Thin layer chromatography-matrix assisted laser desorption ionisation-mass spectrometry of pharmaceutical compounds.
Thin-layer chromatography (TLC) is of great importance for the pharmaceutical industry as a simple, quick, and low cost analytical method. Considerable effort has been made over the past decades to combine the simplicity of TLC with the selectivity and sensitivity of mass spectrometry (MS) detection. In the pharmaceutical industry sensitivity is an especially important factor, since the allowed impurity level of most drugs is under 0.1%.The aim of the present thesis was to develop methods for the direct examination of pharmaceutical compounds from TLC plates by matrix-assisted laser desorption/ionisation time-of-flight mass spectrometry (MALDI TOF MS). The study was started by comparing several approaches for the application of the matrix for direct TLC-MALDI including a newly developed electrospray matrix deposition method. This new method was found to be superior to the other techniques studied. It produced a stable signal, minimised analyte spreading, and hence allowed the scanning of a TLC plate to obtain chromatographic as well as mass spectral data. The plotted mass chromatograms assisted in spot location, and allowed the calculation of Rf-values. These showed good agreement with the Rf -values determined by UV detection. The decrease in mass resolution and mass accuracy commonly observed in TLC-MALDI TOF MS due to the uneven nature of the silica gel layer was corrected by internal recalibration on selected matrix ions during the scanning of the TLC plate. To enhance the signals recorded directly from a TLC plate the use of an extraction solvent prior the matrix application was explored. Further improvements in sensitivity were obtained by modifying a robotic x-y-z axis motion system to act as an electrospray deposition device and by use of special Si 60 F[254] HPTLC-MALDI targets. Using both approaches sensitivities in the high fmol range were obtained. To minimise matrix interference, which can suppress analyte signals, the application of suspensions of particles of different materials and sizes (Co-UFP, TiN, TiO[2], graphite and silicon) onto eluted TLC plates were investigated. The structural analysis of pharmaceutical compounds was achieved by post-source decay - matrix-assisted laser desorption/ionisation (PSD-MALDI) mass spectrometry performed directly on the separated spots. TLC-MALDI MS is not only applicable to the qualitative analysis of pharmaceutical compounds. The generation of quantitative data by using a structural analogue as an internal standard is also described. Different approaches to the incorporation of the internal standard into the TLC plate were tested. The most successful approach was to develop the TLC plate in the mobile phase to which the internal standard was added. Good accuracy, precision, linearity and sensitivity was obtained using this approach.
Networked Digital Library of Theses and Dissertations
Sheffield Hallam University (United Kingdom).