I tried yesterday the Shinoda Test for flavonoids. I followd the protocol. I extracted direct from the yellow flower Tithonia sp with ethyl acetate, later I evaporated and ressolubilized with etanol. Then I put the Mg powder an the HCl and was negative three times. The literature is says there is a lot of flavonoids in this flower, but the test just didn't work. I tried oxalic-boric reaction with aceton in Uv light, but it was negative too.
Daniel Ceccon , I suggest to repeat the test with positive control. This will confirm your procedural accuracy. If its negative with standard flavonoid or flavonoidal extract, then there is some error in your process.
1. I agree with validating your procedure with a positive control. Shinoda test yields different colours depending on the type of flavonoid being tested, but if I'm not mistaken, it will be somewhere around the yellow to red spectrum.
2. What is the colour of your extract? I'm thinking it might be possible that the colour confounds the result of your Shinoda test.
3. Can you look up biogenesis of flavonoids in Tithonia? It should give you a better clue whether flavonoids are abundant in Tithonia flowers. It's just I have my reservations -- their flowers are yellow to red coloured (I'm talking about T. diversifolia and T. rotundifolia), which to me, looks more like carotenoids than anthocyanins (which can be considered as flavonoids)
4. Look up ontogenic factors, which may affect flavonoid content.
I hope these are helpful rather than not!
Realmente é ideal que o teste seja refeito com uma amostra controle, já que o teste de Shinoda também é eficiente na detecção de flavonóides em flores, afirmo isso por ter realizado recentemente o teste para avaliar o perfil fitoquímico da flor da Turnera ulmifolia e deu positivo.
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