Sample introduction gaseous

As SFC provides gaseous sample introduction to the plasma and thus near-100 % analyte transport efficiency, coupling SFC with plasma mass spectrometry offers the potential of a highly sensitive, element-selective chromatographic detector for many elements. Helium high-efficiency microwave-induced plasma has been proposed as an element-selective detector for both pSFC and cSFC [467,468] easy hyphenation of pSFC to AED has been reported [213]. 

Gaseous sample introduction into an ICP-MS presents different problems. Owing to its extremely sensitive nature, Dean et al. [13] introduced the sample as the gaseous hydride by a flow-injection approach. This was reasonably effective because lower volumes of samples and reagents were in use. They utibzed nitric acid as a carrier stream to prevent the formation of argon chloride species in the plasma. Argon chloride has the same mass as arsenic which is mono-isotopic, and this severely bmits arsenic determination. An additional problem was that the sensitivity was extremely dependent on the purity of reagents. 

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]. 

The gaseous sample introduction line is connected directly to the central tube of the plasma torch, eliminating the need for the conventional nebulizer and spray chamber. Additional equipment to handle gas mixing and dilution at controlled flow rates may be required. 

Fundamentally, introduction of a gaseous sample is the easiest option for ICP/MS because all of the sample can be passed efficiently along the inlet tube and into the center of the flame. Unfortunately, gases are mainly confined to low-molecular-mass compounds, and many of the samples that need to be examined cannot be vaporized easily. Nevertheless, there are some key analyses that are carried out in this fashion the major one i.s the generation of volatile hydrides. Other methods for volatiles are discussed below. An important method of analysis uses lasers to vaporize nonvolatile samples such as bone or ceramics. With a laser, ablated (vaporized) sample material is swept into the plasma flame before it can condense out again. Similarly, electrically heated filaments or ovens are also used to volatilize solids, the vapor of which is then swept by argon makeup gas into the plasma torch. However, for convenience, the methods of introducing solid samples are discussed fully in Part C (Chapter 17). 

Every mass spectrometer consists of four principal components (Fig 1) (1) the source, where a beam of gaseous ions are produced from the sample (2) the analyzer, where the ion beam is resolved into its characteristic mass species (3) the detector, where the ions are detected and their intensities measured (4) the sample introduction system to vaporize and admit the sample into the ion source. There is a wide variety in each of these components and only those types which are relevant to analytical and organic mass spectrometry will be emphasized in this survey. The instrumentation... 

Elements such as As, Se and Te can be determined by AFS with hydride sample introduction into a flame or heated cell followed by atomization of the hydride. Mercury has been determined by cold-vapour AFS. A non-dispersive system for the determination of Hg in liquid and gas samples using AFS has been developed commercially (Fig. 6.4). Mercury ions in an aqueous solution are reduced to mercury using tin(II) chloride solution. The mercury vapour is continuously swept out of the solution by a carrier gas and fed to the fluorescence detector, where the fluorescence radiation is measured at 253.7 nm after excitation of the mercury vapour with a high-intensity mercury lamp (detection limit 0.9 ng I l). Gaseous mercury in gas samples (e.g. air) can be measured directly or after preconcentration on an absorber consisting of, for example, gold-coated sand. By heating the absorber, mercury is desorbed and transferred to the fluorescence detector. 

Mass spectrometry (MS) is an analytical technique of great interest, one that provides structural information and quantitative data not easily obtained by other techniques. In view of these advantages, mass spectrometers have been widely used as detectors in gas chromatography however, adapting them for use with HPLC systems has been more difficult, because the sample is not in the gaseous phase and the solvent must be removed prior to ionization. These difficulties have been overcome by the development of a number of sample-introduction and ionization tech-... 

The introduction of gaseous samples is another possibility in ICP-based spectrometric techniques. The clear advantage is excellent transport efficiency however, the applications are limited to the volatile forms of elements. There are several reviews available on gas chromatography... 

Hydride generation is a sample introduction technique exclusively for elements that form volatile hydrides (e.g. As, Se, Sn). An acidified sample solution is reacted with sodium borohydride solution, liberating the gaseous hydride in a gas-liquid separator. The generated hydride is then transported to... 

Kantor T. (2000) Sample introduction with graphite furnace electrothermal vaporization into an inductively coupled plasma effects of streaming conditions and gaseous phase additives, Spectrochim Acta, Part B 55 431-448. 

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