Quantifying plasma chemistry products at atmospheric pressure

Atmospheric plasmas are a gas of ionised atoms or molecules, which occur at or near atmospheric pressure. These plasmas can produce a range of reactive species, many of which can be very valuable to us in a number of applications. But which species can be produced by atmospheric plasmas? How can they be of use? We asked Gert Willems to talk to us about his recent JPhysD publication “Absolutely calibrated mass spectrometry measurement of reactive and stable plasma chemistry products in the effluent of a He/H2O atmospheric plasma“, which works to quantify the plasma chemistry products produced in atmospheric plasmas. A number of these species are biologically relevant, and so this work has important implications. Read on to find out what Gert Willems had to say about his recent paper, and the significance it has in the important, emerging field of plasma medicine.

Low-temperature gas discharges, known as plasmas, have been around for quite a while. They are formed in gas at low or atmospheric pressures where a part of the atoms or molecules is ionized through the application of strong electric fields. The generated free electrons and ions can be further manipulated by electric fields to produce, for example, the fine nanostructures of modern processors. In this case, this is done under a pressure 100 times lower than atmospheric pressure. Cold atmospheric plasmas are operated at atmospheric pressure, and have been used already in the nineteenth century by Werner von Siemens to generate ozone. They are however still investigated as an attractive source of reactivity, for example in the new research field of plasma medicine. Next to the charged species and electric fields, the cold ionised gas also generates many biologically relevant reactive species, such as NO (a known signaling molecule in human body), ozone, OH radicals, hydrogen peroxide, and energetic UV light. Importantly, this unique combination of reactivity is generated in a cold gas (temperature below 40°C), meaning it can be brought into contact with living tissues such as skin or chronic wounds. As a result, bacteria can be inactivated, and the biological activity of tissues or the healing process of a wound can stimulated. Many studies have also demonstrated that plasma treatment preferentially damages cancer cells.

MBMS measured H2O2 and OH densities as function of distance. The full lines represent results of the 2D axially symmetric fluid model of the species transport and reaction kinetics in the plasma effluent. The dotted, dashed and short dotted lines represent the model results for nini,H = 1 x 1013cm-3, nini,H = 1 x 1014cm-3 and nini,H = 2 x 1014cm-3 respectively. The line indicated by "diffusion only" shows the effect on nH2O2 due to radial diffusion losses only. Due to experimental constraints, HO2 was only measured for one distance point. Image taken from Gert Willems et al 2017 J. Phys. D: Appl. Phys. 50 335204, © IOP Publishing, All Rights Reserved.

MBMS measured H2O2 and OH densities as function of distance. The full lines represent the results of the 2D axially symmetric fluid model of the species transport and reaction kinetics in the plasma effluent. The dotted, dashed and short dotted lines represent the model results for nini,H = 1 x 1013cm-3, nini,H = 1 x 1014cm-3 and nini,H = 2 x 1014cm-3 respectively. The line indicated by “diffusion only” shows the effect on nH2O2 due to radial diffusion losses only. Due to experimental constraints, HO2 was only measured for one distance point. Image taken from Gert Willems et al 2017 J. Phys. D: Appl. Phys. 50 335204, © IOP Publishing, All Rights Reserved.

In our research we focus on quantitative measurements of the densities of plasma-generated reactive species and photon fluxes, and on the characterisation of their isolated and combined effects on a variety of biological samples; ranging from small proteins, to human cells. Molecular beam mass spectrometry (MBMS) is a very good tool for this research, because it can quantitatively analyse the composition of the plasma, including highly reactive species and ions. The investigations are also performed at the Ruhr-University Bochum developed reference jet, which is used in several laboratories in Europe in plasma medicine studies, making our results highly relevant.

In our latest publication, we studied with molecular beam mass spectrometry a cold plasma generated in a He/H2O gas mixture. This measuring system places reactive and neutral species from atmospheric pressure into a very low-pressure chamber, thereby creating a collisionless environment, instantaneously freezing all reactions. The sampled species can then be analysed according to their mass, and their concentrations can be calibrated with a detection limit down to 0.1 parts per million. The obtained data provides insight into plasma chemical generation pathways of these species. When correlated with the biological effects induced by the plasma treatment, the data also provides an understanding of the fundamentals of the interaction of these species with the studied samples. In our study, we were also able to determine the direct electron impact ionisation cross-section of the H2O2 molecule, which was not reported before.

The He/H2O plasma is important for its use in biomedical applications, because the production of reactive oxidising species such as atomic oxygen or hydroxyl radicals seems to be essential in treating bacteria or cancer cells. Additionally, biomedical systems typically consist of humid interfaces, or are immersed in aqueous environments.

About the Authors

Gert Willems is a Marie Curie Fellow at the Research Department “Plasmas with complex interactions” at Ruhr-Universität Bochum, Germany, and is about to complete his PhD. He studied physics engineering at Ghent University, Belgium and focuses on understanding the reaction chemistry of low temperature, atmospheric pressure plasmas.

Prof. Dr. Jan Benedikt is full professor at Kiel University, Germany. He studied physics at University of West Bohemia in Plzeň, Czech Republic and obtained his PhD degree in 2004 at Eindhoven University of Technology in The Netherlands. He was research assistant and junior professor at the Ruhr-Universität Bochum between 2004 and 2017. His main research fields are plasma diagnostics, with a focus on molecular beam mass spectrometry, plasma-solid interaction, and atmospheric pressure plasmas for future applications. He is an author of eighty research papers and he received the Hans-Werner-Osthoff Plasma Physics Prize in 2009.

Prof. Dr. Achim von Keudell is full professor at the Experimental Physics Institute for Reactive Plasmas at Ruhr-Universität Bochum, Germany. He studied physics at the Technical University of Munich, got his PhD at the University of Bayreuth, and was staff scientist at the Max-Planck-Institute for Plasma Physics before he became Professor at the Ruhr-Universität Bochum. His research focuses on plasma-surface interactions in low pressure magnetized and atmospheric-pressure plasmas.


CC-BY logoThis work is licensed under a Creative Commons Attribution 3.0 Unported License. Image taken from Gert Willems et al 2017 J. Phys. D: Appl. Phys. 50 335204, © IOP Publishing, All Rights Reserved.



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