The substrates used are Syringaldazine, ABTS (2,2'-azino-bis-(3-ethylbenzthiazoline-6-sulfonic acid) or 2, 6 Dimethoxy phenol, guaiacol. The best one is ABTS and I think 420 nm is most prefereable depending on which substrate are you using. You may also test at both wavelengths and find out which gives more adsorption. Also consider optimal pH and temperature for best activity results. 

To calculate Extiction coefficient may be the following ;available from researchgate answered questions:

Beer–Lambert law, A=abc

absorbance, A,

molar absorption coefficient, molar extinction coefficient, or molar absorptivity ,a,

pathlength, b,

the concentration, c,

i.e;

a=A/(b*c)

where b is fixed and constant.

So,a proportional to A/c

i.e, Slope of Absorbance (A) v/s Molar Concentration (c) devided by b (cell lenght) give you value a (molar absorption coefficient)

Good information by Kemal. At 420 nm the sensitivity is higher, since the epsilon is higher, but you can have some interference of other colored compounds at the assay. This is why some authors follow the ABTS oxidation at 436 nm. You can try what you choice in your conditions.

Hi there,

Absorbance of ABTS+ at 420nm is maximal so it presents more sensitivity and more accuracy compared to the absorbance at 436nm which actually lies in the slope of the peak absorption (see the following document: https://openi.nlm.nih.gov/detailedresult.php?img=PMC3117622_aci-1-2011-037f3&req=4). In that case, a slight shift on the wavelength scale (due to the spec) would result in a big variation in absorbance which is a further source of error. So measuring peak or valley is always better as the signal is more stable.

Thank you all for your answers. It has been really helpful.