As a new material characterization technology, PDF (Pair Distribution Function) is useful in the study of the local structure of both crystal and amorphous materials. Many drug crystals will change into different crystalline forms or even amorphous states at different temperatures or humidity. This makes XRD ineffective in determining the active composition of the drug. For example, in the PDF maps of crystalline and amorphous ketoprofen (as shown below), we can analyze that in the PDF signal interval of more than 4 angstroms, the amorphous sample and the crystal sample are significantly different, but in the nearby atomic region (less than 4 angstroms), the PDF maps of different samples are basically the same. This shows that the composition of the same ketoprofen compound, although the crystal type is different, but the neighbor atomic bond length and coordination number are exactly the same. This provides another possible means of characterizing different compound apis.