Prevention of evaporation of small-volume sample solutions for capillary electrophoresis using a mineral-oil overlay

The suppression of evaporation of water from small volumes of sample soluti ons or reagents for capillary electrophoresis by the use of a mineral-oil o verlay was investigated in affinophoresis applications, in which the affini ty constant of a mutant protein of recombinant human galectin-1 to a lactos e affinophore, a triply negative charged ion having a lactoside as an affin ity ligand, was determined. When an injection was carried out from a minimu m of 20 muL of an aqueous solution beneath the oil overlay, no oil contamin ation inside the capillary was observed, provided the capillary was cleanly cut so that the end was flat, and the polyimide coating had been removed f or a distance of about 2 mm from the end. Affinophoresis was carried out us ing 20 PL of an affinophore solution covered with an oil overlay. The abnor malities in the electropherograms as the result of the evaporation of the w ater from the solution during storage prior to use in an automatic operatio n of a capillary electrophoresis instrument were suppressed, with respect t o the formation of a base line gap, an increase in the detection time of a marker ion and an increase in the initial current. A solution in a vial cou ld be used repeatedly for a longer period of time when overlaid with minera l oil than in the absence of an overlay. The use of a mineral-oil overlay i s a simple but very efficient technique for solving the problem of the evap oration of water from small volumes of aqueous solutions for use in capilla ry electrophoresis.