When and How Should I Change My Electron Multiplier Horn in My Mass Spectrometer?

Video Notes

This video only applies to the 5977 single quadrupole MSD.


An electron multiplier is a device used to multiply the signal of electrons on the mass spectrometer. The signal is sent to a digitizer, which produces the abundance seen on the computer. When an ion reaches the end of the ion beam, it is strongly attracted to a high-energy dynode (HED) which carries a strong opposite charge to the charge of the ion. For example, if the ion is positive the dynode is charged in negative. When the ion strikes the dynode, electrons are released and are attracted to the electron multiplier horn. A single electron striking the horn will release additional electrons, the number of which are dependent on the gain charge. Because of the cone shape of the horn, the released electrons bounce to the opposite side of the horn and release even more electrons. The voltage differential from the entrance to the end of the horn, funnel the electrons to the digitizer. The digitizer translates the signal into a format that can be understood by your computer, which then generates peaks on the screen.

As the electron multiplier is used, the ability of the horn to multiply the signal is degraded. This requires a higher charge to be placed on the horn to maintain the same results. That charge is read in the tune report as “EM volts.” The tune determines what the EM volts setting should be.

When to Change the Electron Multiplier Horn

Use the stoplight system to determine when to change your horn. The following values are EM volts read on your tune report.

Red = >2000

Yellow = 1800-2000

Green = <1800

Electron multiplier horn (5977B/C: part number G7002-80103, 5977A: part number G3170-80103)


  • The maximum value in the software is 3000V.  The EM usually starts out with quite a low required voltage, sometimes as low as the mid 800s to 1000V or so, jumps up sort of quickly, then stabilizes and goes up slowly until near the end of its life when the required voltage begins to increase quickly again.  The graph shown is in Paul draw just for illustration. The timing of the flat to slowly increasing part of the curve is related to sample throughput, gain/EM voltage settings, cleanliness, and more.

    The tune EM voltage is one of the parameters to look at in every autotune report.  

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