Background Spectral Features for Moderate-Power Nitrogen Microwave-Induced Plasma-Mass Spectrometry

Abstract

The mass spectral background features of a moderate-power nitrogen microwave-induced plasma are presented. In general, the mass spectral background obtained from a nitrogen plasma is less congested at low mass than is the case with argon ICP-MS. In addition, there are no significant (less than 20 counts per second) background species above mass 60. At normal multiplier voltage, the major background species found in a dry nitrogen plasma include ¹⁴N⁺, ¹⁴NH⁺, ¹⁴N₂⁺, ¹⁴N₂H⁺, ¹⁴N¹⁶O⁺, ¹⁴N¹⁶OH⁺, ¹⁴N₃⁺ and ¹⁴N₄⁺. This background is unchanged by the nebulization of deionized water, 5% (v/v) acids (nitric, hydrochloric, and sulfuric acids), and 5% (v/v) methanol solutions. These background species do not interfere with the determination of K, Ca, and Se at their major isotopes, which commonly have interfering background species in Ar ICP-MS. On the basis of the nebulization of 5% HCl and methanol in the nitrogen plasma, As and Cr do not have isobaric interferences at their major isotope masses 75 and 52. In addition, preliminary studies on the effect of Na as a concomitant element on analyte signals indicate severe enhancement or suppression of analyte signals.