The structural elucidation of iridoid glycosides dimers was very difficult even using nuclear magnetic resonance (NMR) spectroscopy. The reason is that NMR spectra might overlap and influence the quality of corresponding spectra. Thus, a rapid and sensitive method for the confirmation of iridoid glycosides dimers in complicated mixture remains desirable.  

In order to establish a reliable and fast analysis method, researchers in Dr. WU Zhijun's lab at the Chengdu Institute of Biology, Chinese Academy of Sciences firstly investigated the structure characteristic using electrospray tandem mass spectrometry method. Dr. WU and his colleagues found that the tandem mass spectra of the three isomers are clearly different, although their structures are very similar. Interestingly, the product ions at m/z 469 and 487, in the MS/MS spectra of [M + Na]⁺ for saprosmoside E and saprosmoside D, respectively, have two possible structures.

The processes forming product ions were investigated by comparing their tandem mass spectra to that of paederoside and paederosidic acid. It was found that the structure of the ion at m/z 469 is different from paederoside, although their m/z is the same. However, the structure of the ion at m/z 487 is the same to that of paederosidic acid (m/z 487). These characteristic ions can be used to distinguish unambiguously these isomers.

The method is a rapid and new analysis method for the identification of isomeric iridoid glucoside dimmers and solves an important problem perplexing analyst. This research entitled "Electrospray ionization tandem mass spectrometry of isomeric iridoid glucoside dimers" was published in Rapid Communications in Mass Spectrometry on Jan. 6, 2013.