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Address
196084, St.-Petersburg
Parkovaya str. 4
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| т/ф: (812) | 388-3814 388-7924 388-3433 |
TOF Mass-spectrometer (mass-reflectron type)
Basic characteristics
| Parametres | TOF mass-spectrometer |
| Resolution | 180 |
| HWD | 220*100*100 мм |
| Mass | 4,8 kg |
| Time required for analysis | 0,1 sec |
| Detection limits | 10-12 torr |
Range of measurements of the volume fraction of components
| Hydrogen | 0.2 */100 |
| Carbon monoxide | 0.2*/100 |
| Nitrogen | 0.2*/100 |
| Oxygen | 0.2*/100 |
| Argon | 0.2*/100 |
| Carbon dioxide | 0.2*/100 |
| Methane | 0.2*/100 |
| Ethane | 0.2*/100 |
| Propane | 0.2*/100 |
| Helium | 0.2*/100 |
| Sulphur dioxide | 0.2*/100 |
| Sulphuretted hydrogen | 0.2*/100 |
Theoretical bases
Operation of the TOF mass-spectrometers.
First let us consider the operation of TOF mass-spectrometers. While ionized gas molecules are drifting in field free drift region they are divided into groups of relatively close values of Mi/e. The general scheme of TOF mass-spectrometer is presented in Figure 1.
In an ionic source (1) ionization of gas molecules is carried out by electron bombardment (energy is about 100eV). A straight cathode (tungsten string) is used for gaining electrons (1а). Ions are pulsed into the flight tube (free of fields region) (2) of the mass-spectrometer analyzer. While passing free field region the ions become separated into individual packets and get to the detector (3), and each packet becomes a mass peak. Peaks of mass-spectrum are exposed to the corresponding processing, allowing to obtain data about structure of gas sample.
However, the temporal distribution of one packet with similar ions (with the given attitude Mi/e) while moving in free of field region is volatile because of different energy of ions that leave the mass-analyzer’s source. Therefore when moving in free of field space ions of one packet group, because faster ions leave last, but are moving faster. After traveling some distance faster ions just catch up with the slower ones. The plane where the temporal distribution of ionic packets is minimal is called spatio-temporal focusing plane (STF plane). Resolution of mass-analyzer is maximal in this plane. At strong spread of ionic packet it can ally with close ionic packets, therefore the capability of mass-analyzer to analyze gaseous components of the tested gas reduces.
The main issue of ionic focusing in TOF mass-analyzers is to gain ionic packets with minimal temporal distribution, that are located as far as possible from each other.
The electrostatic mirror, called a reflector, is applied to equalize the spread in energy values of the same ions and to increase the resolution of mass-analyzers. It is mass-analyzers of mass-reflectron type..
In mass-spectrometers of mass-reflectron type the focusing of ion on energy is provided due to the consecutive moving of particles in free if field region and in the field of an electrostatic mirror (a set of electrodes). Such scheme allows to reduce the volume of the analyzer unit and to increase its focusing properties essentially. By selecting of electric potentials on electrodes of mass-analyzers the focusing of ions is maximal. So, in TOF mass-analyzer the time difference of movement of ions in free of field space is compensated by the time they stay in a reflector. You can see the mechanical trajectory of ions in TOF mass-analyzer in the Fig.
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