The microstructure of combination DPI formulations is complex and is related to the surface interfacial interactions between components of the formulation, which define the structure and thereby performance of the formulation (1, 2, 3). Previous studies have attempted to investigate the microstructure of combination DPI formulations using a combination of atomic force microscopy (AFM) and Raman chemical imaging (RCI) (4). These studies, however, have focused primarily on the surface interaction between two different drugs and their aerodynamic particle size distribution.