Adding sugar to nitrates to improve conversion of NO3 into N2 is, in my opinion not the best solution to the problem of analysing nitrates for 15N by EA-IRMS.
Nitrate nitrogen is in the highest state of oxidation (+V) nitrogen can possibly assume. So, immersing NO3 in an oxygen rich environment and expecting to lower its oxidation state does not really compute. Formation of any gaseous nitrogen compounds under standard EA conditions is caused by thermolysis and probably to some degree by reactions of nitrate with the tin capsule.
Sugar (i.e. sucrose) contains per mol 12 mol equivalents of carbon and could theoretically mop up oxygen from 8 mol equivalents of nitrate - if nitrate would be the only source of oxygen (which it ain't in an EA combustion reactor run in standard mode; buzz word: oxygen pulse). Based on the reasoning behind the addition of sugar to nitrate to improve 15N analytical performance graphite or charcoal would seem more appropriate choices as "additives".
However, there is a simple, yet elegant solution - drum roll please: switch off the oxygen pulse. Thermolytic break-up of nitrate in the absence of an oxygen pulse is sufficient for efficient conversion of the resulting thermolysis products into N2 to take place during passage through the reduction reactor.
This elegant solution was published by Rolf Siegwolf's group in 2013 in Analytical & Bioanalytical Chemistry (N Gentile et al. (2013), Anal Bioanal Chem, 405, 159-176; DOI 10.1007/s00216-012-6471-z).
The effect on nitrate conversion and resulting N2 peak (shape) and 29/28 ratio trace when running samples with oxygen pulse vs. without are easily seen in the IRMS traces shown in Figures 4 and 7 of this paper.
From: Stable Isotope Geochemistry [mailto:[log in to unmask]] On Behalf Of Keisuke Koba
Sent: 07 March 2015 03:20
To: [log in to unmask]
Subject: Re: [ISOGEOCHEM] IAEA-N3
Hi Jennifer and Arndt;
You can find the effect of "sugar" to have good results for nitrate with EA-IRMS from this article;
S.R. Silva, C. Kendall, D.H. Wilkison, A.C. Ziegler, C.C.Y. Chang,R.J. Avanzino 2000 A new method for collection of nitrate from fresh water and the analysis of nitrogen and oxygen isotope ratios J of Hydrology http://www.sciencedirect.com/science/article/pii/S002216949900205X
At 12:13 15/03/07, Schimmelmann, Arndt wrote:
>An oxidative EA environment can quantitatively form N2 from ammonium
>sulfate, but not from nitrates. Nitrates need to be reduced to N2.
>Some time ago it was suggested to pre-mix some sucrose with the nitrate
>reference material to solve the problem, namely the sucrose reduces the
>nitrate to N2. Unfortunately I forgot the reference. It had something
>to do with a 'sweet solution'.
>On 3/6/2015 3:03 PM, Jennifer McKay wrote:
>> Hi Everyone,
>> Has anyone noticed "odd" behaviour of IAEA-N3 (potassium nitrate)
>> when running it through the EA-IRMS? I have only used it a couple of times
>> and in both instances the standards at the end of the run were "off".
>> In the first instance the IAEA-N3 values a the end of the run were a
>> tad heavier than those at the start and in the other instance (after
>> sitting in the autosampler for >24 hr) much lighter. No problems
>> (i.e., strange drifts) for the USGS 40 in the same runs.
>> Before starting to run all sorts of tests to figure out what's going
>> on I thought I'd ask.
>> Bye for now,
>Arndt Schimmelmann, Ph.D. | Senior Scientist Indiana University |
>Department of Geological Sciences
>1001 East 10th Street | Bloomington, IN 47405-1405 | USA Ph (812)
>855-7645 | Home (812) 339-3708 | FAX (812) 855-7899
>e-mail: [log in to unmask]
Keisuke Koba [log in to unmask]
Department of Environmental Science on Biosphere, Institute of Agriculture, Tokyo University of Agriculture and Technology Saiwai-cho 3-5-8, Fuchu-city, Tokyo 1838509, Japan
Building #2, rm116 +81-42-367-5951 (tel/fax)
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