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is really measured is the mass-to-charge ratio (m/z) for each ion.) At any given moment,

ions of a particular mass are allowed to pass through the analyser where they are counted

by the detector. Subsequently, ions of a different mass are allowed to pass through the

analyser and again the detector counts the number of ions. In this way, the analyser scans

through a large range of masses.

In the majority of instruments, a particular type of ionisation is coupled to a particular

mass analyser that operates by a particular principle. That is, EI, CI and FAB are combined

with magnetic sector instruments; ESI and its derivatives with quadrupole (or its variant

ion-trap) and MALDI is coupled to TOF detection.

9.3.2 Quadrupole mass spectrometry

The quadrupole analyser consists of four parallel cylindrical rods (Fig. 9.5). A direct

current (DC) voltage and a superimposed radio frequency (RF) voltage are applied to

each rod, creating a continuously varying electric field along the length of the

analyser. Once in this field, ions are accelerated down the analyser towards the

detector. The varying electric field is precisely controlled so that during each stage

of a scan, ions of one particular mass-to-charge ratio pass down the length of the

analyser. Ions with any other mass-to-charge value impact on the quadrupole rods

and are not detected. By changing the electric field (scanning), the ions of different

m/zsuccessively arrive at the detector.

Quadrupoles can routinely analyse up tom/z3000, which is extremely useful for

biological MS since, as we have seen, proteins and other biomolecules normally give

a charge distribution ofm/zthat is centred below this value (see Fig. 9.6).

Note thathexapoleandoctapoledevices are also used, to direct a beam into the

next section of atriple quadrupoleor into the ion trap for example, but the principle

is the same.

Glass capillary ESI needle ±5 kV

Sample solution

Solvent evaporation

Mass

analyser

Heated capillary

Fig. 9.4Electrospray ionisation source. The ESI creates very small droplets of solvent-containing analyte

by atomisation ornebulisationas the sample is introduced into the source through the fine glass (or

other material) hollow needle capillary. The solvent evaporates in the high-vacuum region as the spray of

droplets enters the source. As the result of the strong electric field acting on the surface of the sample

droplets, and electrostatic repulsion, their size decreases and eventually single species of charged analyte

(free of solvent) remain. These may have multiple charges depending on the availability of ionisable

groups.

360 Mass spectrometric techniques