2009-Electrophoresis-Formamide as modifier in MEKC
The effect of formamide (FA) as a modifier on the retention in MEKC with SDS as the
detergent was investigated. The mobility of a series of alkylphenones and of a zwitterionic
fluorescent compound as a function of the FA and the SDS concentration was
determined for this purpose. Buffering electrolyte was borate, pH 9.23, with total ionic
strength of 50 mM. The dependence of the mobility on the FA content – up to 63% w/w
– of the BGE (at 10mM SDS) allows the conclusion that the micelles are destabilized,
and the CMC is shifted to higher values. In the system containing 33% FA or more no
micelles are present anymore, and the retention factors of all compounds tend to zero. In
an MEKC system with 27% v/v FA the CMC of SDS is increased from 2.4mM in the
aqueous BGE with the same buffer composition to 9.7 mM, a behavior that is in contrast
to electrolyte-free FA–water systems. The partition constants of free analytes and the
formation constants of the adduct between analyte and detergent monomer (assuming
1:1 stoichiometry) were derived from the dependence of the mobility on the SDS
concentration. In addition, the involved equilibria were extended by that from the
distribution of the analyte–monomer adduct between aqueous and micellar phase, and
the according partition constants were derived as well. A selective change in the extent of
partitioning was observed for the zwitterionic compound. In general, all binding
constants were decreased upon addition of FA, though to a different extent. Although the
binding constants of the analyte–monomer associate were only slightly influenced, the
most pronounced decrease is found for their partitioning into the micelles.
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