Atomic fluorescence development

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). 

Glow discharge is essentially a simple and efficient way to generate atoms. Long known for its ability to convert solid samples into gas-phase atoms, GD techniques provide ground-state atoms for atomic absorption or atomic fluorescence, excited-state atoms for atomic emission, and ionised atoms for MS [158], Commercial instrumentation has been developed for all these methods, except for GD-AFS and pulsed mode GD. 

It can be seen from the above that the sample stream emerging from the plasma will be rich in free ions and atoms of the elements from the sample. Thus, the ICP could provide an attractive source for analytical methods other than those based upon straightforward emission. Instruments using the ICP source as a basis for atomic fluorescence have been developed. 

Recent developments in the determination of elements in this group have been very much linked to the use of atomic fluorescence detection systems rather than AAS (see section 8.7). ICP-AES and ICP-MS can also be used but they are generally inferior in sensitivity. Best sensitivity is obtained from AFS detection. It should also be noted that the analysis may also be required to detect and measure organic compounds of these elements because of the toxicity in the organic form. Separation by one of the methods reviewed in Chapter 4 may thus be used in sample processing prior to analysis. 

The importance of trace elements is manifold and. unfortunately, previously hampered by relatively insensitive analytical methods. Good methods for determining concentrations of I ppm or less have been available for relatively few dements yet these may be the optimum concentrations for a particular trace element. When the responses of living organisms are more sensitive than the laboratory black boxes," the chemist naturally develops an inferiority complex. Fortunately, the recent development of analytical techniques capable of determining parts per billion has opened new vistas for the study of these problems. Some of these techniques are atomic absorption, atomic fluorescence, activation analysis, and X-ray fluorescence. 

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... 

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. 

Simon, S., Barats, A., Pannier, F., Potin-Gautier, M. Development of an on-line UV decomposition system for direct coupling of liquid chromatography to atomic-fluorescence spectrometry for selenium speciation analysis. Anal. Bioanal. Chem. 383, 562-569 (2005)... 

Figure 2 shows an example of separating organomercury using supercritical CO2. A 10-m X 50- tm-in-ner diameter SB-Methyl 100 column was used for the separation. Due to their poor solubility in supercritical carbon dioxide, monoorganomercury compounds were derivatized by diethyldithiocarbamate. An interface for a system consisting of SFC and atomic fluorescence spectrometry was developed for the detection of organomercurials. 

Many researchers have attempted to determine mercury levels in the blood, urine, tissues, and hair of humans and animals. Most methods have used atomic absorption spectrometry (AAS), atomic fluorescence spectrometry (AFS), or neutron activation analysis (NAA). In addition, methods based on mass spectrometry (MS), spectrophotometry, and anodic stripping voltametry (ASV) have also been tested. Of the available methods, cold vapor (CV) AAS is the most widely used. In most methods, mercury in the sample is reduced to the elemental state. Some methods require predigestion of the sample prior to reduction. At all phases of sample preparation and analysis, the possibility of contamination from mercury found naturally in the environment must be considered. Rigorous standards to prevent mercury contamination must be followed. Table 6-1 presents details of selected methods used to determine mercury in biological samples. Methods have been developed for the analysis of mercury in breath samples. These are based on AAS with either flameless (NIOSH 1994) or cold vapor release of the sample to the detection chamber (Rathje et al. 1974). Flameless AAS is the NIOSH-recommended method of determining levels of mercury in expired air (NIOSH 1994). No other current methods for analyzing breath were located. 

Atomic fluorescence with conventional hollow-cathode or electrodeless-discharge sources has not shown significant advantages over atomic absorption or atomic emission. As a consequence, the commercial development of atomic fluorescence instrumentation has been quite slow. Sensitivity advantages have been shown, however, for elements such as Hg, Sb, As, Se, and Te. 

Laser-excited atomic fluorescence spectrometry is capable of extremely low detection limits, particularly when combined with electrothermal atomization. Detection limits in the femtogram (10 g) to attogram (10 g) range have been shown for many elements. Commercial instrumentation has not been developed for laser-based AFS, probably because of its expense and the nonroutine nature of high-powered lasers. Atomic fluorescence has the disadvantage of being a singleelement method unless tunable lasers with their inherent complexities are used. 

Atomic absorption, optical emission and atomic fluorescence as well as plasma mass spectrometry and new approaches such as laser enhanced ionization now represent strong tools for elemental analysis including speciation and are found in many analytical laboratories. Their power of detection, reliability in terms of systematic errors and their costs reflecting the economic aspects should be compared with those of other methods of analysis, when it comes to the development of strategies for solving analytical problems (Table 20). 

The application of microtron photon activation analysis with radiochemical separation in environmental and biological samples was described by Randa et al. (2001), and both flame and plasma emission spectroscopic methods are also widely used. A more recently developed technique is that of laser-excited atomic fluorescence spectrometry (LEAFS) (Cheam et al. 1998). 

Winefordner and co-workers (9, 16, 20, 22) have developed the theory of atomic fluorescence flame spectrometry most extensively. The integrated intensity of atomic fluorescence. If, in w-sec./cm. -ster. for low concentrations of absorbing atoms is given by the following equation ... 

Atomic fluorescence flame spectrometry is receiving increased attention as a potential tool for the trace analysis of inorganic ions. Studies to date have indicated that limits of detection comparable or superior to those currently obtainable with atomic absorption or flame emission methods are frequently possible for elements whose emission lines are in the ultraviolet. The use of a continuum source, such as the high-pressure xenon arc, has been successful, although the limits of detection obtainable are not usually as low as those obtained with intense line sources. However, the xenon source can be used for the analysis of several elements either individually or by scanning a portion of the spectruin. Only chemical interferences are of concern they appear to be qualitatively similar for both atomic absorption and atomic fluorescence. With the current development of better sources and investigations into devices other than flames for sample introduction, further improvements in atomic fluorescence spectroscopy are to be expected. 

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