There are two main classes of silylating

reagents: those producing trimethylsilyl (TMS) derivatives and those producing tert-butyldimethylsilyl (TBDMS) derivatives. TMS derivatives can be produced by a wide variety of reagents, including N,O-bis(trimethylsilyl)trifluoroacetamide (BSTFA) and N-methyl-N-(trimethylsilyl)trifluoroacetamide (MSTFA). TBDMS derivatives are prepared by the reaction with N-methyl-N-(tert-butyldimethylsilyl)trifluoroacetamide (MTBSTFA) [59,60]. now Higher reproducibility is realized with GC-MS than with LC-MS, and better compound separation is achieved. However, the main advantages of LC-MS/MS Inhibitors,research,lifescience,medical over the classical GC-MS procedures are the reduced time of analysis due to the elimination of derivatization steps prior to the chromatographic Inhibitors,research,lifescience,medical separation, and the fact that LC-MS/MS allows us to determine the ratio of the conjugated fraction (glucuronide and sulfate) to the free fraction, thus reducing the risk of false-positive or misleading results [61]. 3.3. High Performance Liquid Chromatography Compared to GC, HPLC is a powerful tool for determining highly polar compounds in several matrices [62]. In particular, reversed-phase Inhibitors,research,lifescience,medical liquid chromatography (RPLC) run on an octadecyl silica column is an indispensable technique, i.e., RPLC enables use of both aqueous and organic solvents. HPLC is the most widely used analytical technique

in the pharmaceutical industry because of its versatility and ability to retain and resolve a number of compounds. However, the retention Inhibitors,research,lifescience,medical of polar analytes often requires a highly aqueous mobile phase, which can cause a number of problems, such as decreased small molecule sensitivity in electrospray ionization mass spectrometry (ESI-MS) [63]. Although it is a powerful

separation mode, one major limitation of RPLC is its inability to adequately retain extreme polar compounds (i.e., more highly polar compounds). Inhibitors,research,lifescience,medical In particular, the major focus of untargeted LC-MS-based metabolomics studies in recent years has been to improve the separation of water-soluble compounds. In this case, normal-phase liquid chromatography (NPLC), which includes silica or alumina as the stationary phase, is used. NPLC is generally used to separate highly polar compounds with no water in the mobile phase, e.g., hexane, chloroform, or benzene, under isocratic elution conditions [64]. However, because it is difficult to dissolve hydrophilic compounds, Anacetrapib such as peptides and nucleosides, in a non-aqueous mobile phase, the application of NPLC to biological samples is limited. On the other hand, despite the versatility of RPLC, serious retention and/or selectivity problems may arise for highly polar compounds. Ion pair chromatography offers a robust method for separating highly polar compounds, such as sugar phosphates [65], nucleotides [66], and carboxylic acids, by using an RP column.