If you do not know the absorptivity of a particular biomass, you can determine it using the Beer-Lambert Law. A=ε⋅c⋅l ;A = Absorbance (unitless)

• ε = Molar absorptivity (also called molar absorption coefficient), in units of L·mol⁻¹·cm⁻¹ (for molecular solutes) or often L·g⁻¹·cm⁻¹ (for biomass samples)
• c = Concentration of the biomass (mol/L or g/L, depending on your units)
• l = Path length of the sample cell (in cm, typically 1 cm)

You will need to take your soluble lignin sample and dry it so that you can get the mass of your lignin. Using that solid sample, prepare several solutions of specific concentration. Take UV-VIS measurements, then you can use the Beer-Lambert equation to solve for absorptivity by plotting absorbance vs concentration. The slope of your line will be ε × l.

Hope that helps.