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Transducers for Mass Spectroscopy: Detectors

! Electron Multipliers

An incident ion beam causes two e- to be emitted from the first dynode. These electrons are accelerated to the second dynode where each causes two more electrons (four in all) to be ejected. These in turn are accelerated to a third dynode and so on, eventually reaching, say, a tenth dynode by which time the initial two electrons have become a shower of 29 e-'s.

! Faraday Cup

Ions travelling at high speed strike the inside of the metal (Faraday) cup and cause secondary e- to be ejected. This production of electrons constitutes a temporary flow of electric current until the electrons have been recaptured. The Faraday cup detector is simple and robust and is used in situations in which high sensitivity is not required.

! Scintillator ('Daly' detector)

A fast ion causes electrons to be emitted and these are accelerated towards a second ‘dynode’. In this case, the dynode consists of a substance (a scintillator) which emits photons (light). The emitted light is detected by a photomultiplier and is converted into an electric current. Since photon multipliers are very sensitive, high gain amplification of the arrival of a single ion is achieved. These detectors are also important in studies on metastable ions.

MS/MS : Hybrid Mass Spectroscopy

IONIZATION & FRAGMENTATION

mass analysis

141

Advantages of using Tandem Mass Spectroscopy:

Allows for more powerful

Structure elucidation

129

Selective detection of a target compound

Greatly reduce interferences

from MS #1

Study of ion-molecule reactions

m/z

to collision cell

DECOMPOSITION

Conceptual representation of MS/MS In this case ions of m/z 129 is selected by the first MS and these ions are directed into a collision chamber and analyzed by a second MS.

mass analysis

129

100

114

from MS #2

m/z

7

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