Methods of gas purification and effect on the ion composition in an RF atmospheric pressure plasma jet investigated by mass spectrometry

Abstract

The analysis of the ion chemistry of atmospheric pressure plasmas is essential to evaluate ionic reaction pathways during plasma-surface or plasma-analyte interactions. In this contribution, the ion chemistry of a radio-frequency atmospheric pressure plasma jet (μ-APPJ) operated in helium is investigated by mass spectrometry (MS). It is found, that the ion composition is extremely sensitive to impurities such as N 2 , O 2 and H 2 O. Without gas purification, protonated water cluster ions of the form H + (H 2 O) n are dominating downstream the positive ion mass spectrum. However, even after careful feed gas purification to the sub-ppm level using a molecular sieve trap and a liquid nitrogen trap as well as operation of the plasma in a controlled atmosphere, the positive ion mass spectrum is strongly influenced by residual trace gases. The observations support the idea that species with a low ionization energy serve as a major source of electrons in atmospheric pressure helium plasmas. Similarly, the neutral density of atomic nitrogen measured by MS in a He/N 2 mixture is varying up to a factor 3, demonstrating the significant influence of impurities on the neutral species chemistry as well.

Vacuum ultraviolet spectroscopy of cold atmospheric pressure plasma jets

“…If OH and H originate from water impurities, they should show a somewhat similar behavior. However, N can effectively react with OH to form NO and H. [71] This is in agreement with MBMS measurements of N densities as already mentioned above [67] indicating that N is effectively transformed into NO in reactions with impurities in the He/N 2 gas mixture with He 5.0 purity. In this article, we have given a detailed overview about windowless VUV spectrometry applied to APPs.…”

“…Additionally, the N density is very sensitive to O‐containing impurities (H 2 O, O 2 ) in the feed gas: Further purification of the He 5.0 purity gas with a cold trap and an oxygen absorber leads to 60% higher N densities at 0.3% N 2 admixture and up to factor three higher N densities at 0.65% N 2 admixture (see fig. 6 in Große‐Kreul et al [ 67 ] ). A more detailed insight into helium–nitrogen plasma chemistry, in particular, with a focus on additional admixture of O 2 and N 2 , for example, by air impurities, can be found in Bibinov et al [ 68 ] and Murakami et al [ 69 ]…”

Chemical kinetics in an atmospheric pressure helium plasma containing humidity

“…Water is also usually present as a feed gas impurity. 29 Therefore, the investigation of RS directly produced from water vapor, such as O and the hydroxyl radical OH, is of interest for biomedical applications. These species can act as precursors for longer-lived species such as hydrogen peroxide (H 2 O 2 ), an important signaling agent in cells, 19 , 30 and O 3 .…”

Numerical simulation of the effect of water admixtures on the evolution of a helium/dry air discharge

“…[6]. In such discharges the presence of water impurities is unavoidable and has been shown to highly affect the plasma chemistry and dynamic evolution [20][21][22][23][24][25][26][27][28][29][30][31][32][33]. In practice, water impurities are due to the operation of the plasma in the ambient humid air and also due to the plasma interaction with biotic surfaces in wet and moist environments.…”