User`s manual

3-10 Calibration
SRS QMS Gas Analyzer
General Procedure
Peak tuning is a simple procedure that requires the introduction of two known gases into the vacuum
system. A low mass gas (1-20 amu recommended) is used to adjust the low end of the mass axis, a high
mass gas, with a mass-to-charge ratio close to the upper limit of the instrument’s mass range, is used to
adjust the high end of the mass scale.
Several analog scans are performed on the sample and the peak positions and widths are checked and
adjusted as necessary. Changes in resolution affect the sensitivity of the RGA, and a Sensitivity tuning
procedure should always be performed at the end of the peak tuning process.
The entire procedure can be carried out with the help of the Peak Tuning command (Head Menu) of
RGA Windows. The program guides you through the calibration procedure and automatically updates
the Peak Tuning Parameters in the RGA Head based on the results of the calibration.
The overall adjustment procedure is very simple and must follow the order described below:
1. Low mass peak position adjustment
2. High mass peak position adjustment
3. Repeat 1 & 2 in that order one or two more times until no more changes in peak positions are
observed.
4. Low mass resolution adjustment
5. High mass resolution adjustment
6. Repeat 4 &5 in that order until no more changes in peak width are observed.
7. Repeat 1 & 2 in that order one or two more times in the case steps 4 & 5 caused changes in
peak positions.
Important: Collect a fresh analog scan for each step. The peak positions are adjusted such that all
peaks throughout the scanning range of the RGA fall within +/- 0.25 amu of their known mass-to-
charge ratio (This is needed to make sure the Peak-locking algorithm used for single mass
measurements always finds the mass peak within its search window). The peak width, ∆m
10%
, must be a
constant, and less than 1 amu throughout the whole scan range.
Example: The following figure shows the result of peak tuning the RGA based on the
18
H
2
O
+
(low
mass= 18 amu) and
86
Kr
+
(high mass=86 amu) calibration peaks. All peaks are at their correct mass
settings and show absolute resolution values of about 0.9 amu. Also note a 20 amu peak in the low
mass spectrum corresponding to
40
Ar
++
at 20 (i.e. 40/2) amu.