Applied mineral exploration methods, hydrothermal fluids, baro-acoustic decrepitation, CO2 rich fluids
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Thermodynamics shows Au is insoluble in CO2 fluids

Do IOCG deposits form from CO2 rich fluids?

Inclusion shapes can prove heterogeneous trapping

Disproportional FI trapping from heterogeneous fluids explains gas-dominant systems

A discussion of H2 analysis by mass spectrometry

A mechanism to form H2 in the MS ioniser during analyses

Why don't Exploration geologists understand fluid inclusions?

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Studies of 6 Pegmatite deposits

A study of the Gejiu tin mine, China

Data on MVT Pb-Zn deposits, Tunisia

Data from Hall and Mt Hope Mo, Nevada

A magnetite study - Bergslagen region, Sweden

Exploration using palaeo-hydrothermal fluids

Using opaque minerals to understand ore fluids

Decrepitation using Fe-oxide opaques

Understanding baro-acoustic decrepitation.

An introduction to fluid inclusions and mineral exploration applications.



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A proposed mechanism for the generation of H2 from H2O

 in the electron impact ioniser of Mass Spectrometers

To explain the erroneous H2 analyses of aqueous Fluid Inclusions

Kingsley Burlinson  April 2013


Many researchers have analysed fluid inclusion fluids using quadrupole mass spectrometers (MS) to identify the chemical species contained. Some or these researchers have reported the presence of hydrogen in these fluids. But as discussed in another page on this website, such analyses are probably incorrect. Laser Raman analyses of fluid inclusion fluids never find any hydrogen present in these fluids. The problem is that spurious hydrogen is generated in the ioniser of mass spectrometers from water and all other hydrogenous species and this contamination is being ignored. It is assumed that all the ioniser-produced hydrogen is single atoms of mass 1, which do not interfere with measurement of hydrogen molecules at mass 2. But this assumption is unsafe and many spectra of moist air show the presence of hydrogen at mass 2 (example shown here). This hydrogen must be generated within the ioniser but there is no discussion of a mechanism for this.

This discussion suggests a mechanism to explain the origin of spurious hydrogen of mass 2 from the ionisation of water, in the electron impact ioniser of mass spectrometers.

The ionisation of water produces copious quantities of H* and H+ with mass 1. (I use the superscript *  to indicate a neutral free radical atom) It is assumed that because the mean free path of particles in the ultra-high vacuum of the mass spectrometer is very long, these particles have no chance to interact to form H2 with mass 2. Consequently, any H2 (with mass 2) is assumed to have been present in the original analyte as hydrogen gas molecules, rather than being generated in the ionizer. But these assumptions fail to recognize that fluid inclusion analyses introduce large amounts or water into the spectrometer vacuum, which greatly affects the instrument operation.

  1.  The MS vacuum chamber walls and electrodes are all coated with copious amounts of water.

Water is a serious problem in ultra-high vacuum systems as it is "sticky" and adheres to all the chamber walls and prevents attainment of high vacuum. To get a good vacuum, operators resort to baking their chambers at some 200 C for hours to desorb water after any opening of the chamber to change samples! The water adheres to the chamber walls in a layer several molecules thick. This quote is from Kurt. J. Lesker Co, who make residual gas analysers for ultra-high vacuum equipment.

"Water vapor  clings to every surface, many molecular layers thick. As the pressure is reduced, water vapor molecules enter the gas phase but when they hit another surface they are again strongly bound. This makes it very difficult to pump away
"

Because fluid inclusion volatiles are almost always dominated by water, their analyses are done in a "wet" vacuum, in which all the chamber surfaces are coated with water, which is being continually introduced during the analyses. The adhesion of water to surfaces prevents its removal by the vacuum pumps.

  2.  Ionisation of water produces copious amounts of H+.

Electron impact ionisation of water produces H2O+ , which is unstable and can then fragment to produce OH+ and H, or alternatively OH and H+ as either fragment might claim the positive charge. King & Price (Electron ionization of H2O, Simon J. King, Stephen D. Price, International Journal of Mass Spectrometry, 277 (2008) 84–90) document the partial ionisation coefficients for the ionisation of water and show that the coefficient for H+ is 0.240 (relative to the coefficient of formation of H2O+) at an ioniser operating voltage of 100 volts. H+ is also produced during ionisation of all other hydrogenous species in the analyte, such as hydrocarbons and H2S. There is an abundance of hydrogen ions of mass 1. What happens to all this H+?  (The table of partial ionisation coefficients from King & Price is in the appendix, below)


  3.  H+ hits the walls of the chamber and interacts with the adsorbed water.

The H+ is attracted by the electric field towards the entrance aperture of the mass spectrometer, as is the case for all positive ions. But first there are several focusing and collimation stages for the ions to traverse. This cross section of an electron impact ioniser shows 5 focusing stages before entry to the mass spectrometer path. Many ions will hit the focusing and collimation electrodes and not enter the spectrometer path at all. If this were a dry vacuum, the ions would merely stick to the surfaces and cause no trouble. But remember (from discussion point  #1 above), this is a wet vacuum system and all the focusing electrodes and chamber surfaces are coated with water molecules. So the H+ is actually impacting a target of water molecules coating all the electrode surfaces. This is a BIG problem!

This is a diagram of a typical electron impact ioniser. Note the abundant ion impacts (shown in red) with electrodes during focusing and collimation. Mass spectrometer manufacturers estimate that only 1 to 3% of the positive ions generated in the ioniser enter the mass spectrometer aperture. The overwhelming majority of ions are "off-axis" and impact the apparatus electrodes and walls, which are all coated with water molecules.


electron impact ionizer
           (image modified from   http://www.defensetechbriefs.com  )


  4.  Hypothesis: H+ interacts with water adhering to the electrodes to generate H3O, then H2 .

When the H+ ions impact the water molecules adhering to the electrode surfaces, there is a strong possibility that various mechanisms could lead to the formation of H2. One possibility is that it could form hydronium, H3O and then decompose generating H2. The hydronium ion is common in interstellar space (an environment quite similar to a mass spectrometer vacuum!) and decomposes to give OH and H2. Subsequent H+ impacts could convert the OH back to H2O. The H2 generated is not strongly adsorbed on the electrodes and would return to the vacuum where it could be ionised into H2+ (mass 2) and then wrongly assumed to have been an original component of the analyte, when it is in fact spurious contamination. This H2 would have to travel back into the electron beam to be ionised and this step is problematical, but not impossible and would depend on the mechanical configuration of the ioniser. Over the long time frame of fluid inclusion analyses, which can be several hours, the H2 level in the vacuum could become quite substantial as it is only poorly removed by the turbo-molecular vacuum pumps commonly used. The pumping efficiency of these pumps varies exponentially with the square root of the molecular weight, so heavier molecules than H2 would be extracted preferentially, leading to a gradual increase in concentration of the H2 during the analysis.



 Conclusions

The assumption that ionisation of water in the electron impact ioniser produces only H*  or H+ of mass 1 is unsafe as it ignores the very real problem of the presence of copious quantities of water in the analyte. The water adheres to all the walls and electrodes in the vacuum chamber. These aqueous coated and electrically charged surfaces attract H+ ions whereupon interactions can occur, probably resulting in H2. Because the interactions do not occur in the gas phase but on electrode surfaces, the very long mean free path of particles in the ultra-high vacuum is irrelevant. The target water molecules are conveniently attached to all the electrode surfaces awaiting the inevitable impact of the abundant H+ ions, a perfect configuration to produce complex fragments including H2.


It is wrong to assume that the ionisation of water only produces hydrogen atoms or ions of mass 1. Although the electron impact ioniser appears to be simple, the chemistry and interactions of all the ion and free radical fragments it produces is far from simple and must not be ignored as has been done too often.

H2 (mass = 2) is a common by-product from the ionisation of water by the electron impact ioniser of mass spectrometers. This analytical technique cannot be used to determine hydrogen in aqueous analytes because of the generation of spurious H2 by the instrument.


An additional discussion of the errors of mass spectrometric analyses has been published in Economic Geology, August 2013, V108 #5, p1211, together with a reply by Hofstra and Landis.


 APPENDIX


Table of coefficients from the study by King & Price.


Relative partial ionisation cross-sections following electron ionisation of H2O, expressed relative to the cross-section for forming H2O+ , as a function of electron energy E.


E (eV)

µ [H+]

102 * µ [H++ ]

102  * µ [O++ ]

µ [O+ ]

µ [OH+ ]

200

0.261 (11)

0.118 (6)

0.173 (30)

0.0671 (18)

0.315 (3)

175

0.263 (9)

0.119 (18)

0.149 (20)

0.0667 (14)

0.315 (1)

150

0.261 (12)

0.117 (10)

0.109 (11)

0.0651 (18)

0.313 (3)

125

0.255 (12)

0.116 (11)

0.067 (17)

0.0615 (15)

0.310 (2)

100

0.240 (11)

0.113 (8)

0.024 (3)

0.0540 (13)

0.305 (3)

85

0.225 (12)

0.112 (5)

0.005 (3)

0.0466 (17)

0.299 (2)

75

0.209 (9)

0.113 (12)

0.000 (3)

0.0402 (15)

0.293 (3)

65

0.185 (8)

0.109 (8)

0.001 (1)

0.0328 (11)

0.285 (2)

60

0.174 (8)

0.109 (16)

0.000 (1)

0.0292 (18)

0.279 (2)

55

0.160 (8)

0.107 (12)

0.000 (1)

0.0255 (16)

0.273 (2)

50

0.141 (8)

0.107 (12)

0.000 (1)

0.0202 (11)

0.262 (2)

45

0.124 (7)

0.109 (8)

0.000 (1)

0.0159 (10)

0.252 (4)

40

0.109 (6)

0.104 (11)

0.000 (1)

0.0101 (25)

0.239 (5)

35

0.089 (6)

0.096 (12)

0.000 (1)

0.0052 (8)

0.218 (3)

30

0.068 (5)

0.076 (5)

0.000 (1)

0.0013 (9)

0.184 (4)


The value in parenthesis indicates two standard deviations in the last figure.