Microdialysis uses the principle of dialysis mentioned above. The microdialysis probe containing a dialysis membrane is used to be implanted in various tissues and organs such as liver, heart, skin, blood and so on. Then it is perfused with the acceptor stream that has similar ionic strength and composition to the donor phase (extracellular fluid). Due to the concentration gradient between the perfusate and extracellular fluid, the analytes diffuse across the dialysis membrane, which are transferred into LC system. In order to get high temporal resolution of monitoring, the sample
volume has to be usually less than 1 L. Off-line MD-LC method would consequently encounter evaporation and degradation problems. The problems can be overcome by using the online MD-LC method, since the sample transfer and injection procedures are performed in a closed flow system. However, temporal resolution is often defined by the LC analysis time for the online system because it is usually longer than the MD sampling time.