Try this method. However, I would use LC/MS if I have the intrument.

Unfortunately, I do not have acces to a LC/MS instrument, yet.

Well, then use what you have.

To be able determine flavonoids by GC-MS you need to derivatise the OH groups by some method and trimethyl silylation with BSTFA and TMCS is a good procedure.

It is possible to analyze flavonoids by silanized derivatives in GC-MS. Just beware that under a common oven temperature program the flavonoids elution-time can be increased up after 65 minutes (due to the size of flavonoid molecules), so this is recommended to increase the helium flow at 1.5 or 2 ml / min or the runtime to 75 mins, at maximun temperature of 300° C.

Thanks a lot for the very useful tip Mr. Juan Vazquez-Martinez!

Try add 50 uL of BSTFA+1%TMCS to your dry plant extract, seal the bottle and incube at 70ºC for 25 minutes. After derivatization, inject on GCMS.

You should keep in mind that the derivatization will change the mass of flavonoids. So, one should be careful with the identification of compounds.