Editorial
CE + HPLC = CEC
T
here is a new separations kid on the block named "capillary electrochromatography" (the operative acronym is CEC). CEC is an assortment of electrochemical, fluid dynamic, and chromatographic phenomena. In methods language, CEC is a hybrid of capillary electrophoresis (CE) and highperformance liquid chromatography (HPLC). The hybrid is a bit amorphous because all the essential processes that operate individually in CE and HPLC—for example, pressurized and electroosmotic flow (EOF), and stationary-phase partition (or adsorption) and electrophoretic migration—can be simultaneously present in a CEC experiment. In the "minimalist" form of a CEC experiment, an eluting solution containing a dissolved electrolyte in an HPLC-type packed column (e.g., a silica column packed with C18-modified, micron-sized silica beads) is induced into EOF by applying a voltage gradient down the column. EOF replaces the HPLC pump and interactions of different sample constituents with the C surface produce the differential band velocities that separate the mixture. The amorphous part comes with ionic samples for which electrophoretic migration occurs concurrendy with partition and when pressurized flow is used in combination with EOF Electroosmosis is, of course, a long-known phenomenon. EOF's emergence in CEC was, at least in part, prompted by researchers gaining familiarity with its important role in band motion in CE and its newfound role in solution mixing and flow injection analysis experiments on micromachined silica plates. It's easy to see why CEC is getting a lot of attention. The analytical chemist can control a literal summation of CE and HPLC experimental parameters, including flow rate and the choice of a wide range of solvents and stationary phases. The stationary phase can be surfacemodified silica beads or some other polymeric material, relatively hydrophobic or a polar ion exchange layer, or a "monolith" formed from a sol-gel or molecularly imprinted polymer phase. Gradient elution can be accomplished using either solvent mixtures or applied
voltage bias gradients. This overall flexibility greatiy aids the researcher well versed in HPLC materials and can be applied to chiral separations, pharmaceutical drug discovery projects, environmental analysis, and more. Although the presence of die stationary phase and the attendant mass transport effects degrade the plate count relative to CE, effective applications still seem possible. I saw a plethora of interesting examples of CEC earlier this summer at the HPLC '99 conference, which clearly came from this transposition of knowledge—it's a good illustration of how science can work in leaps and bounds. However, some aspects of the CEC effort require a critical look. It is important to realize that, although the EOF process can produce stable flow rates just like a pump, EOF is more than a pump; ;t is a complex and chemically variable phenomenon. The fundamentals of the electroosmotic effect in the complex, three-dimensional context of a packed column can be outlined today in only very general terms. The electroosmotic effect is both spatial and chemical. It is spatial because the diffuse double laver is larger than molecular dimensions and sensitive to the solution composition the topoloffv of the underlyint? surface and the charge of that surface which is itself a chemicallv based eline- them is goinff to be an interesting scientific storv
CEC might prove to be useful in other ways. For example, it might be useful in materials chemistry studies of charged surfaces, including organic, solventswollen, and radiation-damaged. A germane issue in such an application would be an assessment of the sensitivity of the EOF velocity to small changes in surface charge. This CEC hybrid is fascinating. Its flexibility is matched only by its complexity; both will be ore for analytical chemists to mine for years to come.
Analytical Chemistry News & Features, August 1, 1999 4 9 9 A
