5.1 This test method allows the simultaneous determination of total nitrogen and total phosphorous from one sample digestion step.

5.2 This test method measures oxidized ammonia and organic nitrogen (as nitrate) and soluble nitrate simultaneously. By subtracting the nitrate + nitrite value from a non-digested sample gives a TKN:

When using this test method:

where:
TN	=	total nitrogen, and
TKN	=	total Kjeldahl nitrogen.

1.1 This test method is applicable for the analysis total nitrogen (organic nitrogen + ammonia-N + nitrate-N + nitrite-N) as nitrate and total phosphorus as orthophosphate in unfiltered water samples by alkaline persulfate digestion followed by ion chromatography (IC).

1.2 Total Kjeldahl nitrogen (TKN) is determined by the calculation. To determine TKN subtract the nitrate-N and nitrite-N in a digested sample from a non-digested sample (see Section 4, Summary of Test Method).

1.3 The limit of detection (LOD), limit of quantitation (LOQ), and reporting range in Table 1 are based on the two-step process for this test method: digestion and analytical step. Because the digestion step requires a sample dilution, the LOD and LOQ are higher than undigested samples. The reporting range, LOD, and LOQ can be modified and perhaps improved depending on several factors (see Section 6, Interferences).

(A) Ten calibration levels, each injected in duplicate.

(B) LOD calculated as 3 × S/N.

(C) LOQ calculated as 10 × S/N.

(D)

where:
A	=	the average method blank concentration,
	=	the student’s t-value for the single-tailed 99th percentile t statistic a standard deviation estimate with n – 1 degrees of freedom, and
Sb	=	the sample standard deviation of the replicate blank analyses.

(E) Phosphate LOD/LOQ was calculated based on a dilution factor of 15× relative to the system concentrations.

1.4 The method detection limits (MDL) are shown for reference. The digestion reagent contains background nitrate and results in higher detection limits. MDL will be shown after the interlaboratory study (ILS) is completed.

1.5 The values stated in SI units are to be regarded as standard. No other units of measurement are included in this standard.

1.6 This standard does not purport to address all of the safety concerns, if any, associated with its use. It is the responsibility of the user of this standard to establish appropriate safety and health practices and determine the applicability of regulatory limitations prior to use.