Microplasma

A laser pulse is used to knock material from the surface of a solid sample the laser pulse creates a microplasma that ionizes molecules in the sample. 

A laser pulse can ablate material from the surface of a sample, and create a microplasma which ionizes some of the sample components. The laser pulse accomplishes both vaporization and ionization of the sample [366,534,535]. This method is called laser ionization mass spectrometry (LIMS). 

The nonconverted part of the laser output is focused into a gas-filled cell. A microplasma is produced in the focus (see Section III.9), and this emits a wavelength continuum between 2000-8000 A having sufficient intensity to be used as lightsource for the analysing pulse. Its pulse duration, as well as the spectral intensity distribution, depends on the gas used in the cell. With 1 atmosphere oxygen, for instance, the pulsewidth is 30 nsec. 

One advantage of this method is the automatic synchronization of both pulses to within a few nanoseconds. The second pulse can easily be delayed by changing the length of the light path before generating the microplasma. 

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. 

Eijkel, J. C.T., Stoeri, H., Manz, A., A molecular emission detector on a chip employing a direct current microplasma Anal. Chem. 1999, 71(14), 2600-2606. 

A number of configurations of microplasma reactors will be described here. Classification will be based on the power sources, the electric field switching frequency ranging from DC to GHz, and electrode geometries and materials, extending from DBDs to micro hollow cathodes and microcavity discharges. 

Figure 4 (a) Cross-sectional diagram of a silicon-based microcavity discharge device with an inverted square pyramid microcavity and (b) an SEM (scanning electron microscopy) image of a single microplasma device with 50 x 50 pm2 emitting aperture (Becker et al, 2006 reproduced with permission). 

Novel applications have been developed from the combination of microreactor technology and nonequilibrium microplasma chemistry. Here we discuss a selection from the recent literature on this topic to illustrate several main trends. We will focus on microplasmas in confined microchannels for the purpose of chemical synthesis and environmental applications. 

Figure 7 Schematic diagram of microplasma reactor for the synthesis of silicon nanoparticles. A microdischarge forms at the cathode tip and extends a short distance toward the anode (Sankaran et al, 2005 reproduced with permission).

Figure 8 Schematic diagram of experimental setup and image of microplasma reactor with VHF source developed for the synthesis of photoluminescent silicon nanocrystals at room temperature (Nozaki et al, 2007a reproduced with permission). M.B. is a matching electrical circuit.

Figure 9 Capillary head of UHF microplasma reactor developed for the synthesis of molybdenum oxide nanoparticles (Bose et at, 2006 reproduced with permission).

Another example of inner wall modification of microchannels with a microplasma is the deposition of uniform platinum films in microchannels... 

Through the generation of highly reactive species such as energetic electrons and active radicals, microplasma reactors create novel process windows for C-C and C-H bond cleavage involved in the decomposition of harmful gaseous pollutants at atmospheric pressure. As an example of... 

Figure 11 Photos of a microplasma in (a) a capillary and (b) a microchannel in a Pyrex chip, developed for plasma CVD of platinum films (Kadowaki et al, 2006 reproduced with permission).

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