USGS Test Method I-2515-91

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Method Name:
Ammonium plus organic nitrogen, dissolved. Official Name: Ammonium plus organic nitrogen, dissolved

Organic nitrogen is converted to ammonium ions at a temperature of 370oC in a reaction medium of sulfuric acid, potassium sulfate, and mercury (II). In principle, nitrate and nitrite are not reduced to ammonium ions under these conditions; USGS nomenclature refers to Kjeldahl nitrogen as ammonium plus organic nitrogen to emphasize this distinction. In practice, however, nitrate and nitrite might interfere positively or negatively.An air-segmented continuous flow analyzer is used to automate the photometric determination of ammonium ions in resolvated Kjeldahl digests by the salicylate analog of the Berthelot reaction. Resolvated Kjeldahl digests often contain suspended particulates and ions that interfere with the photometric finish. Both classes of interferents are eliminated by means of an on-line gas diffusion cell, which consists of a continuous-flow, parallel-plate dialyzer assembly that replaces the dialysis membrane with a hydrophobic, microporous, polypropylene membrane. Gases pass through the microporous polypropylene membrane; particles and ions do not. Before passage through the gas diffusion cell, ammonium ions in acidic resolvated Kjeldahl digests mix with the alkaline donor stream and are converted to ammonia: NH4+(aq)plus-OH --> NH3(g) | + H2O.Inside the diffusion cell, gas-phase ammonia in the donor stream passes through the polypropylene membrane and is trapped in the interferent-free recipient stream.

This method is used to determine Kjeldahl nitrogen in water, drinking water, wastewater, brines, and water-suspended sediment. The suitability of this method for determination of Kjeldahl nitrogen in bottom materials has not been investigated.

Patton, C.J., and Truitt, E.P., 2000, Methods of analysis by the U.S. Geological Survey National Water Quality Laboratory -- Determination if ammonium plus organic nitrogen by a Kjeldahl digestion method and an automated photometric finish that includes digest cleanup by gas diffusion: U.S. Geological Survey Open-File Report 00-170.

In-line digest cleanup by gas diffusion eliminates all particulate and potential ionic interference in the photometric finish. Thus, resolvated digests containing suspended particulates do not require filtration prior to analytical determinations. Likewise, colorimetric reagents do not contain complexing agents, such as citrate, tartrate, or EDTA, that are required to prevent precipitation of calcium (II) and magnesium (II) in the alkaline analytical stream of methods that lack a gas-diffusion cleanup step.Once samples have been acidified, they are subject to contamination by ammonia in the laboratory atmosphere. The digestion process, therefore, must be performed in a hood that is located in an ammonia-free area of the laboratory. Other analytical or housekeeping procedures with potential to contribute ammonia vapor to the laboratory atmosphere may not be performed in or near this hood. Avoid delays between sample preparation, sample digestion, and digest analyses to minimize the risk of ammonia contamination.Nitrate can exert both a positive and negative interference in Kjeldahl nitrogen determinations. As stated by the American Public Health Association (1992, p. 4?94): During [Kjeldahl] digestion, nitrate in excess of 10 mg/L can oxidize a portion of the ammonia released from the digested organic nitrogen, producing N2O and resulting in a negative interference. When sufficient organic matter in a low state of oxidation is present, nitrate can be reduced to ammonia, resulting in a positive interference. The conditions under which significant interferences occur are not well defined and there is no proven way to eliminate the interference.

QC Requirements:
Calibrate instrument using calibration standards (CAL); quality control samples (QCS); and laboratory blanks (LB) analyzed at a minimum of 1 for every 10 samples.

Maximum Holding Time:
30 days



0.1 to 10.0

Within-run precision was determined on the basis of 55 replicate determinations of the same high suspended-solids sample. Between-day precision was based on 26 replicate determinations of the same sample between April 8 and April 26, 19991. Duplicate data for 564 randomly selected samples were determined for Kjeldahl nitrogen was also used to assess between-day precision.

The MDL was estimated by using the protocol set forth in USEPA Appendix B, Part 136.

Revision Number:

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