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9.4 Detectors

9.4.1 Introduction

The ions from the mass analyser impinge on a surface of a detector where the charge is

neutralised, either by collection or donation of electrons. An electric current flows that is

amplified and ultimately converted into a signal that is processed by a computer. Thetotal

ion current(TIC) is the sum of the current carried by all the ions being detected at any given

moment and is a very useful parameter to measure duringon-lineMS. A plot of ion current

versus time complements the ultraviolet trace that is also normally recorded during the

chromatography run. Unlike the ultraviolet trace which depends on the absorbance of each

component at the particular wavelength(s) set on the ultraviolet detector, the TIC is of course

independent of the light-absorbing properties of a substance and depends only on its

ionisability in the instrument.

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Trapping plate

(at back)

Trapping plate

(at front)

Ions

Excitation

(transmitter) plate

Detector plate

Detector plate

Fourier transform

Excitation (transmitter) plate

Magnetic field

Radio frequency

Fig. 9.18Schematic diagram of the Fourier-transform ion cyclotron resonance (FT-ICR) instrument. The

technique involves trapping, excitation and detection of ions to produce a mass spectrum. Thetrapping plates

to maintain the ions in orbit are at the front and back in the schematic. Theexcitationortransmitter

plateswhere the radio frequency (RF) pulse is given to the ions are shown at each side and thedetector

platesthat detect theimage currentwhich is Fourier transformed are shown at the top and bottom.

The sample source is normally electrospray (described in Section 9.2.4) or MALDI (see Section 9.3.8 and

Fig. 9.10). The ions are focussed and transferred into theanalyser cellunder high vacuum. The analyser

cell is a type of ion trap in a spatially uniform strong magnetic field which constrains the ions in a circular orbit,

the frequency of which is determined by the mass, charge and velocity of the ion. While the ions are in

these stable orbits between the detector electrodes they will not give a measurable signal. In order to achieve

this, ions of a givenm/zare excited to a wider orbit by applying a RF signal of a few milliseconds’ duration.

One frequency excites ions of one particularm/zwhich results in the ions producing a detectableimage

current. This time-dependent image current is Fourier transformed to obtain the component frequencies which

correspond to them/zof the different ions. The angular frequency measurements produce values form/z.

Therefore the mass spectrum is determined to a very high mass resolution since frequency can be measured

more accurately than any other physical property. After excitation, the ions relax back to their previous

orbits and high sensitivity can be achieved by repeating this process many times.

377 9.4 Detectors