Many modern material systems contain heterogeneous mixtures of components, and it is important to be able to characterize the structures of the components in such mixtures. Working with low concentrations is often desirable, either to achieve the proper pharmacokinetic properties of a drug molecule or to prevent particle aggregation and settling. To study structure in these systems, difference atomic pair distribution function (dPDF) methods are often used, in which a signal from only the component of interest is extracted by subtracting scattering contributions from other components, background, and environment.

By studying small nanoparticles of a proprietary active pharmaceutical ingredient (API), we recently showed that dPDF methods can be used to detect nanoparticles at very dilute concentrations, in this experiment as low as 0.25 wt%. This allows for components to be studied in situ rather than in contrived situations with exaggerated concentrations. In this case, the API can be studied at the actual concentration of the marketed drug product.

Read more: Maxwell W. Terban, Matthew Johnson, Marco DiMichiel and Simon J. L. Billinge, Detection and characterization of nanoparticles in suspension at low concentrations using the x-ray total scattering pair distribution function technique, Nanoscale 7, 5480-5487 (2015).