What preservation method and storage time are specified for TKN (Organic Kjeldahl) samples?

What preservation method and storage time are specified for TKN (Organic Kjeldahl) samples?

• A. Filter Immediately; Cool <=6 C; 48H
• B. Analyze Immediately; 0.25h
• C. Add H2SO4 to pH<2, Cool <=6 C, 7D/28D
• D. Add HNO3; 28D

Answer: (C)

Preserving TKN samples hinges on stopping transformations that can change nitrogen content before analysis. For Organic Kjeldahl nitrogen, the standard approach is to acidify with sulfuric acid to bring the pH below 2, which suppresses microbial activity and chemical changes. Then keep the sample cool, at or below 6°C, to slow any remaining processes. The hold time is specified as 7 days, with some protocols allowing up to 28 days depending on the approved method. This combination—acidification to pH < 2, refrigeration, and the 7/28-day hold time—ensures the sample stays representative until analysis.

Preserving TKN samples hinges on stopping transformations that can change nitrogen content before analysis. For Organic Kjeldahl nitrogen, the standard approach is to acidify with sulfuric acid to bring the pH below 2, which suppresses microbial activity and chemical changes. Then keep the sample cool, at or below 6°C, to slow any remaining processes. The hold time is specified as 7 days, with some protocols allowing up to 28 days depending on the approved method. This combination—acidification to pH < 2, refrigeration, and the 7/28-day hold time—ensures the sample stays representative until analysis.