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Following on from Tim and Wolfram's contributions, an investigation by Abe and Yoshida (Rapid Commun. Mass Spectrom. 2003; 17: 395-400), of whether direct introduction of air into the mass spectrometer could be used for precise 17O/16O and 18O/16O measurements of the molecular oxygen component, also showed (using prepared mixtures of N2-O2 and Ar-O2) that increasing the Ar/O2 ratio resulted in increases of both delta-33 and delta-34 values to a much greater extent than was observed with N2-O2 mixtures. This demonstrated that formation of NO in the ion source could not be a major factor in causing the increase in delta values.

The authors speculated that, when interfering gases such as N2 or Ar are present with O2, the mean free path of ionized O2 molecules may be reduced and isotope fractionation during admission to the flight tube may consequently be decreased. It was pointed out that this hypothesis can also explain why the presence of Ar ('heavier' than N2) results in greater shifts in delta-34 and delta-33 values than does the presence of N2.

Regards to all,
Martin

Martin F Miller
British Antarctic Survey,
High Cross, Madingley Road,
Cambridge CB3 0ET, UK
Tel: +44 (0)1223 221537
E-mail: [log in to unmask]<mailto:[log in to unmask]>


From: Stable Isotope Geochemistry [mailto:[log in to unmask]] On Behalf Of Wolfram Meier-Augenstein
Sent: 24 June 2011 11:39
To: [log in to unmask]
Subject: Re: Nitrogen isotopes on volcanic gases


One of the reasons why the concurrent presence of O2 in the ion source causes problems with the isotope ratio measurement of N2 is the fact that both gases will react with each other in the ion source to form NOx.

The effect of argon on the isotope ratio measurement of nitrogen as mentioned by Tim cannot be explained as easily. However, the same (?) effect can be observed when analysing CO2 while argon is present. Depending on amount of argon present, on can observe d13C values as low as -60 o/oo for a CO2 sample with a known d13C value of -30 o/oo.

I suspect the same effect is at work when the concurrent presence of N2 in the ion source interferes with the isotope ratio analysis of H2.  Not only become observed d2H values more negative than they should be, the ion yield of H2+ (i.e. peak / signal size) decrease dramatically.

We had a look at this some time ago and named it the "IQ effect" as in "ionization quench" since this seemed to be the most plausible explanation for an effect not caused by isobaric interference with the actual isotope ratio measurement.

The paper was published by RCM in 2009, Vol. 23, pages 2011-2016


Best,

Wolfram

________________________________
From: Stable Isotope Geochemistry [[log in to unmask]] on behalf of Heaton, Timothy H.E. [[log in to unmask]]
Sent: 24 June 2011 10:07
To: [log in to unmask]
Subject: Re: [ISOGEOCHEM] Nitrogen isotopes on volcanic gases
On the same theme, the presence of trace amounts of argon mixed in with atmospheric N2 also has an effect (see Mariotti 1984 in Nature, 311, 251-252). Argon being much more difficult to remove than oxygen.

Tim H.E. Heaton
NERC Isotope Geosciences Laboratory
British Geological Survey
Keyworth, Nottingham NG12 5GG, England
(www.bgs.ac.uk/nigl/index.htm<http://www.bgs.ac.uk/nigl/index.htm>)
Tel. +44(0)115 936 3401
Email: [log in to unmask]<mailto:[log in to unmask]>

From: Stable Isotope Geochemistry [mailto:[log in to unmask]] On Behalf Of Eleazar Padrón
Sent: 24 June 2011 00:06
To: [log in to unmask]
Subject: Re: [ISOGEOCHEM] Nitrogen isotopes on volcanic gases

Thank you Joe for this valuable information,

Regards

Eleazar

El 17/06/2011 20:21, Joe Orchardo escribió:
Eleazar

The isotopic analysis of mixed gases by dual inlet mass spec was pioneered by Michael Bender and his group at the University of Rhode Island, Graduate School of Oceanography. An overview of the method can be found in "Elemental and isotopic composition of occluded O2 and N2 in Polar Ice" By Todd Sowers, Michael Bender and Dominique Raynaud. JGR 94 pages 5137-5150. 1989. Also, details can be found in John Kiddon's dissertation at that school.

Basically, as you change the elemental ratio of O2/N2, the measured isotopic value of either N2 or O2 changes, in a linear manner. For instance, if you took air (as your standard) and and a sample of pure N2 and measured the isotopic value of the N2, and then added pure O2 to the pure N2 and measured that sample, the measured 15N value would be different. Add more O2 and you measure another different 15N. Same N2 different measured 15N.

Conversely, the same effect occurs for 18O measurements.

if you want to measure the isotopic values of mixed gases, you need to constrain these effects and make corrections for them.

These relationships of the isotopic value (of either N2 or O2) with respect to the relative amounts of N2 and O2, are unique for each mass spec (we had one MAT 251 and 2 MAT 252's, each with a unique slope (the relationship between Isotope value and changing N2/O2 ratios).

Regards,

Joe Orchardo


On Jun 16, 2011, at 11:15 AM, John Eiler wrote:

The isotope ratios you measure on these sorts of instruments reflect the abundance ratios of the sample, but modified by fractionation that occurs in the source (easily on order of a few percent).  They obviously also reflect differences in detector gain if you are making a multi-collector measurement, but let's set that aside for now.

The source fractionation varies with pressure, chemistry of the gas in the source, and potentially other things (e.g., whether your filament is old and oxidized or fresh; whether the walls of the source are wet or dry). The reason we normally don't have to think about these details is that we standardize by analysis of a chemically identical gas at the same pressure.  It sounds like you are analyzing chemically dis-similar gases. There are groups with experience doing this (e.g., the Scripps lab), but my understanding is they try to make the sample and standard at least similar in chemistry and perhaps apply corrections for mis-matches.

As for interferences, I suspect you are measuring 29/28 ratios.  Both of these are influenced by isotopologues of CO, which is boiled off the filament under any condition and will certainly be made in the presence of a gas of mixed chemistry. I'm sure there are other possible interferences (perhaps fragments of hydrocarbons).

John

On Jun 16, 2011, at 7:59 AM, Eleazar Padrón wrote:

Dear John,

Thank you for your answer. My standard is atmospheric air and the sample is purified in N2, with small amounts of O2, He, Ar, and H2O. I am trying firstly to analyze it without any purification, because the concentration of the main interference, CO2, is around 0.04% in the standard and much lesser in the sample. I do not understand which interference or which fractionation could produce errors in the analysis.

Eleazar

El 16/06/2011 15:48, John Eiler escribió:
Perhaps I misunderstand your post, but from your description it sounds like your samples and standards might differ greatly in chemical composition (i.e., O2, H2O, Ar, etc.). If so, they would undergo significantly fractionations in the source, and might be subject to different sorts of isobaric interferences. Have you made measurements in which both sample and standard are purified N2?

John

On Jun 16, 2011, at 7:29 AM, Eleazar Padrón wrote:

Dear colleagues,

I am starting the analysis of 15N/14N ratio measurements on volcanic gases by means of Thermo MAT 253. I would like to receive some advices about the use of Dual Inlet system to make this type of analysis. I have used atmospheric air as standard, but the results I am obtaining are much heavier than expected. The sample is taken in a evacuated bottle where a 5N NaOH solution retain the CO2 of the volcanic gas.
Can anybody help me on this?

Thank you

Eleazar

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Eleazar Padrón González

Instituto Tecnológico y de Energías Renovables, S.A. (ITER, S.A.)

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Santa Cruz de Tenerife - España (Spain)

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