Dear Kinga,

About the carbonate content I realized later on - I had a quick look at
the message first and answered on that - has some danger in it...
You are right on the temperature there - if only analysing volume of
carbonate or carbon isotopes. But normally, if analysing carbonates, one
wants both the carbon and oxygen - while oxygen gives the more
'sensitive' info, e.g. for the paleotemperature if analysed for that
purpose...

Probably I am raised in the 'old-fashioned' way - just flushing a
powdered sample with a dry inert gas does not remove the adsorbed water
from the sample. Amount of water adsorbed is depending on the humidity
of air and the total surface (grain size) of the samples, and can be
considerable in volume. This worries me in more of the automated
systems, if samples are not degassed correctly and biassed by adsorbed
water.
Maybe I am too much a 'perfectionist' in avoiding all possible
errors/deviations when analysing - this was the philosophy I was raised
with...

Indeed if working with the less dense phosphoric acid water is
introduced anyway, so then you do not need to care on adsorbed water
then. I do not support that idea. It is an old discussion about
phosphoric acid - on isogeochem list this already was discussed without
leading to any argreement in the preparation of acid to use - I just
remind on the remark (I thought it was by Bob Clayton or was it Herman
Craig?) that the original fractionation calibration is on the 'dense'
phosphoric acid prepared according to the way by McCrea and with the
'Cr' (the green acid). Any deviations from that acid might change the
fractionation of the process of carbonate digestion - the fractionations
of all the processes occuring are not known, if the processes are
anyway. I think it is very important to have a standardized density for
the phosphoric acid prepared in a similar way - the acid polymerizes by
releasing water when becoming denser. Continuous extraction of water
just results in more polymerization and thickening of the acid,
rendering it useless in the end. An agreed and fixed density at least
will create a standard reaction, making comparison of results from
different laboratories less a question mark.
Funny everybody tries to have a solid calibration in sense of MS
measurement without caring about any standardization or inter-laboratory
calibration on the techniques used - different techniques may result in
diferent values, and if a linear deviation it only needs a simple
correction, but if not, things become more complicated..... And what is
a calibration anyway if realizing in the on-line systems becoming
increasingly favourable (for good reasons) nowadays the reference gas is
introduced at different parts of the system by different workers! Just
an example....

If the proper acid (say the 103 percent one) is too thick for the
automated system, then the automated system should be re-thought and
changed, not the acid to make use of the tool possible... It is
remarkable that carbonates analysed by other tools - e.g. by laser (the
Sharp and Cerling method) or in the newly published EA-py-CF-IRMS sytems
give values different by about a permil (if I remember correctly - have
to look that up again for the correct value - it is certain they deviate
from the acid digestion method).

I hope I am not kicking to much against some 'holy houses' with this - I
try to bring forward the unsolved problem of the phosphoric acid -
everybody seems just to do as he/she likes... not a good situation for
proper comparison of analytical values.

Best wishes,
Pier de Groot.
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Dr. P.A. de Groot
University of the Witwatersrand
Economic Geology Research Unit
Department of Geology
Private Bag 3
2050 Johannesburg
South Africa
Tel. +27 11 7162564
Fax. +27 11 3391697
E-mail <[log in to unmask]>

Visit the combined HOME-PAGE of EGRU-Geology on the Internet:
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