Inficon HAPSITE ER - Mass Spectrometer

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HAPSITE ER Operating Manual
2.4.2 Mass Spectrometer
The Mass Spectrometer is comprised of three basic physical systems: ionizer,
mass selector, and ion detector. These are mounted together in a vacuum manifold
which also includes: an inlet, two vacuum pumps, and a portion of the vacuum
interconnect valve, as shown in Figure 2-1 on page 2-3. Figure 2-3 is a
representation of the three sub-systems of the Mass Spectrometer.
Figure 2-3 Three Subsystems of the Mass Spectrometer
The inlet flow from the membrane isolation valve is brought directly to the ionizer.
Within the ionizer, the compound introduced from the inlet flow is subjected to a
bombardment of electrons which are boiled off the hot filament. Collisions with the
energetic electrons remove one electron from some of the gas molecules, leaving
them with a net positive charge. This process is termed ionization. Other gas
molecules are fractured into smaller molecules, some of which are also ionized.
The remaining stream of gas is pumped away by the vacuum pump system.
The ionized molecules, or ions, are driven from the ionizer toward the mass
selector by the different voltages on the ion volume and focusing plates. As the ions
move through the orifices in these plates, the ions are formed into a nearly parallel
beam of mixed ions of nearly the same energy.
The mass selector (or mass filter) is a quadrupole analyzer. The quadrupole
analyzer is comprised of four parallel rods, mounted with precise alignment and
spacing. Opposing rods are electrically connected together. The two pairs of rods
are connected to a radio frequency (RF) voltage 180°
out of phase with each other.
In addition, the two pairs of rods have a direct current (DC) voltage applied to them;
positive on one pair, negative on the other.
The ion beam is directed down the center of the array of rods. At any specific
combination of RF and DC fields, some ions are light enough to oscillate
harmonically with the RF field. This oscillation causes them to increase in energy
and in speed until the ions impact one of the rods and are neutralized. The DC field
acts upon the heavier ions resulting in their movement from the center towards the
rods. Once on the rod, the heavier ion is neutralized. At a specific combination of
RF and DC fields, ions of a specific mass will be able to transit the rod structure
and emerge at the exit where detection occurs.
Mass Selector
Detector
Ionizer