Atom reservoir

The intrinsic drawback of LIBS is a short duration (less than a few hundreds microseconds) and strongly non-stationary conditions of a laser plume. Much higher sensitivity has been realized by transport of the ablated material into secondary atomic reservoirs such as a microwave-induced plasma (MIP) or an inductively coupled plasma (ICP). Owing to the much longer residence time of ablated atoms and ions in a stationary MIP (typically several ms compared with at most a hundred microseconds in a laser plume) and because of additional excitation of the radiating upper levels in the low pressure plasma, the line intensities of atoms and ions are greatly enhanced. Because of these factors the DLs of LA-MIP have been improved by one to two orders of magnitude compared with LIBS. 

Ross, R. T., Gonzalez, J. G., and Segar, D. A. "The Direct Determination of Chromium In Urine by Selective Volatilization with Atom Reservoir Atomic Absorption". Anal. Chlm. Acta (1973), 63, 205-209. 

Fluorescence Spectroscopy with a Carbon Filament Atom Reservoir". Anal. Chlm. Acta (1969), 27-41. 

Te. Instruments based upon the use of a chemical flame as the atom reservoir have not proved to be generally successful. The introduction of the ICP torch renewed interest in atomic fluorescence and new instruments based on the ICP torch as a source of free atoms were constructed. However, these seem to have been only slightly more satisfactory than earlier instruments and have not come into widespread use. Some detection limits are included in Table 8.6. 

ATOM RESERVOIR MONOCHROMATOR PHOTOMULTIPLIER READ OUT SYSTEM... 

Nonflame atom reservoirs have been developed for specific atomic spectrometric techniques. Electrothermal atomizers (carbon rods, carbon furnaces, or tantalum ribbons) have been developed for AAS or AFS since they require the generation of ground state atoms, whereas... 

Photomultipliers are generally used to convert the spectral radiation to an electrical current and often phase-sensitive lock-in amplifiers are used to amplify the resulting current. AES and AFS require similar read-out systems because both methods are measuring small signals. The difficulty associated with both these methods is the separation of the signal for the atomic transition of interest from the background radiation emitted by excited molecular species produced in the atom reservoir. AFS phase locks the amplifier detection circuit to the modulation frequency of the spectral source. Modulation of the source is also used in AAS. 

AES quantifies the deactivation of excited atoms. Atom reservoirs will also produce excited molecules that could interfere with the subsequent analysis since emission from excited molecular species is broad... 

The selection of a technique to determine the concentration of a given element is often based on the availability of the instrumentation and the personal preferences of the analytical chemist. As a general rule, AAS is preferred when quantifications of only a few elements are required since it is easy to operate and is relatively inexpensive. A comparison of the detection limits that can be obtained by atomic spectroscopy with various atom reservoirs is contained in Table 8.1. These data show the advantages of individual techniques and also the improvements in detection limits that can be obtained with different atom reservoirs. 

On the basis of the preceding discussion, it should be obvious that ultratrace elemental analysis can be performed without any major problems by atomic spectroscopy. A major disadvantage with elemental analysis is that it does not provide information on element speciation. Speciation has major significance since it can define whether the element can become bioavailable. For example, complexed iron will be metabolized more readily than unbound iron and the measure of total iron in the sample will not discriminate between the available and nonavailable forms. There are many other similar examples and analytical procedures that must be developed which will enable elemental speciation to be performed. Liquid chromatographic procedures (either ion-exchange, ion-pair, liquid-solid, or liquid-liquid chromatography) are the best methods to speciate samples since they can separate solutes on the basis of a number of parameters. Chromatographic separation can be used as part of the sample preparation step and the column effluent can be monitored with atomic spectroscopy. This mode of operation combines the excellent separation characteristics with the element selectivity of atomic spectroscopy. AAS with a flame as the atom reservoir or AES with an inductively coupled plasma have been used successfully to speciate various ultratrace elements. 

Backstrom and co-workers [1] have demonstrated significant improvements in nebuHzer efficiencies by increasing analyte transport efficiency, at a solvent load acceptable for the atom reservoir in question, and therefore improving detection Hmits. Conventional nebuHzer systems do not allow this because an increased analyte transport efficiency will give a too high a solvent load in the atom reservoir. 

Recent advances in instrumentation have been primarily in improved atom reservoirs and automation. By far the most commonly used atom reservoir in AAS is the flame, either air acetylene (2400°K) or nitrous oxide acetylene (3200°K) for more... 

Segar DA, Gilio JL. 1973. The determination of trace transition elements in biological tissues using flameless atom reservoir atomic absorption. Int J Environ Anal Chem 2 291-301. 

The general construction of an atomic absorption spectrometer, which need not be at all complicated, is shown schematically in Fig. 1. The most important components are the light source (A), which emits the characteristic narrow-line spectrum of the element of interest an absorption cell or atom reservoir in which the atoms of the sample to be analysed are formed by thermal molecular dissociation, most commonly by a flame (B) a monochromator (C) for the spectral dispersion of the light into its component wavelengths with an exit slit of variable width to permit selection and isolation of the analytical wavelength a photomultiplier detector (D) whose function it is to convert photons of light into an electrical signal which may be amplified (E) and eventually displayed to the operator on the instruments readout, (F). 

Cases have been observed where the isotopic line absorption profiles completely overlap, e.g. boron-10 and -11 in a krypton-filled lamp at 249.7 nm [244]. Hannaford and Lowe [245] later showed that this was caused by an unusually large Doppler half-width induced by the fill-gas, and, if neon is used, the 208.9 and 209.0 nm lines can allow the determination of boron-10 and boron-11 isotope ratios. The 208.89/208.96 nm doublet was found to be more useful than the 249.68/249.77 nm doublet. Enriched isotope hollow-cathode lamps were used as sources. A sputtering cell was preferred to a nitrous oxide/acetylene flame as the atom reservoir, as it could be water-cooled to reduce broadening and solid samples could be used, thus avoiding the slow dissolution in nitric acid of samples of boron-10 used as a neutron absorber in reactor technology. 

The determination of calcium by the nonflame methodology was found to be impossible so a nitrous oxide-acetylene flame was used as the atom reservoir. 

The key to the success of GD is the ease with which it can create an atom reservoir of the sample material directly from the solid state. There are many types of glow-discharge [130], including the common neon light. 

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