There are currently no promulgated U.S. EPA methods for the determination of total nitrogen in ambient water and wastewater. There are approved methods for the determination of inorganic nitrogen (NO3-N, NO2-N and NH3-N) and for organic nitrogen (TKN-NH3–N).1 The U.S. EPA Clean Water Act (CWA) requires that all methods used for CWA compliance reporting be EPA approved.2 While there are no methods for the determination of total nitrogen, it is still a required parameter in many U.S. EPA permits, including monitoring of nutrient pollution for ambient water criteria.3 Because Part 136.3 Table 1b methods are required, laboratories have no recourse but to measure total nitrogen as the sum of TKN, NO3-N and NO2-N.4 Measurement of total nitrogen by calculation presents several problems, one with TKN determination, and the second with the need to perform separate tests.
A new technique, high-temperature catalytic combustion with chemiluminescence detection, uses a laboratory TOC analyzer fitted with a nitrogen module to accurately obtain results for total nitrogen in 5 minutes or less.
Problems with the TKN method
The TKN method presents problems to the high-throughput environmental laboratory attempting to analyze large numbers of samples for trace concentrations of total nitrogen by requiring a preliminary manual digestion with concentrated sulfuric acid, a metal catalyst and potassium sulfate. Highest recoveries are obtained using a mercury catalyst, but because of the toxicity of mercury many laboratories choose to use copper sulfate instead. The mercury catalyst results in a clear digest solution while the copper catalyst results in a green-colored solution. The classical TKN procedure distilled ammonia nitrogen (the product of the digestion) separating the analyte from the matrix prior to analytical determination by titration or colorimetry. The added distillation step is very time-consuming and severely limits laboratory throughput.
Miniaturized methods for the determination of TKN by semi-automated block digestion followed by continuous flow colorimetric methods have been developed.5 These continuous flow methods omit the distillation step, speeding the analysis, but suffer from difficulties that result from the color of the sample digest absorbing light at the analytical wavelength, improper matrix matching causing both positive and negative deflections of the baseline due to differences in refractive index between the sample solution and the carrier solution and excess acid in the digestion solution causing reagents to precipitate within the continuous flow analyzer chemistry cartridge.
The new total nitrogen method, which is in development at both ASTM International and Standard Methods for the Examination of Water and Wastewater, measures total nitrogen directly, has a low enough detection limit for ambient water quality monitoring, has a large dynamic range allowing analysis of clean and polluted samples in one batch and does not experience a high degree of carryover (contamination) from sample to sample.
New total nitrogen method to replace TKN
The proposed method couples a high-temperature catalytic oxidation (or combustion) total organic carbon analyzer, such as described in ASTM D7531 or Standard Methods 5310B, with a chemiluminescent nitrogen detector. The combustion tube of the TOC analyzer is packed with a catalyst (platinum on an alumina support) and capped with a small amount of ceramic fiber. The combustion tube is assembled in a furnace and heated to ≥720 ºC. Zero carbon air is used as a carrier gas and as a supply of oxygen to the ozone generator of the nitrogen detector. The sample stream passes through a thermoelectric cooler immediately after exiting the combustion tube.
When a sample is introduced into the combustion tube at ≥720 ºC, the nitrogen in the sample converts to nitrogen monoxide (reactions 1 and 2). Nitrogen gas in the carrier gas (air) does not interfere. The carrier gas containing the nitrogen monoxide (NO) is cooled and dehumidified in a thermoelectric cooler. The cooled gas then enters the chemiluminescence analyzer where the NO reacts with ozone (O3) and converts to a combination of nitrous oxide (NO2) and excited nitrous oxide (NO2*) (reactions 3 and 4). As the NO2* returns to the ground state it emits radiation, which is measured photoelectrically (reaction 5). The detector signal generates a peak that is proportional to the nitrogen concentration in the sample.
To determine selectivity, a comparison of intensity was made between NO3-N, NH4-N and a 1+1 mixture of NO3-N and NH4-N to determine if there are differences in response (Figure 1).
Figure 1 – Response of different inorganic nitrogen compounds (50 mg/L-N).
The average response of the NH4-N and the mix of NH4-N and NO3-N were near equal with a slightly lower response of the NO3-N standard. A test was then made to determine the variation in recovery of inorganic nitrogen compounds when using different calibrants. This data is shown in Figure 2.
Figure 2 – Recovery of inorganic nitrogen with different calibrants.
The results in Figure 2 suggest that if NH4-N is used as a calibrant, results for NO3-N could be biased low, and if NO3-N is used as the calibrant, results for NH4-N could be biased high. However, using a mixture of NO3-N and NH4-N seems to compensate. Recoveries for all compounds tested, regardless of calibrant, were within 10% of the true value.
A calibration curve was prepared using the mixed (NO3-N + NH4-N) calibrant and a series of inorganic and organic nitrogen compounds at 100 mg/L N were analyzed for recovery (Figure 3). All compounds were recovered well within 90–110%.
Figure 3 – Recoveries of various nitrogen compounds.
A study of recovery was made on commercially available quality control check standards comparing catalytic combustion to TKN. This study (results summarized in Figure 4) obtained near equivalent results to TKN.
Figure 4 – Comparisons of TKN and catalytic combustion TN.
Another study6 compared catalytic combustion total nitrogen in soil extracts to total nitrogen determined by a manual heated persulfate digestion followed by flow injection analysis. This data is shown in Figure 5.
Figure 5 – Comparison of persulfate digested (TN) and catalytic combustion (TNb).
Alternative methods, such as the one used by Doyle with the data shown in Figure 5, rely on alkaline persulfate digestion followed by colorimetric detection7,8 or ion chromatography detection.9
Nydahl studied the persulfate oxidation method extensively and determined that the results of 10 sewage treatment plant effluents were equivalent to the results obtained by the TKN method.10 Kroon compared TKN with an automated UV persulfate digestion method and found no significant differences in surface waters or wastewaters.11 Bronk and others compared the persulfate oxidation method with the high-temperature catalytic combustion method and found that they provide reproducible results that are consistent with each other.12 Daughton, in testing the applicability of the high-temperature catalytic combustion with chemiluminescence detection method as a substitute for TKN in oil shale retort waters, found that for 12 oil shale wastewaters the results did not differ significantly. Daughton also reported that some heterocyclic nitrogen compounds that are not quantitatively recovered by TKN are recovered by the high-temperature combustion method.13
High-temperature catalytic combustion with chemiluminescence detection is a technique capable of replacing TKN plus nitrate nitrite for the determination of total nitrogen in environmental samples. The proposed method requires the addition of a nitrogen module to an existing TOC analyzer and obtains results equivalent to or better than other total nitrogen methods in as little as 5 min per sample. Once the technique is validated and becomes an official consensus standard method, it will be submitted to the U.S. EPA for consideration for approval in Clean Water Act reporting.
- 40 CFR Part 136.3, Table 1b.
- http://water.epa.gov/scitech/methods/ cwa/index.cfm
- http://www2.epa.gov/sites/production/ files/documents/rivers1.pdf
- Doyle, A.; Weintraub, M. et al. Persulfate digestion and simultaneous colorimetric analysis of carbon and nitrogen in soil extracts. SSAJ 01 2004, 68(2).
- http://nwql.usgs.gov/pubs/WRIR/WRIR-03- 4174.pdf; accessed July 16, 2014.
- Method 4500-P J. Persulfate Method for Simultaneous Determination of Total Nitrogen and Total Phosphorus. Standard Methods for the Examination of Water and Wastewater, 22nd ed., 2012.
- http://www.astm.org/DATABASE.CART/ WORKITEMS/WK44300.htm; accessed July 16, 2014.
- Nydahl, F. On the peroxodisulfate oxidation of total nitrogen in waters to nitrate. Water Research 1978, 12, 1123–30.
- Kroon H. Determination of nitrogen in water; comparison of a continuous-flow method with on-line UV digestion with the original TKN method. Anal. Chim. Acta 1993, 276, 287–93.
- Bronk, D.A.; Lomas, W.L. et al. Total dissolved nitrogen analysis: comparisons between the persulfate, UV and high temperature oxidation methods. Marine Chemistry 2000, 69, 163–78.
- Daughton C.G. and Jones, B.M. Chemiluminescence vs. Kjeldahl determination of nitrogen in shale retort waters and organonitrogen compounds. Anal. Chem. 1985, 57, 2320–5.
William Lipps is environmental chemical business unit manager, Shimadzu Scientific Instruments, Inc., 7102 Riverwood Dr., Columbia, Md. 21046, U.S.A.; tel.: 410-381-1227; e-mail: [email protected]; www.shimadzu.com