Liquid chromatography coupled with tandem mass spectrometry (LC/MS/MS) has been frequently utilized for the sensitive and selective determination of the trace level compounds in biological samples. In particular, LC/MS/MS equipped with electrospray ionization (ESI) as the ion source is most often used method, since ESI can ionize wide range of the compounds including the polar compounds or large molecular weight compounds. Furthermore, ESI requires lower temperature for ionization compared with other ionization methods such as atmospheric pressure chemical ionization (APCI), and thus it can be used for the thermally unstable compounds. In LC/ESI-MS/MS, the analytes having the following properties can be sensitively detected. Firstly, the analytes must be ionic or ionizable forms in the solution phase, since, in the ESI, gas phase ions are mainly generated by transferring the ions in solution into gas phase in the presence of a strong electrical field. Secondly, it is preferable for the analytes to have the appropriate hydrophobic structures, since (i) the hydrophobic ions prefer to reside at the droplet surface generated by electrospray and these ions enter the gas phase more readily than those in the droplet interior and show the higher signal intensities. (ii) The hydrophobic compounds can be well separated on the reversed phase column from salts and interfering compounds possessing suppression effects on ESI. (iii) The hydrophobic compounds are eluted by the mobile phase with the higher organic solvent content. The higher organic solvent content is suitable for the stable generation of charged droplets by electrospray and thus gives the higher signal intensities. Thirdly, it is desired for the analytes to have the suitable structure for MS/MS detection (selected reaction monitoring (SRM)), i.e., to fragment efficiently upon collision induced dissociation (CID) and generate an intense and particular product ion. However, not all the compounds can be favorably analyzed by LC/ESI-MS/MS. Thus, chemical derivatization of the analyte is often used to enhance the detection sensitivity. So far, a number of derivatization reagents for LC/ MS/MS have been reported and they are summarized in several review papers.
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