Open-tubular electrochromatography of organic phosphates on a sapphyrin-modified capillary

Sapphyrin coating of the inner wall of the capillary results in a distinct interaction of the phosphate residue-possessing compounds as proven by a seven-membered model mixture of nucleoside mono- and diphosphates and ATP. Modification of the inner surface of the capillary not only alters the endoosmotic flow (as would have been expected) but brings about an electrochromatographic effect based on the interaction of tested phosphate moiety-bearing solutes with the immobilized sapphyrin layer. Elution of the sample can be achieved by using either 25 mM borate–acetate buffer in which monophosphates are not only separated from each other, but also selectively separated from di- and triphosphates (ATP). With the other two buffer systems tested, i.e. borate–phosphate and Tris–HCl, better selectivity (though smaller interaction with the capillary coating) was observed. The coating is relatively stable (can be used for 20 subsequent runs at least), simple to materialize, and in spite of a strong UV absorbancy of sapphyrin at the wavelength used (254 nm), decreases the limit of detection by no more than one order of magnitude as compared to the untreated capillary. Resolution factors (calculated to the preceding peak) are in most cases better in the electrochromatographic separation mode as compared to the separation in the untreated capillary, which reflects both the decrease in the electroosmotic flow and the interaction with the capillary wall coating.