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mass resolution;> 106 is observable with most instruments. The instrument also

allows tandem MS to be carried out. The ions can be generated by a variety of

techniques, such as an ESI or a MALDI source. FT-ICR MS is based on the principle

of ions, which while orbiting in a magnetic field, are excited by radio frequency

(RF) signals. As a result, the ions produce a detectableimage currenton thecellin

which they are trapped. The time-dependent image current is Fourier transformed

to obtain the component frequencies of the different ions, which correspond to their

m/z(Fig. 9.18).

9.3.13 Orbitrap mass spectrometer

The resolving power of FTICR-MS is proportional to the strength of the magnetic

field therefore superconducting magnets (3–12 tesla) are required, which makes for

high maintenance cost. The Orbitrap mass analyser is a lower-cost alternative to the

high magnetic field FTICR mass spectrometer. Ions are trapped in theorbitrap,where

they undergo harmonic ion oscillations, along the axis of an electric field (see

Fig. 9.19). Theirm/zvalues are measured from the frequency of the ions, measured

non-destructively, using Fourier transforms to obtain the mass spectrum. The instru-

ment has high mass resolution (up to 150 000), high mass accuracy (2–5 p.p.m.), an

m/zrange of>6000 and a dynamic range greater than 10^3 with sub-femtomol

sensitivity.

Collision cell

Ion source

(electrospray,

ion trap or MALDI)

Reflector

Quadrupoles Collision

gas

Detector

Linear detector

Resolving

quadrupole

Fig. 9.17Diagram of a hybrid quadrupole TOF MS. The diagram shown here does not represent

any specific instrument from a particular manufacturer. The source may be an ion trap device,

an electrospray or even a MALDI source (such as in the ‘MALDI Q-TOF’ from Micromass). Other hybrid

instruments include the Bruker Daltonics ‘BioTOF III, ESI-Q-q-TOF System’ and the ‘QSTAR’ Hybrid

LC/MS/MS from Applied Biosystems with an electrospray, nanospray or an optional MALDI source.

The Shimadzu Biotech ‘AXIMA MALDI QIT TOF’ combines a MALDI source with an ion trap and

reflectron TOF mass analyser.

376 Mass spectrometric techniques