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HPLCremains the workhorse technique in liquidphase separations. The method has two fundamental roots. One is gel-permeation chromatography for the characterization of polymers (1); the other is GC. HPLC began to emerge when GC researchers turned their attention to liquid-phase separations (2, 3). The original LC columns were typically 50-100 cm long, with a 1-2 mm i.d., and packed with 50-200-µm-diam particles. Shorter diffusion distances were known to provide better performance. However, it was not possible to prepare high-performance columns with particles with diameters <30 µm by using the dry-packing techniques that had been applied successfully in GC. One improvement was the development of superficially porous particles that resulted in better column performance at low retention factors (4,5).A breakthrough was achieved in the early 1970s, when slurry packing techniques were developed for fully porous particles <30 µm in diameter (6-8). Researchers rapidly learned to pack efficient columns with 10-, 5-, and 3-µm particles (9-11). The first commercial columns packed with 10-µm particles became available in 1973 (12). During these early investigations, it became clear that the pressures required to operate long, small-particle columns at suitable velocities (above the minimum of the plate-height-velocity curve) heat up the mobile phase because of friction (13,14). This heat would ultimately limit the benefits of using smaller particle sizes unless measures were taken to compensate for this effect.Today, two driving forces continue to test the limits of HPLC. One is the need for faster separations, such as analyses of either simple samples or a few constituents in a complex sample (15-17 ). The second is the desire to achieve greater separation power to quantify or identify all the constituents of a complex sample or to compare the contents of complex samples with each other (18)(19)(20)(21)(22) A new separati on techni que takes advantage of sub-2-µm porous parti cl es.

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