A valid working method must first be available. For pharmaceutical compounds and their subsets, there is a procedure called Pharmacopoeia, or for compounds related to the food, food industry, etc., reliable sources are available. Sometimes a valid method of analyzing a substance by purchasing a shipment or sample is requested from the company that produces that substance.
However, the first step is to have a valid and appropriate method and agenda.
If the working method is not available, the work becomes a little more difficult and requires experience and skills, as well as research on the desired composition, system and methods of analysis, etc., which itself has a detailed description.
Suppose we have a valid method available. For example, the :

Determination of the effective substance of Ceftriaxone drug using chromatography and HPLC
This method is based on the method provided in Pharmacopoeia (USP).

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