Naturally occurring molecular species used for plasma diagnostics and signal correction in microwave-induced plasma optical emission spectrometry†
In the present study, we evaluate the N2+/OH emission intensity ratio as a diagnostic tool for identifying the best instrumental operating conditions in microwave-induced plasma optical emission spectrometry (MIP OES). This molecular species signal ratio is compared with the traditional Mg II/Mg I ratio. Aluminum, Ba, Mn, Sr and Zn (analytes), and high concentrations of C, Na, Ca, HNO3 and HCl (sample matrices) are used as models to investigate the effects of complex matrices on analyte recoveries. The N2+/OH signal ratio is more sensitive to changes in plasma conditions than the Mg II/Mg I ratio. Some other advantages include real-time monitoring capabilities, and the possibility of independently tracking variations in both plasma and sample introduction. For less complex matrices, the N2+/OH signal ratio may be used for instrument optimization, which ensures plasma conditions are as similar as possible when analyzing standard solutions and samples. For analyses involving severe matrix effects, molecular species such as CN, N2, N2+ and OH are used for signal correction. Significant improvements in accuracy are achieved by employing the analyte-to-molecular species signal ratio, or their product, for calibration. Both the use of the N2+/OH signal ratio as a diagnostic tool, and of molecular species for signal correction to minimize matrix effects are simple strategies that may significantly contribute to expanding the analytical capabilities of MIP OES and facilitating its application in routine analysis.