Design and development of a Reflectron Time-of-Flight Mass Spectrometer (RTOF -MS) for Resonance Ionization Mass Spectrometry (RIMS)

Applications of Time-of-Flight Mass Spectrometer (TOF -MS) for RIMS are quite well known. Enhancement in mass resolution of RIMS signals is expected by replacing the TOF-MS with RTOF -MS. We have designed such a RTOF instrument as part of our RIMS set up. The mass spectrometer designed is based on optimization using an ion simulation software with improved resolution and sensitivity. The mass spectrometer consists of electrodes for ion accelerator with two stages of homogeneous electric fields, and an ion reflector with a single stage of homogeneous electric field. The ions after acceleration can be focused as well as steered in both vertical and horizontal directions with the ion optics incorporated in the system. A novel grid design for uniform electric field and high transmission at extraction, acceleration and reflectron stages is included. The accelerator and ion reflector are separated by the first field free drift space. The ion reflector and the micro channel plate (MCP) ion detector are separated by a second field free drift space. Optimized electric fields on the electrodes ensure that a spatial distribution of ions initially at rest in the ion extraction zone are brought to a temporal focusing of first and second orders in the detector plane. Simultaneous energy focusing is also possible along with spatial focusing. Therefore, mass resolution is enhanced over the linear TOF-MS that provides only for longitudinal focusing of first order, while the optimized novel grid design ensures that ion transmission and instrument sensitivity are improved. The great success of TOF instruments during the last decade is due to many important features that are some or other way unique in mass spectrometry, e.g., virtually unlimited upper mass limit, very small sample size ~ 10 g, unitary transmission (no small apertures), acceptance of high initial energy spread of ions (>100eV), ideal compatibility with pulsed or spatially confined ionization methods, complete mass spectrum at single laser shot and in only few microseconds, spatial resolution with laser ionization for solid samples ~ few μm, and simplicity and ease of fabrication with few moving parts. We will discuss few important aspects related to the time-of-flight before describing the simulated configuration.