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This web page's content and links are no longer actively maintained. It is available for reference purposes only. NASA Official: Dr. Paul Mahaffy

Gas Chromatograph Mass Spectrometer

Mass Spectrometer: Detector

Electron multiplier

The fragments that survive the journey through the analyzer shoot into the electron multiplier - the detector of the GCMS. The electron multiplier detects every ion of the selected mass that passes through the quadrupole analyzer.

Electron multipliers use a process known as secondary electron emission. When the ions hit a surface, it causes the electrons in the outermost area of the atom to be released, which are known as secondary electrons. The number of secondary electrons released depends on several factors, such as the type of particle, the angle at which it strikes the surface, and the energy and characteristics of the surface struck.

Electron Multiplier Schematic

The GCMS uses a continuous dynode electron multiplier, also known as a channel electron multiplier. It is comprised of "the channel," a hollow, cornucopia-shaped tube made of semiconductive glass. Semiconductive glass is glass that has a limited ability to conduct (or transmit) electricity. On the GCMS, lead silicate glass is used.

When the ions hit the inner surface, secondary electrons are emitted. These electrons are then accelerated through an electric field, which is generated by applying the proper voltage to the surface of the tube. The electric field forces the emitted electrons to hit the wall, and these electrons, like the ion, also cause electrons to be emitted. This process continues until there are enough electrons to emitted to create a measurable current. Because the electrons are depleted by the process, the tube wall needs time to "recover." The period of recovery time is known as dead time.

Electron Multiplier

The multiplier tube is curved so as to prevent "ion feedback." Ion feedback occurs when residual gas molecules are inadvertently ionized and accelerated so they produce ions. A curved structure helps prevent this.

By generating a large number of electrons, the electron multiplier amplifies the signal that was initially sent to the detector. This is important, since often times the signals received are fairly weak. The GCMS uses 2 electron multipliers - one in high-sensitivity mode and another in low-sensitivity mode. The high-sensitivity multiplier enables scientists to gather data about the trace gases present in Titan's atmosphere.

Huygens will radio the data it receives to Cassini, and Cassini will then radio the information back to Earth. The data the scientists will receive will create a mass spectrum that looks similar to this:

Sample Mass Spectrum

The scientists will then analyze this spectrum to understand what is happening in Titan's atmosphere.

Next: GC Intro

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"The Channel" image and Electron Multiplier Schematic are courtesy of the Burle Channeltron Handbook