Evaluation of microchannel plate gain drops caused by high ion fluxes in time-of-flight mass spectrometry: A novel evaluation method using a multi-turn time-of-flight mass spectrometer.

Use of a two-stage microchannel plate (MCP) detector is common in time-of-flight (TOF) mass spectrometry because it shows excellent time resolution with sufficient gains. However, the gain drops significantly when the detector detects intense ion fluxes, such as matrix ions, by matrix-assisted laser desorption/ionization mass spectrometry. As a result, significant ion signals corresponding to analytes, such as proteins, are hidden, thereby hampering the mass spectral interpretation. However, details of this phenomenon have not previously been investigated using ions because of the lack of suitable measurement methods and apparatus. Thus, we herein report a novel method for controlling the TOF of two selected ions, as a function of time differences between each other using a multi-turn TOF mass spectrometer. This method involves the use of an isotope cluster of ions that fly in a figure-of-eight orbit and the extraction of an ion at a given lap number. A series of time differences (∆t) between two ions in a TOF spectrum can be generated using this method. We evaluated the time constants of gain recovery after high ion-flux detection for two sets of two-stage MCP detectors to obtain values of 1,600 and 180 μs for channel plate resistances of 500 and 71 MΩ, respectively. The obtained time constants from the gains determined at various values of ∆t were 0.48 and 0.38 fold (for 500 and 71 MΩ, respectively) of the values suggested from the channel plate resistance and capacitance.