oleanolic acid has Different types such as oleanolic acid glucuronic, oleanolic acid glycosides and free oleanolic acid. How can measure different types of acid? And how can we distinguish them?
Depends if it is a mixture and what is the matrix that you are operating from. If the sample is near to pure I would suggest LC-MS. If you are working from biological matrix or plant extracts you might want to try HPTLC-gradient elution with fluorescent detection or spray reagent. Triterpens are a bit tricky. I am attaching two papers that might helps for inspiration Regards
http://www.uj.ac.za/EN/Faculties/science/departments/biochemistry/research/ResearchGroups/Documents/JournalOfPlanarChromatography11(24)82.pdf
http://www.sciencedirect.com/science/article/pii/S1044030506001875
I agree with Anke's suggestion.
You should use MS, because the information achievable by HPLC-DAD is merely indicative, since the spectra from several glucoronides or glycosides are nearly equal. In MS, you should compare the relative percentages of each fragment and compare with literature dealing with the same matrix that you are studying. It is not an easy job, but the results are worthy.
I have some papers regarding this subject. See for instance:
Phenolic profiling of Veronica spp. grown in mountain, urban and sandy soil environments. Barreira et al., 2014, Food Chemistry, 163, 275-283.
I think the alternative should be GC-MS after derivatization if necessary. It's a very useful technique for both determination and quantification.
HPLC-DAD could be also used but less sensible and less accurate for isomer identification. LC-MC techniques could afford sensibility but the problem for separation could not be solved. At least, you should consider UHPLC-MS/MS techniques for both sensibility and efficiency for structural identification.
The good choice will depend on several considerations such as quantity of available material, techniques availability, needed acuracy, etc...
Try the method in the following paper:
Chloe Townley, Rhea C. Brettell, Richard D. Bowen,,* Richard T. Gallagher and William H.C. Martin,* (2015) Application of positive mode atmospheric chemical ionisation to distinguish epimeric oleanolic and ursolic acids, European Journal of Mass Spectrometry, 21:433-442.
This allows a determination of the different forms of free oleanolic and ursolic acids, although it will not detect your glucosides unless your mass spec can measure quite high masses.
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