This way of the spectrophotometric assay of flavonoids employs that those compounds (flavonoids) posses similar molar attenuation coefficient in long wavelengths. General its good assumption due to groups of flavonoids have the same moiety in their chemical structure.

However, this is not the same when using LC-MS. The ionization of two similar compounds can be very different. The coefficient between those two compounds probably should be determined. I would be very careful when giving such result as You described.

No.

It is not correct to identified any similar compound by LCMS as quercetin.

Generaly, flavonoid compounds have very similar structure and Mw.

Hello Fatma, the "quercetin equivalent" notation for flavonoid concentrations should be used only for the total flavonoids assay. I agree with Adam´s recommendation for your particular case as the way you can determine the concentration of your specific compound.

Regards.

Thanks a lot for your comments. I don't have this compound in the pure form (it is a glycoside of quercetin). I agree that it may not be ionized as quercetin, but I optimized the ion and I picked a product ion to be included in the LC-MS method. This is just a primary step but I have a plan to isolate and use it in the quantification. I cannot be sure that it is well-separated in the chromatography to use LC-UV as you know plant extracts are so complex mixtures.

The Quercetin 3-glucoside is avaiable as a standard in Sigma Aldrich. (cat number 16654-10MG).

The Quercetin 4'-glucoside is also available http://polyphenols.com/flavonols/quercetin-4-glucoside-article165-171.html

If You compound is glucoside of quercetin in different OH moiety is better to use Your described approach with isomers than with different compounds.

Another perhaps wild suggestion would be to hydrolyze the glycoside as to obtain the aglycone. In this way you could quantify your compound as quercetin. This approach may be useful if you are not interested in the determination of the glycoside concentration. Regards.

use standard quercetin in hplc &find out its retention time with peak are you can quantify your sample for quercitin

Thanks a lot for the suggestions. I am giving an example but the flavonoid I am interested in contains bi-glycosides and I have a standard for the same flavonoid linked to different two sugars. Adam Zmysłowski, I am doing a similar approach by using the commercially available standard with closely similar structure. Thanks again!