Quantification of a trace amount of polymorphic impurities is critical especially in the quality control of Active Pharmaceutical Ingredients (APIs). Conventional quantitative analysis by X-ray diffraction (XRD) requires calibration curve preparation, reference intensity ratio (RIR) or crystal structure (Crystallographic Information File, CIF). However, the materials of APIs are often missing from the database. Therefore, their quantification is not straightforward. Rigaku have developed a novel quantitative analysis method called “Direct Derivation (DD)” that requires only XRD data and chemical formula. Here, quantitative analysis of trace amounts of the hydrate phase of API is performed using the DD method and the limit of quantification (LOQ) is calculated.