| Quadrupole
Mass Spectrometers, because of their nature, require an operating vacuum
of 10-4 mBar or better. However most processes which need monitoring occur
at a pressure in excess of 10-4 mBar. It is necessary therefore to have
some form of interface between the mass spectrometer and the process
vessel. These interfaces are called Inlet Systems and the type required is
dependent upon the application.
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| Capillary Inlet
This
system is a form of continuous gas sampling, and may be used across a
pressure range of 10-4 mBar to atmospheric pressure (1000 mBar). It
consists of a narrow tube which connects the sampling point to the mass
spectrometer vacuum system. The length and internal diameter of the tube
have to be selected according to the pressure at which the sample is
taken.
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| The
disadvantage of this system is that at higher sampling pressures ( above
100 mBar) the response time becomes very long ( of the order of a few
minutes ) due to the length of the tube. This can be overcome by a bypass
type inlet system.
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| Capillary Inlet with bypass
This
type of inlet system is better suited to sampling over a pressure range
from 100 mBar to atmospheric pressure. The operating principle is that a
capillary tube is connected to a T-piece which is attached to a rotary
pump. To the third arm is fitted a molecular flow leak or leak valve. Gas
is drawn along the capillary by the rotary pump and a small portion of the
gas which flows through the T-piece, passes across the leak and into the
mass spectrometer.
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| This
inlet requires a gas flow of approximately 20 ml min and achieves a
response time of better than 100 ms. Capillary inlets are also available
with a heater, which improves response time, and with an auxiliary pump to
draw the sample through the bypass line. A crimp at the sample end to
restrict the sample gas flow and a throttle valve on the bypass pump will
allow much smaller sampling rates and maintain good responce times.
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| Leak Valve Inlets
A
good, continuous sampling, leak valve can provide the flexibility to
sample across a range of 10-5 torr to well above atmospheric pressure. The
operating principle is the precise control of a knife edge biting into a
surface. The knife edge position is controlled by a fine screw operating
mechanism with a calibrated dial so that a particular leak rate may be
reproducibly set. The main disadvantage of this method of gas sampling is
that fractionation of the gas occurs across the valve seat and the degree
of fractionation depends on the leak rate setting. It therefore makes
calibration of this inlet difficult. Response times when sampling from
higher pressures may also be long unless attention is paid to producing a
continuous gas flow across the valve seat. The inlet is available either
as a general purpose, polymer sealed valve or as an all metal precision
valve.
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| Membrane Inlets
A membrane system consists of a tube terminating
in a special semi-permeable membrane stretched over a grid.
The
application and the components of interest in the sample determine the
type of membrane selected, as the membrane can exert considerable
selectivity. Perhaps the most common types of membrane material employed
are Teflon and silicone rubbers. Teflon is used where gases are to be
monitored in the absence of solutes and where water is in high abundance.
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| Silicone
is highly permeable to oxygen and can be used for the monitoring of
organic compounds in situ. The major application of this inlet is in
sample enrichment, particularly of organic materials, where the low levels
would fall below the normal detection limit of the mass spectrometer.
Enrichments by a factor of 200 are possible giving detection levels down
to the low ppb.
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| Batch Inlet
Continuous sampling techniques are satisfactory
provided the mass spectrometer can be sited close to the sampling point.
As this is not always possible a sample must be collected in a suitable
vessel and transferred to the mass spectrometer. Introducing the gas into
a vacuum system requires a batch inlet
Batch
inlets can be specially designed to meet particular applications but they
generally consist of a reservoir that is connected by valves to a vacuum
system.
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| The sample can be
introduced through a septum port (if the sample is in a gas syringe) or
through a valve port (if contained in a glass or metal collection flask).
Options
might include the ability to purge the reservoir with inert gas or heat
the chamber to reduce background levels between samples (especially useful
if water is being measured). Another useful option is the addition of a
pressure gauge.
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| Dual
inlets.
Dual
Inlets consist
of a high conductance isolation valve, and a leak valve placed in parallel
across a single inlet line. The wide range in the respective conductance's
means that this single system can sample from atmospheric pressure to high
vacuum. This capability is particularly useful in applications such as
sputtering where the pressures can alter dramatically during different
stages in the process. |
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MULTI_STREAM GAS SAMPLING.
The instruments are not expensive but we still accept that most users
would prefer to introduce multiple samples into one mass spectrometer rather
than buy a mass analyser for each sample stream. The multi stream gas sampling
inlet allows the connection of up to 32 gas streams to one capillary inlet.
Which is then used to introduce each selected sample into the source of the
analyser.
All streams not being sampled flow through their own individual inlet and
outlet connections. Both the operation of electrical switching to select the
channel (which can be one or more calibration gases) and determination of dwell
time per channel can be under computer control. This inlet is very versatile but
a typical application is for the sampling of dissolved gases (see membrane
inlet) and off gases from multi-vessel fermentation and cell culture
installations.
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Residual
gas Analysers 
