A micromethod for concentration and desalting utilizing a hollow fiber, with special reference to capillary electrophoresis

Concentration of solutes in small volumes offers special problems. The difficulties are particularly great when the sample concentration is low, because the adsorption onto the surfaces with which the solutes come into contact may be very strong. The adsorption not only decreases the original sample concentration but also often alters the relative concentrations of the solutes, causing serious deviations from the original composition of the sample. A novel concentration method is described using a hollow fiber filled with the dilute sample solution. The technique is based on the transport of water out of the fiber through the pores in its wall and has the advantage that solute adsorption is negligible, at least when the solute concentration is not extremely low. The water transport can be achieved by passive or active evaporation or by the Donnan effect, accomplished by immersing the fiber filled with sample into a polymer solution. The method has the advantage that it permits concentration of all substances independently of their molecular weight. Proteins, for instance, were enriched 3000-fold in one step. By using an array of fibers and collecting all of the concentrated zones for a second concentration, the degree of concentration can theoretically become 3000(2) = 9 x 10(6)-fold, provided that the limit of solubility is not exceeded. The speed of enrichment of proteins;was 14-fold per minute for a fiber with an inner diameter of 0.2 mm, length 280 mm, and wall thickness about 0.01 mm in the dry state (the speed can be increased considerably by blowing air over the fiber, e.g., by means of a fan). Simultaneously with the concentration, some purification of the sample can often be achieved by using a fiber with an appropriate molecular weight cutoff value.