On-line preconcentration in capillary electrochromatography using a porousmonolith together with solvent gradient and sample stacking

Preconcentration effects of solvent gradient and sample stacking are invest igated on a photopolymerized sol-gel (PSG) in capillary electrochromatograp hy. The porous PSG monolith has a high mass-transfer rate. This characteris tic promotes preconcentration of dilute samples. Plugs of samples more than 2 cm in length prepared in the separation solution (nongradient condition) are injected onto the PSG column. The extent of preconcentration is quite significant showing up to a 100-fold increase in peak heights of the separa ted analytes. Even larger preconcentrations are achieved under gradient con ditions by dissolving the sample in a matrix with a higher concentration of noneluting solvent (water). For eight alkyl phenyl ketones and four polycy clic aromatic hydrocarbons that serve as neutral test analytes, improvement s in peak heights obtained under gradient conditions can be more than a 100 0-fold. Indeed, injection of a 91.2-cm plug, which is more than 3 times the total length of the capillary, was possible with only a minor loss in reso lution. Five peptides serve as charged test analytes. Nongradient condition s in which the sample is hydrodynamically injected onto the PSG column show sizable preconcentration because of sample stacking. The use of a solvent gradient with the same ionic strength, however, does not appear to have pra ctical value because of destacking caused by the changing organic compositi on that affects the conductivity. As an alternative preconcentration method , we demonstrate that electric field-enhanced sample injection on the PSG y ielded up to a 1000-fold improvement in detection sensitivity for the test peptides.