Arc nebulization

Arc nebulization can be used to produce an aerosol from a conducting solid sample. The aerosol is then introduced into the plasma for elemental or isotope ratio analysis. Powdered samples can be mixed with graphite and pressed to get a conducting pellet. The only matrix related interfering ions detected are ArC , ArO and Ar2 . A precision of about 1% has been reported for isotope ratio determinations. 

This technique uses an electrothermally heated graphite furnace, a carbon rod, or metal filament to atomize the sample. The sample is placed in the form of a solution or a solid in the atomizer. 

A transport efficiency of greater than 80% has been achieved by a rhenium filament ETV system with a glass housing geometry. Optimum sensitivity is obtained with a moderate heating rate and it is dependent on the carrier gas flow. 

The requirements for ETV-ICP systems (both ICP-AES and ICP-MS) differ significantly from those for ETA-AAS. When using ETV-ICP, it is necessary to introduce a volatile sample into the gas stream. The atomization stage is then performed in the ICP unit. This is the reason why the technique is referred to as ETV in the case of ICP-AES and ICP-MS, but ETA (electrothermal atomization) in the case of AAS. The chemical matrix effects observed in AAS are negligible using ETV-ICP, whereas the transport efficiency of the sample from the ETV unit to the ICP unit is critical to the analytical performance in ETV-ICP-MS. 

Using ETV-ICP-MS detection limits improved by a factor of 10 to 10 compared with pneumatic nebulization methods. Detection limits using pneumatic nebulizers with ICP-MS are typically 1 to lOOngP, and those for ETV-ICP-MS 1 to lOpgP (Table 34). This is due to more efficient nebulization (the nebulization efficiency of pneumatic nebulizers is only 1 to 2%) and removal of the solvent matrix by thermal pretreatment. Thus, using this technique it is possible to analyse samples containing high levels of dissolved salts. Also the ability to operate with very small sample volumes (10 to 100 fil) is useful, especially when the amount of sample is limited. 

GLASS ball joints GLASS SHUTTER LEAD SCREW GUIDE SCALE 

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Fig. 8.21. (A) Scale diagram of an arc nebulizer. (B) Three-dimensional view of the DSI device of Horlick et al. (Reproduced with permission of the Royal Society of Chemistry.)...

As Stated in the section on sample introduction (section 2.1), dissolution of materials containing rare earths is often difficult and/or time-consuming. Therefore, some research has addressed various ways of circumventing sample dissolution by introducing solids directly into ICPs. Mass spectra for rare earth analytes are shown for arc nebulization of an ore (fig. 26) and laser ablation of a ceramic (fig. 27). Because of the rapid scanning possible with the mass spectrometer, virtually all the elements can be identified and determined semi-quantitatively (i.e., to within a factor of two to five) quickly by ICP-MS. For these studies, ICP-MS is particularly attractive because of the high and fairly uniform sensitivity for most elements, the simple, readily interpreted spectra, and the ability to follow a transient sample introduction pulse. The combination of a solid sampling... 

Emission spectrometer incorporating a sample nebulizer, grating monochromator, photomultiplier detection system and microprocessor controller. Excitation by dc-arc plasma jet, or inductively coupled plasma. Laser excitation sometimes used. 

Flames used in analytical measurements arc similar to those produced by Bunsen burners with the added provision of a means of introducing the sample directly into the combustion zone. Support (oxidant) and fuel gases are fed to a nebulizer along with the sample solution. The mixed gases and sample aerosol then pass through the jets of the burner where ignition occun. 

This may be either a continuous process, used when the sample size is relatively large (1 ml or more), or a discrete process, used with samples of less than 20 /il. Continuous-flow systems are simpler to use and more precise, but they are less sensitive. They employ a nebulizer in association with a flame or gas plasma, and either a rotating electrode (Rotrode) or drip-feed to the electrode with the arc or spark. The pneumatic nebulizer has an efficiency of 5-10% and generates an inhomogeneous aerosol. Efiiciency can be improved by proper design of the nebulizer and spray chamber (N4), by use of heated nebulizer gas (R6) or ultrasonic devices (S23). The maximum improvement is a 5- to 10-fold increase in sensitivity. There is also an increase in the complexity and cost of the instrument which usually offsets these benefits. The effect... 

A cross-flow nebulizer for dc arc solution and AAS work was described by Valente and Schrenk [113]. For ICP work the nebulizer should have capillaries with diameters < 0.2 mm and a distance of 0.05-0.5 mm between the tips. The capillaries also can be made of metal, glass or plastic and they have similar characteristics to the concentric nebulizer. Types made of Ryton for example enable the aspiration of solutions containing higher concentrations of HF. Then aerosol gas flows are around 1-2 L/min at 2-5 bar. This has been confirmed by optimization measurements described by Fujishiro et al. [114]. They found that by varying the inside and the outside diameters of the capillary from 0.15 to 0.9 mm and from 0.5 to 1.5 mm, respectively, the pressure drop across the sample tube decreases from 150 to 50 mbar. By increasing the gas flow from 0.75 to 1.75 L/min, the pressure drop across the sample tube was found to increase from 150 to 350 mbar. As shown in Fig. 43 [114], the variations in the drops in pressure considerably influence the droplet size. 

Dc plasma jets were first described as useful devices for solution analysis by Korolev and Vainshtein [366] and by Margoshes and Scribner [367]. The sample liquids were brought into an aerosol form by pneumatic nebulization and arc plasmas operated in argon and with a temperature of about 5000 K were used. 

Emission spectra were first utilized in analytical chemistry as they were simpler to detect than absorption spectra. Flames, arcs, and sparks are all classical radiation sources. Lundegardh first applied a pneumatic nebulizer and an air-acetylene flame. The development of prism and grating instruments was parallel. Photography was employed to detect the spectral lines. The first commercial flame photometers came on the market in 1937. 

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