The electron number density of atmospheric-pressure argon and helium microwave-induced plasmas operating in the power regime of 100 to 450 W has been examined. The resulting data demonstrate a trend of increasing electron density, <i>n</i><sub>e</sub>, for both the Ar and He microwave-induced plasmas as forward power is increased. An examination of <i>n</i><sub>e</sub> vs. plasma observation position demonstrates a maximum in <i>n</i><sub>e</sub> at the central plasma observation position for both plasmas. Further, spatial dependence of electron density appears to be more pronounced at high power levels. Nebulization of aqueous solutions containing varying concentrations of an easily ionizable element into the Ar microwave-induced plasma, MIP, demonstrates little if any effect on <i>n</i><sub>e</sub>. Moreover, this observation can be explained by the fact that there is a far greater quantity of water than easily ionizable element being introduced into the plasma in a given time period. Thus the electron contribution resulting from water degradation products in the plasma far outweighs that from the relatively small amount of easily ionizable element present. This last point is further substantiated by an examination of the Ar MIP with and without solution nebulization.

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