can be seen from spectrum A (a product ion scan) that m/z 107 is the most abundant product ion, which
confirms the initial loss of CH 3 and not C 3 H 7. To identify the neutral loss, m/z 137 is then selected as the
precursor ion as this will contain either an^18 O atom and/or two^13 C atoms. The most abundant product
ion seen in spectrum B, another product ion scan, is m/z 109 which shows that the^18 O atom must still
be present in the m/z 109 ion and therefore^16 O in the m/z 107 ion. Thus, the neutral loss is C 2 H 4 and not
CO.
Ion Detection and Recording System
Ions from the analyser pass through a slit and impinge either on an earthed electrode called a collector
plate or on a type of photomultiplier tube known as an electron multiplier. The currents produced are
amplified and monitored with a high-speed recorder employing mirror galvanometers and ultraviolet
sensitized chart-paper which operates at several different sensitivities simultaneously to avoid the need
for multiple scanning of each sample. The spectra may also be viewed on a VDU screen. Many mass
spectrometers are interfaced with a dedicated mini- or microcomputer to facilitate the processing and
presentation of the very rapidly accumulated data and to make comparisons with library spectra. This is
virtually essential where the spectrometer is also to be interfaced with a gas chromatograph (p. 114) as
rapid scanning of eluted peaks can generate hundreds of spectra in just a few minutes. The reliability of
mass spectral data is ensured by frequent calibration of the instrument with a standard of known
fragmentation pattern, a long-chain fluorinated hydrocarbon such as a perfluorokerosene being a
common choice.
Principle of Mass Spectrometry
The principle of separation by a magnetic analyser can be rationalized in terms of the kinetic energy of
the fragment ions, the accelerating voltage and the magnetic field. The kinetic energy of fragments of
mass m and charge z accelerated in a potential gradient V may be equated with the electrical force
acting on them,
where ν is the velocity of the fragments. The magnetic field, acting at right angles to the direction of
motion, causes them to be deflected into a circular trajectory in which the centripetal force due to the
magnet is balanced by the centrifugal force due to the kinetic energy. Thus,
where B is the magnetic field and r the radius of curvature of the trajectory. Eliminating ν from
equations (9.26) and (9.27) gives: