Efforts to Standardize TKN Measurements and Add Total Nitrogen to 40 CFR Part 136.

Optimizing Laboratory Operations
Oral Presentation

Prepared by W. Lipps
Shimadzu Scientific Instruments, 10330 Old Columbia Rd, Columbia, MD, 21046, United States


Contact Information: [email protected]; 443-539-6216

ABSTRACT

The Total Kjeldahl Nitrogen (TKN) procedure has been in use in some form or other for over 100 years. Initial methods accepted for environmental testing used a macro digestion, distillation, and titrimetric analysis. Methods have evolved to use block digestors with steam distillation, or no distillation followed by various determinative techniques including titration, Ion Selective Electrode (ISE), direct colorimetry, continuous flow colorimetric, and gas diffusion with colorimetric or fluorescence detection techniques. Catalysts for the digestion include selenium, mercury and copper. The methods and QC practices vary highly between labs with no real standardization. Performance testing (PT) shows reasonable agreement between labs; however, PT samples are generally interference free and not necessarily reliable to capture differences between method conditions that may affect results in real samples. With lack of a total nitrogen (TN) parameter at 40 CFR Part 136, laboratories in the US have been forced to measure TN as the sum of TKN and nitrate + nitrite (TN Calc) using any Part 136 approved TKN method and any Part 136 approved nitrate plus nitrite method. The nitrate methods are generally well defined, using ion chromatography, or colorimetric reactions that are specific to nitrate or nitrite. As we will show, the TKN methods are not standardized. If or when the new TN methods are compared to TN Calc, differences in results are generally attributed to the TN method and not to the lack of standardization or potentially inherent inaccuracies of the TKN methods. Thus, we are standardizing TKN methods at Standard Methods and ISO to better define interferences, and to achieve global harmonization between the methods, with better comparability between labs. This presentation will show some of the differences between the techniques and the variability between the TKN methods. Steps to achieve standardization will be presented.