SELF-CONSISTENT FRAMEWORK FOR STANDARDIZING MOBILITIES IN FREE SOLUTION CAPILLARY ELECTROPHORESIS - APPLICATIONS TO OLIGOGLYCINES AND OLIGOALANINES

A theoretical analysis of deviations from ideality in ionic transport is presented to correct mobilities, mu, measured in free solution capi llary electrophoresis (CE) to mobility at infinite dilution, mu degree s (limiting mobility). Non-ideality is treated at the same level of ap proximation as in equilibrium, using a correction factor for the sum o f the analyte and counter-ion radius originally suggested by Robinson and Stokes (Electrolyte Solutions, 1961). Unlike previous corrections using Debye-Huckel-Onsager theory, which are strictly applicable only at very low ionic strengths, this treatment is expected to be valid fo r univalent ions migrating in a uni-univalent background electrolyte f or ionic strengths up to 0.075 mol kg(-1), a range typical of CE exper iments. The analysis is applied to the determination of mu degrees in acidic and basic buffers for oligoalanines and oligoglycines with degr ee of polymerisation 2 to 6. Limiting mobilities for the fully protona ted and deprotonated peptides are found to be numerically equal but op posite in sign, consistent with a change in charge from +1 to -1. In a ll uni-univalent buffers studied (berate, citrate, low pH lithium phos phate and sodium phosphate) mu degrees values established using data o ver a range of pH and ionic strength are found to be identical and in excellent agreement with previous values from isotachophoresis. Values of mu degrees 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 ma y be attributed to limitations of the model for a buffer with 1+:2- an d 1+:3- ions. This self-consistent framework for standardising mobilit ies in free solution CE is expected to be widely applicable to univale nt analytes migrating in a 1:1 background electrolyte.