Elemental Analysis and the Dumas Method

The Dumas method for nitrogen determination, developed in 1831, is older than the Kjeldahl, 1883, but more convenient in many aspects such as speed, safety, cleanliness, productivity, and cost per analysis.

The problem in the past was that it was not easy to reproduce the conditions required by the Dumas method and for this reason, the Kjeldahl technique took the lead and became considered as the classical method for nitrogen/protein determination.

Nowadays, thanks to steps forward in technology, the Dumas nitrogen determination is becoming more widespread.

Results obtained with the Dumas nitrogen determination are usually a little bit higher than with Kjeldahl since even the heterocyclic compounds and nitrogen compounds (e.g. nitrites and nitrates) are detected.

In the Kjeldahl method, such compounds are converted into the ammonium ion incompletely or not at all. The opposite could also happen (rarely) because in this kind of analysis there are lots of variables that could influence the final result.

Indeed, there are many minor variants of the Kjeldahl method, involving the use of different catalysts, heating times, volumes, distribution of sulfuric acid, and masses of test portion: this shows that the Kjeldahl procedure may be influenced by experimental errors. Recovery is the same for both the methods (≥ 99.5%), whilst the detection limit is lower for Dumas than for Kjeldahl (0.001 mg N absolute vs. ≥ 0.1 mg N absolute).

Combustion
Once the sample is weighed and purged of any atmospheric gases, it is heated in a high-temperature furnace and rapidly combusted in the presence of pure oxygen at about 1,000 ºC. This leads to the release of substances such as carbon dioxide, water, nitrogen dioxide and, above all, nitrogen as several oxides (NyOx).
Sample + O₂  → CO₂  +  H₂0 + NxOy + O₂ + other oxides

Reduction and Separation
The combustion products are collected and allowed to equilibrate. The gas mixture is passed over hot copper to remove any oxygen and convert nitrogen oxides into molecular nitrogen. The sample is passed through traps that remove water and carbon dioxide.
CO₂  +  H₂0 + NxOy + O₂ + Cu →  CO₂  +  H₂0 + N₂ → N₂

Detection
The measured signal from the thermal conductivity detector for the sample can then be converted into total nitrogen content.
EDTA, aspartic acid, acetanilide, urea, atropine and other reagents can be used to create calibration curves in the DUMASoft^TM software in order to translate in mgN the signal received from the TCD.