Hi all,

From: Lars Lofgren <[log in to unmask]>
Subject: Good method for CO2 from carbonate?


> Dear listmember
>
> I was asked to determine carbonate content in a solid powder and did the
> following with my VG Optima GC-IRMS normally in use for simple breath
> analysis.
> Is this a good method?
> Improvements suggested?
>
> *       100mg solid powder put in 1.8 ml gas tight glass vials
> *       Vials flushed with pure helium in a glove chamber to reduce
> background CO2
> *       200 ul 0.1M phosphouric acid (>10x molar excess) added with a
> syringe via the septa
> *       mix 5 min wait 30 minutes
> *       Headspace gc-irms to get total CO2 and isotope ratios as a bonus

We've been doing basically exactly this with our AP Carbonate Prep for just
about a year now.  You'll probably get good carbon data with this method,
but you could run into some problems with reaction between the acid and the
septa, depending on the septa material, and the temperature they get to.
We've found that the septa used on Labco Exetainers provide no discernible
reaction at 70 degrees over a couple of days.

In order to get good oxygen data, you'll need to (1) get the reaction
temperature controlled.  [The AP system has a very nice hot block which can
take 120+ tubes at 70 degrees and automatically overflush with helium AND
dump in the acid]  (2)  Make your phosphoric acid up to an SG of about 1.93
by adding P2O5 on a hot plate.  This is tricky, and pretty dangerous, so if
you want a technique for it, I can email it to you.  Just ask nicely, 'cause
I've got to type it from paper copy :-)

At 100 degrees, calcite will be fully reacted in about 5 minutes, depending
on grain size.  At room temperature, about 3 hours.  Other minerals, eg
dolomite, ankerite take up to 3 days at 25, and about 1-3 hours at 100.

> Got nice results anyway, but have to be sure I did not do any major
> methodology or practical mistakes

I started out doing this on the AP2003 Breath Prep back in '97, basically
using the technique you are using with the refinement of a hot block and
100, and got great data for both C and O from calcite.  About 0.07 permil
errors on C and 0.09 permil on O.

Good luck!

Andrew

PS   There's some info on AP's system on www.analyticalprecision.co.uk under
Products then Carbonate Prep System if you want a look :)