Merits of microwave plasmas for optical emission spectrometry – characterization of an axially viewed microwave-sustained, inductively coupled, atmospheric-pressure plasma (MICAP)

Abstract

Considerable technological improvements in the field of microwave plasma technology intrigue to reconsider this technology as a promising alternative to the inductively coupled plasma (ICP) as emission-source for analytical spectrometry....

Effects of argon on the analytical properties of a microwave-sustained, inductively coupled, atmospheric-pressure plasma

“…The line selection was based on previous work on MICAPs in axial viewing. 3 The excitation temperature T exc was calculated from the slope of a Boltzmann plot 26 of 22 Fe(I) emission lines located in the spectral region $367-377 nm. Emission signals of the 400 mg L À1 Fe solution (solution #3) were used for this purpose.…”

“…1. 3 The microwave energy was provided by a 2M262A 1.5 kW 2.45 GHz magnetron connected to a Magdrive 2000 (Dipolar AB, Sweden) high voltage power supply. A conventional one-piece ICP-OES torch (for Ciros Vision EOP, Spectro, Germany) having the following dimensions was used: outer quartz glass tube: 17 mm inner diameter (ID), 19.4 mm outer diameter (OD); intermediate quartz tube: 14 mm ID, 16 mm OD, injector tube: 2.5 mm ID, 6 mm OD.…”

“…As krypton and xenon are too expensive for routine laboratories even when consumed only at flow rates of a few ml min −1 , this work focuses on argon/nitrogen mixed gas microwave plasmas, extending our previous work on air as the plasma gas. 3 As already noted, the use of nitrogen as the plasma gas entails an intense, highly structured optical plasma background over most of the spectrum relevant for optical emission spectrometry as well as degraded analyte excitation and ionization when compared to an argon ICP. Clearly, any improvement of one or both shortcomings of nitrogen plasmas can be expected to narrow the gap between MIPs and ICPs in terms of LODs.…”

“…1 However, when it comes to the actual number of instruments used in routine laboratories, the inductively coupled plasma (ICP) is the dominating plasma source for both optical emission (ICP-OES) and elemental mass spectrometry (ICP-MS). Reasons for this are diverse, but the fact that limits of detection (LODs) in ICP-OES are about one order of magnitude lower compared to those in MIP-OES, even when using the same spectrometer, 2,3 should not be underestimated. While there are several ways of constructing a microwave-powered plasma source, 4 nitrogen has been commonly used as a plasma gas in high power (≥1000 W) MIPs rather than argon due to plasma filamentation – which is the collapse of a diffuse, toroidal discharge into an arc.…”

“…Moreover, the optical plasma background is significantly smaller and less structured in an argon plasma when compared to a nitrogen plasma. 3 All the above mentioned factors contribute to the degraded LODs of microwave-based plasma sources, operated with nitrogen as the plasma gas. While nitrogen is generally cheaper and can even be produced on-site from compressed air in a nitrogen generator, argon is preferred from an analytical point of view.…”