Strategy for on-line preconcentration in chromatographic separations

In chromatographic separations, the heights of peaks are proportional to th e concentrations of sample components present in an injected mixture. In ge neral, an increase in the peak height cannot be achieved by simply increasi ng the injection time or the sample plug length. An exception occurs if som e form of on-line preconcentration is possible. We present a new strategy f or achieving on-line preconcentration by the use of a porous chromatographi c material that acts as a solid-phase extractor as well as a stationary-pha se separator. We are able to realize significant on-line preconcentration u sing capillary columns filled with a photopolymerized sol-gel (PSG). More t han 2-cm plugs of sample solution can be loaded into the capillary and conc entrated using a running buffer that is the same as the injection buffer (t o avoid solvent gradient effects). As a demonstration, mixtures of three di fferent polycyclic aromatic hydrocarbons, eight different alkyl phenyl keto nes, and five different peptides in solutions of aqueous acetonitrile have been injected onto the PSG column and separated by capillary electrochromat ography. The preconcentration is marked in terms of peak heights, with up t o 100-fold increase for the PAH mixture, 30-fold for the alkyl phenyl keton e mixture, and 20-fold for the peptide mixture. Preconcentration takes plac e because of the high mass-transfer rates possible in the highly porous str ucture, and the extent of preconcentration follows the retention factor k f or a given analyte.