Elementary Analysis

The term quantitative elementary analysis is understood to mean the quantitative determination of the elements carbon (C) and hydrogen (H) as the main constituent of all organic substances, as well as the most important heteroatoms: nitrogen (N), oxygen (O), sulfur, more rarely also phosphorus (P) and the halogens (F, Cl, Br, I).

Commercially available devices of the CHNS elementary analysis work according to the principle of the so-called combustion analysis. A precisely weighed sample quantity is transferred into a combustion tube via a sample sluice and burnt at constant temperatures of up to ~1400 °C (due to exothermal reactions up to 1800°C) with the addition of pure oxygen (O2). The liberated gaseous oxidation products (analyte gas) are subsequently passed through the complete analyzer with the aid of an inert carrier gas. In order to reduce the nitrogen oxides (NOx) formed during the combustion to nitrogen (N2) and to remove possible O2 excesses from the gas sample, the gas sample is then passed through a pack of hot copper (chips or granulate). As a result, only the analysis gases N2, CO2, H2O and SO2 remain in the He carrier gas stream. At this point, the ULTRA.sens and the INFRA.sens are used for the selective and quantitative determination of carbon (as CO2) and sulfur (as SO2). In the following schematic diagram diagram, such a structure can be seen.

Principle Eelental Analyser

 

ICARUS2                                    ICARUS Tiegel 2

Automatic analyzer ICARUS (Bruker Elemental GmbH)                        A metal sample is inserted into the sluice

 

Calculation

With the aid of an example, the basic calculation basis for an elementary analysis will be shown below.

 

The weight of the unknown sample should be m1 = 0.1754 g.  Assume that the sulfur content in this example is cS = 0.178%. With Gl. 1, the mass can be calculated as follows:

Gl.1

The sample therefore contains mS= 0.0003122g S. The ratio of the molar masses of sulfur and sulfur dioxide is approx. 2, so that the following relation exists for the mass of sulfur dioxide which results from a given mass of sulfur:

Gl.2

From 0.0003122 g S thus ≈0.0006244g SO2 originate. The molar mass of SO2 is ≈ 64 g/mol. Under normal conditions, 64 g of SO2 occupy a volume of 22.4 l. This results in the SO2 volume (= 0.000219L) in the gas phase:

Gl.3

During the peak process, the 02 volume flow leads to a total volume VN2 in which the whole volume of S02 is located:

Gl.4

This relationship is shown in the following figure. With GL. 4, the yellow, rectangular course is described. The integral is identical in both cases.

Peak

The SO2 concentration in the gas phase detected by the sensor can then be calculated as VSO2<<VO2 as follows:

Gl.5

If we take the expressions from Gl. 1-4 in GL. 5, a relationship between the concentration in the gas phase and the S content in the solid sample becomes apparent:

Gl.6

Since, in this method of analysis, the concentration in the solid sample should be concluded from the concentration in the gas phase, this equation must then be transposed for the desired variable. The following relationship then results:

Gl.7

The constant quantities or controlled variables can be combined into a device factor or calibration factor f, which is determined experimentally. Certified solid samples are used for this purpose.

Calculation

Literatur:

Paplewski, P.: Gasanalyse von Metallproben (C, S, H, N, O). Chapter 15.7 aus  Gasmesstechnik in Theorie und Praxis (Wiegleb, G.) Springer-Vieweg Verlag 2016 S. 1057-1075

Wiegleb, G.: A novel fast response, low level gas sensor system for detection of SO2 and CO2, based on combined NDIR- and NDUV-technology. PEFTEC Conference 2015 (19. Nov.) in Antwerpen PEFTEC 2015