Hi Carolyn,
	Whenever I see the mass 30 background increasing I suspect the water 
trap.  Especially a lingering mass 30 background that eventually goes 
away.  Although you have changed out the water trap are you sure that 
the magnesium perchlorate that you are using is really dry.  Here in 
Florida it goes bad even in the bottle if it is not really tightly 
capped and stored in a desiccator.  (The other thing that I suspect is 
the Cu but you have controlled for that also).   Hope this helps, Jason


On Aug 2, 2006, at 12:08 PM, Carolyn L K Colonero 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
>
>

Jason Curtis, Ph.D.
Stable Isotope Mass Spec Lab manager
Associate-In Geochemistry
Department of Geological Sciences
241 Williamson Hall
University of Florida
Gainesville, FL 32611
[log in to unmask], office 352-392-2296
lab 352-392-3344, fax 352-392-9294