Applied Mineral Exploration
Discussion and research relevant to mineral
exploration. A Discussion about H2 analyses reported by Rabiei et. al., 2017
Hydrogen analyses by mass spectrometry are probably incorrect;
laser raman analysis is preferred.
Kingsley Burlinson 2017
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. 14831507.
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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