ATTENUATION OF METAL-OXIDE IONS IN INDUCTIVELY-COUPLED PLASMA-MASS SPECTROMETRY WITH A GRAPHITE TORCH INJECTOR

Desolvated solution aerosols are injected into the axial channel of an inductively coupled plasma through a graphite tube inserted directly into the plasma. This hot injector constricts the stream of analyte and prevents it from widening excessively as it travels through the plasma. Thus, the sampling orifice for the mass spectrometer can be positioned several millimeters downstream from the tip of the initial radiation zone without substantial loss of analyte ion signal. The signal ratio for LaO+/La+ can be reduced to 0.05% with conventional desolvation or 0.01% with cryogenic desolvation under operating conditions that yield maximum metal ion signal. These values are greatly superior to those obtained with conventional torch injectors. The graphite injector also improves sensitivity for atomic ions by factors of 1.5-15, with the best improvements seen for elements such as As and Zn that have high ionization energies. This modification to the plasma does not compromise other analytical figures of merit such as rinse-out time or the suitability of a single set of operating conditions for multielement analysis.