Thermodynamic modeling for inductively coupled plasma has opened new horizons in analytical chemistry.

The work has been published in the Journal of Analytical Chemistry. Inductively coupled plasma is a plasma that forms in an inert gas within a discharge chamber, burner, or other plasma reactor when a high-frequency alternating magnetic field is applied. The plasma used in spectrochemical analysis is nearly electrically neutral because the positive ionic charge is balanced by the negative charge of free electrons.

Understanding the role of primary background ions in thermochemical processes became the focus of research in the 1980s and 1990s. Scientists noted how ions generated in the plasma can influence recombination, ionization, and interactions with neutral particles.

New articles by Russian researchers have published findings based on a multi-component quasi-equilibrium thermodynamic model. In these studies, the authors analyze the thermochemical processes occurring in inductively coupled plasma and the details of background ion formation, which traditionally causes significant spectral interferences in analyses. The research utilized thermodynamic modeling methods to determine the plasma composition based on temperature and the composition of its working medium.

During the experiments, the concentration of major background ions, such as H+, O+, OH+, H2O+, H3O+, NO+, Ar+, ArH+, ArO+, and others, was studied in relation to plasma temperature, water aerosol concentration, and the presence of nitric and hydrochloric acids in test systems.

Thermodynamic calculations for cold plasma were conducted at atmospheric pressure of 0.1 MPa over a temperature range from 2000 to 5000 K in increments of 500 K. Similar calculations for normal plasma covered a temperature interval from 5000 to 8000 K. The obtained data show that increasing the plasma temperature in the cold region to 3500 K leads to a significant rise in the concentration of primary ions, while for normal plasma above 5000 K, a decrease in the concentrations of H2O+ and H3O+ ions was observed alongside an increase in the concentrations of other background ions. All of this aligns well with experimental data.

The results indicated that the use of thermodynamic modeling allows for a high degree of accuracy (determination coefficient R² = 0.92) in predicting the gas-kinetic temperature of the plasma. At the same time, wide variations in parameters, such as mass ratios of system components, did not lead to significant changes in concentration dependencies.

“The closeness of theoretical and experimental data confirms that primary background ions in cold plasma are generated directly in the discharge plasma, highlighting the importance of understanding these processes for analytical applications,” said Maxim Maltsev, a researcher from the Department of General Physics at MIPT. “We proposed a method for unequivocally estimating the gas-kinetic temperature by comparing calculated and experimental data on primary ions. The thermodynamic modeling algorithm can be enhanced with conditions to account for complex interactions among all primary background ions.”

The research findings demonstrated agreement between theoretical calculations and real experimental data, confirming the effectiveness of the thermodynamic model used for analyzing thermochemical processes in inductively coupled plasma. This agreement opens the door to further calculations and applications in solving practical analytical tasks. The studies involved scientists from MIPT, UrFU, the Institute of Metallurgy of the Russian Academy of Sciences, the Institute of High Temperature Physics of the Russian Academy of Sciences, and KubSU.