The Kjeldahl technique determines serum total protein by quantifying which element?

• A. Peptide bonds
• B. Refractive index
• C. Ultraviolet absorption at 280 nm
• D. Nitrogen content

Answer: (D)

The essential idea is to quantify nitrogen content in a sample to estimate how much protein is present. Proteins contain a fairly constant amount of nitrogen, about 16% by weight, so measuring nitrogen and applying a conversion factor lets us infer protein concentration. In practice, the sample is digested with acid to convert organic nitrogen into ammonium, then the ammonium is quantified by titration after neutralization. The result is the nitrogen amount, which is then converted to protein using a factor (commonly around 6.25).

This method specifically targets nitrogen, not the peptide bonds or aromatic residues. That’s why it’s used to reflect total protein content rather than relying on bonds or optical properties. For comparison, ultraviolet absorption at 280 nm relies on aromatic amino acids, and refractive index measurements depend on overall solute content, neither of which directly measures nitrogen.