How to identify and discriminate between the methyl quinates of chlorogenic acids by liquid chromatography–tandem mass spectrometry

Abstract

The methyl esters of chlorogenic acids, methyl quinates, are widely distributed in plant materials and frequently appear as extraction artifacts in plant samples. This is the first time when liquid chromatography-tandem mass spectrometry methods have been used for the identification and characterization of the methyl quinates. For this purpose, methyl quinates of mono caffeoylquinic acids and mono feruloylquinic acids were synthesized as authentic standards. The methyl quinates of mono and diacyl chlorogenic acids have shown characteristic fragmentation pattern in their tandem mass spectra. MS(n + 1) spectra of the methyl quinates of diacyl chlorogenic acids were identical to MS(n) spectra of mono acyl derivatives. These quinates do not produce any methyl quinate peak at m/z 205 if compared with quinic acid peak at m/z 191 in negative ion mode. In the MS(n) spectra of these quinates, cinnamic acid part or cinnamoyl part was detected as a base peak in negative ion mode. The retention time, order of elution and fragmentation pattern were completely different if compared with LC-MS(n) methods developed for chlorogenic acids. These LC-MS(n) methods have been applied for the identification and regioisomeric characterization of the methyl quinates of chlorogenic acids in maté tea and woodruff (Galium odoratum). Two methyl caffeoylquinates (2 and 4) were identified as methyl 3-caffeoylquinate and methyl 5-caffeoylquinate.