15. Measure the absorbance of samples in a 5 cm cuvette at 630 nm
and calculate the ammonium concentration by the calibration
curve (see Note 4).
3.3 Dissolved
Organic Nitrogen
(Total Dissolved
Nitrogen, TDN)
1. Combust the vials and filters for 4 h at 450
C, and cool them
to room temperature before use.
2. Prepare 2 mol/l HCl by dissolving 33.2 ml concentrated
hydrochloric acid in ultrapure water and fill up to a volume of
200 ml. Store in a glass bottle.
3. Filter the sample with the syringe and syringe filter. Rinse the
syringe with the sample before filtering. Discard first 5 ml of the
filtrate and collect 20 ml into the glass vial. Prepare filter blank
by filtering ultrapure water.
4. Add 80 μl 2 mol/l HCl to the vial.
5. Put on the lid and place the vial in the refrigerator or freezer
until it can be run in the nitrogen analyzer (see Note 6).
6. Prepare a stock standard solution containing 1000 mg N/l by
dissolving 4.716 g ammonium sulfate (pre-dried at 105
C for
1 h) in ultrapure water and fill up to a volume of 1 l. Store the
stock solution in a glass flask in the refrigerator (stable for
1 month).
7. At the time of measurement, prepare suitable working standards (e.g., 0–1 mg/l) for the calibration of the analyzer
(see Note 7). Prepare an intermediate standard solution
100 mg/l from the stock solution and pipette appropriate
amounts of this solution and 2 mol/l HCl (200 μl/100 ml)
for working standards into volumetric flasks, and fill with ultrapure water (for blanks only ultrapure water and HCl). Calibration standards should be made weekly.
8. Fill each calibration vial about 1/2 to 2/3 full, put on a plastic
film (see Note 8), and make holes on the film with a clean
syringe needle.
9. Prepare the TDN analyzer for analyses according to the operation manual (see Note 1).
10. Place standards in the autosampler of an analyzer and run the
standards for TDN calibration curve and linearity test
(see Note 2).
11. On the day of analyses, take samples out of refrigerator/freezer
and let them get room temperature before further processing.
12. Place the samples in an ultrasonication bath for 10 min.
13. Remove the lids, put on a plastic film (see Note 8), and make
holes on the film with a clean syringe needle.
14. Run samples with the analyzer (see Note 9). Verify proper
operation of the instrument with reference material
Determining Inorganic and Organic Nitrogen
77
and calculate the ammonium concentration by the calibration
curve (see Note 4).
3.3 Dissolved
Organic Nitrogen
(Total Dissolved
Nitrogen, TDN)
1. Combust the vials and filters for 4 h at 450
C, and cool them
to room temperature before use.
2. Prepare 2 mol/l HCl by dissolving 33.2 ml concentrated
hydrochloric acid in ultrapure water and fill up to a volume of
200 ml. Store in a glass bottle.
3. Filter the sample with the syringe and syringe filter. Rinse the
syringe with the sample before filtering. Discard first 5 ml of the
filtrate and collect 20 ml into the glass vial. Prepare filter blank
by filtering ultrapure water.
4. Add 80 μl 2 mol/l HCl to the vial.
5. Put on the lid and place the vial in the refrigerator or freezer
until it can be run in the nitrogen analyzer (see Note 6).
6. Prepare a stock standard solution containing 1000 mg N/l by
dissolving 4.716 g ammonium sulfate (pre-dried at 105
C for
1 h) in ultrapure water and fill up to a volume of 1 l. Store the
stock solution in a glass flask in the refrigerator (stable for
1 month).
7. At the time of measurement, prepare suitable working standards (e.g., 0–1 mg/l) for the calibration of the analyzer
(see Note 7). Prepare an intermediate standard solution
100 mg/l from the stock solution and pipette appropriate
amounts of this solution and 2 mol/l HCl (200 μl/100 ml)
for working standards into volumetric flasks, and fill with ultrapure water (for blanks only ultrapure water and HCl). Calibration standards should be made weekly.
8. Fill each calibration vial about 1/2 to 2/3 full, put on a plastic
film (see Note 8), and make holes on the film with a clean
syringe needle.
9. Prepare the TDN analyzer for analyses according to the operation manual (see Note 1).
10. Place standards in the autosampler of an analyzer and run the
standards for TDN calibration curve and linearity test
(see Note 2).
11. On the day of analyses, take samples out of refrigerator/freezer
and let them get room temperature before further processing.
12. Place the samples in an ultrasonication bath for 10 min.
13. Remove the lids, put on a plastic film (see Note 8), and make
holes on the film with a clean syringe needle.
14. Run samples with the analyzer (see Note 9). Verify proper
operation of the instrument with reference material
Determining Inorganic and Organic Nitrogen
77