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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.


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.


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.


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.


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.


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.


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.


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. 


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.


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.


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.



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.


Solids Probe.

Analysis of solids that can be attached to a small cup at the probe tip. A built in heater at the probe tip heats the sample to slowly evaporate the sample. The probe usually is inserted into the vacuum chamber via a vacuum lock. The higher temperature probes are normally water cooled. This probe is used in conjunction with a cross beam Ion Source.




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+44 (0)1805 625 205
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  Anglo Scientific Instruments,
  3, Devon Units, Hatchmoor Industrial Estate,
            Torrington, Devon. EX38 7HP. UK.
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