Radio-frequency generator

Emission spectroscopy utilizes the characteristic line emission from atoms as their electrons drop from the excited to the ground state. The earliest version of emission spectroscopy as applied to chemistry was the flame test, where samples of elements placed in a Bunsen burner will change the flame to different colors (sodium turns the flame yellow calcium turns it red, copper turns it green). The modem version of emission spectroscopy for the chemistry laboratory is ICP-AES. In this technique rocks are dissolved in acid or vaporized with a laser, and the sample liquid or gas is mixed with argon gas and turned into a plasma (ionized gas) by a radio frequency generator. The excited atoms in the plasma emit characteristic energies that are measured either sequentially with a monochromator and photomultiplier tube, or simultaneously with a polychrometer. The technique can analyze 60 elements in minutes. 

The apparatus consists primarily of an ICP source and a spectrometer. The spectrometer may be of simultaneous type (polychromator) or a sequential type (monochromator). The ICP source consists of a radio frequency generator that can produce at least 1.1 kW of power. It also has other components, which include torch, coil, nebulizer, spray chamber, and drain. 

An additional power source is required for the radio-frequency-generating system. 

Figure 7.2 Schematic diagram of a parallel-plate cold-plasma reactor representing the plasma-generated species. RF radio frequency generator R radical species M metastable excited species hv. UV and VUV emitted radiation.

In an alternative approach (42) the sample is coated on a ferromagnetic wire in the inlet of the gc machine, surroimded by a coil connected to a radio frequency generator. A short pulse of radio frequency power causes the temperature of the wire to rise rapidly to its Curie point, where it remains imtil the power is switched off. In theory the great advantages of this approach are its self-thermostat action and the absence of direct contact between the power imit and the pyrolysis wire and this has made Curie point p5Tolysis popular, with commercial imits available. However, it has been pointed out (12) that neither the speed of temperatime rise nor the self-thermostat action is as reliable as is often claimed. 

Hgure 5.43 A plasma torch for ICR A radio-frequency generator ionizes argon in the tube and accelerates the ions, maintaining heat by sustained collisions. Once the generator is off, the plasma stops forming. 

Until recently, chemical vapor deposition of cracked hydrocarbons was a source only of pyrolytic carbon with a graphite structure but when deposited in a glow discharge produced by a radio frequency generator of 0.5-1.0 kW from a gas pressure of 0.5-3 kPa and at 950°C, microcrystals of diamond have been deposited. The diamond film is formed from 111 and 100 crystal faces and has a high hardness as shown in Table 6.15. 

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