That really depends on the reaction (or more complex process) you wish to follow. It will also be influenced by your solvent (if it is transparent in the area of interest) and other media properties. Please, start by having a look at the starting material (i.e. 4-nitrophenol) UV/vis spectrum: https://webbook.nist.gov/cgi/cbook.cgi?ID=C100027&Units=SI&Mask=400%23UV-vis-Spec#UV-Vis-Spec and determine whether your spectrometer is able to measure this spectrum under your conditions. You might also want to have a look at the following: Article ChemInform Abstract: Nitroarene Reduction: A Trusted Model R...
Article Biogenic synthesis and characterization of gold nanoparticle...
Also, please, what exactly do you mean by "initial loss"?
you don ´t formulate your question in a scientific manner and your question remains too vague for a good answer.
I agree with the answers of Maksim and Peter and I ask , like Peter, what do you mean by " initial loss". Please give infos about your solvent and your reaction and pH of the solution.