GCMS - Mass Spectrometer

"A mass spectrometrist is someone who figures out what something is by smashing it with a hammer and looking at the pieces."

A mass spectrometer produces ions (charged particles) from chemical substances and then uses electric and/or magnetic fields to measure the mass (or weight) of the ions. Using the mass and relative abundance of ions in a mass spectrum, scientists can determine the molecule's structure and elemental composition.

There are several different types of mass spectrometers: time-of-flight, neutral, and ion, to name a few. The mass spectrometer on the Huygens probe is a quadrupole mass spectrometer. Scientists chose to build a quadrupole mass spectrometer because it could withstand the extreme temperatures of space and the harsh vibrations experienced during the spacecraft launch.

All mass spectrometers, regardless of their specific type, have three basic parts. Below is a schematic of a quadrupole mass spectrometer.

The source is the part of the mass spectrometer where the chemical samples are taken in. If the sample is not already ions, it is converted into ions at this time. The chemical sample for Huygens' GCMS is Titan's atmospheric gases. It will be continuously sampled throughout Huygens' mission, which will last somewhere between 2 to 3 hours.

Huygens' GCMS source accepts the chemical sample through leaks, which are channels of tiny holes. Leaks can be comprised of various materials; the leaks in Huygens' GCMS are comprised of glass. Each hole is only a few microns (about 0.0000394 inches) in diameter. Because the holes are so small, they can easily get clogged - even a piece of space dust is big enough to do it! Consequently, Huygens' GCMS has several different holes for one leak.

Leaks control the pressure inside the vacuum by limiting the flow of gases into the instrument. If the pressure gets too high, it interferes with ion analysis. The leaks are designed to prevent this from happening by only letting in certain pressures.

The mass spectrometer takes the atmospheric gas in through the leaks into a vacuum, which is an airtight chamber. Creating a vacuum is easier said than done. If the GCMS were on earth, scientists could simply run a vacuum pump and wouldn't have to worry about the size, weight, and power-they would have continual access to as much power as they needed. In space, however, the pumps must run off of a battery. To overcome this difficulty, this instrument uses 2 different types of pumps.

At the left are getter pumps. They use chemical reactions to remove gases from the vacuum chamber. These pumps run continuously throughout the voyage.

At right are the ion sputter pumps, used to ionize the gas coming in. The ion is then accelerated through an electric field so it gets physically buried into the walls of the vacuum chamber. These pumps are turned on at specified times during the mission, and, just like the getter pumps, their primary purpose is to get rid of any unwanted chemicals in the vacuum chamber.

Unfortunately, using the ion sputter pump is an imperfect solution to purifying the chamber. First of all, a fraction of the ions that have been physically buried in the chamber wall may work their way out of the wall later and interfere with tests being run. In addition, chemical pumps work well only for some compounds. These factors limit their use, but these limitations are not insurmountable problems. The instrument's designers developed the mission timeline to work around the challenges.

In addition, although getter and ion sputter pumps work well in creating a vacuum, it is impossible for scientists to create a complete vacuum. There is always some mixture of gases inside the chamber, which scientists call residual or background. The background tends to be gases of a low molecular weight, such as hydrogen or helium, because lightweight gases are extremely difficult for pumps to remove.

Although the amount of background is very small, it can influence the results of tests run by the GCMS if scientists do not account for it. Consequently, when the scientists turn on the GCMS, the first 50 seconds are devoted to a background scan, during which the scientists record the residual gases present in the instrument. Scientists then use this data to differentiate background from actual atmospheric gas let into the instrument.

Once the gas sample is obtained, it is then converted into ions (charged particles) by bombarding the gas with an electron beam. Huygens' GCMS has two different energies that it can use for the electron beam. Using different energies allows the instrument to detect and analyze a larger variety of chemicals.

How does bombarding atmospheric gas with an electron beam change the gas into ions? The gas is comprised of molecules, and molecules are built from atoms. Atoms consist of electrons (negatively charged particles), neutrons (charge-less particles), and protons (positively charge particles). Atoms are electrically neutral; they have no charge because the number of electrons is the same as the number of protons.

Pictured below is an atom. The protons and neutrons are located in the center. They make up the nucleus of the atom. The electrons orbit around the nucleus. Since electrons are on the outside of the atom and are lighter than protons, they can easily be moved from atom to atom.

When the electron beam hits the atoms of a molecule, it dislodges an electron. The atom that lost the electron now has more protons than electrons, and this imbalance gives it a positive charge. These positively charged particles are ions called cations.

Once the electron has been dislodged, it may hit a different atom of a molecule. When the electron hits the atom, the atom captures it into its electron cloud, thus giving the atom more electrons than protons. The atom is the negatively charged, and these types of ions are called anions.

The electron bombardment transfers such a large amount of energy to the molecules that the ions cannot stay together. They fragment - that is, they break apart into smaller pieces. The mass spectrometer "smashes" the atom by overwhelming it with so much energy that it breaks apart. Some fragments created are ions; some fragments are neutral. The neutral fragments will not be filtered by the analyzer and detected, but the ion fragments will.

The ion fragments are guided from the ion source into the quadrupole analyzer using lenses. Lenses are tiny tabs of metal or small, hollow cylinders that have a voltage running through them. By either attracting or repelling the ions, they guide the ions into the quadrupole analyzer.

The quadrupole mass spectrometer used here is made with 4 rods of hyperbolic surfaces. A direct current field is applied to 2 rods and a radio frequency (RF) field is applied to the other 2 rods. These rods generate an electric field through which the ions can move.

The strength and frequency of the RF field determines whether or not an ion of a certain mass passes through the rods (and is counted by the detector) or smashes into a nearby surface. For example, in a 120 volt field at a radio frequency of 2 MHz, only ions of 16 Daltons (Da) will navigate through the rods and into the detector. Heavier or lighter ions do not survive the journey to the detector. In this manner, scientists can control the mass of the ions that the detector collects. View the animation here.

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.

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 process depletes the electrons, the tube wall needs time to "recover." The period of recovery time is known as dead time.

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:

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

Gas Chromatograph

Descent Sequence

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