Unexpected Signals in Gas Chromatograph/Multiphoton Ionization/Time-of-Flight Mass Spectrometer Designed for the Detection of Negatively Charged Species

摘要

A time-of-flight mass spectrometer was constructed for use in detecting negatively charged species induced by laser multiphoton ionization. Mass spectra, collected for a variety of compounds, contained several signal peaks. Their flight times were, however, an order of magnitude shorter than the values predicted based on the length of the flight region. To clarify this curious phenomenon, a simple molecule, namely, nitric oxide was measured for calibration of the flight times and for assignment of the signal peaks. A careful investigation led to a possible mechanism, in which the signals do not arise from negative ions but arise from electrons generated at the surface of the repeller electrode by collisions of positive ions that are formed by multiphoton ionization of the analyte in a molecular beam.