INFICON HAPSITE ER Chemical Identification System - Manual (Page 35)

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HAPSITE ER Operating Manual
molecules are fractured into smaller molecules, some of which are also ionized.
The remaining stream of gas continues out the far side of the ion volume and 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 the focusing plates. As the
ions move through the holes 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. Opposite rods are electrically connected together. The two pairs of rods
are connected to a radio frequency (RF) voltage 180
o
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 pick up energy and
increase 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 end to be detected.
When the ions emerge from the mass selector, the ions are directed to the detector.
The active element of the detector is an electron multiplier. The electron multiplier
responds to the arrival of each individual ion with a cascade of electrons, each of
which generates more electrons. The result is a small burst of electrical current in
response to each ion emerging from the mass selector. The signal from the
electron multiplier is connected to the electronic amplifier and data-handling
system outside the vacuum.
In order to determine the constituents of the gas mixture, the ratio of RF to DC field
strengths is varied (swept) to permit progressively heavier ions to transit the mass
selector. The sweep, or scan, over the full range of masses (from 1 to 300 AMU)
only takes about 100 milliseconds; the sweep is usually repeated many times to
statistically improve the quality of the data. This produces the mass spectrum, a
plot of the partial pressure (or population or intensity or amplitude) of each mass.
The mass spectrum is compared with a library of mass spectra characteristic of
many individual compounds, and the HAPSITE reports the compounds which
match the observed spectrum.
Alternatively, the mass spectrometer can remain tuned to a specific mass or set of
masses. The instrument measures the partial pressure of only those masses as a
function of time. Operation in this mode, termed selected ion monitoring (SIM),
permits very sensitive measurement of the presence of one or a few compounds
which have already been identified.