Microwave Plasma Systems in Optical and Mass Spectroscopy

Abstract

The art and science of microwave plasma (MWP) optical and mass spectroscopy is briefly presented including very recent advances in the field up to 2015. The use of MWPs as radiation sources for optical emission (OES) and atomic fluorescence (AFS) spectroscopy and as atom reservoirs for atomic absorption (AAS) and cavity ringdown (CRDS) spectroscopy as well as ion sources for both elemental and molecular mass spectrometry (MS) is treated. Devices for producing both E‐type capacitively coupled microwave plasma (CMP)‐electrode and microwave‐induced plasma (MIP)‐electrodeless MWPs, including ICP‐like H‐type plasmas, are classified and discussed, in addition to techniques of their diagnostics, and results for the analytically relevant plasma parameters are presented. The means of generation of voluminous symmetrical plasmas and uses of microplasma devices are also presented with an effort to comment on general classification of microwave cavities. Further, the use of microwaves for boosting of glow discharges (GDs) and laser‐induced plasmas (LIBS) is treated along with other tandem sources. Methods for introduction of gaseous, liquid, and solid samples into the MWP are discussed. They include direct vapor sampling (DVS), chemical vapor generation (CVG) and hydride generation (HG) techniques, dry aerosol generation techniques (electrothermal vaporization (ETV), spark (SA) and laser ablation (LA) and continuous powder introduction (CPI)), and wet aerosol generation techniques using both solution and slurry nebulization. Special reference is made to coupling with gas chromatography (GC) and also with various separation techniques for liquids including high‐performance liquid chromatography (HPLC), supercritical fluid chromatography (SFC), and size‐exclusion chromatography (SEC). The analytical figures of merit in the case of OES with MIPs, CMPs, microwave plasma torch (MPT), MWP‐electrode sources including rotating‐field‐sustained plasma, and H‐type MWP as well as microplasmas are given. There are also described cases of atomic absorption and fluorescence with these sources. The developments in both elemental and molecular MS in the case of both cold and hot MWPs and in the case of various types of sample introduction techniques are discussed. Applications of MWP analytical spectroscopy are in the fields of biological, clinical, environmental, and industrial samples with special reference to multielement analysis, element speciation, on‐line monitoring, particle sizing, and direct solids analysis. A critical comparison of the methodology with other spectroscopic methods for the determination of the elements and their species is given.

Replacing the Argon ICP: Nitrogen Microwave Inductively Coupled Atmospheric-Pressure Plasma (MICAP) for Mass Spectrometry

“…He-MWPs are well-suited for the detection of high-IE elements, such as the halogens and nonmetals. However, due to relatively low gas temperatures and power densities, He-MWPs are not capable of efficient vaporization and atomization of liquid or solid aerosols, and are thus not a viable replacement of the ICP …”

Quantification capabilities of N2 MICAP-MS with solution nebulization and aerosol desolvation

“…However, these high-power N 2 MIP-MS were only sold in Japan 7 and have thus not been widely distributed. 8 The emerging MICAP is another high-power nitrogen plasma that can be used as an alternative ion source for OES 9 and MS. 10 Compared to the conventional argon-based inductively coupled plasma (ICP) employed for these analytical techniques, [11][12][13] the nitrogen microwave plasmas are more costeffective in their operation and reduce the occurrence of argon-containing spectral interferences. 6,14 In addition, the MICAP should exhibit a negligible plasma potential since the microwave coupling is purely inductive.…”