• Q:  (i) What is the optimum value of Multiplier Gain needed in a RGA?

    (ii) Some manufacturers mention Electron Multiplier gain 200A/mbar & some write 0.1 A.mbar?   Which RGA is better and if so what is basic criteria/parameter gets affected?
    A:  (i) Most manufacturers provide a calibration routine that adjusts the multiplier voltage to give a gain of 104 so that the reading from the Faraday detector is equal to the reading from the multiplier detector.   The gain of the multiplier will gradually reduce over its lifetime and thus there is plenty of scope to increase the multiplier voltage keeping the gain at 104 constant. [Typically a new SEM will have a gain of 106 to 107 at maximum voltage].
    (ii) This is confusing terminology from the manufacturer.  In the first example the RGA probably has a source sensitivity of 2x10E-4A/mbar (see Answer below) and an Electron Multiplier set to 106 gain.   The second example probably uses a Closed Ion Source, which has lower sensitivity, but is operated at a higher pressure than an Open Ion Source and will give much better signal to noise performance when sampling an atmospheric gas into the vacuum system.

  • Q:  Generally in RGA specifications, manufacturers mention sensitivity (for Nitrogen) as 1E-4 A/mbar with Faraday Cup and 0.61 to 200 A/mbar with electron multiplier. Some manufacturers give data for Argon gas.
    A:  1x10E-4A/mbar is the typical sensitivity quoted for many RGA ion sources.   A faraday cup gives no amount of amplification, therefore 1x10E-4A/mbar is often quoted as the sensitivity figure for an RGA with a Faraday detector.   The sensitivity figure for an RGA with an electron multiplier is often simply assumed to be the source sensitivity multiplied by the gain of the electron multiplier.
    It’s usually assumed that such sensitivity figures are given for nitrogen.   Sensitivity figures given for Argon can be confusing!  A European Working Party on the calibration of RGA’s has devised an ISO standard, ISO/DIS 14291: Definitions and specifications for quadrupole mass spectrometers.   The ISO standard specifies nitrogen, however it’s not yet gained wide acceptance.

  • Q:  How to convert sensitivity of RGA from Nitrogen gas to Argon gas?
    A:  The relative sensitivity factor for Argon (with respect to nitrogen) is typically quoted in literature as something between 1.2 and 1.4.
    Thus a sensitivity figure for argon should be divided by 1.2 or 1.4.

  • Q:  What is the general acceptable sensitivity value for any RGA with Electron Multiplier?
    A:  A typical figure for source sensitivity is 1x10E-4A/mbar and a sensible gain of an electron multiplier is x10,000.   Therefore a reasonable sensitivity value for an RGA with Electron multiplier is 1x10E-4 x 10,000 = 1 A/mbar.

  • Q:  What parameter decides the sensitivity value for any RGA with and without electron multiplier?
    A:  In practice there are several parameters that can affect an RGA’s sensitivity . Therefore you may find it more helpful to simply specify minimum detectable partial pressure.   All manufacturers will be able to give you a figure for this and it’s easier to demonstrate.

  • Q:  What is the exact principle of "Total Pressure Measurement" by RGA?
    A:  Different RGA manufacturers use different methods to measure and display a Total Pressure reading.
    In essence the RGA instrument is switched into a mode where it behaves as an ‘imperfect’ Bayard Alpert Gauge.   Ions are produced by the RGA electron impact ion source, or a hot filament used only for the total pressure measurement.   These ions are then collected on a Faraday Plate/Cup detector and the current is measured by the RGA preamplifier.   Thereby, a total ion count is equated to total pressure measurement.
    This method of total pressure measurement lacks the sensitivity and symmetry of a typical Bayard Alpert gauge but can provide a good approximation to the total pressure within the vacuum system.

  • Q:  Why does the sum of partial pressures by RGA spectrum not match with the Total Pressure Value measured by RGA and the Total Pressure Gauge (ie BAG/IMG)?
    A:  There are many factors that influence the partial pressure measurement of an RGA:-
    1. Ionisation cross-section (also known as relative sensitivity) which depends upon gas species.   [Typical RGA is ‘calibrated’ for nitrogen at mass 28; Rel Sens = 1.0]
    2. Sensitivity of RGA ion source – this can be affected by cleanliness of vacuum system.
    3. Transmission through mass filter.
    4. Cracking Pattern (fragmentation) of gas species.
    5. Vacuum system pressure – above 10-4 mbar the mean free path is usually around the same order of magnitude as the RGA analyser leading to ion-molecules collisions and reduced sensitivity.
    Therefore, the RGA is a useful instrument for measuring the relative partial pressures of the various gas species in the vacuum system and should not be relied upon for absolute measurements.   In common with all vacuum gauges the reading should only be used a guide unless the gauge has recently been calibrated.
    The calibration of RGAs is a complex subject that has been taken up by a European Working Party.  A Workshop was held 13 – 16 April 2012, Lake Bled, Slovenia, entitled “Measurement characteristics and use of QMS for vacuum applications"