SRS Labs QMS 100 Series, Руководство пользователя

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Pressure Reducing Inlet 1-5
Technical Reference
Pressure Reducing Inlet
The gas handling subsystem is designed to achieve several goals:
•
reduce the pressure of the sample gas to the operating range of the mass spectrometer
(<10
-5
mbar)
•
provide a quick response time to changes of sample composition at the inlet
•
allow for easy connection to system being measured
•
use conventional materials
If only the first goal where important a single stage pressure reduction would be suitable.
For example, about 50 cm of 50
µ
m capillary would perform the required
pressure reduction. In a single stage design, all the gas that enters the capillary is
delivered to the spectrometer chamber. For a 70
l
s
-1
pump, the flowrate at the
capillary inlet is 70 n
l
s
-1
! The velocity of the gas near the capillary inlet is very
small. To demonstrate, the figure at the right shows the end of a 1/16 OD capillary
and a hemi-spherical boundary of the same diameter. The time it would take to
drain the volume inside the boundary is a measure of the response. For the small
volumetric flowrate in this example, it would take 30 seconds to drain the tiny
volume. This low speed implies that the capillary relies on diffusion or forced
convection to respond to concentration changes.
The effect of a low flowrate on leak detection is catastrophic. Imagine trying to
locate a leak by moving the capillary tip around a fitting on a gas line. With the capillary in the example
above, you could be 1/16 of an inch away from the leak source and not detect it for 30 seconds. To
locate the leak, you would either have to move incredibly slow, or be exactly at the leak.
Low flowrates also severely restrict the inlet. At the left is shown a capillary
with a syringe tip filter installed. The gap between the filter element and the
end of the capillary (about 1/16 inch in this example) creates a dead volume.
If this volume is approximated to be well mixed, the time constant is over 5
seconds. The time constant relates to the rate at which a system responds to
a step change and is characterized by the familiar
[ ]
1
− −
exp t t
c
response.
In this example, a sudden change outside the filter would not be completely
detected for over 15 seconds.
To achieve fast response, the QMS uses a bypass flow configuration, which
draws 200-1000 times larger flowrates through the capillary. The same
amount of sample is delivered to the spectrometer. As the name implies, most
of flow is bypassed around the spectrometer. Also the pressure drop across
the capillary is now only a factor of 1000 (as opposed to 1 billion). This
means the capillary can either have a larger bore or be shorter than in a single stage system. The lower
pressure drop makes feasible many capillary materials commonly used in gas chromatography. The
following sections discuss details of the flow of the sample gas within the instrument.