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Elemental Analysis and the Dumas Method

Elemental Analysis and the Dumas Method
The Dumas method, also known as combustion method, is a primary method of determination of Nitrogen and Protein ensuring fast results, ease-of-use and safety. However, it is considered 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 by AOAC, AACC, ISO, DIN, ASBC, AOCS, OIV, etc. Nevertheless, in some countries the local guidelines recommend one method rather than the other.
This is the reason why many decision-makers and users are undecided between the two methods and between following the local guidelines or the International Standards.

The growing trend of Dumas method
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.
Since 2011 VELP Scientifica has introduced nitrogen analyzers 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.

Dumas and Kjeldahl method

Dumas and Kjeldahl 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 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 (0.001 mg N absolute vs. ≥ 0.1 mg N absolute).

Principles of Operation

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 passes through a reduction chamber containing copper heated to around 650 °C. This stage 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.

Steps of the Dumas method

Steps of the Dumas method
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 DUMASoftTM software in order to translate in mgN the signal received from the TCD. 

Recommendations

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

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