Applied Mineral Exploration
Discussion and research relevant to mineral
exploration. Mass spectrometric analysis of water confirms the generation of H2 (mass 2)
H2 is an instrument by-product
in the electron impact ionization of water in mass spectrometers
With contributions by Dr. Thomas J. Barrett of the University of Sussex.
May 2026
In other discussions on this site I have shown that H2
(m/z=2) is a by-product of the presence of water in the analyte
during electron-impact ionization in the mass spectrometer and
have proposed a mechanism for its formation. H2 from water
Consequently mass spectrometry cannot be used
to measure H2 in aqueous analytes.
In discussions with Dr. Thomas J. Barrett of the University
of Sussex we have further examined the potential source of the H2
and Dr. Barrett also carried out a mass-spectrometric analysis of
water at various humidities during a 4 hour analysis.
It was pointed out that H2 is known to be generated by
reaction between water and the ionizer filament. This does occur
when using a tungsten filament, however only minimal H2
(or none) is generated when using the now almost universal Yttria
(or thoria) coated rhenium filaments.
Dr. Barrett performed an analysis of water at humidities of 22%,
35%, 56% and 86% preceded and followed by background measurements
of dry nitrogen for an hour. His data included mass fractions 1,
2, 16, 17, 18, 20 and 32. However the humidification water used
was tap water and surely contained dissolved oxygen, which
prevented an attempt to observe the relation between H2
and O. Consequently I have re-plotted his data here with only
masses 1, 2 and 18. Masses 16 ( O+ ) and 17
(OH+ ) give mostly identical results as mass 18 (H2O+).
This spectometer experiment was carried out by Dr. Thomas J. Barrettt of the University of Sussex, November 2025.
Clearly there are substantial amounts of H2 mass 2 present at all humidities and none in dry nitrogen at the start of the experiment. This H2 must be a by-product of the ionization of the water and was not a component of the analyte. This H2 is a function of, but not directly related to the humidity. Any attempt to measure H2 in an aqueous analyte cannot merely use a "correction" adjustment to compensate for this instrumental artifact. The presence of H2 in the analytical result cannot be used to imply that molecular H2 (mass 2) is present as a separate species in an aqueous analyte.
Because the H2 result shows delayed response to changes in the humidity, I infer that the generation of H2 occurs by a multiple stage mechanism, consistent with the mechanism I have proposed here. (proposed mechanism for the generation of H2) At the zone 1->2 change, there was an accidental excess of water introduced. The H2 response was delayed from the water response and took some time to increase and overshot the eventual equilibrium point for 35% humidity. At the increase to 86% humidity from zone 3 ->4 the H2 response was again delayed from the water response. At reductions in humidity between zones 2->3, 4->5 and 5->6, there was also a delayed decay in the H2 response. Note that there was an instrumental problem in changing from zone 4->5. There was a single point analysis of mass 32 O2 (not shown on the plot) with relative ion current off scale and above 3, suggesting accidental introduction of oxygen (from air?). This diluted the humidity and reacted with H2 and destroyed the expected decay response of H2. At the change back to dry nitrogen from zone 5->6 there was a very distinct slow decay of the hydrogen response for some 15 minutes until the vacuum pump eventually removed the molecular H2. (Turbo-molecular vacuum pumps are much less effective on low molecular weight species such as hydrogen.) The slow increase in H2 at humidity increases and the extended slow decay at humidity decreases is consistent with my proposed mechanism in which water molecules must first be adsorbed or desorbed from the instrument electrodes, where they are subsequently impacted by H+ ions generating hydronium which then dissociates giving H2 (molecular) which can then be ionized and appear to have been part of the analyte when it is in fact an instrumental artifact resulting from the ionization of water.
In an attempt to better understand the mechanism of formation of H2, this plot of ion current ratios was prepared.
Here, ionization products formed by direct ionization of water plot at ratio 1. This is true for ions of mass 1 and 17 and of course, water at mass 18. However H2 mass 2 shows an indirect relationship with water at equilibrium humidities. It does not resolve the actual mechanism of formation of H2 mass 2 but does suggest that my proposed mechanism which incorporates an intermediate "hydronium" step may be correct. I also deduce that H2 is not the result of a reaction of water with the ionizer filament as that would give a ratio of 1.
Conclusions
H2 (mass 2) is formed
during the ionization of water. The observation of
H2 in spectra from aqueous analytes does not imply the
presence of molecular H2 in the analyte as it is an
instrumental artifact generated by an indirect (complex) mechanism
from the ionization of water in the electron impact ionizer.
The H2 is not caused by a reaction between the
filament and water.
Claims by various authors of the existence of H2 in
aqueous analytes, measured using mass-spectrometry, are erroneous
and misleading. It is astonishing that authors have completely
failed to consider the chemical formula of water and the
likelihood of derivation of H2 from the water. The
absence of H2 in the NIST tables for ionization of
water is the result of a narrow understanding of the ionization
process and failing to consider associated indirect mechanisms.