Extend the Dynamic Range of Time-of-Flight Mass Spectrometer
Date Issued
2006
Date
2006
Author(s)
Shiao, Kai-Yu
DOI
en-US
Abstract
Our laboratory is developing an ultra-sensitive mass spectrometer nick-named “Dust-buster”. Its purpose is to study isotope ratio variations in individual dust grains collected by the STARDUST spacecraft from comet Wild-2. The conclusion of our preliminary study was that the best apparatus for our research is to combine laser ionization (LI) of secondary neutrals (SN) and TOF-SIMS (Time of Flight Secondary Ion Mass Spectrometry). Such an instrument will have the full mass spectrum capability of TOF while achieving 10-1000 times higher sensitivity than SIMS across the periodic table because vast majority of the sample mass loss is as neutral not ion when sputtered by primary ions.
The detector is a major part of the “Dust-buster” which is responsible for recording the flight time and the intensity of each ion species. The detector we plan to use in the “Dust-buster” is micro-channel plate (MCP) detector. A potential problem is the limited dynamic range for the MCP detector which otherwise has many desirable characteristics. Three possible ways to extend the dynamic range are (1) beam-switching method, (2) using simultaneous detector which has much longer dynamic range than MCP detector, and (3) gain-switching method which was proposed by our collaborators at Argonne National Lab, Dr. Wallis Callaway.
The objective of my MS thesis research is thus to check whether these methods work. The first work is the computer simulation of the beam-switching method. The second is measuring the switching time of the simultaneous detector used in the Element Ⅱ® ICP-MS manufactured by Thermo Electron Corporation. The third is to implement the idea of the gain-switching method. The approach I took was to use a Behlke switch to fast change the voltage difference across the MCP surfaces. The gain of MCP is mainly controlled by the difference of the bias voltages applied to its front and rear surfaces. Behlke switch is a type of MOSFET switch that can switch high voltage signal thousands of volts in less than 30 nano-seconds. Thus we should be able to change the voltage difference hence the gain of the MCP at high speed thus increasing its dynamic range.
From the computer simulation and experiments, we know that these methods are capable of extending the dynamic range. Also from the works we can explore the out comings and short comings of each methods.
Subjects
飛行時間
質譜儀
動態範圍
微通道偵測器
TOF
Mass spectrometer
Dynamic range
MCP
Type
thesis
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