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Total Nitrogen by In-line UV Digestion  

FIAlab standard method for total nitrogen through in-line UV digestion using the FIAlab-2500/2600/2700  and FIAlab's IL-UV In-line Digestion Unit.  Note, this method is also compatible with Sequential Injection Systems (please inquire).

Assay Typical Throughput Concentration Range Notes
Total Nitrogen (High) 30 samples/hour 0.1 to 5 ppm 1 cm flowcell
Total Nitrogen (Low) 30 samples/hour 0.01 to .5 mg (N)/L 10 cm flowcell

 

 Principle:

Adapted from:
 Cerda, Oms et al., Analyst 1996, 121, 13-17
Oms, Cerda et al. Talanta  2003, 59, 319-326

In the in-line total nitrogen method, nitrogen compounds are oxidized to nitrate using alkaline persulfate/UV digestion with FIAlab's IL-UV In-line Digestion Unit.    Then the standard nitrate method is used to measure the post-digested sample.  Total Nitrogen includes dissolved organic nitrogen, ammonia, nitrate and nitrite.

To compare the results to the alternate batch "TKN" method, the nitrate and nitrite component of the total nitrogen results must be measured separately and removed.  

This can be achieved by measuring each sample with the UV lamp on (for total nitrogen) and re-measuring with the UV lamp off (for nitrate plus nitrite).   TKN will compare with the difference of these measurements.

 Comments:
 

               

Warning: The in-line total nitrogen method works well with a maximum sample concentration of around 3-5 ppm N before the persulfate begins to be depleted.  At higher concentrations the digestion process slows/stops prior to near 100% conversion, and can result in understated measurements.  Higher concentration samples must be pre-diluted (prior to digestion) with DI water to < 3-5 ppm N, or diluted in-line with an automated dilutor.

Adapted from:
Cerda, Oms et al., Analyst 1996, 121, 13-17
Oms, Cerda et al. Talanta  2003, 59, 319-326
Interferences:
  1. Chlorine can interfere by oxidizing the reduction column.
  2. High concentrations of iron, copper or other metals cause negative interference. EDTA is added to the buffer to reduce this interference.
  3. Oil and grease will coat the surface of the cadmium. This interference is eliminated by pre-extracting the sample with an organic solvent.
  4. Sample turbidity/color may interfere. Pre-filtration with a membrane filter should help.

Special Hardware:

FIAlab - IL-UV In-line Digestion Unit for In-line digestion of total nitrogen, total phosphorous, total cyanide. Includes secondary two channel peristaltic pump.

Programming: 

Please contact FIAlab for details on the programming/timing of the sample digestion period within the IL-UV coil.

Reagents:

Carrier: 
The carrier should be matrix matched to the sample, i.e., salt content for seawater samples, and KCL content for soil extracts.

Persulfate Digestion Solution:

7.6 g of sodium tetraborate decahydrate

16.0 g of potassium persulfate

Dissolve in 1 L DI water


Reagent 1: 1.6 M Ammonium Chloride Buffer (for nitrate only).

Note - For nitrite, plug this R1 port with an included Teflon Plug

500 ml TOTAL: Balance with Degassed DI water

43 grams Ammonium chloride

0.50 grams disodium ethylenediamine tetraacetic acid (EDTA) [372.24 FW]  (only if metals may be present)
 ~1 grams Sodium Hydroxide Pellets as needed

Place 400 cc of water into a volumetric flask and add the ammonium chloride and EDTA.  Mix until dissolved.  Adjust the pH of this solution to ~8.5 using sodium hydroxide pellets.  Add water to fill flask to 500mL.

CAUTION: Ammonium chloride sometimes may contain significant amounts of nitrate.

Reagent 2: Colorimetric Sulfanilamide Solution.

 20 grams Sulfanilamide [172.21 FW]

50 ml 85% Conc. Phosphoric Acid  (H3PO4)
 0.50 grams N-1-naphthylethylenediamine dihydrochloride [259.18 FW]

500 ml TOTAL: Balance with Degassed DI water

Mix well and store in dark bottle.

Cadmium Column: 
Cadmium column can be prepared/packed by user, or purchased ready for use.  For nitrite assays, use a bridge of 0.03" ID tubing (~4 inches) in place of the column.

 

1)     CC120 Cadmium Column for nitrate reduction column  (or)

2)     Prepare your own Cadmium

 

 Standards:

100ml ICNAT-100 (nitrate standard)

100ml ICNIT-100 (nitrite standard)

Source: 727-524-7732 - www.exaxol.com 

Please email or phone FIAlab Instruments for additional product information.
 Email: fialab@flowinjection.com, Voice: 425-376-0450 or 1-800-963-1101,  Fax: 425-376-0451