Another method for determining nitrogen content of organic materials was developed by Jean-Baptiste Dumas, a French chemist. He found that nitrogen could be analyzed by first combusting the material at high heat in the presence of oxygen and subsequently measuring the volume of gaseous nitrogen released. Although his method was originally described in 1833, the results were somewhat variable and the Kjeldahl method proved to be much more reliable. In recent years, the Dumas combustion method has made a comeback. Its main advantage is that results may be
Combustion GC Detection
Combustion GC Detection
Figure 4.2 Diagram of instrumentation and steps in a Dumas protein combustion. (From http://www.uga.edu/~sisbl/udumas.html, University of Georgia. With permission.)
obtained more quickly (about 5 min for a single sample versus 2—3 h for the Kjeldahl method). Disadvantages still remain: the equipment required is more expensive than the Kjeldahl method, and the Dumas method analyzes single samples prepared sequentially. Thus, using any of the methods, the same total time is needed to obtain results from a group of 15—20 samples.
Figure 4.2 shows the various steps in the combustion and analysis process. A small sample (-0.2—1 g) is wrapped in a piece of foil and placed in a multiplace sample holder (Figure 4.2a).
The sample is dropped into a combustion tube (Figure 4.2b) heated to ~1000°C where the meat sample is mixed with pure oxygen, and the organic components are totally converted into CO2, N2, H2O, and nitrogen-containing oxides. Helium gas then flushes these products through a copper-containing column (Figure 4.2c) where the nitrogen oxides are fully converted into N2. Water and CO2 are removed by a trap (Figure 4.2d) and the remaining volatiles pass through a gas chromatograph (Figure 4.2e). The nitrogen gas then flows through a thermal conductivity meter where the signal is compared to a separate parallel stream of pure helium. The area under the nitrogen peak is determined (Figure 4.2g), and the nitrogen content calculated by comparing with results from known standards (such as ethylenediaminetetraacetic acid [EDTA] or lysine) run at the same time as the unknown. As with the Kjeldahl, the protein content is calculated using the conversion factor of 6.25 X N.
A comparison of the results using the Kjeldahl digestion and Dumas combustion methods has been made.4 Using both methods, 12 laboratories analyzed the same set of 15 meat samples. The samples included both raw and cooked products with varying fat content. The nitrogen values obtained by both methods were in remarkably good agreement. Only 2 of 360 determinations by the Dumas combustion method were considered outliers. Combustion instruments from three different commercial manufacturers gave similar precision, repeatability, and reproducibility. Thus, both the AOAC Kjeldahl method 928.08 and the AOAC combustion method 992.15 are now accepted for total nitrogen and estimated protein in whole meat samples. It should be mentioned that accurate analysis by both methods requires adequate mixing and careful sampling. The U.S. Department of Agriculture's Chemistry Laboratory Guidebook (1986) provides directions for sample preparation. Emulsified meat products need to be ground through a 1/8 in. plate twice, and coarse ground products passed through the grinder three times to ensure adequate mixing. Since only 200—900 mg of sample is typically used for combustion, replicates should be used because sampling errors can be easily made.
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