Optimization and Characterization of a Nitrogen-Sustained Microwave Inductively Coupled Atmospheric Pressure Plasma Quadrupole Mass Spectrometer (N2-MICAP-MS) for Elemental Analysis

The performance of a nitrogen (N2)-sustained Microwave Inductively Coupled Atmospheric Pressure Plasma (MICAP) ionization source integrated with a Quadrupole Mass Spectrometer (QMS) was characterized.Instrument properties known to influence performance in ICP-MS analysis such as plasma forward power, gas flows (nebulizer and auxiliary), torch injector internal diameter, and water load into the plasma were examined.Additionally, LODs and linear dynamic range for selected analytes were determined for MICAP-QMS with solution sample introduction.The instrument was optimized for maximum signal intensity while maintaining the oxide-to-element ratio (i.e.CeO + /Ce + ) at ≤ 3% and double charged ion formation (i.e.Ba ++ /Ba + ) at ≤ 2%.Changes in plasma forward power show no or little effect on ionization of low-ionization-energy elements; however, higher power is required for ionization of elements with first ionization energies greater than ~9 eV.The nebulizer and auxiliary gas flows of the MICAP were optimal at 1.15 and 1.50 L min -1 , respectively.A comparison of wet and dry (desolvated) sample introduction with injector diameters of 0.8, 1.5, and 2.4 mm showed that optimal conditions are obtained with a dry plasma and a 1.5 mm injector diameter.A linear dynamic range (LDR) of 10 8 for the MICAP-QMS is demonstrated; this LDR is 2-3 orders of magnitude lower than conventional ICP-QMS.The LDR of the MICAP-QMS is limited at low concentrations by a high continuous background, likely from metastable ions or photons reaching the detector.Limits of detection span from parts-per-trillion to parts-per-billion.