Water sorption and near IR spectroscopy to study the differences between microcrystalline cellulose and silicified microcrystalline cellulose before and after wet granulation.

Silicified microcrystalline cellulose (SMCC) has been shown to have advantages over conventional microcrystalline cellulose (MCC). These advantages are (i) improved tablet strength compared to that achieved with MCC, (ii) the retention of compressibility after wet granulation, whereas MCC produces weaker tablets after wet granulation, and (iii) superior flow properties than MCC. In this study gravimetric and calorimetric vapour sorption data and near IR spectroscopy have been used to study MCC and SMCC before and after wet granulation. It was found that MCC, SMCC and wet granulated SMCC had essentially identical physical structures (except for a size increase due to granulation). Wet granulated MCC had a different enthalpy of water sorption at low RH, and its near IR spectrum was different from the other samples in the region which relates to C-H bonding. It can be concluded that MCC and SMCC are of very similar structures, thus these analytical techniques cannot provide an explanation for the improvements in compressibility. However the change in compressibility in MCC after wet granulation may relate to the observed differences in internal bonding in this sample.

Optimisation of sample presentation for the near-infrared spectra of pharmaceutical excipients.

The effects of sample presentation on near-infrared (NIR) reflectance spectra were examined. Using a Foss NIRSystems Rapid Content Analyzer, which uses sample cups for sample presentation, four important parameters were identified: cup diameter, sample thickness, cup material and packing method. Below a critical diameter of 20 mm, which is dependent on the detector geometry, the spectra became increasingly distorted (i.e., changes in spectral intensities and spectral shape, shifts in peak positions and occurrence of Wood's peak). The minimum sample thickness not to cause spectral distortion was dependent on the physical and chemical nature of the substance. A thickness > or = 10 mm was found to be adequate for most pharmaceutical excipients. The method of packing was also important. Tapping a powdered sample sometimes caused significant changes (P < 0.05) in the spectral absorbance values compared with simply pouring the sample into the sample cup. Standard sample cups made from quartz were to be preferred owing to their lack of background absorptivity. However, the two commercially available flat based vials examined, which were made from soda glass and clear neutral glass, proved to be as suitable for all except applications of the most exacting nature. The spectral distortions resulting from variations in cup diameter, sample thickness and cup material were also shown to alter significantly the values of two commonly used identification algorithms, correlation coefficient (< 0.95) and maximum distance (> 3.0 standard deviation distance), sufficiently to cause misidentifications.