9780521516358book.pdf

9.4.2 Electron multiplier and conversion dynode

Electron multipliersare used as detectors for many types of mass spectrometers. These are

frequently combined with aconversion dynodewhich is a device to increase sensitivity.

The ion beam from the mass analyser is focussed onto the conversion dynode, which emits

electrons in direct proportion to the number of bombarding ions. A positive ion or a

negative ion hits the conversion dynode, causing the emission of secondary particles

containing secondary ions, electrons and neutral particles (see Fig. 9.20). These sec-

ondary particles are accelerated into the dynodes of the electron multiplier. They strike

the dynodes with sufficient energy to dislodge electrons, which pass further into the

electron multiplier, colliding with the dynodes, producing more and more electrons.

9.5 Structural information by tandem mass spectrometry

9.5.1 Introduction

As mentioned above, the newer ionisation techniques ESI and MALDI are soft ionisa-

tion techniques (as is FAB and its derivative techniques). In contrast to EI, they do not

produce significant amounts of fragment ions. Therefore in order to obtain structural

information on biomolecules and sequence information (in the case of proteins and

peptides), tandem MS has been developed. The technique can also be applied to obtain

sequence information on oligosaccharides (see Sections 9.5.5 and 9.5.6) and oligo-

nucleotides. Although it is unlikely that this method will ever replace DNA sequencing

(a)

Ions
from
MS

Scintillator or

phosphor screen

Photomultiplier

Electrons released

Conversion dynode

e–

e–

Ions

from MS

Electrons released

Amplification

To amplifier

e–

e–

e–

Conversion dynode

(b)

Fig. 9.20Conversion dynode and electron multiplier. (a and b) Each ion strikes the conversion dynode (which

converts ions to electrons) which emits a number of electrons that travel to the next, higher-voltage dynode.

The secondary electrons from the conversion dynode are accelerated and focussed onto a second dynode,

which itself emits secondary electrons. Each electron then produces several more electrons. Amplification is

achieved through the ‘cascading effect’ of secondary electrons from dynode to dynode that finally results in

a measurable current at the end of the electron multiplier. The cascade of electrons continues until a

sufficiently large current for normal amplification is obtained. A series of up to 10–20 dynodes (set at

different potentials) provides an amplification gain of 10^6 or 10^7.

379 9.5 Structural information by tandem mass spectrometry