Sensitivity forming elements

Auger electrons are not just a by-product of XPS. These highly surface-sensitive, primarily element-specific electrons form the basis of a spectroscopy which is highly appreciated in the fields of materials and surface science, namely Auger electron spectroscopy (AES) [5,17,19,20]. 

Results presented by Stockwell [9] for some of the hydride-forming elements and for mercury illustrate the enormous increase in sensitivity achieved with automated analytical chemistry methods (Table 5.2). Earher developments centred on the batch approach. These methods have recently been dropped (in favour of continuous-flow techniques) because they were not easy to use, were very dependent on operator abihty, and were difficult to automate. 

Introduction into a DC plasma requires rather more care and attention owing to its inherent design features. As the hydride is being introduced into the plasma, it is necessary to provide a controlled sheath of argon to contain the hydride and direct it into the plasma. This chimney effect significantly improves the sensitivity for hydride-forming elements. This interface has also formed the basis of an introduction system for mercury vapour into an atomic-fluorescence spectrometer as described by Godden and Stockwell [12]. 

The analysis of rock samples was conducted in the chemical laboratory in Ust-Kamenogorsk, Kazakhstan. by inductively coupled plasma mass spectrometry, and the equipment used was an ELAN-6100 (US) mass spectrometer. In the present abstract the distribution of two elements zinc (as the basic ore-forming element) and titanium (of the siderophile element) is examined. The sensitivity of the analysis is 5 ppm for Zn and 0.05% for Ti. The analytical results for Zn and Ti are presented on contoured... 

Unstable radionuclei result on subjecting the nuclei of some elements to neutron bombardment. During the decay process, in which the radionuclei return to more stable forms, characteristic radiation is emitted. The energy of the radiation is characteristic of the element, and its intensity forms the basis for quantitative elemental analysis. The advantages of NAA for trace analysis include low detection limits, good sensitivity, multi-element capability and relative freedom from matrix effects. However, for successful application of this technique skilled personel are required and because of the low sample throughput the amount of work involved in the analysis of column fractions, for example, is prohibitively high. In addition, it may take up to several weeks before the results are available. Further, only few laboratories have easy access to a neutron source. 

AFS is based on the absorption of radiation of a certain frequency (the energy transition from the outermost electronic orbitals to a higher energy state) and the subsequent deactivation of the excited atoms with the release of radiation. The most useful type of fluorescence, resonance fluorescence, involves a fluorescence emission radiation of the same wavelength as that used for excitation. Because of the inherent sensitivity of the fluorescence emission process, AFS is one of the most sensitive atomic techniques. All the benefits of AFS are enhanced when this spectromet-ric technique is used in combination with vapor generation methods, especially for covalent-hydride-forming elements. 

It will be clear from the above examples that hydride-forming elements especially are often subjected to such speciation studies. There are, of course, good reasons for this. These are the very elements which tend to form toxic organometallic compounds, and they are elements which may be determined with excellent sensitivity. Moreover, interferences are not usually a problem following a separation process. 

Direct analysis of solids for selenium by XRF has a detection limit of —0.5 mgkg and so is often insufficiently sensitive. Rock, sediment, and soil samples can be dissolved using wet chemical methods (HF-HCl-etc.) followed by La(OH)3 co-precipitation to separate hydride-forming elements including selenium. This is present as Se(IV) following acid dissolution (Hall and Pelchat, 1997). The methods described above for aqueous samples can then be used. 

Shortening the collection time is recommended when other hydride-forming elements present in the sample react more slowly than the analyte forming the volatile species. This fixed-time kinetic approach is less sensitive than its steady-state counterpart but provides better selectivity and shorter analysis times [32,33]. 

Over the atomization temperature range ( 1200°-3000°C), carbide formation between many metal oxides and graphite is favored thermodynamically (28). While carbide formation has been used to explain poor sensitivity for elements such as boron or tungsten, which form refractory carbides, little attention was initially devoted to the interference caused by carbide formation with other elements. However,... 

Platelets differ from other formed elements of blood in expressing mechanisms for uptake, storage, and endocytotic release of 5-HT. 5-HT is not synthesized in platelets, but is taken up from the circulation and stored in secretory granules by active transport, similar to the uptake and storage of NE by sympathetic nerve terminals (see Chapters 6 and 12). Measuring the rate of Na -dependent 5-HT uptake by platelets provides a sensitive assay for 5-HT-uptake inhibitors. 

In order to prepare thin-film samples, the desired elements must usually be chemically or physically separated from the host compound. The elements being determined are collected in a physical form suitable for x-ray analysis using such methods as ion exchange, solvent extraction, or precipitation. Metallic ions, for example, may be collected on resin-loaded paper, which also serves as the mechanical support in the x-ray spectrometer. The absolute sensitivity for elements isolated from the host compound is 0.01 to 1 /ug analysis of elements present in trace concentrations is possible with this preconcentration approach, if the total sample size is sufficiently large. 

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