An ultra-micro method for the determination of total nitrogen in biological fluids based on Kjeldahl digestion and enzymatic estimation of ammonia.

An ultra-micro method for the determination of the total nitrogen-content of biological fluids and suspensions is described, based on a digestion in sulphuric acid and a enzymatic determination of the ammonia formed with glutamate dehydrogenase (EC 1.4.1.3). The proposed method yields the same results as the classical Kjeldahl procedure, but is less time-consuming. The detection-limit of the nitrogen, without loss of precision and accuracy, is much lower than in the original Kjeldahl procedure, and is in the order of 35 ng N per sample.