User Manual SERVOPRO PLASMA Trace N
2
Analyzer
APPENDIX 1 55
RECOMMENDED GAS SAMPLING HARDWARE AND TECHNIQUE
FOR LOW TRACE N
2
MEASUREMENTS IN ARGON
Revision 1 , 200307
By Servomex, Québec, Canada.
1.0 Introduction
Servomex produces very sensitive equipment for low level measurements in the range of ppm
and ppb. In such types of measurements, the stability, accuracy and response time of the system
are greatly dependent on the gas sampling lines, regulators, valves and cylinders in use; operator
manipulations are also crucial, one must be careful . Servomex guarantees the performance
within analyzer specifications, if the system is installed according to this application note.
The cost of such gas sampling systems is much lower than the cost entailed by loss of production
or by shipping bad products to customers, - especially for customers operating electronic wafer
plants.
This application note is based on our experience in the use of our equipment. The
hardware listed here was tested by us.
1.1 Leaks
Before going further we must explain what "leak" means. The most frequent cause of
malfunctions in analytical systems is leakage. Here, by a leak we mean introduction of outside
contaminants originally not present in the system.
Introduction of outside contaminants inside a gas line may happen through permeation. For
example, different types of plastic lines, polymers, neoprene, etc. are permeable to oxygen,
nitrogen and moisture. You will not detect any leak, but, oxygen and other contaminants will in
fact get into your line, even if the line pressure is much higher than atmospheric pressure. We
may define a leak in the following way: a leak occurs in a system where the mass flow velocity is
less than the molecular velocity.
When this situation occurs, gas molecules move in both directions through the leak. The net flow
of a particular gas (contaminant) will depend on the relative partial pressure of that gas on each
side of the leak. In a sample line of argon having only 0.5 ppm of nitrogen, there will be a net
flow of nitrogen inward unless the sample pressure is several thousands of pounds.
To avoid such leaks, never use plastic lines. Use stainless steel lines with compression-type
fittings or better (VCR type). Avoid pipe thread fittings in analytical equipment.
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