FAKULTÄT FÜR CHEMIE
Mikroanalytisches Laboratorium


Universitaet Wienhttp://www.univie.ac.at

C/H/N-Analysis

Here you find a short introduction to the principles, a characterization of the working range and a compilation of known interferences in C/H/N elemental analysis.

... and always mind this important advice!


Introduction

Elemental analysis on carbon, hydrogen and nitrogen is the most essential - and in many cases the only - investigation performed to characterize and/or prove the elemental composition of an organic sample. Numerous compounds include no additional elements besides C, H and N except oxygen, which is seldom determined seperately (although it can be done!).

Determination of C/H/N is done using a "2400 CHN Elemental Analyzer" by Perkin Elmer. This apparatus was introduced in 1989. Prior C/H/N-analysis was performed on the precursor model "240 CHN Elemental Analyzer" developed by the same supplier. This instrument is now in use for the oxygen determination.


Measuring Principle

The sample under test is weighed in using a tin capsule. The required amount is 2 to 3 mg of organic material and can hardly exceed 10 mg, if inorganic matter with little carbon content is investigated. After folding the capsule (looking rather like wrapped tin foil) the sample is placed in the autosampler.

The tin capsule enclosing the sample falls into the reactor chamber where excess oxygen is introduced before. At about 990 °C the material is "mineralized". Formation of carbonmonoxide is probable at this temperature even under these conditions of excess oxygen.

The complete oxidation is reached at a tungsten trioxide catalyst which is passed by the gaseous reaction products. The resulting mixture shoulöd thus consist of CO2, H2O und NOx. But also some excess O2 passes the catalyst.

The product gas mixture flows through a silica tube packed with copper granules. In this zone held at about 500 °C remaining oxygen is bound and nitric/nitrous oxides are reduced. The leaving gas stream includes the analytically important species CO2, H2O und N2. Eventually included SO2 or hydrohalogenides are absorbed at appropiate traps.

High purity helium (Quality 5.0) is used as carrier gas. Finally the gas mixture is brought to a defined pressure/volume state and is passed to a gas chromatographic system. Separation of the species is done by so called zone chromatography. In this technique a staircase type signal is registrated. Step height is proportinal to the substance amount in the mixture.

Blank values are taken from empty tin capsules

Calibration is done by elemental analysis of standard substances supplied by the instrument's manufacturer for this purpose.


Working Range

Mineralization and detection covers every species of the analyte elements in the sample. Beside purely organic samples various metal organic compunds and even inorganic samples as carbides and nitrides have successfully been characterized.

The detection limit for carbon and nitrogen at sample amounts of 2 to 3 mg was found to be at about 0,05 w-% (500 ppm) in what case the uncertainty stays at about 0,02 w-%.

According to the apparate's supplier the instrument's uncertainty in the medium range stays below 0,3 w-% as required by journals to prove the expected composition. With very carbon rich samples we found this tolerance was slightly exceeded (still within 0,5 w-%).


Problems and Interferences

The weighing of oily or fluid substances is impossible using the thin walled tin capsules. For this purpose alumina pans with a lid are available. These pans are tightly closed by cold welding to prevent loss of sample by ... and minimize evaporation. As the blank value for nitrogen is dramatically increased be the enclosed volume of air the determination limit for N in liquid samples is increased to 0,1 to 0,2 w-%.

With highly viscous or even glassy materials elemental analysis is even impossible with the above method.

It has long been known that phosphorus can interfere in the mineralization of organic material. In literature the formation of glassy P2O5.x H2O.y C has been described. Elemental analysis of phosphorus contaning compunds can thus suffer from systematic deviations in the determined carbon content exceeding the tolerance limit of 0,3 w-%. This effect can be controlled by the addition of vanadium pentoxide (V2O5).

Fluorine is mineralized to form HF which reacts at the wall of the silica tubes which form the main part of the reaction zone. The gaseous products (SiF4 and relatives) can cause systematic errors which rarely become significant with respect to the 0,3 w-% tolerance.

The mineralization of metal containing samples can also be affected by interferences. By modification of the method most of these can be compensated - but this requires that we know about the composition ...

Therefore ... mind this important advice!

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