Low pressure ICPs

Whereas a microwave plasma is most commonly used for the PE-CVD of diamond films, an ECR is the only plasma that is used for diamond deposition below 1 Torr [27-29]. Although Bozeman et al. [30] reported diamond deposition at 4 Torr with the use of a planar ICP, there have been a few reports that describe the synthesis of diamond by low-pressure ICP. Okada et al. [31-33] first reported the synthesis of nanocrystalline diamond particles in a low-pressure CH4/CO/H2 ICP, followed by Teii and Yoshida [34], with the same gas-phase chemistry. 

Inductively coupled plasma Plasmas generated by application of radiofrequency power to a nonresonant inductive coil and maintained by an inductive electromagnetic field. Low-pressure ICP is a high-density plasma source. 

Also low pressure ICPs and MIPs, often used as ion sources (see e.g. Creed et al. [606]) are very similar in their performance to glow discharges, except for them being electrodeless discharges. They have shown particular merits for element-specific detection in chromatography, where detection limits down to the pg/s level... 

Figure 4-83. Radial distributions of plasma density ( p), electric field (E ), and potential (cp) in low-pressure ICP discharge.

In speciation, glow discharges are excellent detectors for GC work, as shown earlier. In addition to the low-power and low-pressure ICPs, they can be successfully used for element-specific detection for gas chromatography. An RF-GD-MS system has been used as a detector for GC by Olson et al. [832]. The set-up should consist... 

The intrinsic drawback of LIBS is a short duration (less than a few hundreds microseconds) and strongly non-stationary conditions of a laser plume. Much higher sensitivity has been realized by transport of the ablated material into secondary atomic reservoirs such as a microwave-induced plasma (MIP) or an inductively coupled plasma (ICP). Owing to the much longer residence time of ablated atoms and ions in a stationary MIP (typically several ms compared with at most a hundred microseconds in a laser plume) and because of additional excitation of the radiating upper levels in the low pressure plasma, the line intensities of atoms and ions are greatly enhanced. Because of these factors the DLs of LA-MIP have been improved by one to two orders of magnitude compared with LIBS. 

Applications Basic methods for the determination of halogens in polymers are fusion with sodium carbonate (followed by determination of the sodium halide), oxygen flask combustion and XRF. Crompton [21] has reported fusion with sodium bicarbonate for the determination of traces of chlorine in PE (down to 5 ppm), fusion with sodium bisulfate for the analysis of titanium, iron and aluminium in low-pressure polyolefins (at 1 ppm level), and fusion with sodium peroxide for the complexometric determination using EDTA of traces of bromine in PS (down to 100ppm). Determination of halogens in plastics by ICP-MS can be achieved using a carbonate fusion procedure, but this will result in poor recoveries for a number of elements [88]. A sodium peroxide fusion-titration procedure is capable of determining total sulfur in polymers in amounts down to 500 ppm with an accuracy of 5% [89]. 

There are many different ways to obtain information from either AAS or ICP alone however when combined with MS, the mass spectrometer becomes the detector. Flame gases are taken into a mass spectrometer through a port into a low-pressure compartment and then transferred, once the pressure is low enough, to the mass spectrometer (see Figure 15.4).The interface between the AAS or ICP and the MS has been a problem in the past, but these problems have largely been overcome [8,9],... 

The influence of various gas pressure conditions within the laser ablation cell on the particle formation process in laser ablation has also been investigated.69 In LA-ICP-MS studies at low pressure (down to 2kPa) a small particle size distribution and a reduction in elemental fractionation effects was obtained. But with decreasing pressure and transport volume of ablated material, a significant decrease in the ion intensities was observed as demonstrated for uranium measurements in the glass SRM NIST 610.69 However, the laser ablation of solid materials at atmospheric pressure in LA-ICP-MS is advantageous for routine measurements due to lower experimental effort and the possibility of fast sample changing in the ablation chamber. Fractionation... 

The interface used today between the atmospheric-pressure plasma and the low-pressure mass spectrometer is based on a differentially pumped two-stage interface similar to those used for molecular beam techniques [89-91]. The key to successful development of ICP-MS instruments was the use of a relatively large ( l-mm-diameter) sampling orifice so that continuum flow was attained with an unrestricted expansion of the plasma to form a free jet. When small orifices were used, a cold boundary layer formed in front of the orifice, resulting in substantial cooling of the plasma, including extensive ion-electron recombination and molecular oxide formation. The smaller orifices were also susceptible to clogging. 

Figure 10.2 is a schematic diagram of a helium MIP-MS system, with gaseous sample introduction, developed by the Caruso group. This is the most popular method of sample introduction to date for MIP-MS analysis as the MIP at low pressures is not tolerant to liquid samples. A commercial ICP-MS system may be modified by mounting an MIP discharge source in place of the ICP source. A Beenakker cavity is commonly used as the microwave source and serves to focus the microwave energy. Cavity construction and dimensions have been described in detail by Evans et al. [18]. 

Low-pressure He-ICPs have been used as chromatographic detectors for mass spectrometry [111,112,114], Organotin compounds [112] were speciated by using a plasma operated at 100 W. A low-pressure torch must be constructed from a quartz tube of dimensions approximately 150 mm long and 6 mm outer diameter to sustain such a plasma. This torch is connected at one end to the GC interface and to the sampler cone at the other. Heated transfer lines must be used for reproducible transfer of the GC analytes. As the low-pressure system may not completely at-... 

Low-pressure He-ICP-MS Pg This technique may provide information on both elemental composition and molecular structure. These instruments are not commercially available. 112, 115, 111... 

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