|CRPG Noble Gas Laboratory||
High precision helium isotope measurements in air
Jennifer Mabry, Tefang Lan, Pete Burnard and Bernard Marty
There are several interesting questions which require the ability to measure helium isotopes at a precision of 2 permil or better. For example, the atmospheric composition of helium isotopes (3He/4He=1.382±0.005×10-6) may vary due to the release of excess crustal helium (3He/4He ~10-8) during fossil fuel extraction. This could produce a variation over time as more and more crustal helium accumulates in the atmosphere. It may also lead to geographic variations in the helium composition since fossil fuel production and consumption is not homogeneous around the globe.
Measuring atmospheric helium isotopes at 2 permil precision is not a trivial task. Helium is only present at values of 5.24 ppm in the atmosphere. This means that large quantities of air need to be collected and purified in order to have a reasonable sample of helium to measure. There are six orders of magnitude difference in abundance of the two isotopes of helium, making simultaneous detection challenging. Also, 3He has several isobars, which must either be resolved or corrected for. We developed a method to mitigate these challenges and measure helium isotopes in air samples at better than 2 permil precision.
First, we purify around 20 cm3 of air, removing everything except for the neon and helium. We store this purified sample air in an adjustable volume. Then we take a just part of the sample air and expose it to a cryogenic trap which is cold enough to trap the neon. Then the fully purified sample (helium only) is let into the mass spectrometer for measurement. We can repeat this process 10 more times for each sample, measuring a standard in between each sample measurement. Before taking a new portion of sample gas, we decrease the volume in order to maintain the same pressure for every measurement. This ensures that every measurement is made under the same conditions.
The mass resolution of the multiplier is ~700 which is sufficient to resolve 3He isobars (HD and 3H), and the resolution of the Faraday cup is 425. An electrostatic filter before the multiplier helps to further improve isotope separation and block out stray ions.
With this method, after making many repeat measurements of air from Nancy, France, we find 0.016 % (1σ) long term reproducibility. On average, when making 4 (typical number of samples collected) repeat measurements we have 0.065 % (1σ) average standard error.