Assessing changes in the surface properties of a crystalline material

A suitable material has been identified for use as a system check for Inverse Gas Chromatography (IGC) to ensure the any variability in results seen was not due to the instrument. The selected material, alpha-alumina, was found to be stable over time (no marked change after four years). Results from five columns of a single lot had a mean ysD of 35.2 mJ/m2 with a coefficient of variation of 2.75% demonstrating repeatability. Deliberate manipulation of the material by sieving resulted in a small change in ysD of 1.7 mJ/m2 by IGC i.e. alpha-alumina was not prone to sampling error. Results from three subsequent lots were not significantly different to the first and no significant difference between two IGC instruments seen, alpha-alumina has been demonstrated as a suitable system check for IGC. Salbutamol sulphate has been chosen as the material to be studied to investigate change over time for a number of batches under normal storage using a variety of analytical techniques. The first sample (B1) was shown to be crystalline by Gravimetric Vapour Sorption. Two columns analysed simultaneously by IGC yielded unexpected differences in ysD of 72.0 and 80.4 mJ/m2, which decreased with time. Storage at elevated temperature and humidity to accelerate change caused the ysD to reduce to approximately 51 mJ/m2. Subtle but repeatable differences were observed by Thermal Gravimetric Analysis, Specific Surface Area and Scanning Electron Microscopy. Disproportionation of the sulphate (loss of SO3) was hypothesised to be the cause based on data from TGA-Mass Spectroscopy. When three additional samples were analysed by the same techniques, the same trends were not observed. Additional tests using InfraRed and head space analysis / Principal Component Analysis were performed. The data generated across all samples indicated the first sample (B1) to be atypical. Overall this research has shown that batch to batch variation can be due to changes in the surfaces of crystalline materials but that it is not always due to amorphous content. It has shown that the combination of IGC with complementary techniques can provide useful insight for comparing batches of the same material. Furthermore that IGC can be used to monitor changes in a material under conditions representative of typical material storage.