Atoms as ions

This is the basic process in an inductively coupled plasma discharge (ICP). The excited ions can be examined by observing the emitted light or by mass spectrometry. Since the molecules have been broken down into their constituent atoms (as ions) including isotopes, these can be identified and quantified by mass spectrometry, as happens with isotope ratio measurements. 

The distinction in sensitivity between measurements of radiation and atoms (as ions at a given mass-to-charge ratio by MS) can be demonstrated by comparing intensity in terms of the radioactive decay Eqs. (2.4) and (2.7) for a radionuclide with an atomic mass of 100 amu and a half-life of 10 y (3.16 x 108 s). If the sample under assay emits beta particles at the relatively low rate of 1 x 10-2 d/s, this corresponds to 4.6 x 106 atoms or, for a 100 amu radionuclide, 7.6 x 10 14g. Alpha particles can be measured at an approximately 1,000-fold lower decay rate, for which the number of atoms and the mass correspondingly are 1,000-fold lower. 

Probably the first to recognize the possibility that atoms, as ions, might be associated with a definite fundamental unit of... 

Atomic absorption spectroscopy (AAS) is used to determine the chemical composition of the metal loading of a supported catalyst. In a sample preparation procedure the catalyst is treated with very strong and often oxidizing acids to extract all metal atoms as ions in solution. This solution is injected into a spectrometer that gives a quantitative analysis of all metal components in the solution based on the spectral absorption (or emission in the case of Auger electron spectroscopy, AES) in a flame. Note that in this method all dissolvable metal atoms are analyzed not only the catalytic active surface atoms. 

Measuring the electron emission intensity from a particular atom as a function of V provides the work function for that atom its change in the presence of an adsorbate can also be measured. For example, the work function for the (100) plane of tungsten decreases from 4.71 to 4.21 V on adsorption of nitrogen. For more details, see Refs. 66 and 67 and Chapter XVII. Information about the surface tensions of various crystal planes can also be obtained by observing the development of facets in field ion microscopy [68]. 

These equations indicate that the energy of the scattered ions is sensitive to the mass of the scattering atom s in the surface. By scanning the energy of the scattered ions, one obtains a kind of mass spectrometric analysis of the surface composition. Figure VIII-12 shows an example of such a spectrum. Neutral, that is, molecular, as well as ion beams may be used, although for the former a velocity selector is now needed to define ,. ... 

In many ion-atom and ion-molecule collisions, one is often only interested in the projections on various charge states, which can be given a very simple treahnent. The Thouless determinant at separation of the two product fragments can be expressed as... 

Using your optimized expression for W, calculate the estimated total energy of each of these atoms and ions. Also calculate the percent error in your estimate for each ion. What physical reason explains the decrease in percentage error as Z increases ... 

An uneventful coupling of two hemispherical cavitand molecules — a tetrameth-anethiol and a tetrakis(chloromethyl)precursor (see p. 169) — yielded D.J. Cram s (1988) carcerand . ft entraps small molecules such as THF or DMF, cesium or chloride ions, or argon atoms as permanently imprisoned guests . Only water molecules are small enough to pass through the two small pores of this molecular (prison) cell. 

Amount of substance mole mol Amount of substance which contains as many specified entities as there are atoms of car-bon-12 in exactly 0.012 kg of that nuclide. The elementary entities must be specified and may be atoms, molecules, ions, electrons, other particles, or specified groups of such particles. 

As indicated in the previous section, the adsorption of a gas by a solid is the outcome of the forces of attraction between the individual molecules of the gas and the atoms or ions composing the solid. These forces have been studied theoretically over a number of decades, and though impressive advances have been made in recent years these remain more in the nature of refinements than of fundamental changes in the ideas themselves. And since. 

A big step forward came with the discovery that bombardment of a liquid target surface by abeam of fast atoms caused continuous desorption of ions that were characteristic of the liquid. Where this liquid consisted of a sample substance dissolved in a solvent of low volatility (a matrix), both positive and negative molecular or quasi-molecular ions characteristic of the sample were produced. The process quickly became known by the acronym FAB (fast-atom bombardment) and for its then-fabulous results on substances that had hitherto proved intractable. Later, it was found that a primary incident beam of fast ions could be used instead, and a more generally descriptive term, LSIMS (liquid secondary ion mass spectrometry) has come into use. However, note that purists still regard and refer to both FAB and LSIMS as simply facets of the original SIMS. In practice, any of the acronyms can be used, but FAB and LSIMS are more descriptive when referring to the primary atom or ion beam. 

The momentum of a fast-moving atom or ion is di.ssipated by collision with the closely packed molecules of the liquid target. As each collision occurs, some of the initial momentum is transferred to substrate molecules, causing them in turn to move faster and strike other molecules. The result is a cascade effect that ejects some of the substrate molecules from the surface of the liquid (Figure 4.2). The process can be likened to throwing a heavy. stone into a pool of water — some... 

By using a beam of fast atoms or ions incident onto a nonvolatile liquid containing a sample substance, good molecular or quasi-molecular positive and/or negative ion peaks can be observed up to about 4000-5000 Da. Ionization is mild, and, since it is normally carried out at 25-35°C, it can be used for thermally labile substances such as peptides and sugars. 

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