Solid solution quantification from full powder X-ray diffraction profile: novel application of multivariate calibration

Non-stochiometric crystals, such as solid solutions, represent a valid strategy for fine-tuning the properties of materials from inorganic pigments to active pharma­ceutical ingredients. The change is achieved by substitution of structurally similar compounds within the crystal structure, resulting in a minor variation of cell parameters, and it can be experimentally observed as a shift of the X-ray diffraction profile, with the shift depending on the substitutional amount. After proper alignment of the diffraction data, chemometric methods can detect the evolution of the solid solution profile and correlate it to the molar composition, creating rapid quantitative models. The solid solutions chosen for this work are NA2·FAxSA1-x and IN2·FAxSA1-x, i.e. co-crystals of nicotin­amide (NA) and isonicotinamide (IN) with fumaric acid (FA) and succinic acid (SA) in different proportions; they are used as model systems for the development of principal component regression and partial least-squares models for the quantification of solid solution composition. Different alignment strategies are presented, and the results obtained from test samples show the prediction efficacy of the proposed approach.