Elemental Combustion Analysis
Total Nitrogen and Carbon
For total nitrogen and total carbon analysis, sample materials in solid or liquid states must be converted into N2 and CO2 gases.
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, elemental 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, and measurement of concentrations via gas chromatography (GC).
Safety Warning: Magnesium perchlorate poses health and fire hazards. For more information see www.fishersci.com/msds
Sodium hydroxide poses a hazard to eyes, skin, and internal organs. For more information see www.fishersci.com/msds
Acetanilide poses a hazard to eyes, skin, and internal organs. For more information see www.fishersci.com/msds
Atropine poses a hazard to skin, eyes, and internal organs. For more information see http://www.sciencelab.com/xMSDS-Atropine-9922981
In the apparatus, a rotating autosample changer: (a) delivers one tin-encapsulated sample at a time into the top of a quartz combustion tube (b). This tube contains granulated chromium oxide combustion catalyst and is held at 900 degrees C. A pulse of pure O2 is admitted with each sample, which is enclosed in an ultra-pure 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 non-reactive 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.
Calibration and reference materials The Elemental Combustion Analyzer at F&M is a Costech ECS 4010 CHNS-O system. It is calibrated by including five solid-phase reference materials in the tin capsule stage at the beginning of each run, and at fixed intervals thereafter (usually one reference standard per ten unknowns.)
Ultra-high purity acetanilide (four samples in ca. 0.25, 0.50, 0.70 and 1.00 micrograms increments) and atropine (at ca. 0.1 micrograms) were used to generate the calibration curve; total C and total N contents of these materials are calculated from their chemical formulae.
Empty tin-capsule blanks are included ever tenth sample, and any detectable N or C in these blanks was 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.
Analytical Procedures
1) Soil were oven-dried (80 degrees C, 24 hours).
2) Dried samples are ground to talcum powder consistency (250 um or less) using a ball mill or ceramic mortar and pestle 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 is representative of the total sample. Poor precision can often be traced to visible granules in the sample.
3) Roughly 25 micrograms of soil samples are weighed into pure tin capsules using an automated and computer controlled microbalance.
4) In addition to the calibration standards, a certified standard reference soil (ECA 542) and an internal soils standard (BS-1 13) were analyzed every ten unknowns.
5) All samples, standards and blanks we loaded in a 50-slot auto-changer carousel.
6) Automated analyses were controlled by Windows-based EAS Software with a multichannel 24 bit A/D interface connected to the electronic detection system in the ECA.
7) The ECS software compares the elemental peak to the calibration standard data, and generates a report for each element on a weight basis.
Bibliography
Costech Instruments, 2006, Elemental Combustion System CHNS-O, www.costechanalytical.com
Hauck, R. D. 1982. Nitrogen-Isotope Ratio Analysis, sec.36-3.2.2, Conversion of total nitrogen to ammonium-nitrogen. pp.744ff. In Methods of Soil Analysis, Part 2, Chemical and Microbiological Properties. American Society of Agronomy, Madison, Wisconsin.
Kirsten, Wolfgang. 1983. Organic Elemental Analysis: Ultramicro, Micro, and Trace Methods.Academic Press/Harcourt Brace Jovanovich, New York.
University of Georgia Institute of Ecology, Elemental Analysis by Micro-Dumas Combustion. www.uga.edu/~sisbl/
