In the world of forensic investigation, Detective Martinez was assigned a challenging case involving a series of counterfeit currency circulating in the city. The counterfeit bills were so meticulously crafted that they closely resembled genuine currency, making it difficult to detect the fraudulent notes. Determined to uncover the truth, Detective Martinez turned to chromatography as a valuable tool in their investigation. In the context of investigating counterfeiting, what is chromatography primarily used for?