Stanford Research Systems RGA100, Operating Manual And Programming Reference

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Basic Operating Modes of the SRS RGA 2-5
SRS Residual Gas Analyzer
Basic Operating Modes of the SRS RGA
The SRS RGA is a mass spectrometer that analyzes residual gases by ionizing some of
the gas molecules (positive ions), separating the resulting ions according to their
respective masses and measuring the ion currents at each mass. Partial pressure
measurements are determined with the help of previously calculated sensitivity (i.e.
calibration) factors by reference to the abundance of the individual mass numbers
attributed to each gas type.
During analysis, positive ions are formed within the ionizer and directed towards the
spectrometer’’s
quadrupole mass-filter . The mass filter determines which ions reach the
detector at any given time. It is operated by a combination of RF and DC voltages and the
filtering action is based on the mass-to-charge dependency of the ion trajectories on the
RF and DC fields. The magnitude and frequency of the RF determine the
mass-to-charge
ratio of the ions that can pass through the filter without striking the rods (i.e. with stable
oscillations). The RF/DC ratio determines the filter selectivity. Ions that successfully pass
through the filter are focused towards the
detector and the resulting analog current is
measured by the very sensitive electrometer.
A brief note on Mass Units in Mass Spectrometry:
Since molecules are so small, it is convenient to define a special type of mass units to
express the masses of individual ions. The
atomic mass unit, amu , defined as 1/12 of the
mass of a single carbon atom, isotope 12 (i.e.
12
C), is the unit of molecular mass most
commonly used in mass spectrometry ( 1 amu = 1.660 540 x 10
-27
kg ). To a very accurate
approximation, the mass of a molecule in atomic mass units (amu) is equal to its mass
number M , defined as the sum of the number of protons and neutrons in the molecule.
Mass spectrometers do not actually measure the molecular mass directly, but rather
the mass-to-charge ratio of the ions.
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.
For singly charged ions, the mass to charge ratio is numerically equal to the mass of the
ion in atomic mass units (amu).
RGA users often use the term ““mass of an ion”” when they really mean the mass-to-
charge ratio. This convenient way of speaking is strictly valid for singly charged
ions only.
The SRS RGA as a Mass spectrometer
The SRS RGA can perform both analog and histogram scans over its entire mass range.
Residual gas analysis relies on the interpretation of the spectral data generated by these
two modes to completely characterize, both qualitatively and quantitatively, a vacuum
environment.