We routinely run carerra marble in the 15-30 ug range using an Optima and
auto common acid bath system. Some time ago, we obtained a couple of
'hockey pucks' of carerra marble from a quarry core and we seived chips
down into the 150-250 ug range. 2-3 chips gives approximately 10-20 ug. In
the 15-20 ug size range, we get precision of better than +/-0.08 and
+/-0.05 per mil for oxygen and carbon respectively. Above 20 ug, the
precision runs +/-0.05 and 0.03 for O and C (on a good day). I've found
that sieving smaller than this can cause a problem with adsorbed water onto
the marble grains. We now roast our stds with our samples at 360C under
vacuum to eliminate any adsorbed water. This has helped to get our external
precision down. Good luck.

Howie Spero

P.S. Ironically, our in-house carerra marble seems to be more homogeneous
at the low end than does NBS-19.

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   *  Prof. Howard J. Spero     *   e-mail:   [log in to unmask]  *
   *  Department of Geology     *   Office:   (916) 752-3307             *
   *  University of California  *   Lab:      (916) 752-3311             *
   *  Davis CA 95616-8605 USA   *   FAX:      (916) 752-0951             *
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