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Dumas nitrogen and total nitrogen content

Nowadays, thanks to steps forward in technology, the Dumas method is becoming more and more widespread.

Stringent standards on the composition of food and feed are being applied in ever widening markets. Precise analysis of these components is now essential to any company wishing to market its products, particularly on an international basis.
VELP Scientifica has introduced a new nitrogen analyzer based upon the Dumas nitrogen determination. Unlike the commonly used Kjeldahl method, Dumas method, also known as elemental analysis or combustion method, detects the total nitrogen content, not only that which is contained within proteins.

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 and 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 (0.003 mg N absolute vs. ≥ 0.1 mg N absolute).

Principles of Operation
Dumas nitrogen determination requires well homogenized samples, heated in a high-temperature furnace where the combustion takes place rapidly at over 1000 °C in the presence of pure oxygen. This produces mostly water, carbon dioxide and nitrogen as several oxides (NyOx). This gas mixture (most water is removed) is passed through a reduction chamber containing copper heated to around 650 °C. This converts nitrogen oxides into elemental nitrogen and collects the oxygen in excess. Different traps remove the residual water and carbon dioxide. Total nitrogen content is measured by a thermal conductivity detector.

The first Dumas apparatus

Original apparatus as designed by the French chemist Jean-Baptiste Dumas (1800-1884) for total nitrogen determination.

An automated instrumental technique has been developed which is capable of rapidly measuring the total nitrogen content of food samples and is beginning to compete with the Kjeldahl method as the standard method of analysis for nitrogen content for some foodstuffs, and more.

The sample, both solids and liquids, is weighed in tin foils (maximum capacity 1 gram) and introduced into autosampler holes.

The method has three steps:

  • 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.

Generally, VELP Dumas Nitrogen Analyzer NDA 701 uses EDTA (9.57% N) as the reference standard thanks to its high oxygen content that means that less oxygen is required for the combustion. However, aspartic acid, acetanilide, urea, atropine and other reagents can be used to create calibration curves. Their choice depends upon the sample types. Some users will even select one of their own products with a well-established profile as a reference standard. A good calibration curve requires five/six standard points at different quantities (in milligrams) covering the mgN range of expected nitrogen.

The small sample size raises the risk of obtaining an unrepresentative sample. For this reason, if using the Dumas technique, is strongly recommended to perform an efficient homogenization of the sample, in order to obtain and test a representative sample.

Currently, the elemental analysis according Dumas method is considered as an alternative to the classical Kjeldahl method. In fact, both of the methods are validated by different international organizations: Kjeldahl is officially recognized by AOAC, EPA, DIN, ISO, etc., whilst Dumas nitrogen determination by AOAC, AACC, ISO, DIN, ASBC, AOCS, OIV, etc.  However, in some countries the local guidelines recommend one method rather than the other.

Dumas nitrogen related products