Emst, Glen,

I have run many samples for enriched 15N, and have never noticed any
problem with using KHSO4 as the trapping acid.  I find it very surprising
that any SO2 would actually get through a normal purity copper column.  I
have always assumed the KHSO stays precipitated as a salt in the combustion
tube, otherwise there would be a huge SO2 peak in the instrument that would
affect the analysis eventually, and I have never seen evidence of this.  In
ou combustion tubes I have alway used chromium oxide, followed by cooper
oxide, with some silver wool at the end.  Is it possible that this
combination does not let SO2 through?

I would recommend paper disks if possible.  We have generally used a
Whatman #3 which is the thickest, and therefore the most absorbent.  The
glass filters work well too but build up molten glass in the furnace, which
is hard to remove.

Paul.


At 08:38 AM 6/13/01 -0500, you wrote:
>Ernst Witter,
>
>In the past, I looked at microdiffusion methods, and for a number of
>reasons decided not to use them. The acid problem you have had is one of
>those reasons. I am sorry but I do not have a solution for you.
>
>Glen Martin
>
>
>
>
>                     Ernst Witter
>                     <ernst.witter@M        To:     [log in to unmask]
>                     V.SLU.SE>              cc:
>                     Sent by: Stable        Subject:     Suitable acids for
>                     Isotope                15N microdiffusion traps
>                     Geochemistry
>                     <ISOGEOCHEM@LIS
>                     T.UVM.EDU>
>
>
>                     06/13/01 04:33
>                     AM
>                     Please respond
>                     to Stable
>                     Isotope
>                     Geochemistry
>
>
>
>
>
>
>We have been trying out several variations of the microdiffusion method
>for the determination of 15N-labelled ammonium and nitrate in KCl soil
>extracts. Most of the methods published (or discussed on Isogeochem) use
>either H2SO4, KHSO4 or sometimes boric acid to trap the volatilized
>ammonia after addition of MgO (and Devarda's alloy) to the KCl solution.
>Using sulphate-based traps we get an accumulation of sulphur oxides
>(apparantly SO and SO2) in the elemental analyser - IRMS system (Carlo
>Erba coupled to a Finnigan Delta Plus), which are cumbersome to remove
>through heating the GC-column (porapak Q) and changing the combustion
>and reduction tubes.
>I wonder if anyone has tried more instrument-friendly acids, and if so
>which would be most suitable?.
>A related question is the the type of filter paper disc is most suitable
>in this method. Cellulose or glassfibre filter paper?, again having the
>longevity of the combustion tubes in mind.
>
>Grateful for any suggestions,
>
>Ernst Witter
>
>--
>
>----------------------------------------------------
>Dr. E. Witter
>Department of Soil Sciences
>Box 7014
>Swedish University of Agricultural Sciences (SLU)
>75007 Uppsala
>Sweden
>
>Tel +46 18 671237
>Fax +46 18 672795
>E-mail: [log in to unmask]




Paul D. Brooks,
Dept. ESPM-ES,
147 Higard Hall MC 3110,
UC Berkeley, Ca. 94720-3110.

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phone (510)643-1748,
FAX (510)643-5098.