Hongsrisuk, Nathaporn; (2010) Transferability of Near-Infrared Quantitative Calibration Models Between Different Instruments. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

The transfer of near-infrared (NIR) quantitative calibrations between different instruments is difficult and not well understood. This work investigated various methods of calibration transfer and the important factors affecting it. A common method of transfer uses a set of real samples ('transfer set') of a pharmaceutical product. However, because it has a finite shelf-life, this is not ideal. The use of materials which have good reproducibility between batches and suppliers as well as sets of reflectance standards and sets of pharmaceutical tablets were examined as possible alternatives transfer samples. Numerous mathematical procedures for calibration transfer were examined: direct transfer, response surface transformation, mean sample residual spectrum correction, direct standardisation (DS), piecewise direct standardisation (PDS) and model updating. Spectral variability due to physical properties of samples was investigated, using cardboards cut-outs in different sizes and shapes. Additionally, some of cardboard cutouts were also examined as transfer sets. Two calibration transfers of solid samples were undertaken: an assay of paracetamol in Sterwin® 500 mg tablets and a universal quantitative calibration of ibuprofen tablets. Five different NIR spectrometers were used. Model transfers between instruments were best achieved using response surface transformation, DS and PDS based on a subset of real samples. Using use pure substances or a set of pharmaceutical products to facilitate calibration transfer was only partially successful and very dependent on the exact details of the system under transfer. Spectral differences between instruments due to specular and stray radiation effects are believed to be the main problems. Calibration transfer for the liquid system: diethylene glycol-glycerol-water was studied between NIR measurement systems with different amounts of specular and/or stray radiation. The selection of samples used for transfer was found to be a crucial factor influencing successful transfer. The transfer samples needed to be closely matched, both chemically and physically, to the samples being measured.

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