Hi Richard and all,

I suppose there will be a lot of replies to this, and if there aren't, there
should be. Your standard is an excellent idea, but as of now sounds more like a
Guy Fawkes (British) gunpowder plot - far to fine grained.

You are getting into the strong force field between those who think the
reference material is always right, and those who think they themselves, and
their machines, are always right. Those latter ones then blame the reference
material, and rightly so, because it has been shown in many cases that fine
grains exchange or adsorb enough stuff to give  significantly wrong answers.
Before entering this minefield, therefore read the literature. [If we don't read
the literature, everything that has gone before is, unfortunately, wasted -
these days there are apparently not enough employed scientists to pass knowledge
on directly]. You already mentioned Friedman and O'Neil. Also look at the IAEA
reports, for example Gonfiantini, "Advisory Group Meeting on Stable Isotope
Reference Standards .... Vienna 1983". These are far from being outdated. Get in
touch with Manfred Groening, IAEA,who is on this list: and also with Ty Coplen(
who will probably reply anyway!)

As for the actual problem with your fine grained "standard", only briefly.  Of
course you have checked homogeneity of the original material before crushing?
Some are very poor candidates, which is why generally one uses a high grade
marble.
The GRAIN SIZE obviously wants to be "as small as possible", but also free of
the finest fractions, which are subject to exchange with atmospheric moisture,
etc.The official reference materials we now have are therefore SIZED, somewhere
in the region of 0.1 mm I believe. This has to be checked up. Initially
homogeneous crystalline grains of 0.1 or 0.2 mm will be virtually inalterable
under normal conditions. In fact after a spillage, you could almost wash and dry
them.

If you need more info lets know. And have fun!

Peter Blattner
c/ Inst. of Geological and Nuclear Sciences
     PO Box 30368
     Lower Hutt
     New Zealand










"Richard M. Larson" <[log in to unmask]> on 30/04/99 12:31:07

Please respond to Stable Isotope Geochemistry <[log in to unmask]>



 To:      [log in to unmask]

 cc:      (bcc: Peter Blattner/GNS)



 Subject: LS for 13C Standard









Howdy All-
        In the interest of creating my everyday working carbonate standard,
I've ground hand samples from a local limestone to a fine powder .  I guess
geomorphologists would call the resulting texture rock flour.  I figured the
finer the grind, the easier it would be to achieve homogeneity.  It has
since been pointed out to me that too fine a grind might lead to a poor
standard (we haven't had a chance to start analyzing it yet).  Friedman and
O'Neil (1977) make mention on page KK5 that the Solenhofen LS (NBS-20) is
"too fine grained to be a good and lasting standard."  Does this have to do
with potential contamination of atmospheric vapor, difficulties in handling
the standard, or other factors?  Can anyone suggest a suitable "grain size?"
If my lab RH is about 17%, and I store the standard in a dessicator, do I
even need to worry (excessively) about potential atmospheric vapor?  Thanks
in advance for any insights.   Best Regards-  Mark


""""""""""""""""""""""""""""""""""""
  R. Mark Larson
  Research Scientist
  UWYO Stable Isotope Facility
  Dept. of Renewable Resources
  University of Wyoming
  Laramie, WY 82071-3354

  Email:     [log in to unmask]
  Phone:     307.766.5203
  Fax:          307.766.6403