Dear Sir. Concerning your issue about the saponin Standards for estimation of Total saponin by Spectrophotometric method (Vanillin- Sulphuric Acid method) . Total saponins was determined by UV-VIS spectrophotometry after extracts of the sample had been coloured. The methods was linear in the range of 0.05~0.30mg (r=0.9987), and the average recovery was 98.47%, RSD was 3.29% (n=5). The methods is sensitive, reliable, simple and reproducible for the determination of the contents of saponins. You can use oleanic acid as the standard substance. I think the following below links may help you in your analysis:

http://www.tsijournals.com/articles/content-determination-of-total-saponins-from-opuntia.pdf https://file.scirp.org/pdf/OJF_2018010816170152.pdf

Thanks