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Cassini / Huygens

GCMS - Gas Chromatograph

Gas Chromatograph

The gas chromatograph is essentially a filter. It separates compounds present in the gas mixture. There are 3 basic components to a gas chromatograph:

  1. Carrier gas supply:
    2. a gas of specific type and pressure to separate the components present in a gaseous mixture.
  2. Sample introduction system:
    3. where sample is taken into the system.
  3. Detection system:
    4. gives a signal that tells scientists what gases are present.
Hydrogen Manifold
Hydrogen Manifold

The carrier gas of a gas chromatograph is very important. It varies for the gas chromatograph used, but in general the gas must be chemically inert, dry, and free of oxygen. These conditions are required because otherwise the carrier gas might chemically react and interfere with the surface of the gas chromatograph or the gases being studied.

The carrier gas has 2 purposes:

  1. Push the gas through the gas chromatograph column
  2. Clean out the gas chromatograph column after sample analysis

The Huygens GCMS uses approximately 3 standard liters (about 0.8 gallons) hydrogen gas as carrier gas. This quantity allows for the planned 2 to 3 hour descent sequence and provides a 50% reserve. Once the hydrogen gas is turned on, it remains on throughout the mission, continually flowing through the GCMS.

Once the scientists start the gas flow, the hydrogen carrier gas flows into the capillary columns. The Huygens GCMS has 3 columns. These columns are designed to detect compounds that the scientists think might be present:

  1. Column 1:
    2. to separate carbon monoxide, nitrogen, and other stable gases
  2. Column 2:
    3. to separate nitriles (compounds containing nitrogen) and other organics (compounds containing carbon) up to 3 carbon atoms
  3. Column 3:
    4. to separate large organic molecules

The columns are 10 to 20 meters of very tightly coiled silcosteel (as seen in the diagram on the previous page) coated with chemicals on the inside. The coating chemical is specifically chosen so that when gas is injected into the system, certain compounds from the gas will be adsorbed (stick weakly) into the column walls.

A Single GC Column
A Single GC Column

The gas sample is injected into the 3 columns by momentarily opening a valve. The valve is then shut off, and the column is heated to a specified temperature. This releases the chemicals that are adsorbed on the wall, and they are sent to the detector for analysis.

Gas samples are taken approximately every 13 minutes. Of that time, 10 minutes are allotted for analyzing the sample; 3 minutes are allotted for cleaning the column out after analysis.

Mass Spectrometer Descent Sequence Back to GCMS