Your question is very vague and obscure

How do I calculate unknown concentration using UV-Vis spectroscopy?

and it shows that you are not familiar with UV-Vis spectrocopy

Another false formulation:

I am looking for sulfur concentration in untreated diesel oil.

It seems that you mean sulfur as element and you cannot determine it with UV-Vis.

The impurities in untreated diesel  are thiophene derivatives. For his purpose, you must search references and READ them prior you post a question!!!

At first start Google Scholar and not Google and give : "Determination of sulfur   in untreated diesel oil"   in the mask.

https://scholar.google.de/scholar?q=determination+of+sulfur+concentration+in+untreated+diese+oil+%3F&btnG=&hl=de&as_sdt=0%2C5

you 'll earn a lot of hits.

Among them  a pdf RG   can be read free as full text and it will be useful for valuable infos.

Energy & Fuels 2007, 21, 1071-1077

Distribution of Polycyclic Aromatic Sulfur Heterocycles in Three

Saudi Arabian Crude Oils as Determined by Fourier Transform Ion

Cyclotron Resonance Mass Spectrometry

I think  your supervisor is sleeping or very lazy

JRG

see this is my project...u can tell whatever u want ...but why u r telling  about my supervisor...u dont know how to talk with someone ...learn norm and values first ...then give answer ok...

Can you write your answer in correct English. Thank you in advance.

JRG