SRS Labs QMS 100 Series, User Manual

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3-16 Techniques
SRS QMS Gas Analyzer
Techniques
Correcting for the Chamber Background
Even with the sample flow and capillary flow valves closed, their will be a noticeable background in the
mass spectrum. This background in the analyzer chamber is caused by outgassing from the chamber
surfaces, backstreaming through the turbomolecular pump, and gas production from the ionizer of the
RGA. These three processes account for the ever present background of hydrogen, water, nitrogen,
oxygen, and carbon dioxide seen in high vacuum. The outgassing of water can be minimized by
extensive pumping with both valves closed; typically the system can achieve water partial pressures
around 1
×
10
-8
mbar. The other two process (backstreaming and ionizer) are fundamental and cannot
be reduced. The ultimate vacuum of the turbo pump causes nitrogen to be present at no lower than
2
×
10
-9
mbar and oxygen at 1/4 of that level. Carbon dioxide from the ionizer will be present at levels
from 10
-9
to 10
-7
mbar. The software contains a background subtraction feature that allows the
chamber background to be removed from the mass spectrum.
The background spectrum is correctly measured with the sample valve closed and the capillary flow
valve open. When the capillary flow valve is open, the pressure at the exhaust of the turbo pump is the
same as it will be when the sample is measured. Thereby, the backstreaming component of the mass
spectrum will be the same as in the sample measurement. The software can only subtract two spectra
when they cover the same mass range. Set the software to acquire the spectrum (either histogram of
analog) at the speed, range, and schedule you require. To obtain a background subtracted spectrum,
follow these steps:
1. Measure one complete analog or histogram mode spectrum with the sample valve closed. If the
software was set to scan on a continuous schedule, you can select “Stop at End” from the
“Scan” menu to stop when the current scan in progress is complete. The data displayed must
be a complete scan, and be measured with the same parameters as the scans to follow.
2. Under the Utilities menu, select “Background” and select “Scan Data - Background” from the
dialog box. Check the box next to “Enable” and select “OK” to close the dialog box. This
makes the current spectrum the background and all spectra displayed subsequently will have
this spectrum subtracted from it.
3. Open the sample flow valve and start the scan with the “GO” button. The newly acquired
spectra are the background corrected result.
The ability to subtract background is limited by signal proportional noise, which is typically present at
between 1-10% of the signal magnitude. Because this noise originates in the ionizer of the RGA,
subtraction cannot remove much more than 90% of the background. This limits the ability to see small
changes of less than 1% at the same mass as peaks present in the background.