Hi Attila,
I've noticed similar troubles for us, too. We run carbonates nearly every
single day and drift can be VERY problematic. We run about 44 samples in a
run (before rechecking linearity), and actually measure 10 sample peaks.
Four things we've done to reduce the problem:
1) To save time and mental energy, we use LIMS for light stable isotopes
software developed by Ty Coplen at the USGS to make it easy to correct for
drift in our post-analysis corrections. This saves a lot of grief and works
great, provided that the drift is linear, which in our case it usually is.
2) We set our Extraction 1 and Extraction 2 (aka Extraction symmetry) at
about 40%, which helps our linearity considerably, though we lose a bit of
sensitivity.
3) And perhaps the most important thing we do, we pre-treat all our
carbonate samples to eliminate potential contamination from organics that
may be in the samples. This has incredibly improved our repeatability and
yield from samples. Plus, by getting the organics out, our nitrogen peaks
are smaller, if not non-existent, and I'm not concerned about accidentally
getting sulfur or other 'sticky' molecules in the instrument. This allows
us to comfortably measure as much as 2mg of carbonate in a sample. We only
need about 0.1mg of pure carbonate to get a good analysis. I'd be happy to
send you our chemical pre-treatment protocol if you want it.
4) The last thing we did, which required some experimentation, was to quit
using the Labco Exetainers. We now use a 4mL tube and caps with a silicon
septum lined with teflon toward the inside of the tube. The troubles with
the exetainers were, a), the opening at the top of the cap was too narrow
to insure that the sample needle did not pierce too close to where the acid
needle pierced, and b), the septum material of the exetainers actually
reacted with acid to generate a gas. We had continual troubles with drops
of acid remaining on the inside surface of the septum, then acid getting
drawn into the GasBench and making a mess of everything. To further
alleviate this problem, we actually stopped adding acid and sampling in the
same sequence. We now add acid first (either using the acid pump or by
hand), mix the acid and sample using a vortexer, then return the samples to
the GasBench for analysis. I can send you a more detailed explanation of
our protocol if you want.
As is the case for you, waters on the GasBench run great for us. Carbonates
are just difficult.
I hope this is of use to you. ...and if anyone has any comments on what
we've done, I'd also be happy to hear them.
Cheers,
~Penny
At 05:31 AM 12/9/2005, you wrote:
>Dear All,
>
>We struggle with a strange phenomenon with our new delta + XP & Gasbench
>III. It worked perfectly with carbonates, but about a month ago the d13C
>and d18O values started to shift during analyses. In a set of 40 samples
>(including standards) the d13C and d18O values shifted by about 1 permill,
>in opposite directions (d18O positive, d13C negative). Then, the shift
>turned to a sinusoidal drift, again C and O isotopes moving in opposite
>directions. We checked the system with pure CO2, the analyses were
>perfect. We changed the acid, no improvement. After a set of analyses, the
>samples were re-analysed the next day, the shift was much less significant
>(about 0.3 permill, nice linear, correctable).
>
>The system was tuned to linearity, and there is no correlation with sample
>size.
>Originally we analysed 7 sample peaks, then changed to analysing 4 peaks
>(to reduce GC & Nafion saturation). No improvement, the analyses are
>getting worse and worse.
>
>d18O and dD on waters (equilibration with CO2 and H2) work nicely, so the
>problem is related to the carbonates.
>
>Can you give any hint what the cause of this strange behaviour may be?
>Thanks a lot in advance.
>Attila
>
>--
>*********************************************************************************
>Dr. Attila Demeny
>Geokemiai Kutatointezet - Institute for Geochemical Research
>Magyar Tudomanyos Akademia - Hungarian Academy for Sciences
>Budapest, Budaorsi ut 45, H-1112, Hungary
>Tel: (361-)309-2600/ ext. 1223 (office), 1394 (lab)
>Tel/fax: (361-)319-3137
>e-mail: [log in to unmask]
>Http://www.geochem.hu/
>*********************************************************************************
*******************************************************************
Dr. Pennilyn Higgins
Research Associate
"SIREAL"
Stable Isotope Ratios in the Environment Analytical Laboratory
Department of Earth and Environmental Sciences
University of Rochester
227 Hutchison Hall
Rochester, NY 14627
[log in to unmask]
Office: 209b Hutchison Hall Lab: 209 Hutchison Hall
Voice : (585) 275-0601 Outer lab: (585) 273-1405
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