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Here you see the inside of a Dumas apparatus. A rotating multiplace sample dropper delivers one sample at a time into the top of a quartz combustion tube. This tube contains granulated chromium III oxide combustion catalyst and is held at 1200 degrees C. A pulse of pure O2 is admitted with each sample, which is enclosed in an ultrapure tin combustion capsule.

Thermal energy from the combustion of the tin and the sample material can generate an instantaneous temperature of as much as 1800 degrees C at the moment of flash combustion. All combustible materials in the sample are burned and the resulting gas-phase combustion products are swept out the bottom of the furnace by a constant stream of nonreactive helium carrier gas.

All carbon in the sample is converted to CO 2 during flash combustion. Nitrogen-bearing combustion products include N 2 and various oxides of nitrogen NO x; these pass through a reduction column filled with chopped Cu wire (600 degrees C) in which the nitrogen oxides give up their oxygen to the copper and emerge as N 2.

CO2 is removed by a gas trap containing a CO 2 scrubber (sodium hydroxide on silicate carrier granules).

Water vapour from the sample is removed by a second gas trap containing magnesium perchlorate.

The clean sample gases now pass through a gas chromatograph column to separate the N2 from the other gasses . The sample gas pulses and a separate reference stream of helium pass through a detector; differences in thermal conductivity between the two streams are displayed as visible peaks and recorded as numerically integrated areas.

Linear regression applied to combustion of known standard materials yields a regression line by means of which peak areas from unknowns are converted into total element values for each sample. In this way the amount of Nitrogen is measured.

| The Dumas method animation. 1. Sample enclosed in tin combustion capsule 2. Rotating sample dropper 3. O2 Pulse 4. Tube with Catalyst 5. Reduction column 6. CO 2 filter 7. H 2 O filter 8. Chromatograph column 9. Detector |

The Dumas method serves as a more precise and efficient alternative to Kjeldahl. With the Kjeldahl method, digestion, distillation and titration can take up to 1.5 to 2 hours. With the Dumas method, protein analysis time can be reduced to minutes. Precision is also increased.

For food composition databases almost always the Kjeldahl method is used. However, the Dumas method becomes more and more an accepted alternative for the Kjeldahl method.

Advantages:

• Low analysis time.

• Many samples can easily be measured automatically.

Disadvantages:

• Only small and dry samples (5-50 mg) can be analyzed.

• High initial costs.

The Dumas method (or combustion method)

Characteristics:

The combustion method was originally created in 1848 by Dumas. In this method a sample is burned in an oxygen rich atmosphere at high temperatures (about 900C). This leads to the release of CO 2, H 2 O and N 2. The N 2 is measured by use of a thermal conductivity detector and expressed as N. Thereafter these N values are converted to protein values using the same conversion factors as in the Kjeldahl method.

Study this animation to see what happens inside this apparatus.