Please prepare standard curves with gallic acid as well as catechin. You can find out one OD value for both. Based on these values you can express in either gallic acid equivalents or catechin equivalents.

Let us suppose -

1 O.D. of Gallic Acid = X units

1. O.D. of Catechin = Y units

(please note that units have to be same - for instance it can be micrograms or micromoles, mg or mmoles etc.)

In that case Y units of Catechin = X units GAE

or you can say that  X units of Gallic Acid = Y units Catechin equivalents

Hope, I could make you understand.

I fully agree with Pardha-Saradhi.

I agree with Pardha-Saradhi. You should make linear calibration for the gallic acid.

However, as an alternative I suggest you to make correlation between GAE and CE, using simple linear regression rather than simple equation as it was suggested by Pardha-Saradhi . If the coefficient correlation is good, I think you can convert one unit to another unit.

Regards

Hi

First you have to prepare the linear standard calibration curves for both Gallic acid and Catechin.  Check the R2value, it should be more than 0.90 and above. From that you can get the slope (y) value from the plot. This slope value will give you the Absorbance units / concentration of your std used. 

If you use the Gallic acid in microgram to get the linear plot. For example: if your slope value is 0.025 Abs Units/ micro gram of Gallic acid or Catechin. You can use this slope value to calculate and express your unknown sample in Gallic acid equivalence or Catechin equivalence. 

Slope value of your std (OD)/ OD of your unknown sample= Std equivalence.

For example: If you get 0.655 as an OD for your unknown sample then you can calculate 0.655/0.025 = 26.2 micro gram of Gallic acid or Catechin equivalence.

I hope this will be useful and I agree with Pardha Saradhi. 

Good Luck.