Dear Isotopers,

I am almost a newcomer in the stable isotope field, so I do not 
know the traditional background of H3PO4 preparation. However, 
I have had excellent results without using H2O2 or any Cr 
compound. I just heat a batch of H3PO4 to 150 oC and add P2O5 
before every sample set. The acid is introduced to the two-finger 
reaction tubes while it is still warm (70-80 oC). In my experience it 
is quite difficult to store the acid without ANY possibility of water 
contamination, it needs a dry box. The dry box needs space and 
can be costly. Additionally, the acid can take atmospheric moisture 
 when introduced to the vessels, which is not so effective if the acid 
is still hot. And last, it is much easier to pump to vacuum if the acid 
is warm. With this acid preparation my reproducibilities are usually 
better than 0.1 permil. The lack of H2O2 and CrO3 caused no 
fractionation, I obtained exactly the theoretical values for the 
Harding Iceland Spar "standard".

Cheers,
Attila
********************
Dr. Attila Demeny
Laboratory for Geochemical Research
Hungarian Academy of Sciences
Budapest, Budaorsi ut 45, H-1112, Hungary
tel/fax: 36-1-185-1781 or -319-3145
e-mail: [log in to unmask]