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(a)

Orbitrap

High vacuum

Fourier

Mass transform

spectrum

ESI Linear ion trap C-trap

Quadrupoles

(and octopoles)

(b)

Central and outer cylindrical electrodes

r

Z

Fig. 9.19(a) Simplified schematic of an Orbitrap mass spectrometer with an electrospray ionisation (ESI)

source. Ions are transferred from an ESI source (described in Section 9.2.4) through three stages of differential

vacuum pumping using RF guide quadrupoles and octopoles which focus and guide the ions through the

various parts of the instrument. The ions are stored in the linear ion trap then axially ejected to the C-trap

where they are squeezed into a small cloud. ‘Bunches’ of ions are then injected into the Orbitrap analyser.

The third quadrupole, which is pressurised to less than 10^3 torr with collision gas, acts as an ion

accumulator where ion/neutral collisions slow the ions which pool in an axial potential well at the end of

the quadrupole (the C-trap). The linear ion trap operates on a similar principle to the ion trap described in

Section 9.3.3. and Fig. 9.7. MALDI (see Section 9.3.8. and Fig. 9.10) is an alternative ion source. (b) Detail of

the Orbitrap analyser. In the Orbitrap, the ions are trapped in a radial electric field between a central and an

outer cylindrical electrode. They orbit around the central electrode with axial oscillations. The superimposed

harmonic oscillations in theZdirection are detected by measuring the image current at the outer electrode.

The frequency of oscillation is proportional to them/zratio and is detected and processed by fast Fourier

transform, as in FT-ICR MS.

378 Mass spectrometric techniques