UBC Theses and Dissertations
Development and characterization of a next generation, field-free atmospheric pressure photoionization (APPI) source for liquid chromatography-mass spectrometry McCulloch, Ross David
A prototype next-generation, atmospheric pressure photoionization (APPI) source for LC-MS applications has been developed. The device was fabricated through modification of a TurboV ion source. The device incorporates a stainless steel source block and a photoionization lamp mounted to a heated nebulizer. Sample transfer conduits were machined within the source block creating a “flowing reaction chamber” in which ion-molecule reactions, critical to APPI performance, proceed. A potential applied to the source block, provides a field-free environment within. An additional “flight tube” mounted to the outlet of the source block extends the length of the field-free region. The design of the prototype device mirrors the configuration exploited within early, fieldfree APPI sources. The preliminary goal of this research project was to use the prototype device to compare the relative performance of field-free sources with opengeometry APPI sources. These devices provide no extended field-free region. All ionmolecule reactions proceed within an electrical field, altering the processes of ion formation and transmission. To our knowledge, this was the first comprehensive, headto- head evaluation of the two APPI configurations, upon a current MS platform. Factors affecting field-free APPI performance were characterized, leading to a more complete understanding of source optimization. Through an empirical evaluation, the prototype field-free source was found to routinely provide superior performance relative to current open-geometry designs, at times providing order-of-magnitude sensitivity advantages. Significant improvements are tied to the provision of the extended field-free region, enabling efficient analyte ion formation, while enhancing overall ion transmission. The performance of the prototype device was compared to ESI and APCI for the analysis of clinically relevant analytes using high throughput, rapid screening LC/MS workflows. The prototype source was routinely found to provide the greatest sensitivity for the analysis of neutral steroids, including vitamin D, while displaying reduced susceptibility to matrix effects - particularly relative to ESI. This observation was confirmed using a robust linear regression method to build statistical confidence within the results. It was also determined that nebulizer temperature and gas flow rate significantly alter the extent of matrix suppression, realizing new parameters which may impact APPI method optimization.
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