Automatic elemental analysis for the determination of nitrogen and protein - Combustion analysis

While acid digestion according to Kjeldahl destroys the organic sample in a rather rustic way, combustion offers a more elegant and time-saving option. The sample is oxidised in a controlled manner and the resulting gaseous products are subjected to further analysis. The aim is the complete conversion of the sample into the main products carbon dioxide, water and nitrogen dioxide, without the by-products carbon monoxide CO and nitrogen monoxide NO resulting from incomplete combustion.

Principle of automatic nitrogen analysis with the Dumas method (combustion analysis)

Modern combustion instruments such as the DUMATHERM still use the principle of the Dumas method to determine nitrogen and protein content in solid and liquid samples. The entire analysis procedure is performed in 5 steps.

Step 1

The sample enters the Dumas system via the sample changer (AS). In the combustion reactor (CF) an exothermic reaction occurs at 1030 °C between oxygen (O2) and tin capsule, which results in complete combustion of the sample. With the aid of oxidation catalysts it is made sure that the combustion gases are completely oxidized.

Step 2

After combustion, the oxygen flow is switched back to helium (He) which serves as the carrier gas for the combustion products through the rest of the analytical process. The reduction from NOx into N2 takes place on a copper surface in the furnace (RF)

Step 3

The byproduct water is separated then by condensation (CT) and two filters (F1 & F2). F1 is the PermapureTM gasdryer, which separates the majority of the water from the gas mixture. F2 is packed with absorbent to remove the residual amount of water from the gas stream.

Step 4

Carbon Dioxide CO2 is separated from the gas into a CO2 adsorber / desorber system (F3). The CO2 adsorbent is regenerated in the degassing furnace.

Step 5

The remaining gas mixture passes the TC detector (TCD) leading to an output signal which is proportional to the nitrogen concentration in the combustion sample. Standard samples with a known concentration of nitrogen (e.g. EDTA) are used for the calibration of the detector.