Microwave plasma cavity

Another type of microwave plasma cavity which has been used successfully in GC-MIP is the Surfatron which operates by surface microwave propagation along a plasma column [17]. The plasma may be viewed axially or transversely since it extends outside the plasma structure it can sustain a dicharge over a wide pressure range. 

Fig. 1.1. The Beenakker microwave plasma cavity. (Reproduced by permission, from J. Chromatog., 1982, 239, 181 copyright 1982, Elsevier Science Publishers.)...

Richts U., Broekaert J. A. C., Tschopel P. and Tolg G. (1991) Comparative study of a Beenakker cavity and a surfatron in combination with electrothermal evaporation from a tungsten coil for microwave plasma optical emission spectrometry (MIP-AES), Talanta 38 863-869. 

Microwave Plasma Discharge A discharge produced with or without electrodes by microwave fields in stationary or flowing gas streams in a microwave cavity. liquids, gases 1500... 

A more radically different approach for liquid sample introduction is the microwave plasma torch (MPT) (Figure 2). In a recent design three concentric tubes were used where the outer two brass tubes acted as a coaxial waveguide for the microwaves eliminating the need for a cavity. The iimer quartz channel was nonconducting and the heliiun plasma that was formed (at power levels 70-200W) was similar to the ICP plasma, having a central chaimel that was more efficient for sample introduction as described previously. A sheath gas was used to stabilize the MPT and reduce air entrainment. 

A strip-line microwave plasma has been described by Barnes and Reszke [281], [282]. A particular advantage of this design is that it is possible to sustain more than one plasma in the cavity at the. same time. Such a plasma has compared favorably with other designs [282], though it has not yet been widely used. 

Microwave plasmas are operated at 1-5 GHz. They are produced in a magnetron. The electrons emitted by a glowing cathode are led through a cavity with a series of radially arranged resonance chambers to which an ultra-HF (UHF) field is applied. 

An argon or helium plasma is sustained in a microwave cavity which serves to focus or couple power from a microwave source, usually operated at 2.45 GHz, into a discharge cell which is a capillary tube, made of quartz, boron nitride, alumina etc.. Microwave plasmas of different cavity designs may be operated at atmospheric or under reduced pressures [12, 13]. The 50-100 watt power levels for analytical microwave plasmas are much lower than for the DCP or the ICI giving... 

In plasma chemical vapor deposition (PCVD), the starting materials are typically SiCl, O2, 2 6 GeCl (see Plasma technology). Plasma chemical vapor deposition is similar to MCVD in that the reactants are carried into a hoUow siUca tube, but PCVD uses a moving microwave cavity rather than a torch. The plasma formed inside the microwave cavity results in the deposition of a compact glass layer along the inner wall of the tube. The temperatures involved in PCVD are lower than those in MCVD, and no oxide soots are formed. Also, the PCVD method is not affected by the heat capacities or thermal conductivities of the deposits. 

A similar deposition system uses a plasma which is produced by a traveling microwave cavity. No other source of heat is required. The deposition system is shown schematically in Fig. 16.12. The reactants are the same as in the thermal CVD process. Pressure is maintained at approximately 1 Torr. In this case, the deposition occurs at lower temperature, no soot is formed and a compact glass is produced directly. A main advantage of this method is the more accurate grading of the refractive index of the cladding material. 

In a typical MIP-MS instrument, the ICP portion is replaced with one of a variety of microwave discharge sources, usually a fairly standardised (modified) Beenakker cavity connected to a microwave generator. The analytical MIP at intermediate power (<500 W) is a small and quiet plasma source compared with the ICP. The mass spectrometer needs no major modifications for it to be interfaced with the MIP. With MIP used as a spectroscopic radiation source, typically consisting of a capillary (1mm i.d.), a power of 30-50 W and a gas flow below 1 L min 1, multi-element determinations are possible. By applying electrodeposition on graphite electrodes, ultratrace element determinations are within reach, e.g. pg amounts of Hg. 

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