Merits of microwave plasmas for optical emission spectrometry – characterization of an axially viewed microwave-sustained, inductively coupled, atmospheric-pressure plasma (MICAP)†
Considerable technological improvements in the field of microwave plasma technology intrigued us to reconsider this technology as a promising alternative to inductively coupled plasma (ICP) as an emission source for analytical spectrometry. We have investigated and characterized the analytical capabilities of an axially viewed microwave-sustained, inductively coupled, atmospheric-pressure plasma (MICAP) as a potential emission source for spectrometry. In combination with the spectrometer part of a commercial inductively coupled plasma optical emission spectrometer (ICP-OES), limits of detection (LOD) and limits of quantification (LOQ) for 30 elements based on 72 emission lines were determined and compared to ICP as the reference emission source. LODs for MICAP were about a factor of ten higher than those obtained by ICP-OES when using the same spectrometer, detector and data processing software. However, the magnitude of LOD degradation was strongly dependent on the element, the emission line and the excitation energy of the transition. In comparison with a commercially available magnetically excited (Hammer cavity) microwave plasma-optical emission spectrometer (MIP-OES), the LODs were similar with the exception of Ni and Pb. For these elements, lower LODs were attained with the Hammer cavity based instrument due to the more elaborate algorithms available in the control software for background correction and subtraction of the highly structured nitrogen plasma background. The impact of the used plasma gas was investigated for the MICAP with a special focus on air as the sole plasma gas, emphasizing the capabilities of this approach for cost-effective operation, when LODs are not the prime requirement.