Hi Steve
I have seen this before with wood samples where the N qty was about 0.1%.  What you are seeing is an explosion of material where a part of the sample gets blown upwards in the explosion and eventually most the "cloud" gets pushed back in by the helium.  The problem is that some of that cloud gets to the autosampler.  When you go back to C analysis you may get some strange numbers from the samples picking up traces of soot.  This is really bad with the Zero blank Autosampler since it is open all the time to the reactor where the cloud can put a thin, sometimes invisible film, of soot in the tray. Ways around it:
1) Change the sample time to drop earlier than optimum "flash" conditions.  This will cause the sample to glow and burn more slowly but still fairly well.  No explosion.  C-4 must be a blast to analyse in this.
2) Reduce the sample size by a factor of three and triple the gain resistor on your Delta.  This has shown to work very well on the new electronics (Delta XP, V, 253; TOC-IRMS paper in L&O: Methods accepted, Chris Osburn and myself)) that are quieter by 2 orders of magnitude better than the older one.  I have not fully tested this on the older Delta Plus electronics. What you want is:
Mass    Normal    New Gain
28      3E8        1E9
29      3E10       1E11
30      1E11       1E11 (no change)

Chris did some preliminary work with N2 by EA and said it looked really good providing the blanks are low.

Gilles


Gilles St-Jean
Chercheur / Research Scientist
Université d'Ottawa / University of Ottawa
Sciences de la Terre / Earth Sciences
140 Louis Pasteur, Ottawa, Ontario, Canada K1N 6N5
Tel: 1-613-562-5800 xt 6830 (Bureau / Office)
                    xt 6839 (Bureau / Office Lab)
                    xt 6836 (IRMS lab)
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Toile / Web: www.isotope.uottawa.ca

-----Message d'origine-----
De : Stable Isotope Geochemistry [mailto:[log in to unmask]] De la part de Stephen Taylor
Envoyé : 11 avril 2007 11:29
À : [log in to unmask]
Objet : [ISOGEOCHEM] EA - 15N : question for the list

Hi all,

We've recently been analyzing 15N in bitumen samples (it is Alberta 
after all ... ) and I have 2 questions for the list:

1) does anyone have an oil standard for which a d15N value is 
established? Are there any d15N values for NBS 22 out there? FYI: Our 
samples are low in [N], ~0.2 to 0.4%, and to get a decent peak we are 
weighing ~7 - 10 mg of material.

We've noticed a strange phenomena at combustion. (FYI : we are using 
large tin cups, 20ml O2 loop, a relatively fast helium carrier flow 
(~120 ml/min) and to deal with the CO2 & H2O we have inline a Nafion 
drier, a large ascarite trap,  Conflo dilution and a lengthy acquisition time.

2) So ... the Flash is beautiful, but is followed by a dark "cloud" 
followed in ~10 sec by another "softer" flash ... more like a intense 
glow, before returning to the usual orange/red glow. The N peaks are 
symmetric, ~ 700 to 1000 mV, exhibit only a minimal amount of tailing 
(the 29/28 ratio returns to baseline before the diluter kicks in), so 
I'm assuming the burn is sufficient? Solid standards interspersed 
between the bitumen samples show no memory effect.

I'm just curious about the "two stage" burn. Has anyone else seen 
this phenomena when burning material with large C:N?

Cheers,
Steve

Stephen Taylor
Lab Manager, Isotope Science Lab,
University of Calgary, Physics & Astronomy,
2500 University Dr. N.W.
Calgary, Alberta,
Canada, T2N 1N4
(p) 403 220 8268
(f) 403 220 7773
[log in to unmask]
www.phas.ucalgary.ca/isl