 3-4.4 units higher than in water (11.1-11.7).
• Observed deviations from theoretical mobility curves attributed to ion pairing and conjugation phenomena.
• Heteroconjugation with salicylate enabled separation of neutral analytes, with complexation constants around 100-200 L/mol.
• Water content up to 20% in DMF still influenced heteroconjugation and other electrophoretic parameters.

Conclusions:

• DMF is a viable solvent for CE, but its unique properties necessitate careful buffer optimization.
• The significant pK(a) shift in DMF is due to analyte anion destabilization and molecular acid stabilization.
• Ion pairing and conjugation play critical roles in analyte migration and separation in DMF-based BGEs.
• Water content significantly impacts DMF's solvent properties and CE performance, requiring control for reproducible results.