Self-consistent framework for standardising mobilities in free solution capillary electrophoresis: applications to oligoglycines and oligoalanines

Abstract

A theoretical analysis of deviations from ideality in ionic transport is presented to correct mobilities, μ, measured in free solution capillary electrophoresis (CE) to mobility at infinite diluton, μ o (limiting mobility). Non-ideality is treated at the same level of approximation as in equilibrium, using a correction factor for the sum of the analyte and counter-ion radius originally suggested by Robinson and Stokes (Electrolyte Solutions, 1961). Unlike previous corrections using Debye-Hückel-Onsager theory, which are strictly applicable only at very low ionic strengths, this treatment is expected to be valid for univalent ions migrating in a uni-univalent background electrolyte for ionic strengths up to 0.075 mol kg −1, a range typical of CE experiments. The analysis is applied to the determination of μ o in acidic and basic buffers for oligoalanines and oligoglycines with degree of polymerisation 2 to 6. Limiting mobilities for the fully protonated and deprotonated peptides are found to be numerically equal but opposite in sign, consistent with a change in charge from +1 to −1. In all uni-univalent buffers studied (borate, citrate, low pH lithium phosphate and sodium phosphate) μ o values established using data over a range of pH and ionic strength are found to be identical and in excellent agreement with previous values from isotachophoresis. Values of μ o in high pH sodium phosphate buffer are systematically 0.2·10 −8 m 2 V −1 s −1 higher than those in other buffers; this may be attributed to limitations of the model for a buffer with 1+:2− and 1+:3− ions. This self-consistent framework for standardising mobilities in free solution CE is expected to be widely applicable to univalent analytes migrating in a 1:1 background electrolyte.