Nowadays, in some crucial fields such as proteomics, metabolomics, forensics and clinical analysis, the samples are usually only a few microliters or less. To meet the needs in these fields, the use of miniaturized LC systems, for example nano LC is necessary, which have been already commercially available. Accordingly, there has been an increasing trend towards the miniaturization of online SP-LC systems. These miniaturized systems offer some advantages over conventional systems, including lower consumption of solvents and samples, smaller amounts of required stationary-phase material and higher ESI-MS sensitivity due to the lower flow rate and improved ionization efficiency.