Spatial emission profiles for flagging matrix interferences in axial-viewing inductively coupled plasma-atomic...

Spatially resolved measurements of analyte emission along the cross-sectional axis of an axially viewed inductively coupled plasma (ICP) are utilized to indicate the presence of any of the three major categories of matrix interferences (i.e., plasma-related, sample introduction-related, and spectral interferences). Barium at concentrations of 0.05 or 0.1 M was chosen as a prototype element for plasma-related matrix effects, whereas common mineral acids (nitric, hydrochloric, sulfuric, and phosphoric) at volumetric concentrations from 1% to 20% were used to simulate sample introduction-related matrix effects. Three spectrally interfering line pairs (As and Cd at 228.81 nm, Er and Co at 239.73 nm, and Er and Ce at 302.27 nm) were selected for the study of spectral interferences. Due to dependence on the nature of the interference, the analytical bias at the center of the cross-sectional profile varied between -40% and +50%. In all matrix-interference categories, because plasma characteristics and excitation conditions are heterogeneous along this cross-sectional axis, matrix-induced shifts in analyte emission vary accordingly. As a result, the concentrations determined for an analyte along the cross-sectional plasma axis are not constant but exhibit a position dependence that allows the interference to be flagged. With the exception of spectral interference from emission lines whose total excitation potentials (i.e., the sum of ionization and excitation energies of an ionic emission line) are very close, the spatially resolved concentrations provide an effective indicator for flagging any other matrix interference in axial-viewing ICP-emission spectrometry. The method can be employed under the plasma forward power and carrier-gas flow conditions that are common for robust plasma operation.