Melanie Leng at our lab. obtained an off-cut of 'Carrara' marble from a
local stone mason ( I think the term Carrara is just used to describe any
pure white marble). Sieved to retain 90-120 micron grain size we find 30
to 50 microgram sample aliquots in our VG Isocarb can give 1 SD
precisions of less than 0.05 per mil or, on a really good day (and
downhill, with the wind behind you), less than 0.03 permil for 10 samples
in an autorun.

We use this material, calibrated against NBS 19 and 18, as our routine lab
standard to correct our unknown samples to the VPDB scale. I have
seen several people refer to the use of NBS-19 itself for this purpose. I
know the amounts that are used are small, but surely this should be
discouraged?

We haven't systematicaly tried the very small (10 microgram) amounts
you ask about, but Mike Hall at Nick Shackleton's lab. in Cambridge runs a
Carrara marble in small amounts on a Prism - so he might have
information.

Tim Heaton, Melanie Leng
NERC Isotope Geosciences Laboratory
Keyworth, Nottingham NG12 5GG

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