Hi everyone :)

>         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

We've played with this a lot, having had a couple of very fine grained
carbonate standards which have "gone off" over the years.  For our current
working standard, we had the marble ground and sieved, and took out the
fraction between 85 and 160 microns as being: (1) not too big, where you can
get single grains which are slightly heavier/lighter than others, giving an
odd distribution in data where every once in a while you get a number maybe
0.3 heavy, but never anything 0.15 heavy, if you see what I mean; and (2)
not too small.  Unless you overgas your working standard with an inert gas
between weighings, very fine carbonate powders will react with atmospheric
gases, probably atmospheric water being the worst, giving bad,
unreproducable data, especially for O2.

Oh, this isn't speculative.  We HAVE seen both the above effects in previous
standards.

After creating the 85/160 fraction, we split it between 3 bottles, one of
which is used from day-to-day, and the others can be used to periodically
check that nothing has changed.  We have run thousands of analyses on this
standard now on 'conventional' vacuum lines, our PRISM II with IsoCarb
system, and our AP2003 with AP carbonate prep system.

Hope this is of help,

Andrew

--
Andrew Tait
SURRC
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Scottish Enterprise Technology Park
East Kilbride
GLASGOW  G75 0QF

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