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High Pressure Sampling with RGAs
Application Note #8
The types of analysis performed by an RGA are useful in decades or more, the aperture size must be compromised to
many applications other than vacuum systems. But, the RGA tolerate the pressure range. For example, consider a process
is intrinsically a vacuum instrument that operates best near pressure that varies from 10-1 to 10 mbar. The aperture
10-6 mbar. Above 10-5, the response becomes non-linear, and would be designed to drop the pressure from 10 mbar to
above 10-4 the filament will be shut off by the control 10-5 mbar. When the process pressure was at 10-1 mbar, the
electronics. To sample gasses at higher pressures, a pressure pressure at the RGA would be 10-7 mbar. The noise floor of
reduction system is needed. These systems are basically a the RGA does not depend on the process pressure: for a
restriction and a vacuum pump package. Common restrictions Faraday cup detector it is about 10-10 mbar. Therefore, the
are pinholes and capillaries, which can provide pressure dynamic range of the measurement varies from five decades at
reductions of more than six decades. The vacuum pump high process pressure to only three decades at the low
package consists of a turbomolecular pump and a backing pressures. For applications where the full dynamic range is not
pump. In addition to achieving the desired pressure reduction, needed, operating the RGA at low pressure may be acceptable.
the design of a system should provide for a fast response and If the full dynamic range is required over a variety of process
high signal-to-background ratio. pressures, a variable reduction is required. Suitable variable
leak valves are available but are significantly more expensive
At pressures common to vacuum processes, a simple aperture- than a fixed aperture.
based pressure reduction system is suitable. At atmospheric
and higher pressures, a two stage reduction systembased on Another method of increasing the dynamic range and data
a capillary and apertureis used. These two systems will be acquisition rate is to use an RGA with an electron multiplier.
used to illustrate the design of pressure reduction systems for The electron multiplier provides gains from 102 to 106 and
RGAs. lowers the noise floor to as low as 10-14 mbar. This lower
noise floor allows the RGA to provide large dynamic range:
Vacuum Process Sampling (10 to 10-5 mbar) even at low operating pressures.
Figure 1 shows a schematic of a basic pressure reduction A high operating pressure (or throughput of the aperture) at
system. The system has two paths to the RGA: a high the RGA also improves the signal-to-background ratio. In this
conductance path, and an aperture path. The high conductance context, "signal" is the gas that is drawn through the aperture,
path (through valve Hi-C) is provided so that the RGA can and "background" is outgassing from the system plus
monitor the ultimate vacuum of systems before a process backstreaming through the turbo pump. The ultimate vacuum
begins. The Hi-C path is also used when leak testing the
vacuum system with the RGA software's leak test mode. The Signal
aperture path provides the pressure reduction needed for GOOD
operating at pressures up to 10 mbar.
Background
RGA
Ionizer
Hi-C Valve
Process
RGA
Turbo
Pump
Aperature
Hybrid Turbo Pump
Sample Valve Signal
Diaphragm Pump
BAD
Figure 1: Mid-vacuum pressure reduction system Background
Apertures can be readily designed for process pressures in the RGA
range from 10-3 mbar to 10 mbar. If the process always Ionizer
operates within a small range, the aperture can be optimized to
deliver gas to the RGA at about 10-6 to 10-5 mbar. By Turbo
operating the RGA at its optimum pressure, the data Pump
acquisition time is kept to a minimum, and the full dynamic
range in partial pressure is available. For many applications,
the process is operated at one pressure, and the aperture can be
optimized. If the process pressure varies over a range of two Figure 2: Layouts of post-aperature vacuum systems
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High Pressure Sampling with RGAs
of many turbo pump packages is about 10-9 mbar. The
outgassing background will be mostly hydrogen, water and
nitrogen. The backstreaming background will be air. If 1000 mbar 1-5 mbar 10 -6 mbar
RGA
measurements are being made near these background peaks, Inlet Capillary Aperature Sample Valve
the operating pressure should be kept as high as possible. The
background can be minimized by designing the tubing such
that the effective pumping speed at the RGA ionizer is as high Hybrid Turbo Pump
as possible. Figure 2 shows two layouts that both have the Bypass Valve
same "signal" level. The layout with the RGA at the end of a
Diaphragm Pump
small tube has a small effective pumping speed and will show
a larger background level.
Figure 4: High pressure sampling
The system shown in Figure 1 can be assembled as a simple
package. Choosing a small (70 liter/s or less) hybrid turbo pressure would give a time constant of 3.5 seconds in the
pump and a diaphragm backing pump will eliminate any 1/2 inch dead-volume example mentioned above. The second
concern of oil. The use of this pump pair also eliminates method to decrease the time constant is to ensure that any dead
foreline traps and isolation valves. The operation of the system volume is well mixed. After the capillary, the gas is traveling
should be simple: open the Hi-C valve at low pressures, or open at significant velocity (several meters per second). Proper
the sample valve at high pressures. layout of the inlet tubing will use the kinetic energy of the
sampled gas to mix the dead volume (in a sense keeping the
High Pressure Sampling (>100 mbar) volume alive). Figure 5 shows the response to bursts of gas at
the inlet of an atmospheric sampler designed with the above
At high pressure, the aperture assembly is insufficient to considerations. The sub-second response and cleanup are
reduce the pressure while maintaining response time. almost as fast as the RGA data rate.
Consider an aperture that reduces the pressure from 10 mbar
to 10-6 mbar when used with a 70 liter/s turbo pump. The
volumetric flow rate on the high pressure side of the aperture
would be 7 microliter/s. Any dead volume on the high
5.00e-7
pressure side of the aperture (Figure 3) would cause a large
response time-constant (tc = volume/flow rate). If the aperture 4.00e-7
had a small dead volume of 1/2 inch of 0.250 OD tube
(0.028 wall), the time constant would be 35 seconds. This is 3.00e-7
pressure
not an acceptable response time.
2.00e-7
1.00e-7
10 mbar
0.00e+0
0 1 2 3 4 5
time (s)
dead volume Figure 5: Response of bypass pumped system to gas bursts
Glass capillaries are available with small enough bores to
reduce pressure from 1000 mbar to 10-6 mbar without
bypass pumping. While it is possible to build an atmospheric
sampling system based on a 1/4 meter 50 mm glass capillary,
10 -6mbar there are considerable reasons to use a bypass pump
configuration. Bypass pumping improves the operation of a
system by increasing the flow rate of gas through the capillary
Figure 3: Small dead volume slows process response time about 3 to 4 orders of magnitude. The higher flow rates and
smaller pressure drop allow a wider selection of capillaries to
To achieve a fast response time, a capillary inlet is used with be practical. Stainless steel and PEEK capillaries are more
bypass pumping as shown in Figure 4. The system reduces the affordable and flexible than glass capillaries. A large flow rate
pressure in two stages. Most of the sampled gas is drawn means that the volumetric flow rate at the inlet of the capillary
through the capillary and directly to the diaphragm pump, is more reasonable. For a system with 70 liter/s pumping
bypassing the RGA. The pressure at the exit of the capillary is speed, operating at 10-6 mbar, the volumetric flow rate at the
about 1 mbar. A small amount of the sampled gas is diverted inlet would be 70 nliter/s. Any dead volume at the inlet of the
to the RGA through an aperture. This configuration improves capillary would result in an unreasonable response time. With
the response time in two ways. First, the pressure on the high such small flow rates, inlet devices such as filters, valves, or
side of the aperture is held to about 1 mbar. But even this
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High Pressure Sampling with RGAs
connecting hardware cannot be used. Overall, the bypass-
pumped capillary system is more flexible and only requires a
minor addition of hardware (one valve and some tube).
The configuration seen in Figure 4 is made possible by the
recent advances in hybrid turbomolecular/drag pumps and
diaphragm pumps. Traditional designs would have relied on
two rotary-vane pumps and standard turbomolecular pump.
The high compression ratios of the hybrid turbo pumps allow
the two stream (bypass and sample) to be combined. The low
ultimate vacuum of contemporary diaphragm pumps makes
them suitable as a foreline pump. The combination of these
modern technologies means that an atmospheric sampling
system can be constructed into very small packages (less than
8 inch high in a 19 inch rack mount chassis), which are
portable and easy to operate.
Conclusion
Although the RGA is intrinsically a vacuum instrument, inlet
systems are easily designed that allow it to sample gases at
any pressure. A more descriptive name for such systems
would be "online quadrupole mass spectrometer". Mass
spectrometry is a well proven analytical technique, but
traditionally has been expensive and required large machines.
Reduction in cost of quadrupoles and vacuum pumps, along
with the development of easy to use software interfaces,
makes process analysis with mass spectrometry an attractive
technique.
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