We present in this letter how a molecular gas such as nitrogen at different levels of impurity dominates the ionic composition of an atmospheric pressure noble gas plasma such as in helium. The positive charge in the discharge is only determined by helium ions if the discharge gas contains less than 1 ppm of impurity. Above this impurity level, the positive charge is completely determined by the impurity nitrogen. The higher the relative nitrogen concentration, the more N4+ dominates over N2+. If the impurity level is between 1 and about 20 ppm, N2+ is clearly the most abundant positive ion but for higher levels of impurity, N4+ almost completely determines the positive charge.
Martens, T., Bogaerts, A., Brok, W. J. M., & Dijk, van, J. (2008). The dominant role of impurities in the composition of high pressure noble gas plasmas. Applied Physics Letters, 92(4), 041504-1/3. . https://doi.org/10.1063/1.2839613