Carolyn,

I've seen exactly this when the copper in the reduction side was 
nearly consumed (except that it does not recover). Of course, you've 
eliminated this possibility by changing out the reactants but it 
suggests that the oxidation is fine and that something is interfering 
with the reduction.

Paul

At 09:08 AM 02/08/2006, you wrote:
>Hi,
>
>We have been running dC and dN on our EA-IRMS (an NA1500 w/zero 
>blank to a Delta
>plus XP) for about two months.  The first month went great; this 
>month has been
>nothing but headache.
>
>The first month's samples were dC and dN on whole rocks (HCl/HF cleaned) and
>bitumens and fractions thereof (sats, aros, etc).  We took what dN results we
>could get, but there wasn't much N to be had in these.  Some of the whole rock
>sample sizes went as high as 20mg.  Key here though is that through all of
>these very large samples, the background mass 30 remained low, and 
>the standard
>deviations for both the dC and the dN results were good.
>
>This month's samples are kerogens in whole rocks, also HCl/HF 
>cleaned, also with
>low N concentrations, and many with some amount of pyrite.  These samples tend
>to be smaller in size, in the <10 mg range, but as we run a carousel 
>we see the
>mass 30 rise until halfway through the batch, the dN becomes nonsense.  Worth
>noting is that the dC for all of these samples is good, both accurate and
>precise.
>
>Standards run in a batch without unknowns will stay good.  Our 
>blanks are truly
>blank.  Initially I was sure that our Cu reactor was spent, but when I removed
>it, the Cu was only half used.  After another 100 or 200 samples I 
>changed both
>reactors again with no improvement in the conditions.  I have also 
>replaced the
>H2O trap, and baked out the packed column overnight.  I have tried adjusting
>the O2 (timing and amount), but we were already seeing a good flash 
>combustion.
>  Except for the increasing mass 30 background, our peaks are really 
> quite lovely
>(symmetrical and tight).
>
>Any ideas what might be causing the increasing mass 30 background?  It
>invariably arrives after about 10 or so unknowns, making the remaining samples
>garbage.  The high 30 background will go down after a few hours, but standards
>run at this point are not as good, and any unknowns will cause mass 30 to rise
>rapidly.  The only recourse seems to be changing the reactors again, but the
>reagents are not used up.  Is the pyrite poisoning our reactors?
>
>Any thoughts or advice?
>Carolyn
>_______________________________
>Carolyn Colonero
>Lab Manager
>Massachusetts Institute of Technology
>Summons? Geobiology Lab
>E34-510
>42-44 Carleton St.
>Cambridge, MA 02142
>Office: 617-253-7850
>Lab: 617-324-4002