Capillary ultrafiltration: in vivo sampling probes for small molecules.

Capillary ultrafiltration is a novel sampling method convenient for low molecular weight substances in living biological systems. By application of a negative pressure across a hydrophilic membrane capillary, small molecules are actively "pulled" across the membrane and collected. By elimination of large molecules and cellular matter, the ultrafiltrate collected is well suited for further analysis by liquid chromatography, capillary electrophoresis, or mass spectrometry. Ultrafiltration probes (UF probes) provide a simple means to obtain a small-volume sample from subcutaneous tissue, blood, saliva, or any other biological fluid in vivo. The dependence of recovery on flow rate, temperature, membrane dimensions, and vacuum magnitude are considered. The relative merits of capillary ultrafiltration probes and microdialysis probes are considered. UF probe applications presented include in vivo monitoring of drug disposition in human saliva and in the subcutaneous space of awake, freely moving rats.