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Fragment ion
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tarix | 28.07.2018 | ölçüsü | 1,83 Mb. | | #59360 |
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Fragment ion Fragment ion An electrically charged dissociation product of an ionic fragmentation. Such an ion may fragmentate further to produce other electrically charged molecular or atomic moieties of successively lower formula weight.
Typical reagent gases (ex. CH4, isobutane, or NH3) are present in a millionfold excess with respect to the analyte. Typical reagent gases (ex. CH4, isobutane, or NH3) are present in a millionfold excess with respect to the analyte. Analyte is ionized by ion-molecule chemical reactions: - Primary Ion Formation:
- Secondary Reagent Ions:
- CH4 + CH4+ CH5+ + CH3
- CH4 + CH3+ C2H5+ + H2
- Product Ion Formation:
- M + CH5+ CH4 + [M + H] + (protonation)
- AH + CH3+ CH4 + A+ (H− abstraction)
- M + CH5+ [M+ CH5] + (adduct formation)
- A + CH4+ CH4 + A+ (charge exchange)
Material to be analyzed is mixed with a non-volatile chemical protection environment called a matrix Material to be analyzed is mixed with a non-volatile chemical protection environment called a matrix This is bombarded under vacuum with a high energy (4 – 10 keV) beam of atoms. Atoms are typically an inert gas (Ar or Xe)
In this device, positive ions strike a conversion cathode liberating electrons which are then accelerated and multiplied’ via a series of up to twenty dynodes. This type of detector is extremely sensitive, having a gain of up to 108. Aluminium-based dynodes have improved performances of the traditional materials (Cu/Be alloys) which age rather badly in the residual atmosphere of the spectrometers, or during non working periods (returning to atmospheric pressure). In this device, positive ions strike a conversion cathode liberating electrons which are then accelerated and multiplied’ via a series of up to twenty dynodes. This type of detector is extremely sensitive, having a gain of up to 108. Aluminium-based dynodes have improved performances of the traditional materials (Cu/Be alloys) which age rather badly in the residual atmosphere of the spectrometers, or during non working periods (returning to atmospheric pressure).
The ions are directed towards a collector whose entrance, in the form of a horn, is made of a lead doped glass with which acts as the conversion cathode. The ejected electrons are attracted towards a positive electrode and their collisions against the internal walls give rise to multiplication, as with the separated dynodes. The assembly is usually mounted off-axis to avoid the impact of neutral species as well as photons emitted by the filament, equally susceptible to the removal of the electrons. The ions are directed towards a collector whose entrance, in the form of a horn, is made of a lead doped glass with which acts as the conversion cathode. The ejected electrons are attracted towards a positive electrode and their collisions against the internal walls give rise to multiplication, as with the separated dynodes. The assembly is usually mounted off-axis to avoid the impact of neutral species as well as photons emitted by the filament, equally susceptible to the removal of the electrons.
They consist of the union of a large number microchanneltrons arranged like honeycombs. This resembles an electronic version of a photographic plate. Each individual detector is formed from a portion of microtube (25m diameter) whose interior is coated by a semiconductor material acting as a continuous dynode. This system preserves the spatial resolution of the input charged ions. They consist of the union of a large number microchanneltrons arranged like honeycombs. This resembles an electronic version of a photographic plate. Each individual detector is formed from a portion of microtube (25m diameter) whose interior is coated by a semiconductor material acting as a continuous dynode. This system preserves the spatial resolution of the input charged ions.
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