Effect of special samples on organic element analysis tests

The test results of metal/alkali metal samples (content ≤ 15%) have a large error;

When the sample containing metal elements is tested in CHNS mode, some metal particles will flow with the carrier gas and enter the adsorption column, which will affect the lifetime of the adsorption column. At the same time, the presence of metal elements can also deactivate the catalyst, resulting in errors in the test results.

Alkali metal elements will also react with the H₂O produced in the combustion tube to form alkali, which will cause certain corrosion damage to the test instrument;

In addition, the Me-C bond (Me stands for metal element) in the metal sample is difficult to break, which is easy to cause incomplete decomposition of the sample, and will also produce a certain error in the test results.

The test error of the sample containing F is large and the organic matter of high F and all F is prohibited;

In addition to the strong stability of the C-F bond and the fact that it is not easy to break, the more important reason is that the combustion product of fluorine-containing compounds, hydrogen fluoride, can react with silica to form volatile silicon tetrafluoride, which introduces analysis errors, and seriously corrodes the gas pipe wall and shortens the service life of the instrument.

The hydrogen content in high-fluorinated and perfluorinated organic compounds is very low or even zero, and the hydrogen atoms in the compounds are obviously insufficient during the combustion and decomposition process, resulting in the direct combination of fluorine and carbon to form carbon tetrafluoride;

Carbon tetrafluoride is extremely thermally stable, not easy to decompose, and follows the nitrogen peak in the separation process of combustion products, which cannot be distinguished by the thermal conductivity cell detector, resulting in a negative error of carbon value and a positive error of nitrogen value.

The reason for the large test error of the sample containing P and B (P content ≤20%);

Due to the strong stability of the C-P bond and the fact that it is not easy to break, the phosphorus pentoxide generated after combustion and decomposition is easy to encapsulate the carbon particles, so it requires a fairly high temperature to make it completely combusted. B-containing organic compounds also affect the determination results of carbon because the C-B bond is not easy to break. At present, the temperature of the combustion tube (950-1150 °C) can basically ensure the complete decomposition and quantitative conversion of organic phosphorus and organic boride;

However, for some special types of organophosphorus and organoborides that are extremely difficult to decompose, the method of covering the sample with a mixture of 1:1 V₂O₅ and K₂Cro₇ or WO₃ is generally used to promote the cleavage of C-B bonds and C-P bonds. However, this test is cumbersome and complex, so it is not possible to provide such a test method for the time being.

Effect of Si-containing samples on organic element analysis and testing;

It is not recommended to carry out organic element analysis test for silicon-containing samples, because the decomposition temperature of SiO₂ in inorganic silicon is 1600 degrees, while the organic element analysis can only measure 1150 degrees, so the oxygen in it cannot be completely decomposed and detected;

Although silicone can be decomposed, the silicon inside may combine with oxygen and carbon to form SiO₂ and SiC respectively (decomposition temperature 2210°C), resulting in low test results. At the same time, the generated SiO₂ will also enter the catalytic oxidant filling area of the combustion tube with the gas flow in the form of fine dust, poisoning the catalytic oxidant and making it lose oxidation activity.

At present, the most commonly used method is to cover the sample with a layer of oxidant, such as WO₃ or mixed oxidant WO₃+V₂O₅, to prevent the formation of SiO₂ dust and inhibit the formation of SiC. However, such tests are complex and cumbersome, so they are not available for the time being.

Strong acids, alkalis, and explosive samples (such as gasoline, diesel, explosives, and some nitrocelluloses) cannot be tested for organic element analysis;

Samples with strong acids and alkalis will seriously corrode the gas pipe wall during the test process, shorten the service life of the instrument, and the samples that can cause explosiveness are prone to explosion under high temperature conditions, which will cause the combustion pipe to burst at least and cause significant damage to the instrument.

The effect of moisture-absorbing, aqueous (including crystalline water) samples on the test results;

During the test of aqueous samples, the presence of moisture will introduce elements H and O other than the test samples, resulting in the deviation of the content of each element in the test results.

In addition to the effect of moisture introduction, the weighing quality of the sample before the test will be inaccurate, which will also lead to the error of the content of each element.

The element content in the sample is too low, and the test results are inaccurate;

The sample size of the organic element analysis test weighed is 3~5mg, so the content of the corresponding detected substances produced by the cracking of the low element analysis process is very low, and the probability of the uneven distribution of the elements in the sample of the low content element is large, so the error of the test results is large, and in principle, the element results with a test content of less than 1% are for reference only.