I analyzed flavonoid total spectrophotometry, the procedure as below:

2 ml supernatan added 5 ml dd.H2O, then added 0,15 ml NaNO2 5%, 5 minutes later added 0,15 ml AlCl3 10%,

After 6 minutes, added 1 ml NaOH 1 M. Incubate 30', absorbance read at 510 nm.

I use quercetin standard (20,40,60,80,100 mg/l) (y= 0,0005x + 0,0024, r2= 0,9924)

I measure my sample:

2,5 g of biscuit powder added 25 ml metanol HCl. Sentrifuge, filtered. Use the supernatan for analysis. My sample has absorbance 0,018. How to calculate the flavonoid in mg/100 g of sample?