Dear David, Myq, Anna, Lola, Steven, Andrew, Deborah, Philip, Rolf,
Rebecca, and whom it may concern,

I succeeded in digging up the results of the study of effects of oven vs
freeze-drying on the nitrogen isotopic composition of dissolved NH4 or
NO3 ...
My objective was
1) to test whether there is a more rapid, easy to handle method for the
preparation of isolated inorganic N forms for EA-MS by only
precipitating salts
2) to get an idea, if NH4NO3 might be a candidate for determining the
isotopic composition of one of the two forms if only very low sample
amounts are available -  some "isotope dilution technique" (with large
differences in the nitrogen isotopic composition between the to be
determined N form and the other known one) ...

Therefore different N-salts were choosen as standards and (as
triplicates) dissolved in bidest. water
to yield final amounts of either
1)  500 µg (microgramms) N or
2)   50 µg (microgramms) N in the solution.
This solution was evaporated in
a) an ordinary oven at 60°C (sixty degrees celsius, some browsers seem
to have problems with the °)
b) a freeze-dryer at reduced pressure and room temperature.
After complete evaporation the precipitated salts (different crystal
forms and sizes ...) were scraped out and transferred to the tin capsule
... and EA-MS-ed.

The results were ... ambigous ...
Of the standards used, Ammoniumacetate, Ammoniumbenzoate,
Ammoniumdihydrogenphosphate and Ammoniumhydrogencarbonate didn't
precipitate as distinct crystals.
For the nitrates, both oven- and freeze-drying seem to work fine at
higher sample amounts, but freeze-drying seems to result in problems
with only 50 µg (microgramms) N.
For NH4NO3, both methods seem to be o.K. if the final amount is 500 µg
(microgramms), but give erratic results, if only 50 µg (microgramms) are
to be precipitated.

But here you are, the data is in the format: (no nice layout and no
attachment - it is a list to a list ...)
sample name:d15N(number of mesurements/std-dev)
NH4NO3: -1.64(3/0.08)
NH4NO3-500µgN-oven: -1.78(4/0.24)
NH4NO3-50µgN-oven: -0.69(3/0.04)
NH4NO3-500µgN-freeze: -1.86(6/0.13)
NH4NO3-50 µgN-freeze: -2.65(3/0.58)
KNO3: -0.18(3/0.08)
KNO3-500µgN-oven: -0.14(2/0.13)
KNO3-50µgN-oven: -0.20(2/0.11)
KNO3-500µgN-freeze: -0.24(2/0.06)
KNO3-50µgN-freeze: -0.5(1) lower end of linear range of MS...
NaNO3: +2.16(3/0.03)
NaNO3-500µgN-oven: +1.99(2/0.03)
NaNO3-50µgN-oven: +2.06(2/0.08)
NaNO3-500µgN-freeze: +2.05(2/0.02)
NaNO3-50µgN-freeze: +1.7(1) lower end of linear range of MS...
The std-dev of standard gas (90 measurements) before, inbetween, and
after the samples was 0.03, the std-dev of the used IAEA N-1 (8) and
IAEA N-2 (8) was 0.09.
Further steps to include the separation of the inorganic N-forms (by
cation/anion-exchange resins) in the error estimate for the nitrates
will be performed ...

My point of view is that at the moment it is very difficult to give
advise to somebody wheter to oven- or freeze-dry his sediment/soil
samples until every single step in the method (or better: in the
different methods employed today for differing matrices) - i.e.
drying-extraction-separation/destillation-precipitation-measurement - is
documented regarding inherent std-devs and caveats...

Christian.


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Christian Ostertag-Henning          tel: ++49-9131-852-2660 (office)
Institute of Geology                       ++49-9131-852-6243 (lab)
University of Erlangen-Nürnberg         fax: ++49-9131-852-9294
Schlossgarten 5
D - 91054 Erlangen                  email: [log in to unmask]

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