Atomic fluorescence spectrometry radiation, source

Both emission and absorption spectra are affected in a complex way by variations in atomisation temperature. The means of excitation contributes to the complexity of the spectra. Thermal excitation by flames (1500-3000 K) only results in a limited number of lines and simple spectra. Higher temperatures increase the total atom population of the flame, and thus the sensitivity. With certain elements, however, the increase in atom population is more than offset by the loss of atoms as a result of ionisation. Temperature also determines the relative number of excited and unexcited atoms in a source. The number of unexcited atoms in a typical flame exceeds the number of excited ones by a factor of 103 to 1010 or more. At higher temperatures (up to 10 000 K), in plasmas and electrical discharges, more complex spectra result, owing to the excitation to more and higher levels, and contributions of ionised species. On the other hand, atomic absorption and atomic fluorescence spectrometry, which require excitation by absorption of UV/VIS radiation, mainly involve resonance transitions, and result in very simple spectra. 

Atomic fluorescence spectrometry has a number of potential advantages when compared to atomic absorption. The most important is the relative case with which several elements can be determined simultaneously. This arises from the non-directional nature of fluorescence emission, which enables separate hollow-cathode lamps or a continuum source providing suitable primary radiation to be grouped around a circular burner with one or more detectors. 

Atomic Fluorescence Spectrometry. A spectroscopic technique related to some of the types mentioned above is atomic fluorescence spectrometry (AFS). Like atomic absorption spectrometry (AAS), AFS requires a light source separate from that of the heated flame cell. This can be provided, as in AAS, by individual (or multielement lamps), or by a continuum source such as xenon arc or by suitable lasers or combination of lasers and dyes. The laser is still pretty much in its infancy but it is likely that future development will cause the laser, and consequently the many spectroscopic instruments to which it can be adapted to, to become increasingly popular. Complete freedom of wavelength selection still remains a problem. Unlike AAS the light source in AFS is not in direct line with the optical path, and therefore, the radiation emitted is a result of excitation by the lamp or laser source. 

Atomic fluorescence spectrometry (AFS) is the newest of the optical atomic spectroscopic methods. As in atomic absorption, an external source is used to excite the element of interest. Instead of measuring the attenuation of the source, however, the radiation emitted as a result of absorption is measured, often at right angles to avoid measuring the source radiation. 

Laser Fluorescence Noise Sources. Finally, let us examine a technique with very complex noise characteristics, laser excited flame atomic fluorescence spectrometry (LEAFS). In this technique, not only are we dealing with a radiation source as well as an atomic vapor cell, as In atomic absorption, but the source Is pulsed with pulse widths of nanoseconds to microseconds, so that we must deal with very large Incident source photon fluxes which may result in optical saturation, and very small average signals from the atomic vapor cell at the detection limit [22]. Detection schemes involve gated amplifiers, which are synchronized to the laser pulse incident on the flame and which average the analyte fluorescence pulses [23]. 

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. 

The excitation source can be a continuum or a line-like radiation source. Research on atomic fluorescence spectrometry has been connected with the examination of intense radiation sources such as electrodeless discharge lamps and lasers. Various flames, plasmas, and furnaces have been employed as atomizing devices. 

Radiation sources As in AA, line sources are typically used, although high-intensity sources are much more critical in AF. Boosted HCLs and EDLs have been employed. However, tunable lasers certainly provide the optimal sensitivity, i.e., laser excited atomic fluorescence spectrometry. 

In atomic fluorescence spectrometry (AFS), the analyte is introduced into an atomizer (flame, plasma, glow discharge, furnace) and excited by monochromatic radiation emitted by a primary source. The latter can be a continuous source (xenon lamp) or a line source (hollow cathode lamp, electrodeless discharge lamp, or tuned laser). Subsequently, the fluorescence radiation, which may be of the same wavelength (resonance fluorescence) or of longer wavelength (nonresonance fluorescence), is measured. 

Principles and Characteristics The analytical capabilities of the conventional fluorescence (CF) technique (c/r. Chp. 1.4.2) are enhanced by the use of lasers as excitation sources. These allow precise activation of fluorophores with finely tuned laser-induced emission. The laser provides a very selective means of populating excited states and the study of the spectra of radiation emitted as these states decay is generally known as laser-induced fluorescence (LIF, either atomic or molecular fluorescence) [105] or laser-excited atomic fluorescence spectrometry (LEAFS). In LIF an absorption spectrum is obtained by measuring the excitation spectrum for creating fluorescing excited state... 

The basic instrumentation used for spectrometric measurements has already been described in the previous chapter (p. 277). Methods of excitation, monochromators and detectors used in atomic emission and absorption techniques are included in Table 8.1. Sources of radiation physically separated from the sample are required for atomic absorption, atomic fluorescence and X-ray fluorescence spectrometry (cf. molecular absorption spectrometry), whereas in flame photometry, arc/spark and plasma emission techniques, the sample is excited directly by thermal means. Diffraction gratings or prism monochromators are used for dispersion in all the techniques including X-ray fluorescence where a single crystal of appropriate lattice dimensions acts as a grating. Atomic fluorescence spectra are sufficiently simple to allow the use of an interference filter in many instances. Photomultiplier detectors are used in every technique except X-ray fluorescence where proportional counting or scintillation devices are employed. Photographic recording of a complete spectrum facilitates qualitative analysis by optical emission spectrometry, but is now rarely used. 

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. 

The photochemical reactor used for microwave-assisted experiments is an essential tool for experimental work. Such equipment enables simultaneous irradiation of the sample with both MW and UV-visible radiation. The idea of using an electrodeless lamp, in which the discharge is powered by the MW field, for photochemistry was born half a century ago [53, 62]. The lamp was originally proposed as a source of UV radiation only, without considering the effects of microwaves on photochemical reactions. The first applications of EDL were connected with the construction of a high-intensity source of UV radiation for atomic fluorescence flame spectrometry [88-90]. 

Spectrophotometric techniques have been the basis of many coal analysis methods. One of the most widely used techniques for analysis of trace elements is atomic absorption spectrometry, in which the standards and samples are aspirated into a flame. A hollow cathode lamp provides a source of radiation that is characteristic of the element of interest and the absorption of characteristic energy by the atoms of a particular element. X-ray fluorescence is also employed as a quantitative technique for trace element determination and depends on election of orbital electrons from atoms of the element when the sample is irradiated by an x-ray source. 

Adams F, Janssens K, and Snigirev A (1998) Microscopic X-ray fluorescence analysis and related methods with laboratory and synchrotron radiation sources. Journal of Analytical Atomic Spectrometry 13(5) 319-331. 

Some instruments have been developed for both atomic absorption and atomic fluorescence. However, a powerful source e.g. a laser is required for the latter spectrometry. In principle, when a gaseous metal atom is excited by absorption of radiation, it emits fluorescence radiation when it reverts to the ground state. This can be recorded in a monochromator/detector set up, not unlike atomic absorption. (J.Chem. Educ., 59, 1982,909 895 AnalChem., 53,1981,332A 1448A 54, 1082, 553, 1006A). 

---