Rhubarb is an important herbal medicine for the treatment of constipation, inflammation,
and cancer. In this study, a facile method based on liquid chromatography coupled
with electrospray ionization tandem mass spectrometry has been established for the
analysis of bioactive phenolic compounds in rhubarbs. From six rhubarb species, official
(Rheum officinale, R. palmatum, and R. tanguticum) and unofficial (R. franzenbachii,
R. hotaoense, and R. emodi), a total of 107 phenolic compounds were identified or
tentatively characterized based on their mass spectra. These compounds include sennosides,
anthraquinones, stilbenes, glucose gallates, naphthalenes, and catechins. Ion chromatograms
for the identified compounds of different rhubarbs were then compared. Consistent
with previous reports, sennosides and rhein were only detected in official rhubarbs.
Unexpectedly, we found that R. officinale contained very different phenolic compounds
from the other two official species. Sennoside A, which has been considered as the
major purgative component of rhubarb, was only detected in R. officinale, while its
close isomers were observed in R. palmatum and R. tanguticum. In addition, the predominant
anthraquinone glycosides in R. officinale were found to be rhein 8-O-glucoside and
emodin 1-O-glucoside, whereas those in R. palmatum and R. tanguticum were rhein 1-O-glucoside
and emodin 8-O-glucoside. Stilbenes, which are the major constituents of unofficial
rhubarbs, were also different among the species. Our results clarify the chemical
composition of rhubarbs comprehensively for the first time. Due to the significant
differences in chemical components of rhubarbs, we suggest that different Rheum species
be used separately in clinical practice.