Microwave-generated plasma

An alternative approach for the preparation of supported metal catalysts is based on the use of a microwave-generated plasma [27]. Several new materials prepared by this method are unlikely to be obtained by other methods. It is accepted that use of a microwave plasma results in a unique mechanism, because of the generation of a nonthermodynamic equilibrium in discharges during catalytic reactions. This can lead to significant changes in the activity and selectivity of the catalyst. 

It is possible to use a microwave-generated plasma [72], a technique in which the grooves present in an untreated M40 fiber are almost completely removed after about 12 s treatment in air [73]. However, it has been suggested that different results may be obtained, depending on whether RF or microwave generation procedures are adopted. 

Korzec D, Werner F, Winter R and Engemann J 1996 Scaiing of microwave siot antenna (SLAN) a concept for efficient piasma generation Plasma Sources Sol. Technol. 5 216-34... 

Principles and Characteristics The major drawbacks of ICP with argon as the support gas lie in numerous isobaric polyatomic ion interferences and in the lack of sufficient energy to ionise halogens and nonmetals to the necessary extent. With these weaknesses of ICP in mind, the possibility of generating microwave-induced plasmas with alternative gases to argon is of interest. 

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. 

Microwaves have been used to generate plasma in methane at 5-50 Torr. The radicals produced in such a system were then allowed to react over a nickel catalyst, affording a mixture of ethane, ethene, and ethyne [74],... 

Suib et al. [77] used microwaves to generate plasma in an atmosphere containing methane and oxygen. The plasma passing over a metal or metal oxide catalyst led to formation of C2 hydrocarbons and some oxygenates. 

Activities for miniaturizing mass spectrometers (e.g., microplasma on chip or insertion of diode lasers in RIMS), for constructing cheaper and more compact instrumentation with the same performance or improved properties compared to existing instruments are required as the next generation mass spectrometers. The introduction of microwave induced plasmas or of p,-torches to reduce Ar gas consumption involves developments in this future direction. 

Flames and plasmas can be used as atomisation/excitation sources in OES. Electrically generated plasmas produce flame-like atomisers with significantly higher temperatures and less reactive chemical environments compared with flames. The plasmas are energised with high-frequency electromagnetic fields (radiofrequency or microwave energy) or with direct current. By far the most common plasma used in combination with OES for analytical purposes is the inductively coupled plasma (ICP). 

J. M. Costa-Fernandez, F. Lunzer, R. Pereiro, N. Bordel and A. Sanz-Medel, Direct coupling of high-performance liquid chromatography to microwave-induced plasma atomic emission spectrometry via volatile-species generation and its application to mercury and arsenic speciation, J. Anal. At. Spectrom., 10, 1995, 1019-1025. 

Figure 15.8—Coupling of a gas chromatograph with an atomic emission spectrophotometer. Effluents from the capillary column are injected into the plasma and decomposed into their elements. Each chromatogram corresponds to the compound containing the element of interest. For a given retention time, indication as to the elements included in a compound can be obtained. The plasma in this example is generated by heating the carrier gas (He) with a microwave generator confined in a cavity at the exit of the column. A diode array detector system can be used for simultaneous detection of many elements (chromatograms courtesy of a Hewlett Packard document).

Chiba et al. [749] used atmospheric pressure helium microwave induced plasma emission spectrometry with the cold vapour generation technique combined with gas chromatography for the determination of methylmercuiy chloride, ethylmercury chloride and dimethylmercury in sea water following a 500-fold preconcentration using a benzene- cysteine extraction technique. 

The most suitable techniques for the rapid, accurate determination of the elemental content of foods are based on analytical atomic spectrometry, for example, atomic absorption spectrometry (AAS), atomic emission spectrometry (AES), and mass spectrometry, the most popular modes of which are Game (F), electrothermal atomization (ET), and hydride generation (HG) AAS, inductively coupled plasma (ICP), microwave-induced plasma (MIP), direct current plasma (DCP) AES, and ICP-MS. Challenges in the determination of elements in food include a wide range of concentrations, ranging from ng/g to percent levels, in an almost endless combination of analytes with matrix speci be matrices. 

At the present time there are no ETA—AAS methods that can compete with the cold vapour technique for Hg or with hydride generation methods for Sb and Te. Another attractive method for Sb and Te is low pressure microwave induced plasma (MIP) emission spectroscopy [138]. Using low-temperature ashing and solvent extraction as preparation, physiological concentrations of both elements ([Pg.376]

Barnett N. W., Chen L. S. and Kirkbright G. F. (1984) The rapid determination of arsenic by OES using a microwave induced plasma source and a miniature hydride generation device, Spectrochim Acta, Part B 39 1141-1147. 

Tao H. and Miyazaki A. (1991) Determination of germanium, arsenic, antimony, tin and mercury at trace levels by continuous hydride generation-helium microwave-induced plasma atomic emission spectrometry, Anal Sci 7 55-59. 

Schickling C., Yang J. and Broekaert J. A. C. (1996) Optimization of electrochemical hydride generation coupled to microwave-induced plasma atomic emission... 

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