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TIP OF THE MONTH 13 : Mass spectrometer in a helium leak detector

Please explain the mode of operation of the mass spectrometer in a helium leak detector :

TIP OF THE MONTH 13 : Mass spectrometer in a helium leak detector

The Figure shows diagrammatically the heart of a helium leak detection instrument, which is known as the spectrometer cell. From the leak detector’s inlet flange, the gases are passed into this spectrometer cell where the electrically neutral gas particles are bombarded with a high-energy electron beam (shown purple in the Figure) generated by a heated filament at high temperature. If an electron in the envelope of a neutral gas particle is struck, it is removed from the gas particle’s electron shell. This leaves behind a positively charged gas particle, an “ion”. This process is called “electron impact ionisation” and happens to a large proportion of the gas particles that are introduced.

 

 

The positively charged gas particles, still present as a mixture, are now accelerated in an electric field (the rainbow beam at the top in the Figure).

The whole of the analysis cell is in a magnetic field. The charged particles are separated in this magnetic field according to their mass-to-charge ratio. In most cases only one electron was removed, so to a first approximation the separation depends only on their mass. Thus the different masses of the gases from the mixture of air plus the helium test gas cause the mixture to be separated into individual beams. Light gases move in a narrow circular path, whereas the circular path of the heavier gases has a larger radius. Therefore hydrogen (mass 1 as an atom, mass 2 as a molecule) traces out the narrowest circular path (blue in the figure). The heavy gas particles from the air (mainly water, mass 18, nitrogen, mass 28, oxygen, mass 32, carbon dioxide, mass 44 and argon, mass 40) move in a wide circular path and strike against a charged plate.

The magnetic field around the spectrometer cell is constant. The acceleration voltage mentioned above is now adjusted in such a way that the particles of our helium test gas (green in the Figure) fly through several apertures before reaching a signal amplifier. The electric current of helium ions is a measure of the partial pressure of the test gas in the vacuum system and thus of the leak rate of the object being tested.

A vacuum system is needed to enable the helium ions to fly to the detector from the point where they are generated. At atmospheric pressure a helium ion can only fly a distance of about 0.2 µm before it collides with another gas particle. However, at the maximum pressure of 10-4 mbar in the spectrometer cell, our helium ion can fly a distance of about 2 m. This is the only way to ensure that the test gas reaches the detector without being obstructed.

Glossary

Electron impact ionisation: This is a special case of ionisation. In this case a neutral particle is bombarded by a high energy electron beam. A high-energy electron from the beam now strikes an electron that is chemically bonded in the envelope of a neutral atom or molecule. This bound electron is shot out of the chemical bond by the high energy of the impacting particle, leaving behind a positively charged particle (ion).

Heated filament: A heated filament is a thin wire normally consisting of a high melting point metal alloy (possibly with a coating). The heated filament is heated to incandescence in a vacuum by an electric current, causing it to emit electrons.

Ionisation: The process in which an electron is removed from or an electron is attached to an electrically neutral atom or molecule is called ionisation. Because an electron has a negative charge, positively charged ions remain when an electron is removed, and negatively charged ions are formed when an electron is attached.

Mass spectrometer: A mass spectrometer divides up a mixture of particles according to their mass-to-charge ratio. If the charge is always 1, the separation takes place exclusively according to mass. Thus a mass spectrometer can measure a concentration distribution of particles depending on their mass or, when set to a single fixed mass, can follow the time-dependent profile of a particular mass. The latter is what takes place in a helium leak detector instrument.

Spectrometer: A spectrometer is an instrument that indicates the distribution of the measured values of a particular measured parameter. For example the measured parameter can be a wavelength or a frequency (light, sound etc.). Thus the output of a spectrometer consists of a measurement record in which the measured parameter is shown on the X axis and an intensity on the Y axis.

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