Elemental composition, interfaces

Analysis of Surface Elemental Composition. A very important class of surface analysis methods derives from the desire to understand what elements reside at the surface or in the near-surface region of a material. The most common techniques used for deterrnination of elemental composition are the electron spectroscopies in which electrons or x-rays are used to stimulate either electron or x-ray emission from the atoms in the surface (or near-surface region) of the sample. These electrons or x-rays are emitted with energies characteristic of the energy levels of the atoms from which they came, and therefore, contain elemental information about the surface. Only the most important electron spectroscopies will be discussed here, although an array of techniques based on either the excitation of surfaces with or the collection of electrons from the surface have been developed for the elucidation of specific information about surfaces and interfaces. 

In recent decades we have seen an explosion of various spectroscopic techniques for analyzing the elemental composition and chemical states of solid surfaces and films. This explosion has stemmed in part from the large number of surface- or interface-related problems seen in integrated-circuit performance, composite reliability, corrosion, nanostructured components, and so on. Instruments themselves can range from stand-alone units to attachments in national synchrotron facilities or multitechnique systems built around special fabrication sites. However, the basic principle of the technique, and therefore the basic concerns with sample preparation, stay the same. 

Laser ablation ICP-MS can be used to obtain spatial maps of elemental composition in tissues. In one study [253], LA-ICP-MS was used for elemental tagging of fish scales, rat kidney cross sections, and examination of the blood/ bone interface in a pig femur. Ca and Mg were used as natural internal standards. 

Inductively coupled plasma has become the ionization method of choice for elemental mass spectrometry. It was initially developed as the excitation source for multi-element optical spectrometers, because at typical plasma temperatures of 5000-10,000°C virtually all elements on the periodic chart emit detectable light. Most molecules are also atomized at these temperatures, which makes inductively coupled plasma ideal for mass spectrometry monitoring of elemental composition as well. Fassell and co-workers introduced the first inductively coupled plasma interfaced to a mass spectrometer in 1980 (Houk et al., 1980). Elemental mass spectrometry normally requires only low-resolution analysers because unit mass resolution is typically required (i.e. the mass difference between elements, which is always equal to or greater than 1 Da). 

While the input query and search type are all that are necessary to perform a sfmcfure search in PubChem, there are numerous choices by which one may narrow the search to smaller subsets of PubChem. For example, one may search only within a previous Entrez search result, or even a previous stmcture search result, or upload a file of ClDs againsf which fhe search is fo be performed. One may filter based on a wide variety of properties, such as molecular weight, heavy atom count, presence or absence of stereochemistry, assay activity, elemental composition, depositor name or category, etc. Most of these subset operations could be accomplished through appropriate Entrez index queries followed by Boolean operations on structure search results however, the structure search tool provides a convenient one-step interface for chemical search refinement. 

A more convenient and expeditious means of mass measurement with either design is to interface an electronic detector with an on-line computer that acquires and stores all the data, both m/e values and intensity data, while the spectrum is being scanned. After identifying the m/c ratios of the mass standard, the computer calculates the exact masses of all the unknown peaks from the scanning time between standard and unknown and, within a few minutes, prints on a teletype the exact masses and intensities of all the peaks in the mass spectrum. This is possibly the most elegant technique in mass spectrometry, for it provides the analyst with exact masses which can be used to determine the elemental compositions of all peaks in a mass spectrum. 

Libera [99] has presented an alternative for the study of polymer morphology avoiding the staining procedure as a way to induce amplitude contrast. EEe proposed the use of EELS to study different polymer systems to obtain several levels of resolution (related to the radiation sensitivity of the material) when studying interfaces, such as those in polystyrene-poly(2-vinyl pyridine) homopolymer blends, epoxy-alumina interfaces, and hydrated polymers. Polymers could be distinguished from each other on the basis of the energy-loss spectra in their low loss (valence) and core loss (elemental composition). 

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