Abstract: A composition, which has utility as a storage buffer and as a sample diluent before analysis by capillary zone electrophoresis (CZE), is comprised of water, a borate compound, a buffer compound and a pH modifier for maintaining a physiological pH. In addition, a conductivity adjusting compound is present so that the conductivity of the composition is about the same as a prospective capillary electrophoresis running buffer. Containers with storage buffer/sample diluent can be assembled in a kit for conducting serum protein electrophoresis (SPE) or immunosubtraction (IFE/s) analysis. Moreover, the storage buffer/sample diluent can be used in methods of preparing samples for CZE analysis, such as quantitative dilutions and enzymatic or immunological reactions.