Hello Antonio,
for nitrogen and carbon isotopes it should not make any difference.
However, if you evaporate water in a hood, make sure that the ambient air
is free of ammonia vapors.  By the way, I found that the Wedeking et al.
method to prepare kerogens is much more labor-intensive than the one
published by Robl and Davis (Organic Geochemistry 1993, vol. 20, p.
249-255).  The latter has also other advantages, namely that it avoids
some chemical artefacts that occur with the use of concentrated
hydrochloric acid (chlorination of kerogen).
                                   Good luck,
                                            Arndt Schimmelmann

On Thu, 12 Feb 1998, [log in to unmask] wrote:

> Hello,
> I am dealing with some carbon and nitrogen isotopes from kerogen matter
> separated from Jurassic black shales. I am using the procedures of organic
> geochemical analysis from Wedeking, Hayes & Matzigkeit ()  and i would like
> to know if an isotopic fractionation could happen evaporating the sample
> instead of dry freezing it.
> I would be grateful if anybody could give me some tips.
> Thank you
> Antonio
>
> PS: Ciao Zac!
>