USGS Test Method I-2650-03


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Source:
USGS

Method Name:
Nitrogen and Phosphorus, Dissolved, in water by Alkaline Persulfate Digestion. Official Name: Evaluation of Alkaline Persulfate Digestion as an Alternative to Kjeldahl Digestion for Determination of Total and Dissolved Nitrogen and Phosphorus in Water

Description:
Filtered samples are dispensed into glass culture tubes, dosed with alkaline persulfate reagent, capped tightly, and digested in an autoclave at 250oF (121oC) and 17 lb/in2 (117.2 kPa) for 1 hour. The alakaline persulfate digestion procedure oxidizes all forms of inorganic and organic nitrogen to nitrate and hydrolyzes all forms of inorganic and organic phosphorus to orthophosphate. Nitrate and orthophosphate in alkaline persulfate digests are determined in parallel with a 2-channel photometric, air-segmented continuous flow analyzer. Orthophosphate was determined by a phosphoantimonylmolybdenum blue method, which is equivalent to the 2-reagent variants of USEPA method 365.1 and USGS method I-2601-90.

Scope:
This method was validated for determination of total dissolved nitrogen and phosphorus in drinking water, wastewater, and water-suspended sediment. Their applicability to bottom materials was not investigated.

Citation:
Patton, C.J., and Kryskalla, J.R., Methods of Analysis of the U.S. Geological Survey National Water Quality Laboratory -- Evaluation of Alkaline Persulfate Digestion as an Alternative to Kjeldahl Digestion for Determination of Total and Dissolved Nitrogen and Phosphorus in Water, U.S. Geological Survey Water-Resources Investigations Report 03-4174, 40 p.

Interferences:
Chloride concentrations higher than 10,000 mg/L are likely to interfere because of reaction with persulfate to form oxychlorides or chlorine that might deplete persulfate required to oxidize inorganic and organic nitrogen species to nitrate. Resulting active chlorine species can also interfere in colorimetric reactions used to determine nitrate and orthophosphate in digests. Organic carbon concentrations greater than 150 mg/L interfere because of reaction with persulfate to form carbon dioxide, depleting persulfate required to oxidize inorganic and organic nitrogen species to nitrate. Overacidification of samples at collection sites can result in low recovery of inorganic and organic nitrogen, which can be avoided by exclusive use of the sulfuric acid field-amendment solution (see text for details). Nitrate and nitrite do not contribute to Kjeldahl dissolved and total nitrogen ocncentrations in principle, but in practice, positive and negative interferences by these ions are well known. This interference can confound comparison of Kjeldahl nitrogen and alkaline persulfate digestion nitrogen when dissolved nitrate concentrations are greater than about 0.1 mg NO3--N/L. Suspended particles remaining in digests must be removed by sedimentation and decantation or filitration prior to colorimetric analyses. Barium, lead, and silver can interfere by forming insoluble phosphates, but their concentrations in natural-water samples usually are less than the interference threshold. Arsenate, AsO43-, (but not arsenite, AsO33-) can interfere by forming reduced heteropoly acids analogous to those found by orthophosphate. Because of the possibility that aresnite might be oxidized to aresnate by persulfate, both species at concentrations up to 20 mg-As/L in deionized water were digested and analyzed, which determined that interference by either species up to 1 mg-As/L is negligible.

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, as described in OFR 95-443. The method report (WRIR 03-4174) provides detailed instructions for preparing calibrants, matrix spike solution, quality-control check solutions, and digestion check solution.

Maximum Holding Time:
30 days

Media:
WATER

Subcategory:
Inorganic

Precision:
The precision was assessed using statistical and graphical analysis of data for about 2,100 dissolved and whole-water samples that were collected during nominally high- and low-flow conditions and analyzed by alkaline persulfate and Kjeldahl disgestion methods.

Detection:
The MDL was determined using the methodology described in USEPA report Part 136, Appendix B.

Revision Number:
2003

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