Stanford Research Systems RGA100, Operating Manual And Programming Reference

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3-10 Quadrupole mass filter
SRS Residual Gas Analyzer
charged ions which can be detected by the mass spectrometer. The SRS RGA is offered
in three different models with mass ranges of 1 to 100 (RGA100), 1 to 200 amu (RGA
200) and 1 to 300 amu (RGA300). The main difference between the three models is given
by the maximum supply voltage available to the rods.
The terms Resolution and Resolving Power are often used by mass spectroscopists to
express the ability of a mass filter to resolve ions having different masses:
Resolution, or Absolute Resolution =
'
M
10%
: The width
'
M of the pass band
of the filter, defined as the full width at which the ion current falls down to 10%
of the maximum value. Units = amu.
Resolving Power: R= M/
'
M
10%
: Ratio between a particular mass M and the
resolution,
'
M
10%
, at that mass. Dimensionless ratio.
It is common practice in modern RGA’’s to keep
'
M
10%
constant at a value which insures
adequate separation of masses that are 1 amu apart. The SRS RGA quadrupole operates
in that ““Constant Resolution”” or ““Constant
'
m”” mode.
'
M
10%
is preset at the factory to
1 amu, but it can easily be adjusted by the user all the way to the theoretical ultimate
resolution of the filter.
The following figure illustrates the resolution concepts explained above, using a Krypton
mass spectrum as an example. The different isotopes of the gas are well separated from
each other, and a 1 amu peak width (=
'
m
10%
) is measured on the
86
Kr peak at 10% of its
peak height. Also notice the 20% valley between the
82
Kr and
83
Kr peaks.
5.0E-09 .
1.0E-08 .
1.5E-08 .
2.0E-08 .
2.5E-08 .
3.0E-08 .
3.5E-08 .
4.0E-08 .
4.5E-08 .
5.0E-08 .
80 . 82 . 83 . 84 . 86 .
M/Q (amu/e).
100%
0.0E-09
1 amu
100%
20%
10% Peak
Height
Figure 7
Peak Width Measurement
(Torr)