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Basic principles and tests of organic element analysis


What are Organic Elements?

Organic elements usually refer to the most common and widely distributed elements such as carbon, hydrogen, nitrogen, sulfur, and oxygen contained in organic matter; By measuring the percentage content of each element, the elemental composition and chemical formula of the compound can be further determined.

There are many methods for analyzing the content of carbon, hydrogen, nitrogen, sulfur, oxygen and other elements in organic compounds, including gas chromatography, differential absorption and various electrochemical analysis methods (such as conductivity analysis, coulombic analysis), etc.

With the rapid development of science and technology, the organic element analyzer has gradually developed into a new generation of instruments with the characteristics of automation, computer data processing and multi-element joint determination in order to meet people's efficient and convenient testing requirements.

Principles of organic element analysis

The test principle of organic element analysis is very simple, mainly using the high-temperature combustion method to detect the content of each element in the organic compound, and the single test time is only 9min. The test modes can be divided into three modes: CHNS, CHN and O.

In CHNS/CHN mode, the sample is completely burned in an oxidation tube with a pure oxygen atmosphere at 1150 °C to produce CO2, H2O, NOx, SO2, SO3 and other gases, and then the mixture is further reduced to CO2, H2O, N2, SO2 and other gases in a reduction tube (850 °C, reduced copper) and separated by an adsorption-desorption column (Elementar UNICUBE: temperature-programmed resolution-TPD), and then separated by a chromatographic column for thermal conductivity detection, and C, H, N, S element content, as shown in Figure 1.

Temperature-programmed (TPD) is when gases such as CO2, H2O and SO2 are adsorbed at close to room temperature, while N2 is directly detected by TCD unimpeded. After the nitrogen peak detection, gases such as CO2, H2O, and SO2 were desorbed and detected by TCD when the adsorption column temperature rose to 60°C, 140°C, and 220°C, respectively, as shown in Figure 2.

The O mode refers to the pyrolysis of the sample in a mixture of H2/He at 1150°C, followed by the reduction of carbon powder into CO, and the detection of O element content by thermal conductivity.

CₓHᵧNₘSₙ + (x + n + y)/4 O₂ → xCO₂ + nSO₂ + m/2N₂ + y/2H₂O (CHNS mode as an example)

Figure 1:Diagram of the reaction mechanism of organic element analysis and testing
Figure 2:Thermal conductivity detection curve of gas products of organic element analyzer

Test standard: CHNS: aminobenzene sulfonic acid; CHN: acetanilide; O mode: benzoic acid

Selection of test conditions

According to the characteristics of the sample and the type of test element required, different operation modes can be selected from the three modes of CHNS, CHN and O for testing; There are some differences in the test conditions and experimental requirements of each operation mode, as follows:


Sample requirements

Usually powder and bulk tests are the mainstay, but liquid samples with non-aqueous solvents can also be used, but the melting range and boiling range of the sample must be within the allowable range; Powder samples generally need to be more than 30mg, the size of bulk samples is required to be less than 2*2mm, and solution samples need to be provided 2ml;

All test samples are required to contain no elements such as water, metals, Si, P and halogens, which will lead to errors in test results and damage to the instrument;

Samples with strong acids, alkalis or explosive properties (such as gasoline, diesel, explosives, and some nitrocelluloses) are prohibited from organic element analysis tests.

Test accuracy and accuracy

Accuracy: ≤0.2% (measurement accuracy refers to the degree of agreement between the measured values and the degree of proximity to their "true value", that is, the comprehensive concept of precision and correctness);

Accuracy: ≤0.3% (accuracy refers to the degree to which the average value of multiple measurements under certain experimental conditions is consistent with the true value).