Last revised July 22, 1997
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For N, the Kjeldahl-Rittenberg procedure (e.g. Hauck 1982) has a long history and a large following. In this wet-chemistry method an acid digestion is used to produce an ammonium salt, which is then oxidized to N2 gas using hypobromite. This procedure is slow and laborious and involves certain hazards such as hot acid fumes. It also requires care to avoid loss of N during transfer between steps.
The alternative dry Micro-Dumas combustion analysis for total carbon and total nitrogen in solid-phase samples (plant tissue, soils, sediments, etc.) is based on transformation to gas phase by extremely rapid and complete flash combustion of the sample material.
In the apparatus shown above, a rotating multiplace sample dropper (a) delivers one sample at a time into the top of a quartz combustion tube (b). This tube contains granulated chromium III oxide combustion catalist 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 1700 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 CO2 during flash combustion. Nitrogen-bearing combustion products include N2 and various oxides of nitrogen NOx; 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 N2.
Water vapor from the sample is removed by a gas trap (d) containing magnesium perchlorate. If the samples are being analyzed for nitrogen only, CO2 is removed by a second gas trap containing a CO2 scrubber (sodium hydroxide on silicate carrier granules).
The clean sample gases now pass through a gas chromatograph column (E) to separate the N2 and CO2. N2 elutes from the GC column first, then CO2.
The sample gas pulses and a separate reference stream of helium (f) pass through a detector (g); 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.
Ultra-high purity acetanilide is the most frequently used standard material; the total C and total N content of this material can be easily calculated from the chemical formula. In addition, each new lot of standard acetanilide is checked before use on real runs by analyzing samples of National Bureau of Standards NBS1572 Citrus Leaves.
Empty tin-capsule blanks are also included periodically in each batch, and any detectable N or C in these blanks is subtracted from the sample and standard values to give a true zero baseline. Blanks allow correction for traces of C originating from the tin capsules and for the small amount of N2 gas introduced as an impurity in the oxygen pulse.
2) Dried samples are ground to talcum powder consistency (250 um or less) using a ball mill (e.g. Spex Industries 8000) before being sealed into 5 x 9 mm tin capsules. Thorough sample homogenization in the grinder stage is required, to make certain that the tiny subsample taken for analysis (e.g. 2-4 mg for leaf material) is representative of the total sample. Poor precision can often be traced to inadequate grinding that leaves fibrous matter or visible granules in the sample. Wiley mills do not do an acceptable job of grinding for this application. (Nor does that favorite of low-budget improvization, the home coffee mill.)
3) Samples are weighed to the microgram level after grinding, and these weights are recorded and used in the data analysis. The size of the subsample weighed out depends upon the density of the material as well as its N and C content. Typical sample weights are 2-4 mg for plant tissue, 10 mg for straw and 25-30 mg for soil.
4) Another sample-size constraint is related to the absolute amount of material that can be completely combusted in micro-Dumas apparatus. The maximum burnable total C content is around 2500 micrograms (e.g., for NBS 1572 citrus leaf standard, which is 43.27% C, this works out to a maximum sample size of 5.8 mg of ground leaf.) In addition, for element-poor soil samples the sheer bulk of the sample becomes significant. Soil samples of over 50 mg are very difficult to analyze due to rapid ash buildup in the furnace.
5) Combustion capsule formation is critical for successful analytical runs. Please refer to the detailed [sample encapsulation] instructions for further information.