RELAXing in the lab

Crowther et al. (2008) Journal of Analytical Atomic Spectrometry, 23(7) 921-1044

RELAX is an instrument used to measure xenon isotope ratios. Contrary to popular believe, this does not mean we sit around the lab with our feet up all day!

RELAX stands for Refrigerator Enhanced Laser Analyser for Xenon. It is a unique instrument originally designed in the 1980’s, which is continually being improved. It is the world’s most sensitive instrument for measuring xenon isotope ratios. RELAX can detects just a couple of thousand atoms of xenon, a tiny amount when compared to approximately 1,000,000,000,000,000,000 atoms of sodium and chlorine in a single grain of salt.

2 important features contribute to the high sensitivity of RELAX:

1. Xenon ions are made by a process know as resonance ionisation. This is a selective ionisation process which uses a laser tuned to a very specific wavelength so that only xenon atoms will be efficiently ionised. This allows for the detection of very small quantities of xenon in the presence of much larger quantities of other species.
2. There is a cold spot in the spectrometer, held at -193 ^oC (80 K). Xenon in
   the mass spectrometer condenses on the cold spot before
   being released by a pulse from a heating laser. This has the effect of concentrating the xenon, and the ionization laser is then fired through the resulting plume.

RELAX is a time of flight mass spectrometer – the isotopes are separated out according to mass because the lighter isotopes travel faster and reach the deterctors before the heavier isotopes.

Gas in the mass spectrometer condenses on the cold spot, and is then released every 1/10th of a second by a pulse from the infra red heating laser. The ultra violet ionising laser fires through the resulting concentrated plume of gas, selectively ionising only xenon. Xenon ions are separated out by mass as they are accelerated down the fligh tube, with the lighter isotopes arriving at the detectors first.

RELAX highlights:

• Identification of the most ancient water, which formed within 2 millions years of the formation of the Solar System, trapped in tiny bubbles inside salt crystals in the Zag meteorite.
• Demonstrating that the oldest samples of the Earth’s crust incorporated ²⁴⁴Pu as they formed 4.3 billion years ago.
• The first xenon isotopic analysis of material from NASA’s Genesis solar wind sample return mission.
• Analysis of samples returned to Earth by NASA’s Stardust mission.

The RELAX Mass Spectrometer

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