Resonance lines, atomic spectroscopy

Use of glow-discharge and the related, but geometrically distinct, hoUow-cathode sources involves plasma-induced sputtering and excitation (93). Such sources are commonly employed as sources of resonance-line emission in atomic absorption spectroscopy. The analyte is vaporized in a flame at 2000—3400 K. Absorption of the plasma source light in the flame indicates the presence and amount of specific elements (86). 

A schematic diagram showing the disposition of these essential components for the different techniques is given in Fig. 21.3. The components included within the frame drawn in broken lines represent the apparatus required for flame emission spectroscopy. For atomic absorption spectroscopy and for atomic fluorescence spectroscopy there is the additional requirement of a resonance line source, In atomic absorption spectroscopy this source is placed in line with the detector, but in atomic fluorescence spectroscopy it is placed in a position at right angles to the detector as shown in the diagram. The essential components of the apparatus required for flame spectrophotometric techniques will be considered in detail in the following sections. 

As indicated in Fig. 21.3, for both atomic absorption spectroscopy and atomic fluorescence spectroscopy a resonance line source is required, and the most important of these is the hollow cathode lamp which is shown diagrammatically in Fig. 21.8. For any given determination the hollow cathode lamp used has an emitting cathode of the same element as that being studied in the flame. The cathode is in the form of a cylinder, and the electrodes are enclosed in a borosilicate or quartz envelope which contains an inert gas (neon or argon) at a pressure of approximately 5 torr. The application of a high potential across the electrodes causes a discharge which creates ions of the noble gas. These ions are accelerated to the cathode and, on collision, excite the cathode element to emission. Multi-element lamps are available in which the cathodes are made from alloys, but in these lamps the resonance line intensities of individual elements are somewhat reduced. 

We have learned from the preceding chapters that the chemical and physical state of a Mossbauer atom in any kind of solid material can be characterized by way of the hyperfine interactions which manifest themselves in the Mossbauer spectrum by the isomer shift and, where relevant, electric quadrupole and/or magnetic dipole splitting of the resonance lines. On the basis of all the parameters obtainable from a Mossbauer spectrum, it is, in most cases, possible to identify unambiguously one or more chemical species of a given Mossbauer atom occurring in the same material. This - usually called phase analysis by Mossbauer spectroscopy - is nondestructive and widely used in various kinds of physicochemical smdies, for example, the studies of... 

Hyperfine structure measurements using on-line atomic-beam techniques are of great importance in the systematic study of spins and moments of nuclei far from beta-stability. We will discuss the atomic-beam magnetic resonance (ABMR) method, and laser spectroscopy methods based on crossed-beam geometry with a collimated thermal atomic-beam and collinear geometry with a fast atomic-beam. Selected results from the extensive measurements at the ISOLDE facility at CERN will be presented. 

The rest of the apparatus is the same as when operated at the Proton Synchrotron. First tested on cesium [ HUB 78 ], [ THI 81 ] the apparatus was used to uncover the resonance lines of francium for which no optical transition had ever been observed. The CERN on line mass separator, Isolde, makes available a source of more than 10 atoms/sec of chemically and isotopically pure 213 Fr isotope. Such an amount is more than needed for a laser atomic beam spectroscopy. The first step is obviously to locate the resonance line at low resolution, using a broad band laser excitation. In a second step, once the line is located, a high resolution study is undertaken, [ LIB 80] and [ BEN 84]. The observed signal is displayed (fig 3a) at low resolution and(3 b)at high resolution. 

It should be pointed out here that wavelength selection in atomic absorption spectroscopy is largely accomplished by the choice of the monochromatic sharp line source, possessing the wavelength of a resonance line of the element to be determined, a specificity of selection unobtainable by any other means. Any additional wavelength selection can be considered merely secondary and the methods to this end should be examined with this in mind. 

In the following subsections the application of atomic absorption spectroscopy to the determination of the more important elements of biological and clinical interest is presented, and special problems and interferences encountered with individual elements are discussed in detail. The resonance lines given at the beginning of each subsection are those showing greatest absorption, although many elements possess several resonance lines that can be used in analysis. The sensitivity limits quoted are the lowest reported in the literature, usually defined as that concentration of the test element in aqueous solution which produces 1% absorption. The reproducibility of results by most atomic absorption techniques lies... 

Any future development in the study of the reactions of vdW molecules must be related to solving the technical difficulty of sorting clusters according to their size. Several approaches may be taken. In intracluster reactions one may be able to use spectroscopy to initiate reactions in a well-defined complex. For example, in the complexes of HgHj one can excite this complex selectively due to its shifted absorption versus the atomic resonance line. 

Photometers At a minimum, an instrument for atomic absorption spectroscopy must be capable of providing a sufficiently narrow bandwidth to isolate the line chosen for a measurement from other lines that may interfere with or diminish the sensitivity of the method. A photometer equipped with a hollow-cathode source and filters is satisfactory for measuring concentrations of the alkali metals, which have only a few widely spaced resonance lines in the visible region. A more versatile photometer is sold with readily interchangeable interference filters and lamps. A separate fdter and lamp are used for each element. Satisfactory results for the determination of 22 metals are claimed. 

The components included within the frame drawn in dotted lines represents the apparatus required for flame emissions spectmscopy. For atomic absorption spectroscopy there is an additional requirement of a resonance line source. 

For analytical purposes, bismuth can be determined without interference by use of air-acetylene flame atomic absorption spectroscopy (FAAS) (Welz and Sperling 1998, Ju 2002). The characteristic concentration at the 222.8 nm resonance line is 0.2mgL various other analytical lines are compiled in Table 5.1. An improved signal-to-noise (S/N) ratio can be obtained in the air-hydrogen flame with a limit of detection (LOD) of 0.015 mgL . ... 

More than sixty elements can be determined by atomic-absorption or flame-emission spectroscopy, many at or below about 1 ppm [4]. Only metals and metalloids can be determined by usual flame methods, because the resonance lines for nonmetals occur in the vacuum-ultraviolet region however, a number of indirect methods for determining nonmetals have been described. For example, chloride can be determined by precipitating it with silver ion and then measuring either the excess or the reacted silver. Phosphorus (525.9 nm) and sulfur (383.7 nm) species (e.g., Sj) exhibit sharp molecular-band emission in the argon-hydrogen flame. 

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