Method has been developed in which measurements are made only over viscous flow range, eliminating fractionation before the molecular leak and problems due to surface elution

Hughes, D. B.

Upchurch, B. T.

Wood, G. M.

English

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NASA TECH BRIEF 
Langley Research Center El 
Sample
Capillary 
iic '
Ion Source 
Leak 
Cavity	 A 
Ion
Amplifier 
Solenoid	 Ballast 
Valve	 Volume 
Pressure Transducer 
August 1975	 B75-10159 
NASA Tech Briefs announce new technology derived from the U.S. space program. They are issued to encourage 
commercial application. Tech Briefs are available on a subscription basis from the National Technical Information 
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be directed to the Technology Utilization Office, NASA, Code KT, Washington, D.C. 20546. 
Dynamic Delta Method for Trace Gas Analysis 
The problem: 
Trace gas analysis by mass spectrometry is limited 
by instrument background, gaseous elution from 
internal surfaces, precise pressure measurement, and 
molecular diffusion-rate differences through the inlet 
leak. The conventional delta method, in which 
measurements are made at two different sample 
pressures, yields a slope function which alleviates the 
background effects but requires careful determination 
of all other parameters. 
The solution: 
In the new dynamic delta method, measurements 
are made only over the viscous flow range, eliminating 
fractionation before the molecular leak and problems 
due to surface elution.
How it's done: 
As shown in the illustration, the solenoid valve is 
closed and a gaseous sample passes through a heated 
0.006-in. (0.02-cm) stainless-steel capillary to a leak 
cavity. The leak cavity contains an unbonded 
strain-gage pressure transducer and a gold molecular 
leak. The gas proceeds into a ballast volume. The flow 
is approximately 1.2 cm 3/min. The sample pressure 
increases to a final pressure of approximately 3 torr at 
a constant rate. Measurements are made only over the 
viscous flow range. 
Micrometer 
Valve 
Pump ( 150m1 
Axis 
Axis
	 Collector 
Plotter
Gas Analysis System
(continued overleaf) 
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Aeronautics and Space Administration. Neither the United States 	 information contained in this document, or warrants that such use 
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will be free from privately owned rights.
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https://ntrs.nasa.gov/search.jsp?R=19750000159 2020-03-19T20:26:15+00:00Z
Ion current from the resolved mass specific to the 
measured gas is plotted on an X-Y recorder as a 
function of output from the pressure transducer, 
effectively resulting in an infinite number of data 
points. The slope of the plot is therefore a direct 
measure of the partial pressure of the trace 
constituent being determined. As long as output from 
the pressure transducer is linear, a determination of 
absolute pressure is not required. 
The concentration of the trace constituent is 
computed by a comparison of the tangents of the 
unknown (subscript 1) to a known (subscript 2) 
concentration. This can be described by 
tan 01 Q2 I1 102
With the equipment shown, a usable measurement 
range for rare gases in mixtures is from 100 percent 
(pure sample) to parts per million. Measurement 
capability is somewhat reduced for certain reactive 
species. Relative precision over the entire range is 
approximately 1 percent of the measured value. 
Note: 
Requests for further information may be directed 
to:
Technology Utilization Officer 
Langley Research Center 
Mail Stop 139-A 
Hampton, Virginia 23665 
Reference: B75-10159
.. 
Volume (percent) 	
tan 02	 l 122	 Patent status: 
NASA has decided not to apply for a patent. 
where 0 is the slope angle, Q is the electrometer input 
resistance, and 12 is the recorder attenuation. The 
known concentration is obtained by calibration using 
a standard mixture or the pure gas. Isotopic 
abundance determinations may also be made as long 
as standard mass flow corrections for the molecular 
leak are applied. Resistive heating of the capillary is 
controlled by a standard autotransformer.
Source: G. M. Wood
Langley Research Center and
B. T. Upchurch of
Old Dominion University and
D. B. Hughes of
E. I. du Pont de Nemours and Co.
under contract to
Langley. Research Center 
(LAR-1 1800) . 
. 
Categories: 04 (Materials) 
02 (Electronics Systems) 
05 (Life Sciences) B75-10159


Dynamic delta method for trace gas analysis

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