Page 356 - Instrumentation Reference Book 3E

Mass spectrometers 339

potential gradient and may be used to produce the electromagnetic analyzer where a magnetic
ions. Ions can also be formed by the collision of field at right angles to the electrostatic field is
an ion and a molecule. This method can produce applied (Le.: at right angles to the plane of the
stable but unusual ions, e.g., diagram). Moving electric charges constitute an
electric current so that if each carries a charge e
CI-1; + CHq + CH: + CH,
and moves with a velocity v, at right angles to a
and is most efficient at pressures of about uniform magnetic field H, each particle will be
10-'N/m'. subject to a force F where F = Hev in a direction
It is most important to realize that the process given by Fleming's left-hand rule, is., in a direc-
of producing ions from molecules will in many tion mutually at right angles to the magnetic field
cases split up the original molecule into a whole and the direction of the stream. Thus the ions will
range of ions of simpler structure, and the peak of move in a curved path radius r such that
maximum height in the spectrum does not neces- mv2/r = Hey
sarily represent the ion of the original molecule.
For example. the mass spectrum of m-xylene or
C6H4(CH& may contain 22 peaks of different r = mv21Hev = mv1He
mle values. and the peak of maximum height
represents a vnle ratio of 91, while the ions having but
the next highest peak have a mle ratio of 106. mv2 = 2ev or v = J(2evlnz)
r = (m/eH)J(2e Vim)
16.7.4 Separation of the ions
or
The mass spectrometer shown in Figure 16.20
employs the Mattauch-Herzog geometry, but r2 = (m2/e2H2)(2eVIm)
other forms of geometry achieve a similar result. = (2 VlH2)(elm)
The positive ions representative of the sample or
produced in the ion source are accelerated by a
controlled electrostatic field in a simple gun, the mle = (H2r2)12 V
spread of the ions being controlled by the resol-
ving slits. If an ion of mass m and charge e can be At constant values of the electrostatic and electro-
regarded as starting from rest, then its velocity v magnetic fields all ions of the same mlr ratio will
after falling through a potential V volts will be have the same radius of curvature. Thus, after
represented by the equation separation in the electromagnetic analyzer, ions
having a single charge will be brought to a focus
1 along definite lines on the photographic plate
-my2 = eV
2 according to their mass, starting with the lowest
The ion beam then passes through the electro- mass on the left-hand edge of the plate and increas-
ing to the highest mass on the right.
static analyzer where it passes between two The ions will therefore give rise to narrow
smooth curved plates which are at different bands on the photographic plate, and the density
potentials, such that an electrostatic field B exists of these bands will be a measure of the number of
between them which is at right angles to the path ions falling on the band. The sensitivity range of
of the ions. The centrifugal force on the ions will the plate is limited, and it is necessary to make
therefore be given by
several exposures for increasing periods of time to
777 v21r = eB record ions which have a large ratio of abun-
dance. By using long exposure, ions which are
Combining the equations we see that the radius of present in very low abundances may be accurately
curvature I' of the path will be given by measured. The intensity of the photographic lines
r = riz V21eB = 2eC7/eB = 2 VIB after development of the plate may be compared
with a microphotometer similar to that used with
Thus, the curvature of the path of all ions will be optical spectrometers.
dependent upon the accelerating and deflecting As all ions are recorded simultaneously, ion
fields only and independent of the masslcharge beam fluctuations affect all lines equally, and the
ratio. Therefore, if the field B is kept constant the photographic plate also integrates the ions over
electrostatic analyzer focuses the ions at the the whole of the exposure.
monitor slit in accordance with their translational The instantaneous monitor current may be
energies. The monitor slit can be arranged to measured and used to control the sparking at
intercept a given portion of the beam. The the electrodes at optimum by adjusting the gap
energy-focused ion beam is then passed through between the electrodes.