Sample submission - inorganics - NO3- and NH4+ in
soils and water
We use an off-line sample preparation (diffusion) method adapted from
Khan et al. (1998), Sigman et al. (1997), and Holmes et al. (1998) to
extract and collect inorganic nitrogen (NH4+ and/or
NO3-) from water and soil samples prior to stable-nitrogen-isotope
analysis (δ15N) via EA-CFIRMS. Samples
can be prepared here in our lab (for an additional cost), or sent to
us in tin capsules following extraction and diffusion. Below is
the method we use to prepare samples.
Please note: We are in the process of also adopting the anion-exchange
method for nitrate samples (Chang et al. 1997, Silva et al. 2000), so
that eventually we will be able to analyze both δ15N
and δ18O on nitrate.
To see our price list, click here, or contact the lab manager for
more information.
Soil extractions (please scroll down to view Water extractions)
1. Weigh approximately 10 grams of fresh soil into an extraction cup
(we use plastic specimen/urine vials). It is best to start the extraction
process on the same day of collection but, if that is not feasible, the
soil can be stored overnight at 4°C.
2. Using a 1:5 (soil:KCl) ratio, add 50 mL of 1M KCl to each sample.
3. Shake samples at room temperature for 1 hour, or at 4°C for 24
hours.
4. Place a Whatman No.1 filter over a clean extraction cup. Pour the
soil and KCl mixture over the filter to collect the extract in the cup.
Discard the soil (unless using for other analyses).
5. Tightly cap the extractions and diffuse for ammonium and nitrate
within 24 hours, or store the extractions at -20°C until diffusions
can be performed.
Diffusions
In this method, both ammonium (NH4+)
and nitrate/nitrite (NO3- / NO2-)
are converted into ammonia (NH3) and trapped onto glass-fiber
disks (called "acid traps") for isotope analysis. Magnesium oxide is used
to convert NH4+ to NH3 and Devarda’s
alloy converts NO3- to NH3.
Acid traps
1. Place a 1.2-cm diameter pre-combusted glass-fiber disk (made using
a larger diameter glass-fiber filter and a hole-punch) onto a Teflon
membrane (we use two 2-inch pieces of one-inch-wide Teflon tape).
2. Using
a micropipette, add 20 µL of 2.5 M KHSO4 to the glass-fiber
disk.
3. Place another two layers of Teflon tape over the glass-fiber disk.
4. Seal
the layers of Teflon tape by pressing them together using the open end
of a 2-cm diameter glass vial. This should be done on a surface
of aluminum foil with layers of paper towel underneath.
NH4+
1. Add one acid trap to each sample (i.e., soil-extracted KCl in plastic
urine vial).
2. Add heavy powder magnesium oxide (J.T. Baker) to each sample (3 grams
MgO per liter of sample). Close sample lid tightly.
3. Incubate samples at 30°C for 1 week on a shaker table.
4. After one week, remove the acid trap from each sample. Wrap
each trap in aluminum foil, label, and dry in a desiccator for 3-4 days
(or until dry). The desiccator should contain: (1) a beaker of
concentrated sulfuric acid, and (2) silica desiccant.
NO3-
5. Add a new acid trap to each sample.
6. Add ground granular Devarda’s alloy (J.T. Baker) to each sample
(75 mg per 100 mL of sample).
7. Close each sample lid tightly and incubate on a shaker table at 30°C
for 1 week. (It is recommended that you incubate the samples for
the same length of time as was allotted for the NH4+ diffusions.)
8. Remove the acid trap from each sample, wrap in foil and dry in a
desiccator with silica desiccant and concentrated sulfuric acid.
Isotope Analysis
When ready to analyze the samples for stable isotopes of nitrogen (δ15N),
remove the glass-fiber disk from the Teflon tape and fold each disk into
a 5x9-mm tin capsule. Please note that once samples
are folded into the tin capsules, they should be analyzed immediately – as
the tin may react with the reagents used during the prep work. To ship samples to our lab, click here.
Water samples
The diffusion method can also be used for freshwater, estuarine, and marine
samples. Usually, large volumes of water are needed for diffusions; so
appropriate-sized containers should be used. Method details are generally
the same as above for soils, but NaCl is added to freshwater samples prior
to the diffusions (e.g., 50 g NaCl per L) so that the acid traps remain
intact during incubation.
Please note: In order to perform diffusions on water samples,
the concentration of N-NH4+ and N-NO3- must be known so that
enough sample can be used to obtain 20-60 µg of N per
sample. For estuarine and marine samples, salinity should
be known as well.
If you have any further questions regarding the diffusion
technique, please contact the lab
manager for more information.
References
Chang, C.C.Y., Langston, J., Riggs, M., Campbell, D.H., Silva, S.R., and
Kendall, C. 1997. A method for nitrate collection for δ15N and δ18O analysis
from waters with low nitrate concentrations. Canadian Journal of Fisheries
and Aquatic Sciences 56: 1856-1864.
Holmes, R.M., McClelland, J.W., Sigman, D.M., Fry, B., and Peterson, B.J.
1998. Measuring 15N-NH4+ in marine, estuarine
and fresh waters: An adaptation of the ammonia diffusion method for samples
with low ammonium concentrations. Marine Chemistry 60:235-243.
Khan, S.A., Mulvaney, R.L., and Brooks, P.D. 1998. Diffusion methods for
automated nitrogen-15 analysis using acidified disks. Soil Science Society
of America Journal 62: 406-412.
Sigman,D.M., Altabet, M.A., Michener, R., McCorkle, D.C., Fry, B., and
Holmes, R.M. 1997. Natural abundance-level measurement of the nitrogen
isotopic composition of oceanic nitrate: an adaptation of the ammonia diffusion
method. Marine Chemistry 57:227-242.
Silva, S.R., Kendall, C., Wilkison, D.H., Ziegler, A.C., Chang, C.C.Y.,
and Avanzino, R.J. 2000. A new method for collection of nitrate from fresh
water and the analysis of nitrogen and oxygen isotope ratios. Journal Of
Hydrology 228: 22-36.
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