SRS Labs QMS 100 Series, User Manual

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Glossary
SRS QMS Gas Analyzer
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 distance (typically 1 cm) through a gas at
unit pressure (typically 1 mTorr). It depends on the ionization potential of the electrons used for
bombardment.
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.
Ionizer (Probe Component). The portion of the mass spectrometer 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.
Linear response range . The partial pressure range over which linearity in the signal response is
observed. See linearity.
Linearity . The extent to which the change in output signal of the RGA is proportional to the
corresponding change in partial pressure.
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.
Mass peak. The ion current pattern in the vicinity of maximum amplitude by scanning through a small
portion of the mass range containing ions of a single mass-to-charge ratio. Very often, the term
“mass peak” refers to the signal developed from singly charged ions. For example, nitrogen is said
to have a “mass 28 peak”.
Mass Range. The range of mass numbers defined by the mass number of the lightest and the heaviest
singly charged ions which can be detected by the RGA.
Mass spectrometer . An instrument which produces a beam of molecular ions from a sample,
separates the resulting mixture of ions according to their mass-to-charge ratios, and provides
output signals which are measures of the relative abundances of the ionic species present.
Mass spectrum . A graph of ion abundance vs. mass-to-charge ratio.
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