Atomic Fluorescence Spectrometry AFS

The calculation of fluorescence yields for AFS are similar to those for molecular fluorescence (Chapter 5). Ingle and Crouch present an extensive discussion of theory of atomic fluorescence. Given the limited commercial applications of AFS, the theory will not be covered here. It is sufficient to understand that for a resonance transition and low analyte concentration, the fluorescence signal is proportional to the analyte concentration and to the intensity of the source. This assumption is valid for sources that do not alter the population of the analyte states. Intense laser sources can deplete the population of lower-energy states, including the state from which excitation occurs. This condition is called saturation and is discussed under applications of AFS in Section 7.6.3. 

A block diagram for an AFS spectrometer is shown in Fig. 7.58. The fluorescence signal is generally measured at an angle of 90° with respect to the excitation source to minimize 

AES would be expected to suffer from chemical interferences and spectral interferences, as do the other atomic emission techniques we have discussed. The focus will be on the commercial instmments available and on the graphite furnace-laser system. 

Spectral interferences are minimal for cold vapor Hg AFS and for hydride generation AFS, due to the chemical separation step that is used prior to excitation. Background signals in GF-LEAFS are smaller than those from GFAAS, but Zeeman background correction, described in Chapter 6, has been used for GF-LEAFS to provide accurate background correction. 

Graphite Furnace Laser-excited Atomic Fluorescence Spectrometry (GF-LEAFS) 

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The very low Hg concentration levels in ice core of remote glaciers require an ultra-sensitive analytical technique as well as a contamination-free sample preparation methodology. The potential of two analytical techniques for Hg determination - cold vapour inductively coupled plasma mass spectrometry (CV ICP-SFMS) and atomic fluorescence spectrometry (AFS) with gold amalgamation was studied. 

In this work, atomic fluorescence spectrometry (AFS) with vapor generation is used for Hg determination in different types of waters (drinking, surface, underground, industrial waste). 

Principles and Characteristics Atomic fluorescence spectrometry (AFS) is based on excitation of atoms by radiation of a suitable wavelength (absorption), and detection and measurement of the resultant de-excitation (fluorescence). The only process of analytical importance is resonance fluorescence, in which the excitation and fluorescence lines have the same wavelength. Nonresonance transitions are not particularly analytically useful, and involve absorption and fluorescence photons of different energies (wavelength). 

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. 

As noted earlier, USNs have been employed for sample insertion into atomic spectrometers suoh as flame atomio absorption spectrometry (FAAS) [9,10], electrothermal atomic absorption speotrometry (ETAAS) [11], atomic fluorescence spectrometry (AFS) [12,13], induotively ooupled plasma-atomic emission spectrometry (ICP-AES) [14,15], inductively coupled plasma-mass spectrometry (ICP-MS) [16,17] and microwave induced plasma-atomic emission spectrometry (MIP-AES) [18,19]. Most of the applications of ultrasonic nebulization (USNn) involve plasma-based detectors, the high sensitivity, selectivity, precision, resolution and throughput have fostered their implementation in routine laboratories despite their high cost [4]. 

Whatever the analytical method and the determinand may be, the greatest care should be devoted to the proper selection and use of internal standards, careful preparation of blanks and adequate calibration to avoid serious mistakes. Today the Antarctic investigator has access to a multitude of analytical techniques, the scope, detection power and robustness of which were simply unthinkable only two decades ago. For chemical elements they encompass Atomic Absorption Spectrometry (AAS) [with Flame (F) and Electrothermal Atomization (ETA) and Hydride or Cold Vapor (HG or CV) generation]. Atomic Emission Spectrometry (AES) [with Inductively Coupled Plasma (ICP), Spark (S), Flame (F) and Glow Discharge/Hollow Cathode (HC/GD) emission sources], Atomic Fluorescence Spectrometry (AFS) [with HC/GD, Electrodeless Discharge (ED) and Laser Excitation (LE) sources and with the possibility of resorting to the important Isotope... 

Atomic Fluorescence Spectrometry (AFS) (5, 58), quadrupole and double-focusing magnetic sector ICP-MS (59-63) have all been used in the past for the determination of trace element in polar snow and ice. Of those techniques only LEAFS, double-focusing magnetic sector ICP-MS and DPASV have proved capable of direct determination on the analytes in the samples at the required levels. 

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 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. 

Many other methods have been applied - or continue to be applicable - to elemental determinations, but have not been individually covered in this chapter due not only to page constraints but also to their less dominant roles in elemental determinations and current less widespread usage. Methods include thermochemical or thermal analysis, infra-red spectrometry (IR), near-infra-red analysis (NIR), NMR, EPR, kinetic methods of analysis, Mbssbauer spectrometry, gravimetry, volumetry (titrimetry), gas-ometry, fluorescence spectrometry (molecular) (including fluorometry, fluorimetry, spectrophotofluorometry, phosphorimetry, chemiluminescence), atomic fluorescence spectrometry (AFS) (including ICP atomic fluorescence, ICP-AFS, and flame atomic fluorescence). The chapter by Watkinson... 

The related subject of atomic fluorescence spectrometry (AFS), the emission of photons by excited gas phase atoms following excitation by absorption of photons, is also covered in this chapter. 

Atomic fluorescence spectrometry (AFS). An analytical method for the determination of elements in small quantities. It is based on the emission of free atoms when the excitation is performed by the radiation energy. 

Atomic fluorescence spectrometry (AFS) is based on the excitation of gaseous atoms by optical radiation of suitable wavelength (frequency) and the measurement of the resultant fluorescence radiation. Atomic fluorescence is, thus, in principle the opposite process to atomic absorption. Each atom has a characteristic fluorescence spectrum. The wavelength of the fluorescence line may be the same, greater, or smaller than the wavelength of the excitation line. 

A range of chromatographic techniques coupled to element specific detectors has been used in speciation studies to separate individual organometallic species (e.g., butyltins, arsenic species) and to separate metals bovmd to various biomolecules. The combination of a chromatographic separation with varying instrumental detection systems are commonly called coupled, hybrid, or hyphenated techniques (e.g., liquid chromatography inductively coupled plasma-mass spectrometry (LC-ICP-MS), gas chromatography-atomic absorption spectroscopy (GC-AAS)). The detection systems used in coupled techniques include MS, ICP-MS, atomic fluorescence spectrometry (AFS), AAS, ICP-atomic emission spectrometry (ICP-AES), and atomic emission detection (AED). 

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. 

To cite a few examples, HPLC was coupled with SIA for the simultaneous determination of several heavy metals by means of nitro-PAPS (polyfiuoroalkyl phosphate esters) complexes [43]. An SIA—HPLC—atomic fluorescence spectrometry (AFS) system was proposed for As speciation in seafood extracts, implementing standard addition method for simultaneous quantification of four As species [44]. An SIA-HPLC with electrochemical detection was proposed using a homemade microcolumn SPE coupled to SIA in order to automate the sample cleanup, extraction and detection of sulfonamides [45]. 

The sample is generally volatilized by a flame or furnace. The temperature is not usually sufficient to produce ionization, so that the vapor contains largely atoms. These atoms absorb the characteristic incident radiation resulting in the promotion of their electrons to an excited state. They may then undergo transitions to other energy levels and re-emit radiation of another, but still characteristic, wavelength as fluorescence. This allows determination by atomic fluorescence spectrometry (AFS). 

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