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Not right. The Dumas method usually provides slightly higher results than the Kjeldahl method, because it also measures more non-protein forms of nitrogen. In the figure the bias is defined as 'Dumas results minus Kjeldahl results'. The figure shows a positive bias for almost all foods. This means that the Dumas method provides higher values.

Not right. The non-protein nitrogen fraction is higher in seafood and milk than in cereals. The Dumas method detects more of the NPN than the Kjeldahl method. Therefore the difference between Dumas and Kjeldahl is highest for foods with a high NPN fraction. You can see this in the figure most clearly for fish.

Right! Note that in both methods the NPN fraction contributes to measured nitrogen. It is possible to get rid of the NPN fraction by isolating the protein fraction. Total nitrogen measured in the protein fraction gives the exact value for protein nitrogen.

Results of a interlaboratory trial in which foods are analyzed with both Kjeldahl and Dumas method. Figure shows bias (i.e. mean of Dumas results minus mean of Kjeldahl results) between analytical methods, plotted against concentration of nitrogen.

From: Thompson et al. Analyst, 2002, 127, 1666-1668.

These questions will help you to understand the Dumas and Kjeldahl method