PlasmaQuant MS Series
Mass
Spectrometer
131
quadrupole control electronics module supplies the correct DC and radio frequency
voltages to the rods in response to a signal from the system control PCB. The
relationship between the control signal and the mass/charge ratio of the transmitted
ions is calibrated with the aid of a reference solution containing isotopes of known
mass.
Figure 8-25 - Quadrupole mass filter assembly
Each rod is nominally 9.243 mm in diameter and 200 mm long. For correct operation
the geometry of the assembly must be correct to within a few millionths of a meter.
The three insulators are made from a non-conductive material called MACOR®.
Opposite rods are electrically connected via gold plated straps to form a pair.
The quadrupole operates in a vacuum system (VC3) where the pressure is
approximately 5 x 10
-5
Torr during scanning.
Figure 8-26 - The zone suitable for ion travel through the center of the quadrupole is typically 2 mm in
diameter.
The zone suitable for ion travel through the center of the quadrupole is typically 2 mm
in diameter. Figure 8-26 above shows a) shows an axial cross section of the
quadrupole rods and the ion trajectory and b) an ion of the correct mass to charge
ratio traveling the entire length of the quadrupole oscillating a frequency of 3 MHz.
Two 30pF ceramic capacitors provide the coupling between the fringe rods and the
quadrupole. The capacitors act as an RF voltage divider and maintain the fringe rods at
half the RF potential to the quadrupole.
Two 5M
Ω
bleed resistors prevent the fringes from developing a charge from the
passing ion beam.
The resolution of a mass spectrometer is a quantitative measure of its ability to
separate ions of similar mass. The most common measure of resolution is the width of
the peaks in the mass spectrum. Ideally, the resolution should be as small as possible
so as not to interfere with adjacent isotopes. Resolution may be expressed as the peak
width in universal atomic mass units at some specified fraction of the peak maximum,
often 5%. Smaller numerical values indicate a better resolution. Sometimes, however,
the resolution is expressed as the resolving power, m/Dm, where m is the mass of the
ion and Dm is the resolution as defined above. In this case the larger the numerical
value, the better the resolution. To avoid confusion, the resolving power measure of
resolution is not used in Analytik Jena publications.
Resolution