App note 301: An overview of iGC-SEA – a new Instrumental Technique for Characterising the Physico-chemical Properties of Pharmaceutical Materials
The increasing sophistication of pharmaceutical drugs and drug delivery technologies has created the need for new techniques to measure the physico-chemical properties of a wide range of solid pharmaceutical ingredients and formulations. Inverse gas chromatography (iGC-SEA) is a gas phase technique, first developed over 40 years ago, to study the surface and bulk properties of particulate and fibrous materials. iGC SEA has the potential to unlock some of the more difficult to measure physico-chemical properties of pharmaceutical materials such as powder surface energies, acid/base/polar functionality of surfaces, diffusion kinetics, solubility parameters, surface heterogeneity and phase transition temperatures/humidities. These properties affect both the performance and processing of many materials from active drug compounds to excipients and fillers. However, until recently, applications within the pharmaceutical industry have been limited to a few studies of properties such as the surface energy of simple powders. All of these studies have been carried out upon ‘home-built’ pieces of apparatus, often employing manual or semiautomated experimental methods. This has led to a diversity of results in the literature, often seemingly contradictory, due to the differences in instrument design, methodology, sample preparation and individual operator skill.
iGC SEA – The Technique
The principles of iGC-SEA are very simple, being the reverse of a conventional gas chromatographic (GC) experiment. An empty column is uniformly packed with the solid material of interest, typically a powder, fibre or film. A pulse or constant concentration of gas is injected down the column at a fixed carrier gas flow rate and the retention behaviour of the pulse or concentration front travelling down the packed column is then measured by a detector.