In my Biochem labs we would always measure the consumption of NADH, and they always noted that it was useful that NAD+ had essentially no absorbance at 340 nm, which allowed us to directly relate absorbance to concentration by NADH's known molar extinction coefficient (i.e. all of the absorbance at 340 came from NADH).
I'm imagining that it would be possible to measure a reaction rate where the reactants and products both absorb at a given wavelength, but there is a difference between the reactants and the products. Is this as trivial as saying that for every M reacted, the absorbance will change by the difference in molar extinction coefficients?
I'm just wondering about it because they stressed the value of the lack of A340. Was this just a matter of high responsivity/good signal, and maybe convenience? There is no inherent reason why this wouldn't work, right?