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On Earth if you need to collect gases you can use passive hood exhaust under a fusion (convection), or just under hot liquid, or use vacuum pumps for catch gases together with air.
In the space or on Moon there are no convection and atmosphere so we can not use these methods. What is best practices for aggregate gas in open space and on Moon?
The atmosphere of the moon is very rarefied, and is usually considered as vacuum. But if you really want to collect the sparse molecules and atoms present around its vicinity or even in deep space, you could have two approaches:
If you are in movement, for example in orbit around the moon, you could use an "intake" as the one employed in Air-breathing Electric Propulsion or in the Bussard Ramjet. The image below shows the test setup of an intake developed by ESA and SITAEL for an Air-breathing electric thruster.
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If you want a static solution, you could use one of molecular vacuum pump concepts that don't require convection, as the:
A cryopump or a "cryogenic pump" is a vacuum pump that traps gases and vapours by condensing them on a cold surface (...)
These pumps work on the principle that gas molecules can be given momentum in a desired direction by repeated collision with a moving solid surface.
(...) use a high speed jet of vapor to direct gas molecules in the pump throat down into the bottom of the pump and out the exhaust.
A getter is a deposit of reactive material that is placed inside a vacuum system, for the purpose of completing and maintaining the vacuum. When gas molecules strike the getter material, they combine with it chemically or by absorption. Thus the getter removes small amounts of gas from the evacuated space.
or even an
An ion pump ionizes gas within the vessel it is attached to and employs a strong electrical potential, typically 3–7 kV, which allows the ions to accelerate into and be captured by a solid electrode and its residue.
After using one of these devices you could redirect the gas flow to a container or propose some system to collect the condensed material from the Cryopump, for example. Of course there are many other devices capable of performing this capture of particles, and I cited here just some of them in order to give you some ideas!
If you want to study gases on a planet or in near-vacuum:
The main advantage of mass spectrometer (MS) is that it can register very tiny portions of gas/plasma. Also vacuum or near-vacuum is ideal for it - on Earth MS have vacuum pump to be functional.
Very many spacecraft had MS, often even two or more specialised mass spectrometers.
Examples:
Cassini had two
Ion and Neutral Mass Spectrometer (INMS)
Cassini Plasma Spectrometer (CAPS)
Juno have two
Jovian Auroral Distributions Experiment (JADE) for low-energy particles
Jupiter Energetic-particle Detector Instrument (JEDI) for hi-energy particles
MMS mission studuing Earth's magnetoshere have even more:
The Hot Plasma Suite measures plasma particle counts, directions, and energies during reconnection. It consists of two instruments:
Fast Plasma Investigation (FPI), a set of four dual electron spectrometers and four dual ion spectrometers. Hot Plasma Composition Analyzer (HPCA), detects particle speed in order to determine its mass and type. The Energetic Particles Detector Suite detects particles at energies far exceeding those detected by the Hot Plasma Suite. It consists of two instruments:
Fly's Eye Energetic Particle Sensor (FEEPS), a set of silicon solid state detectors to measure electron energy. Between two FEEPS per spacecraft, the individual detectors are arranged to provide 18 different view angles simultaneously; hence the term "fly's eye". Energetic Ion Spectrometer (EIS), measures energy and total velocity of detected ions in order to determine their mass. The EIS can detect helium and oxygen ions at energies higher than that of the HPCA.
Several specialised mass spectrometers are better than one "jack-of-all-kinds" spectrometer. The partcles catched are very different - neutral atoms and molecules, heavy ions, light ions, electrons. They also can be slow and fast (even at relativistic speed).
