Microcolumn sample injection by spontaneous fluid displacement.

The withdrawal of a capillary structure from a sample solution causes a droplet to be formed at the end of the capillary. Because of the interfacial pressure difference across the curved surface of the droplet, the droplet is driven into the entrance of the capillary, thereby causing injection of the sample. Assuming negligible sample penetration by diffusive or convective mixing, this injection is intrinsically the smallest possible for a capillary. Moreover, the injection volume can be varied by changing the shape of the capillary structure, specifically the outer diameter of the capillary. This injection method eliminates the need for external pressure differences, applied fields across the capillary, or precise timing, thus offering several advantages over conventional procedures. Studies using capillary electrophoresis as the separation procedure show that approximately 3.5 nl (66 microns I.D. capillary) sample volumes can be injected by hand with a reproducibility of 5.8 +/- 0.7% R.S.D. Parameters that affect the variability of the injection are discussed.