Nonaqueous capillary electrophoresis of fatty acids derivatized with a near-infrared fluorophore

Saturated linear fatty acids, derivatized with a near-infrared absorbing fluorescent dye, were separated in 100% methanol with 12.5 mM tetraethylammonium chloride added as a charge carrier. Separation at 380 V/cm was acceptable for acids that differed in length by a single carbon. The labeled linear fatty acids behaved as random coils in the nonaqueous separation medium, as shown in a fit to a simple theoretical expression. However, even in 100% methanol with a trimethylsilylated capillary, significant adsorption to the capillary wall occurred, which reduced resolution and slowed the separation. Addition of water to the methanol medium caused significant differences in separation behavior of high molecular weight acids (>C16). Addition of a cetyltrimethylammonium bromide surfactant to the separation medium dynamically coated the capillary and greatly improved the separation. The surfactant also interacted with the acyl tail, apparently causing it to collapse. Resolution in an optimal separation medium (20 mM surfactant) ranged from 1.6 to 1.1, depending on chain length, and theoretical plate heights were under 4 microm (N > 10(5)). Resolution was more than adequate to separate stearic (C18:0) from oleic (C18:1) acid, as well as other unsaturated C18 homologues.