Several people have inquired about a method of diffusing N from solutions onto acid traps for 15N analysis. I use the method of
Brooks et al. (1989, Soil Sci Soc Am J 53:1707-1711), as described by SC Hart, JM Stark, EA Davidson, and MK Firestone in Chapter
42, section 1.2, of Methods of Soil Analysis Part 2: Microbiological and Biochemical Properties. This chapter is written
specifically for sequentially trapping ammonium-N and nitrate-N from KCl extracts of soils for measurement of gross N mineralization
and nitrification. However, I've successfully applied this method with only slight modifications to other types of solutions,
including water samples, TKN digests, and even persulfate digests of K2SO4 extracts. I'm not the ultimate authority on the subject,
but I've been using this method for 10 years for 15N pool dilution work in soils. After you are familiar with the technique, check
out the tips below.

Tips:

1) Though I usually recover 80-90% of N from most solutions, please note that this method is NOT a quantitative for N. The goal is
to get the N out of solution and onto an acid trap for isotopic analysis. Ammonium and nitrate concentrations should be quantified
by a more accurate method, such as with a continuous flow analyzer.

2) This method is more than adequate for enriched samples. However, those concerned about fractionation in natural abundance samples
should diffuse solutions of NIST standards (e.g., ammonium sulfate: 8547 IAEA-N1 delta vs air = +0.4 or 8548 IAEA-N2 delta vs air =
+20.3) alongside their samples.

3) To prepare the cellulose disks, use Whatman #3 filter paper (90 mm D), acid wash it in a vacuum filtration apparatus, then oven
dry it. Use a clean hole punch to cut out about 100 disks per filter paper. Get a spool of SS welding wire (308, 0.035 inch D) and
cut pieces to fit tightly across the inside the top portions of specimen cups (we use Fisherbrand 4 oz nonsterile, cat. no.
14-375-148). Acid wash the wires. Pierce the disks with the wires and stand them up in the perforations of a porcelain dessicator
plate (230 mm Dwith 2 mm holes), with a cardboard disk beneath it, fit inside a large dessicator (we use Nalgene polycarbonate
dessicators) with dessicant in the bottom. Acidify the disks in the dessicator using a repeating pipette to add 10 microliter KHSO4
per disk. If you label every other hole in the dessicator plate in a circular pattern from outside in, you can use it to keep track
of as many as 160 wires when the diffusions are complete.

4) Why use KHSO4 instead of H2SO4 for the acid traps? It's less corrosive to the tin capsules you put the traps in. The trapping
capacity of KHSO4 is ample for direct combustion 15N analysis, so there's no need to use H2SO4.

5) To separate ammonium from nitrate, diffuse ammonium first (by adding MgO to the solution) and remove the trap. Leave the lid off
the sample cup overnight to allow remaining ammonium to vent off before diffusing nitrate (by adding Devarda's alloy).

6) BEFORE starting an experiment, always inquire with the lab that will be running your samples about the target ranges for
micrograms of N per sample and atom % or delta 15N. Then design your experiment to achieve those target ranges. Avoid generating
samples that are excessively enriched - like over 2-3 atom %. Why? There are no NIST standards that go that high and you run the
risk of saturating the 29 cup on the IRMS detector (I speak from experience!).

7) What do you do if you have a solution that has plenty of nitrate but very little ammonium? Avoid spiking samples if at all
possible. One strategy is to use a volume of solution that contains enough ammonium for detection - say 50 microgram N -  and
diffuse all the ammonium onto a single disk. Then diffuse the nitrate onto several disks on a single wire, adjusting the number of
disks so that each one will be in the detection range of the instrument. I've used as many as 7 disks on a single wire.

That's it for now...
-Bill

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Bill Holmes, Research Associate
Terrestrial Ecosystem Laboratory
University of Michigan
School of Natural Resources and Environment
430 East University, Room G540
Ann Arbor, MI 48109-1115
[log in to unmask]   Office:(734)647-0459   Lab: 7-0469/7-0481
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