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