Plasma-torch technique

Solutions of solids may need to be converted into aerosols by pneumatic or sonic-spraying techniques. After solvent has evaporated from the aerosol droplets, the residual particulate solid matter can be ionized by a plasma torch. 

The previous discussion has centered on how to obtain as much molecular mass and chemical structure information as possible from a given sample. However, there are many uses of mass spectrometry where precise isotope ratios are needed and total molecular mass information is unimportant. For accurate measurement of isotope ratio, the sample can be vaporized and then directed into a plasma torch. The sample can be a gas or a solution that is vaporized to form an aerosol, or it can be a solid that is vaporized to an aerosol by laser ablation. Whatever method is used to vaporize the sample, it is then swept into the flame of a plasma torch. Operating at temperatures of about 5000 K and containing large numbers of gas ions and electrons, the plasma completely fragments all substances into ionized atoms within a few milliseconds. The ionized atoms are then passed into a mass analyzer for measurement of their atomic mass and abundance of isotopes. Even intractable substances such as glass, ceramics, rock, and bone can be examined directly by this technique. 

Several authors [386,387] have discussed the spectroscopic and nonspectroscopic (matrix) interferences in ICP-MS. ICP-MS is more susceptible to nonspectroscopic matrix interferences than ICP-AES [388-390]. Matrix interferences are perceptible by suppression and (sometimes) enhancement of the analyte signal. This enhanced susceptibility has to be related to the use of the mass spectrometer as a detection system. In fact, since both techniques use the same (or comparable) sample introduction systems (nebulisers, spray chambers, etc.) and argon plasmas (torches, generators, etc.), it is reasonable to assume that, as far as these parts are concerned, interferences are comparable. The most severe limitation of ICP-MS consists of polyatomic... 

Besides plasmas, which are at the forefront of thermal atomisation devices, other excitation processes can be used. These methods rely on sparks or electrical arcs. They are less sensitive and take longer to use than methods that operate with samples in solution. These excitation techniques, with low throughputs, are mostly used in semi-quantitative analysis in industry (Fig. 15.2). Compared to the plasma torch, thermal homogeneity in these techniques is more difficult to master. 

Trace levels (10 to 10 g/g of sample) of silver can be accurately determined in biological samples by several different analytical techniques, provided that the analyst is well acquainted with the specific problems associated with the chosen method. These methods include high frequency plasma torch-atomic emission spectroscopy (HFP-AES), neutron activation analysis (NAA), graphite furnace (flameless) atomic absorption spectroscopy (GFAAS), flame atomic absorption spectroscopy (FAAS), and micro-cup atomic absorption spectroscopy (MCAAS). 

GC-MIP systems have been investigated in considerable detail. Because of the low power of the plasma, it is easily quenched if the normal, atomic spectrometric sample introduction techniques, such as nebulisation, are used. Capillary columns overcome this problem as they require only low flow rates and small sample sizes more compatible with stable plasma operation. The capillary columns can be passed out of the oven, down a heated line, and the end of the column placed in the plasma torch just before the plasma, thus preventing any sample loss. A makeup gas is usually introduced via a side arm in the torch to sustain the plasma (Fig. 4.1, Greenway and Barnett, 1989). Other dopant gases can also be added in this way to prolong the lifetime of the torch and improve the plasma characteristics. 

Another study on the use of Fe showed that the oxidation rate of acetaldehyde was improved with Ti02 catalysts doped with Fe and Si s)mthesized by thermal plasma (Oh et al., 2003). A Fe content lower than 15% rendered higher activities than the untreated catalyst. The catalyst preparation technique involved a complex procedure using a plasma torch, with all this likely leading to an expensive photocatalyst of mild prospects for large-scale applications. 

The solution obtained from dissolving an oil in a solvent is the simplest sample technique and involves dissolving a known weight of oil or fluid in a suitable solvent that is compatible, stable and noise free, and can be used for nebulisation with an ICP-OES plasma torch. Crude, lubricating oils and hydraulic fluids are soluble in a few solvents that are compatible with ICP-AES, e.g. kerosene, propylene carbonate, tetralin and decalin. The excitation of elements in solutions can be viewed either with radial or axial torches with the latter giving higher intensity readings and lower limits of detection. 

Analysis of cyanoacrylate adhesives was carried out using both non-destructive and destructive techniques in order to study the effect of method selection. Non-destructive methods involved dissolving the cyanoacrylate adhesive in a solvent that is compatible with the sample and the plasma torch of the ICP-OES. A suitable solvent that meets these criteria is glacial acetic acid mixed with propylene carbonate and it also reduces the strong odour effect in the laboratory environment. 

Due to the very high sensitivity of the ICP-MS technique, memory (carry over) effects may occur when analytes from a previous sample are measured in the current sample. In cases where analysis of highly polluted soil digests is carried out, memory effects can occur, they may be indications of problems in the sample introduction system. Severe memory interferences may require disassembly and the cleaning of the entire sample introduction system, including the plasma torch and the sampler and skimmer cones. Due to these memory... 

Thermal ionization. Solid samples are heated to a high temperature (1000-1800 °C) in a vacuum, producing either positive or negative ions. The TIMS method has been used, for example, to obtain records of seawater Sr/ Sr and from biogenic carbonate (e.g. McArthur et al. 2001 Burton Vance 2000). This technique permits precise isotope ratio measurements (external reproducibility <5ppm for Nd/ Nd and Sr/ Sr ratios), but it is restricted to those elements with a relatively low first ionization potential. Inductively-coupled plasma. Sample solutions, or laser ablation products, are ionized in a stream of argon within a plasma torch. The advantage of this technique is that the plasma... 

Mercury may be introduced into the plasma torch by nebulization of the sample, by CV technique, or by GF. By direct pneumatic nebulization of acidified (0.5 % HAc) urine samples into an ICP torch, Lo and Arai (1989) analyzed mercury simultaneously with 10 other metals. The detection limit was 20 fig/L. An improvement in the detection limits for some other metals by a factor of 10 20 by use of an ultrasonic nebulizer has recently been reported (Johnson et al., 1989), and should also be true for mercury. 

In addition to previously mentioned processes, the production of N-doped HO2 nanomaterials has been reported through other methods. These processes are ball milling of Ti02 in a NH3 water solution [413], heating Ti02 under NH3 flux at 500-600°C [414, 415], calcination of the hydrolysis product of Ti(S04)2 with ammonia as precipitator, decomposition of gas-phase TiCU with an atmosphere microwave plasma torch [416], ion implantation techniques with nitrogen [417] and N2 gas flux [418] (see Table 8). 

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