Atomic fluorescence spectral

AFS quantifies the discrete radiation emitted by excited state atoms that have been excited by radiation from a spectral source. There are a number of mechanisms that are responsible for the atomic fluorescence signal resonance fluorescence, step-wise fluorescence, direct-line fluorescence, and sensitized fluorescence. Generally, the lowest resonance transition (l->0) is used for AFS. If a line source is used for excitation and if the atomic vapor is dilute, then the radiant power of the atomic... 

Why are spectral interferences less important in atomic absorption spectroscopy and atomic fluorescence spectroscopy than atomic emission spectroscopy ... 

In flame emission spectroscopy, light emission is caused by a thermal effect and not by a photon, as it is in atomic fluorescence. Flame emission, which is used solely for quantification, is distinguished from atomic emission, used for qualitative and quantitative analyses. This latter, more general term is reserved for a spectral method of analysis that uses high temperature thermal sources and a higher performance optical arrangement. 

The determination of organic selenium compounds is done preferably by GC coupled to element-or molecule-specific detectors, such as GC-AED or molecular mass spectrometric detection (GC-MS).240 In this case, ICP-MS detection does not yield the improvement in sensitivity otherwise seen, which is due to spectral interferences. Dietz et al.241 have compared the analytical figures of merit of three detector systems for GC (AED, atomic fluorescence spectroscopy (AFS), and ICP-MS), arriving at the conclusion that GC-AED is the most sensitive and most practical... 

In the case of atomic absorption and atomic fluorescence the selectivity is thus already partly realized by the radiation source delivering the primary radiation, which in most cases is a line source (hollow cathode lamp, laser, etc.). Therefore, the spectral bandpass of the monochromator is not as critical as it is in atomic emission work. This is especially true for laser based methods, where in some cases of atomic fluorescence a filter is sufficient, or for laser induced ionization spectrometry where no spectral isolation is required at all. 

In atomic fluorescence spectrometry spectral interferences are low as the fluorescence spectra are not line rich. 

Because the atomic fluorescence is measured at a right angle to the source, spectral interferences are minimal and a simple cutoff filter may often be used to isolate the emission line. The intensity of the fluorescence is directly proportional to the analyte concentration. As the analyte concentration within the flame becomes large, self-absorption of resonance fluorescence becomes significant, as it does in flame emission spectroscopy. Under these conditions, the linearity of the instrumental response breaks down and a calibration curve must be used or the analyte solutions diluted accordingly. 

Figure 16. 1 s EELS and NEXAFS spectra of imidazole (a) 4,5 dicyanoimidazole (b), and the EELS spectra of s-trazine (c). Different curves in (a) and (b) correspond to fluorescence yield, electron yield, and EELS (from top to bottom). The hatched lines in each figure are the ionization energies for different nitrogen atoms. The spectral assignments are given in Table 2 (in Appendix). (Reprinted with permission from Apen et al. 1993. American Chemical Society.)... 

For atomic fluorescence, the emission profile of the source can be wider than the absorption profile of the line—even a continuum can be used. A source with a spectral profile narrower than the absorption profile of the line is usually preferred for atomic absorption. 

Atomic fluorescence has many superior features for trace elemental analysis (spectral simplicity, wide dynamic range, and simultaneous multi-element analysis). However, major practical problems of this technique are connected... 

Spectral Interferences. Relatively few examples of actual spectral interferences have been reported in atomic absorption or atomic fluorescence spectrometry. This means that the possibility that a resonance line emitted from a line-like radiation source may overlap with an absorption line of another element present in the atomizer is very small. 

Atomic fluorescence is the most recent development in analytical atomic spectroscopy thus it has not had time to be evaluated as well as other techniques. Further developments in this field with respect to optimizing sources and sample cells, together with improvements in instrumental parameters and development of readily available commercial instrumentation, should lead to this technique serving in the area of analytical spectral methods to supplement the already well-established arc and spark emission, flame emission, and atomic absorption spectroscopy. 

Analytical uses of atomic fluorescence have been developed in recent years. Most of the methods utilize resonance fluorescence, but other types of fluorescence also are useful. For example, spectral emission lines of mercury have been used to produce fluorescence of elements such as iron, thallium, chromium, and magnesium. The instrumentation and techniques for analytical applications of atomic fluorescence are described in Chapter 11. 

The spectral mechanisms involved in atomic fluorescence have been described in Chapter 2 and reference to that chapter should be made to review the various types of atomic fluorescence. Resonance fluorescence is most frequently used for analytical purposes, although other fluorescence mechanisms also are occasionally used. 

A variety of excitation sources may be used for atomic fluorescence since the prime requirements are high intensity and stability. A narrow spectral line source, such as those used for atomic absorption, is not essential. Some of the sources that have been used successfully for atomic fluorescence include the following. 

Philips and Osram spectral discharge lamps have been used as spectral sources for analytical atomic fluorescence. These lamps have internal electrodes and produce intense spectral lines. The spectral lines, however, are subject to line reversal and the lamps are available only for a limited number of elements. Use of Philips and/or Osram lamps require careful control of input energy to produce maximum intensity without line reversal. Under these conditions they have produced satisfactory atomic fluorescence signals for some elements, including cadmium, mercury, zinc, and thallium. 

Continuous spectral emission sources also are useful in atomic fluorescence if they have sufficient intensity. The most commonly used continuous source has been the xenon arc lamp, the 450-W xenon lamp being especially useful. These lamps emit a continuum in the visible and near ultraviolet. Their intensities, however, decrease rapidly below about 2500 A,... 

Atomic fluorescence spectra are simple, with relatively few spectral lines therefore monochromators of extremely high resolution are not required. Either grating or prism instruments may be used. A monochromator... 

Several different types of spectral interferences are possible in analytical atomic fluorescence. If a second, unwanted element emits a fluorescence radiation simultaneously with the analyte element and its wavelength is within the band pass of the monochromator slit width, interference occurs. Not many instances of this type of interference have been identified. Some known examples include cadmium at 2288.0 A and arsenic at 2288.1 A and mercury with iron, thallium, chromium, and magnesium. If such interferences occur, the result will be an erroneous increase in fluorescence signal... 

Other sources of background include spectral line (nonanalyte atomic fluorescence) and spectral band (molecular fluorescence) interferences. Spectral line interferences are caused by the presence of another element that can absorb source radiation and emit fluorescence sufficiently close to the analyte wavelength to be collected by the detection system. Spectral band interferences involve the absorption of source light by a molecule whose fluorescence is collected by the detection system. Nonanalyte atomic fluorescence and molecular fluorescence are minimized by the use of a narrow line source and a nonresonance transition. This is in contrast to AES, where spectral interferences are sufficiently severe that a high-resolution monochromator is required. 

Atomic fluorescence is the process of radiational activation followed by radiational deactivation, unlike atomic emission, which depends on the collisional excitation of the spectral transition. For this, the ICP is used to produce a population of atoms in the ground state and a light source is required to provide excitation of the spectral transitions. Whereas a multitude of spectral lines from all the accompanying elements are emitted by the atomic emission process, the fluorescence spectrum is relatively simple, being confined principally to the resonance lines of the element used in the excitation source. 

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