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In 2018 I published this comment on a paper which presented H₂ analyses made using mass spectrometry of inclusion fluids. (Economic Geology, 2018, V113 #4, pp 997)

Sir: In their paper "Hydrothermal rare earth element (Xenotime) mineralization at Maw Zone, Athabasca basin, Canada and its relationship to unconformity-related uranium deposits" ,  Rabiei et. al. (2107) report the analysis of hydrogen gas within bulk fluid inclusions in quartz samples using mass spectrometry. Although they report only minor levels of H₂ with no impact on their ore formation models or conclusions, it is highly misleading to report such H₂ analyses as H₂ is an instrumental artifact of the mass spectrometer ionizer and not a component of the fluid inclusion volatiles at all. Diatomic hydrogen is generated from a side reaction of monatomic H⁺ and H₂O within the ionizer as explained by Burlinson (2012), which renders any H₂ analysis meaningless. This problem has been further discussed by Burlinson (2013).

Mass spectrographic residual gas analyses are routinely done to determine the quality of high vacuum systems and these show the presence of H₂, mass 2, although H₂ is not present in air. (Hofmann) The H₂ peak is either not discussed, or else attributed to water (without explanation) by the manufacturers of these instruments. It is clear that the dissociation of water in the ionizer is the source of H₂, mass 2, in these analyses.

The ionization of water molecules by electron impact gives H₂O⁺ due to removal of an electron. This species is unstable and decomposes into either ( H⁺ and OH ) or ( H and OH⁺ ). The monatomic H⁺ does not interfere with the analysis of diatomic H₂, so it is wrongly assumed that there is no problem in analyzing for H₂ gas in the original fluid. But there is a side-effect which occurs within the ionizer and which converts some of the monatomic H⁺ into diatomic H₂, causing severe interference and negating any meaningful attempt to measure the H₂ gas content of the fluid inclusion volatiles.

It is well known that water molecules "adhere" to all the surfaces in the instrument including the negatively charged focusing and collimation electrodes, even within ultrahigh vacuum. In fact it is typically necessary to bake the equipment for hours at 200 C to remove water if a "dry" vacuum is required. But fluid inclusion volatiles introduce copious quantities of water and the system is operating in a "wet" vacuum. The H⁺ ions produced in the ionizer have random velocities and must be collimated to produce the beam of particles for use in the mass separation stage. Mass spectrometer manufacturers estimate that only about 1% to 3% of the H⁺ ions contribute to the beam, the remaining 97% to 99% of the H⁺ ions are attracted to and impact the negatively charged surfaces of the apparatus. But these surfaces are all coated with a multi-molecular layer of water, resulting in high energy collisions of H⁺ ions with H₂O molecules on the electrode surfaces. These collisions produce H₃O (hydronium), a species which is common in interstellar space which is an environment not dissimilar to the mass spectrometer vacuum (Wikipedia). Hydronium is unstable and decomposes to produce H₂ (mass 2) which is then ionized, causing H₂ analyses which are nothing but spurious analytical interference caused by the interaction between H⁺ from water ionization and water molecules in the ionizer. The long mean-free-path of ions in the vacuum is irrelevant as the interaction occurs at the water-coated, negatively charged electrodes where interaction between water and H⁺ is certain and frequent.

H₂ analyses of aqueous fluid inclusion volatiles using mass spectrometry are meaningless and misleading. They should not be reported as results and nor should such H₂ results be used to infer or calculate the redox potential of such fluids.

REFERENCES

Burlinson 2012, Hydrogen analysis of fluid inclusions by mass spectrometry is inadvisable: (http://appliedminex.com/h2)

Burlinson 2013, Discussion of Ore genesis constraints on the Idaho cobalt belt from fluid inclusion gas, noble gas isotope and ion ratio analyses: Economic Geology Sept.-Oct. 2012, V107, #6, P1189 (here)

Hoffman, Philip, Residual gas analysis (mass spectrometry): (http://philiphofmann.net/ultrahighvacuum/ind_RGA.html)

Rabiei, M., Chi, G., Normand, C., Davis, W.J., Fayek, M., and Blamey, N.J.F., 2017, Hydrothermal rare earth element (xenotime) mineralization at Maw zone, Athabasca Basin, Canada, and its relationship to unconformity-related uranium deposits: Economic Geology, v. 112, p. 1483–1507.

Wikipedia, Hydronium: (http://en.wikipedia.org/wiki/Hydronium#Interstellar_H3O.2B)

A pdf copy of this discussion is here.

In their reply the authors (here as a pdf)  have misunderstood the issue which is the analytical method, not the existence of H₂. They present laser raman spectrographs to document the existence of H₂ in two uranium deposits, Gryphon and McArthur River. This is very interesting because it appears they prefer to trust H₂ analysis by laser raman over mass spectroscopy, which is exactly my point! The presence of H₂ in uranium deposits is not a complete surprise and I have previously referred to other such work documenting H₂ in Uranium deposits by Dubessy and Pagel, 1988. The H₂ is understood to be caused by the radiolytic decomposition of water.  The problem is the doubtful validity of  mass spectroscopic analyses for H₂. As already discussed, the ionizer of the mass spectrometer generates H₂ as a byproduct of the ionization of water. This seems to be a non-stoichiometric process which cannot be corrected mathematically during the analysis. (Direct production of H₂⁺ during ionization of water does also occur, but at a very low level and the relative coefficient of production of this ion species (about 0.1%) is documented in my previous discussion here. This interference can be corrected mathematically during the analysis.)

I have previously discussed (here) the many observations of H₂ in residual gas measurements of vacuum systems. Residual gas analyzers are mass spectrometers used to determine the "quality" of vacuum systems. These analyses frequently show H₂ although only air is present and the instrument manufacturers state that the H₂ is due to water in the residual air in the vacuum. However traditional ionization doctrine cannot explain the generation of this H₂ from water.  I have also presented and discussed (here) a mass spectrometric analysis of thermally decrepitated inclusion fluids from my own quartz sample, as analysed by professor D. Gaboury of the University of Quebec at Chicoutimi. This analysis has an extreme amount of H₂ which cannot realistically be attributed to primordial H₂. The release curve of the H₂ is strongly correlated with the H₂O release curve which indicates that the H₂ is an ionization product of H₂O within the mass spectrometer.

And I have also previously discussed the work by Norman and Sawkins (1987) who realized that their hydrogen analyses by mass spectrometry were highly suspicious and proposed that a chemical reduction of water during decrepitation generated this hydrogen as an analytical interference. Such a reaction is thermodynamically improbable as I have explained here and it is likely that H₂ was produced by the ionization of water in the mass spectrometer ionizer. Although the authors were astute enough to question the validity of the mass spectroscopic analysis for H₂, they failed to understand that it was simply the ionization of water which produced this H₂.

There are many mass spectrometric analyses which show unexpectedly high H₂ contents in aqueous geo-fluids or in air. These unexplained results point to a previously undocumented problem in the application of mass spectrometry in the analysis of H₂ in aqueous fluids. The existing theory of the ionization of water is not wrong, but is incomplete as it fails to understand a critical side reaction between H⁺ and H₂O in the mass spectrometer ionizer, which converts H⁺ from the ionization of water into H₂, giving spurious and irrelevant analyses for H₂ which cannot be corrected mathematically.

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