Newest
Topics:
For the latest news, see the NEWEST TOPICS page.
Google is too dumb to let me put the list of news in this column and falsely claims that all my pages are self-duplicates.
Google-NONSENSE
Google's so-called "Artificial Intelligence" is an abuse of the concept of intelligence!
Hydrogen analysis of fluid
inclusions by mass spectrometry is inadvisable.
Reported results in the literature
are probably incorrect and should be treated as highly suspicious!
By: Kingsley Burlinson, July
2012
There have been a number of reports in the literature claiming to
have detected hydrogen in fluid inclusions. Some of these have used
mass spectroscopy of fluid inclusions decrepitated into the mass
spectrometer vacuum, either using mechanical or thermal
decrepitation.
But these studies have ignored the
potential for generation of hydrogen within the ionizer of the
mass spectrometer.
Everyone has become so familiar with quadrupole mass spectrometers
as an analytical tool that they have failed to understand the
chemical changes that occur within the ionizer in common use, the
electron impact ionizer. The electron impact ionizer is essentially
a particle accelerator which breaks up molecules. Water,
hydrocarbons and hydrogen sulphide are broken up in the ionizer,
with hydrogen being a byproduct! The monatomic H formed is
converted to diatomic H2 by side effects within the
ionizer. There is a serious risk that any measured hydrogen is
merely an artifact of the analytical method and does not reflect the
hydrogen content in the sample at all. We should NOT accept results for hydrogen analyses by mass
spectrometry without a very thorough consideration of and
correction for the hydrogen generated in the ionizer!
Consequently, the preferred
method to analyse for hydrogen in fluid inclusions should be by
Laser Raman microprobe.
No-one seems to have properly considered the chemical changes
that happen in the ionizer of mass spectrometers. This
ionization process generates hydrogen from water as a byproduct (and
also from other hydrogenous molecules) and this serious oversight
needs to be addressed.
In the analysis of low molecular weight fluids, as found in fluid
inclusions, the ionization required for mass spectrometry almost
invariably uses the common electron impact ionizer, usually run at a
voltage of 70-100 volts. The electron stream of 100 ev electrons
impacts the gaseous molecules, knocking off electrons to generate
the positive ions required by the mass spectrometer. But this often
fragments the parent molecules as molecular bond strengths are much
less than 70 ev, typically only 5 to 10 ev. So the result of
ionization is a mixture of positively charged molecules and numerous
molecular fragments, which may or may not be charged. This
fragmentation is well known of course and is documented in the
commonly used NIST tables which are used in mass
spectrometry analysis. However, these tables do not show molecular
fragments with a mass/charge (m/z) of less than about 15. But just
because hydrogen with m/z of 1 or 2 is not shown in these tables is
no excuse to ignore the fact that it is generated in the ionizer!
The problem is the presence of water and other hydrogenous species,
particularly hydrocarbons. During ionization, these can all fragment
giving rise to hydrogen. Although this has a mass of 1 and may be
uncharged, changes can occur leading to H2+
ions with m/z = 2, which is then wrongly assumed to have come from
hydrogen in the original sample. It is not entirely clear how H*
transforms into H2+ , but it most certainly
does! ( I use the superscript * to
indicate an uncharged free atom.) A possible
mechanism for the generation of H2+ is here.
Ignoring for now the low abundance isotopes of O and H, the
ionization of H2O (molecular mass 18) proceeds by having
an electron knocked off to give 18[H2O]+
.
But this can be unstable and fragments into: 18[H2O]+
>
17[HO]+ + 1H*
.
From the NIST tables for water, we see that the
mass 17 peak is 21% of the intensity of the mass 18 peak, so a large
proportion of 18[H2O]+ undergoes
fragmentation, and there must also be the same large number of
1H* free atoms released as there are of
mass 17 17[HO]+ ions.
There is no doubt whatsoever that abundant hydrogen is generated in
the ionizer from the parent water molecules.
However, these are single atoms with a mass of only 1, rather than
molecules of mass 2 which is used to indicate the presence of H2.
But what happens to all these hydrogen atoms? They are neutral and
so they are not attracted to the electrodes and removed. They are
only removed by the vacuum system. This may not remove them
efficiently, depending on the instrument configuration, and they may
interact and form H2 molecules, which could then be
ionized, measured and wrongly assumed to represent hydrogen in the
parent sample.
This mass quadrupole spectrogram of air, with water, is from "A
user's guide to vacuum technology", Wiley 1989 by John F. O'Hanlon.
The peak at mass 18 is due to water ions. 18[H2O]+
(Used as the 100% reference level)
The peak at mass 17 from 17[HO]+ , (also due
to water) is about 25% of this mass 18 intensity.
And note that there is a substantial peak (about 18%) at mass 2 due
to 2[H2]+ . Surely this peak is
also due to water.
But you clearly must not assume that when measuring an aqueous
fluid, that the hydrogen you measure in a mass spectrometer is
solely due to hydrogen in the source sample fluid.
I have ignored the effect of deuterium of mass 2 in this discussion,
because its abundance is quite low in normal water (only
one atom in 6420, or 156 ppm numerical abundance). Although
this would also contribute to the peak at mass 2, its abundance is
far too small to explain the hydrogen results claimed in too many
fluid inclusion analyses.
How fluid inclusion gases are analysed in the mass spectrometer
To use mass spectrometry for fluid inclusion contents, it is
necessary to open the inclusions trapped within the mineral host
(usually quartz) which releases the volatiles. This must be done
within the vacuum of the mass spectrometer, and the volatiles
released must then be directed into the ionizer of the mass
spectrometer. Two methods of volatile release have been used, either
by mechanical crushing or by thermal decrepitation. In either case,
the release takes some time, up to some 2.5 hours in the instrument described here, which uses thermal
decrepitation at 6° C per minute. The long duration of the
analyses is uncommon in conventional mass spectrometry and could
allow for unusual atomic and molecular interactions in the ionizer
including the generation of H2 from the abundant H
which is present. A possible mechanism is
proposed here.
Hydrogen is an unlikely constituent in fluid inclusions
The main limitation to trapping hydrogen within fluid inclusions is
because hydrogen can easily diffuse through the host minerals and be
lost from the inclusions, even on very short time scales, much less
the many millions of years of inclusion entrapment for geological
samples. It is highly unlikely that substantial hydrogen contents
could remain trapped in fluid inclusions. In addition, hydrogen is
chemically reactive and could easily react with various minerals. It
probably could not even survive unreacted in typical mineralising
systems which are full of oxidants!
Can hydrogen be formed by reactions within fluid inclusions?
Some work (eg D. Norman) has suggested that high hydrogen
results are caused by the generation of hydrogen from chemical
reactions during the analysis. D. Norman suggested that mechanical
decrepitation of the samples is preferable to thermal decrepitation,
to avoid the risk of high temperature reactions, which may
potentially generate hydrogen.
One reaction that D. Norman suggested was:
CH4 + H2O
> CO + 3H2
This reaction can continue further to give CO2 and more H2
This reaction sequence is used commercially to generate hydrogen for use
in ammonia and fertilizer production. HOWEVER, it requires the use
of a Ni catalyst, temperatures of 700° to 1000° C and very
high pressures. The reaction is highly endothermic with
ΔH : +206.1 kJ/mol , and an
unfavourable entropy change of ΔS
: +222 J/°/mol. This reaction is
consequently unfavourable below 940° K (667° C).
It is considered highly
improbable that this reaction is the cause of hydrogen
observed during mass spectrometer analyses of even thermally
decrepitated fluid inclusions, as thermal decrepitation systems
rarely exceed 500° C and there is certainly no catalyst
present!
An example of fluid inclusion volatiles analysis by mass
spectrometry
This analysis was carried out using thermal decrepitation of a
quartz sample known to contain aqueous, CO2 rich fluid
inclusions. (The
instrument is shown here)
Note the astonishingly
high hydrogen analysis (blue graph) which exceeds the CO2
amount! How can this be real for inclusions trapped since the
Proterozoic and from which any original hydrogen must surely have
diffused out? Note that the hydrogen
release mimics the water release with temperature, but NOT the CO2
release! Why would hydrogen preferably co-exist in the aqueous,
liquid phase rich inclusions instead of the gas phase rich (CO2)
inclusions? This is surely
improbable.
The all too obvious explanation is that the hydrogen is being
generated by decomposition of water within the mass
spectrometer ionizer and is not present in the original fluid
inclusions and nor is it the result of chemical reactions during
analysis.
Conclusions
The unexpected high concentrations
of hydrogen in fluid inclusions as measured by mass spectrometer
are very suspicious and unlikely to be real. The most probable
explanation is that this hydrogen is generated as a byproduct of
the ionization of water and other hydrogenous species,
particularly hydrocarbons, in the electron impact ionizer of the
mass spectrometer. A possible mechanism for
this is discussed here.
There seems to be a complete ignorance of this likely problem
during ionization, and all mass
spectrometer measurements of hydrogen content of fluid inclusion
volatiles should be treated as probably erroneous unless
there is a thorough discussion and testing of pure water and
hydrocarbon behaviour in an identical mass-spectrometer and
ionizer for identical analytical durations and temperatures.
But I have seen no such evaluations in the literature and it seems
the fluid inclusion community is ignoring this problem.
None of the discussions in the literature even bother to mention
the ionizer type or operating voltages!
Hydrogen analyses of fluid inclusion contents should only be
analysed by the now more widely available Laser Raman microprobe
method instead of by mass spectrometry. In fact, when fluid
inclusion contents are analysed by laser raman, hydrogen has only
rarely been found, except in some hydrocarbon rich
inclusions (this hydrogen is explained as a laser induced
dissociation of the hydrocarbons) or in uranium deposits (in which
the hydrogen is explained as radiolytic dissociation of water).
Clearly, hydrogen is rarely present in
fluid inclusions and mass spectrometer analyses which claim that
hydrogen is present are probably incorrect and should be
confirmed by laser raman analysis.
Applied Mineral Exploration
Discussion and research relevant to mineral
exploration. 
