Atomic-Fluorescence Spectroscopy

Although atomic fluorescence is not often used in commercial instrumentation, a good deal of research and special applications are performed with this technique. Hence, some details pertinent to atomic fluorescence are summarized here. 

As mentioned before, atomic fluorescence involves excitation of atomic vapor by a radiation source, followed by deactivation by the emission of radiation the emitted radiation is then measured. This process is not unlike molecular fluorescence spectroscopy, described in Chapter 9. Some of the modes of atomic fluorescence were alluded to in Section 10.1 (and in Fig. 10.3) a fluorescent line can be at a wavelength identical to the exciting wavelength, or it can be longer or (very rarely) shorter. There are two main types of fluorescence, resonance and nonresonance. A third type is sensitized fluorescence. 

Resonance Fluorescence. Resonance fluorescence occurs when the atoms absorb and reemit radiation at the same wavelength. The most common examples correspond to transitions originating in the ground state (resonance transitions). For example, resonance fluorescence is observed for zinc at 213.86 nm, for nickel at 

Nonresonance Fluorescence. Nonresonance fluorescence occurs when the exciting wavelength and the wavelength of the emitted fluorescence line are different. There are two basic types direct-line fluorescence and stepwise-line fluorescence. 

In direct-line fluorescence, an atom is excited (usually from the ground state) by a radiation source, and then undergoes a direct radiational transition to a metastable level above the ground state. An example is absorption at the 283.31 nm line by ground-state lead atoms, with subsequent emission at 405.78 nm. Xs with resonance fluorescence, direct-line fluorescence may be excited by absorption of a nonresonance line (e.g., tin fluorescence at 333.06 nm). 

Although fluorescence has been known for a number of years (see Chapter 1), the application of atomic fluorescence to chemical analysis had its start in 1964, when Winefordner and Vickers published a paper describing the principles involved in the method, and Winefordner and Staab published another describing use of atomic fluorescence for the determination of small amounts of zinc, cadmium, and mercury. 

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. 

In analytical atomic fluorescence the sample is reduced to an atomic vapor and excited by radiant energy of a suitable wavelength. The excited atoms emit energy when they return to a lower excited state or the atomic ground state. The intensity of emitted fluorescence energy is measured and is a function of the concentration of the atoms in the sample. Fluorescence spectra of atoms are simple and uncomplicated, in contrast to fluorescence spectra of molecules, since atomic spectra are not affected by vibrational and rotational energy states that exist in molecules. 

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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. 

Instead of employing the high temperature of a flame to bring about the production of atoms from the sample, it is possible in some cases to make use of either (a) non-flame methods involving the use of electrically heated graphite tubes or rods, or (b) vapour techniques. Procedures (a) and (b) both find applications in atomic absorption spectroscopy and in atomic fluorescence spectroscopy. 

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. 

Within the confines of the present volume it is not possible to provide a detailed discussion of instrumentation for atomic fluorescence spectroscopy. An instrument for simultaneous multi-element determination described by Mitchell and Johansson53 has been developed commercially. Many atomic absorption spectrophotometers can be adapted for fluorescence measurements and details are available from the manufacturers. Detailed descriptions of atomic fluorescence spectroscopy are to be found in many of the volumes listed in the Bibliography (Section 21.27). 

Klrkbrlght, G. F. "The Application of Non-Flame Atom Cells In Atomic Absorption and Atomic Fluorescence Spectroscopy. 

Element Atomic emission spectroscopy Atomic absorption spectroscopy Atomic fluorescence spectroscopy... 

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

What is the difference between the shape of the burner supporting a flame in atomic emission, atomic absorption, and atomic fluorescence spectroscopy What is the theoretical basis for these differences ... 

Mercury in natural gas is also measured by atomic fluorescence spectroscopy (ASTM D6350) and by atomic absorption spectroscopy (ASTM D5954). 

Figure 1.2 shows the basic instrumentation necessary for each technique. At this stage, we shall define the component where the atoms are produced and viewed as the atom cell. Much of what follows will explain what we mean by this term. In atomic emission spectroscopy, the atoms are excited in the atom cell also, but for atomic absorption and atomic fluorescence spectroscopy, an external light source is used to excite the ground-state atoms. In atomic absorption spectroscopy, the source is viewed directly and the attenuation of radiation measured. In atomic fluorescence spectroscopy, the source is not viewed directly, but the re-emittance of radiation is measured. 

In atomic fluorescence spectroscopy an intense excitation source is focused on to the atom cell. The atoms are excited then re-emit radiation, in all directions, when they return to the ground state. The radiation passes to a detector usually positioned at right-angles to the incident light. At low concentrations, the intensity of fluorescence is governed by the following relationship ... 

A number of analytical methods were developed for determination of elemental mercury. The methods are reviewed in Refs. [1-4]. They include traditional analytical techniques, such as atomic adsorption spectroscopy (AAS), atomic fluorescence spectroscopy (AFS), and atomic emission spectroscopy (AES). The AAS is based on measurements of optical adsorption at 253.7 or 184.9 nm. Typical value of the detection limit without pre-concentration step is over 1 pg/l. The AEF is much more sensitive and allows one to detect less than 0.1ng/l of mercury... 

Muscat et al. [38] used atomic fluorescence spectroscopy to determine down to 0.6 ng of mercury in 20 - 30 mg samples of wheat. 

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... 

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