Capacitively coupled microwave plasma

Preferred methods in trace determination of the elements include atomic absorption spectrometry (AAS), optical emission spectrometry (OES) with any of a wide variety of excitation sources [e.g., sparks, arcs, high-frequency or microwave plasmas (inductively coupled plasma, ICP microwave induced plasma, MIP capacitively coupled micro-wave plasma, CMP), glow discharges (GD). hollow cathodes, or laser vaporization (laser ablation)], as well as mass spectrometry (again in combination with the various excitation sources listed), together with several types of X-ray fluorescence (XRF) analysis [51]. 

Atsuya and Akatsuka [140] have described a method for determining trace amounts of arsenic. The technique, which uses capacitively coupled microwave plasma with an arsine generation system, has been used to determine arsenic in sewage sludge. 

The capacitatively coupled microwave plasma is formed by coupling a 2450 MHz magnetron, via a coaxial waveguide, to metal plates or a torch where the plasma is formed. Considerable problems have been encountered with this low-cost plasma, particularly from easily ionizable elements which cause dramatic changes in the excitation temperature in the plasma. 

Further designs of ion sources applied in plasma spectroscopy such as electrodeless microwave induced plasmas (MIPs) operating in a noble gas atmosphere at low power (mostly below 200 W) or capacitively coupled microwave plasma using Ar, He or N2 the as plasma gas (at 400-800 W) were described in detail by Broekaert.33 Microwave plasmas produced by a magnetron are operated at 1-5 GHz. Their special application fields for selected elements and/or element species are based (due to the low power applied) in atomic emission spectrometry.33... 

Kitagawa K, Nishimoto N. 1989. Thermal vaporizer - capacitively coupled microwave plasma system for trace mercury determination. J Spectrosc Soc Japan 38(4) 282-287. 

Fig. 103. Capacitively coupled microwave plasma (A) [442] and microwave induced plasma discharges (B, C). (B) MIP in a TMoio resonator according to Beenakker (a) cylindrical wall, (b) fixed bottom, (c) removable lid, (d) discharge tube, (e) holder, (f) coupling loop, (g) PTFE insulator, (h) ...

Seelig M., Bings N. H. and Broekaert J. A. C. (1998) Use of a capacitively coupled microwave plasma (CMP) with Ar, N2 and air as working gases for atomic spectrometric elemental determinations in aqueous solutions and oils, Fresenius ... 

The analytical plasmas are classified according to the method of power transmission to the working gas. There are three dominant types of plasma source in use today (i) Inductively coupled plasmas, ICPs (ii) Direct current plasmas, DCPs (current carrying DC plasmas and current-free DC plasmas) (iii) Microwave plasmas (microwave induced plasmas, MIPs, and capacitively coupled microwave plasmas, CMPs). 

Microwave plasmas are divided into two groups (i) microwave induced plasmas, MIPs and (ii) capacitively coupled plasmas, CMPs. The construction of these two plasma types is very different, and they also differ significantly in their efficiency and analytical possibilities. However, the working frequency of both plasma types is 2450 MHz with few exceptions. 

The equipment used for plasma polymerization, as well as the basic deposition mechanism, is shown in Fig. 1. Although commercial equipment is available, most researchers choose to design and construct their own equipment. AU that is needed is a vacuum chamber with a pump so that a gas or vapour can flow through the chamber and a means for exciting a plasma. Typically, internal electrodes (capacitively coupled plasma) or external coils are used (inductively coupled plasma), as shown in Fig. 1. The power supply can be DC, AC (60 Hz), RF (12.56 MHz), or microwave frequency (2.35 GHz). During deposition the pressure in the chamber is, typically, of the order of 1-20 Pa and powers of 1-100 W are often used. 

A) Arc B) Spark C) Flame D) Plasma sources E) Low-pressure discharges F) Graphite furnace G) Laser plume ICP = Inductively coupled plasma DCP=Direct current plasmajet CMP = Capacitively-coupled microwave plasma MIP = Microwave-induced plasma GDL = Glow discharge lamp HC = Hollow cathode... 

The atomic emission source provides for sample vaporization, dissociation, and excitation. The ideal excitation source will allow the excitation of all lines of interest for the elements in the sample, and do this reproducibly over enough time to encompass full elemental excitation. Excitation sources include but are not limited to (1) inductively coupled plasma (ICP), (2) direct current plasmas (DCP), (3) microwave induced plasmas (MIP), and (4) capacitively coupled... 

Seelig M. and Broekaert J. A. C. (2000) Investigations on the on-line determination of metals in air flows by capacitively coupled microwave plasma atomic emission spectrometry (CMP-OES), Spectrochim. Acta, Part B in press. 

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