Stanford Research Systems RGA100, Operating Manual And Programming Reference

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Glossary of Terms
SRS Residual Gas Analyzer
3
Ion current . The rate of ion flow into the detector. Usually expressed in units of amps.
Ion Energy. The kinetic energy of the ions as they move down the quadrupole mass
filter , and expressed in eV. Note : In the SRS RGA the Ion Energy is equal to the voltage
biasing of the anode grid in Volts and has two possible settings (i.e. Low and High).
Ionization. The process that results in the formation of ions from neutral atoms or
molecules. During ionization, electrons are added or removed from the molecules to form
negative or positive ions respectively.
Note : In the SRS RGA the ionization is caused by
electron bombardment; outer electrons are removed by the impact of energetic electrons
on the molecules.
Ionization efficiency . The ionization probability normalized to the probability of
ionization of a reference gas.
Ionization Potential . The minimum energy per unit charge (often in eV) required to
remove an electron from an atom (or molecule) to infinite distance. Note : In the SRS
RGA the Electron energy must be set above the ionization potential of the molecules for
ionization to occur.
Ionization probability . The number s of ion pairs (equal amounts of positive and
negative charges appear as ions) produced by an electron traveling a unit ditance
(typically 1 cm) through a gas at unit pressure (typically 1 mTorr). It depends on the
ionization potential of the electrons used for bombardment.
Ionizer (Probe Component). The portion of the mass psectrometer probe that generates
the ions and accelerates them as a beam. Note: The components of the SRS RGA ionizer
are: anode grid, repeller, focus plate, and filament.
Linearity . The extent to which the change in output signal of the RGA is proportional to
the corresponding change in partial pressure.
Linear response range . The partial pressure range over which linearity in the signal
response is observed. See linearity.
Mass-to-charge ratio: The mass-to-charge ratio, M/Q , is defined as the ratio of the mass
number M of the ion to its charge Q, measured in units of the electron charge e
-
. For
example: doubly charged ions of argon isotope 36 (
36
Ar
2+
) and singly charged ions of
water,
1
H
2
16
O
1+
, have M/Q = 18, and cannot be differentiated from each other with most
mass spectrometers. Note: Mass spectrometers do not actually measure the molecular
mass directly, but rather the mass-to-charge ratio of the ions. For singly charged ions, the
mass to charge ratio is numerically equal to the mass of the ion in atomic mass units
(amu).
Mass analyzer (Probe component) . The portion of the mass spectrometer probe that
separates the ion beam into its various mass-to-charge ratio components. Also referred to
as Quadrupole mass filter.
Mass Number, M . The mass number, M, is the sum of the number of protons and
neutrons in an atom or molecule.