Are my concentrations varied for this spectroscopic lab?

I am doing a lab on the spectroscopic analysis of antioxidant activity in herbal extracts. The lab has the initial concentrations of the extracts, and in the lab different μL are added to the radical DPPH (kept constant) and absorbance values are then tested using a spectrophotometer. The question I have is that when I am plotting the data, I need the concentration per cuvette (μg/mL) for the herbal extracts. But I don’t understand how the concentrations will vary if different quantities (μL) were tested. Even though the lab says to convert molarity of stock solution to μg/ml, and that each of the concentrations should correspond with the extracts. Does the concentration vary because of the different quantities of the stock solutions placed? And how would I solve for the concentrations then? 

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