Franklin & Marshall College Franklin & Marshall College

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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/

CNS methodology (Total Nitrogen, Total Carbon and Total Sulfur).


For total Nitrogen, Carbon and Sulfur analysis, solid sample materials are converted to N2, CO2 and SO2.


Elemental combustion analysis of solid materials (soil or rock powder) is based on the transformation of solids into the gas phase by rapid and complete flash combustion of the sample material. The concentrations are then measured by gas chromatography.


The general methodology is as follows:

1)      A rotating auto-sample changer delivers one tin encapsulated sample at a time into the top of the quartz combustion tube.

2)      This reaction tube, containing chromium oxide combustion catalyst and thin rods of copper, is held at 980 degrees Celsius. A pulse of pure Oxygen (99.99% purity) is injected with each tin capsule. Thermal energy from the combustion of tin and sample material can generate a temperature of as much as 1700 degree Celsius at the moment of flash combustion. All combustible materials in the sample are burned and the gas phase combustion products are swept out of the bottom of the combustion tube by a constant flow of non-reactive helium carrier gas.

3)      All nitrogen bearing combustion products, including N2 and the various oxides of NOx, pass through the reaction column that has thin copper rods. The nitrogen oxides give up their oxygen and emerge from the column as N2.

4)      All of the carbon is converted to CO2 and the sulfur is converted to SO2. Water vapor is removed from the sample as the gases pass through a magnesium perchlorate vapor trap. The clean sample gases pass through a Gas chromatography column that separates the N2, CO2, and SO2. N2 elutes first, at about 1 minute. CO2 elutes second, at about 1.75 minutes. SO2 elutes last, at about 7 minutes.

5)      The sample gas pulses and the references stream of helium pass through a detector. The differences in the thermal conductivity between the two streams are displayed as viable peaks and are recorded as integrated areas.

6)      Combustion of known standard materials yields a regression line. This regression line is used to convert peak area of unknowns into total element values for each sample.

 

The combustion analyzer used at Franklin and Marshall College is a Costech ECS 4010 CHNS-O system. The calibration curve is generated using 7 aliquots of high purity standards.

Specific CNS methodology

1)      Two Bypass samples are loaded in the auto-sampler first to check peak detection and peak timing.  Sample detection times are verified in the calibration set-up.

2)      A seven-point regression line is established by weighing 4 aliquots of BBOT (0.25, 0.5, 0.75 and 1 microgram approximately) and 3 aliquots of sulfanilamide (0.3, 0.6 and 0.9 microgram approximately). The exact weights are recorded in the sample table and are used to calculate the regression line.

3)      Once the calibration standards, the correlation factors must be checked for the three elements. The factors should be 0.999xx. If they are not, one standard may be removed from the calculation to make the regression better. If more than one standard needs to be removed, then the standards must be rerun.

4)      Samples are weighed into tin capsules. 5-10 micrograms of sample are weighed out. 30 micrograms can be used, but it depends on the carbon levels in the samples. If the carbon peak is too high, the peaks will have a flat top. The concentrations cannot be used if the peak has a flat top. Sulfur peaks can be broad and tend to get broader as the combustion tube gets used. Once the peaks get too broad, the combustion tube must be changed.

5)      The weights of the samples are recorded onto the sample table and are used by the software to calculate the percent of N, C and S in the samples.