CHARACTERIZATION OF THE SURFACE-PROPERTIES OF ALPHA-LACTOSE MONOHYDRATE WITH INVERSE GAS-CHROMATOGRAPHY, USED TO DETECT BATCH VARIATION

Inverse gas chromatography (IGC) was employed to characterise the surf ace properties of four batches of alpha-lactose monohydrate supplied a s chemically and physically equivalent, but reported to exhibit variab le processing performance. These batches were found to be indistinguis hable by FT-laser Raman spectroscopy, quantitative X-ray powder diffra ction, differential scanning calorimetry (DSC), thermogravimetric anal ysis (TGA) and surface area measurement. IGC was undertaken using vapo urs of polar and non-polar adsorbates or probes introduced individuall y, in very small quantities, onto the alpha-lactose monohydrate packed column. The net retention volume, V-N, of each probe was determined, allowing the calculation of various surface properties, including the dispersive component of surface free energy (gamma(D)) and specific in teraction with polar probes (Delta G(A)(SP)). Whilst the gamma(D) valu es were comparable, Delta G(A)(SP) values for polar probes demonstrate d significant differences, indicating batch variation in the polar sur face properties of the four batches of alpha-lactose monohydrate. It i s hypothesised that these differences in surface energetics are caused by minor variation in surface crystallinity or purity.