Detection atomic fluorescence spectrometry

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). 

It has been reported that the differential determination of arsenic [36-41] and also antimony [42,43] is possible by hydride generation-atomic absorption spectrophotometry. The HGA-AS is a simple and sensitive method for the determination of elements which form gaseous hydrides [35,44-47] and mg/1 levels of these elements can be determined with high precision by this method. This technique has also been applied to analyses of various samples, utilising automated methods [48-50] and combining various kinds of detection methods, such as gas chromatography [51], atomic fluorescence spectrometry [52,53], and inductively coupled plasma emission spectrometry [47]. 

Techniques for analysis of different mercury species in biological samples and abiotic materials include atomic absorption, cold vapor atomic fluorescence spectrometry, gas-liquid chromatography with electron capture detection, and inductively coupled plasma mass spectrometry (Lansens etal. 1991 Schintu etal. 1992 Porcella etal. 1995). Methylmercury concentrations in marine biological tissues are detected at concentrations as low as 10 pg Hg/kg tissue using graphite furnace sample preparation techniques and atomic absorption spectrometry (Schintu et al. 1992). 

Figure 6.1 Bar-graph of MeHg in CRM 580. The results correspond to six replicate determinations as performed by different laboratories using various methods. MEANS indicates the mean of laboratory means with 95% confidence interval. Abbreviations-. CVAAS, cold vapour atomic absorption spectrometry CVAFS, cold vapour atomic fluorescence spectrometry ECD, electron capture detection GC, gas chromatography HPLC, high-performance liquid chromatography ICPMS, inductively coupled plasma mass spectrometry MIP, microwave induced plasma atomic emission spectrometry QFAAS, quartz furnace atomic absorption spectrometry SFE, supercritical fluid extraction.

A number of techniques have been used for the speciation of arsenic compounds. The most important has been the formation of volatile hydrides of several species, separation by gas chromatography and detection by AAS. HPLC has been used to separate arsenic species. Several types of detectors have been studied for the determination of arsenic species in the column effluent. These have included AAS both off- and on-line, ICPAES and ICP-MS. An important comparative study of coupled chromatography-atomic spectrometry methods for the determination of arsenic was published (Ebdon et al., 1988). Both GC and HPLC were used as separative methods, and the detectors were FAAS, flame atomic fluorescence spectrometry (FAFS) and ICPAES. The conclusions were (1) that hydride generation and cryogenic trapping with GC-FAAS was the most... 

A combination of IPC and inductively coupled plasma (ICP) MS was extensively explored for the speciation of phosphorus, arsenic, selenium, cadmium, mercury, and chromium compounds [108-118] because it provides specific and sensitive element detection. Selenium IPC speciation was joined to atomic fluorescent spectrometry via an interface in which all selenium species were reduced by thiourea before conventional hydride generation [119], Coupling IPC separation of monomethyl and mercuric Hg in biotic samples by formation of their thiourea complexes with cold vapor generation and atomic fluorescence detection was successfully validated [120]. The coupling of IPC with atomic absorption spectrometry was also used for online speciation of Cr(III) and Cr(VI) [121] and arsenic compounds employing hydride generation [122]. 

IPC-ICP-MS was extensively explored for the speciation of phosphorus, arsenic, selenium, cadmium, mercury, and chromium compounds [4-17] because it provides specific and sensitive element detection. Selenium IPC speciation took also advantage of coupling with atomic fluorescent spectrometry via an interface in which all selenium species are reduced by thiourea before conventional hydride generation... 

Direct nebulization of an aqueous or organic phase containing extracted analytes has been widely used in flame atomic absorption spectroscopy [69-72], inductively coupled plasma atomic emission spectrometry [73-76], microwave induced plasma atomic emission spectrometry [77-80] and atomic fluorescence spectrometry [81], as well as to interface a separation step to a spectrometric detection [82-85]. 

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. 

The samples were analysed for total arsenic by hydride generation-atomic fluorescence spectrometry (HG-AFS) in the BGS laboratories in the UK. The detection limit was generally 0.25 or 0.5 pg L. Additional elements were measured by ICP-AES and in a few cases by ICP-MS. The samples were periodically interspersed with standard reference samples. The analytical data were combined with the site details and entered into a database. 

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... 

M. A. Bolshov, V. G. Koloshnikov, S. N. Rudniev, C. F. Boutron, U. Gorlach, C. C. Patterson, Detection of trace amounts of toxic metals in environmental samples by laser-excited atomic fluorescence spectrometry, J. Anal. At. Spectrom., 1 (1992), 99-104. 

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. 

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. 

Tab. 18. Detection limits in laser atomic fluorescence spectrometry.

About 50 mg were extracted with a mixture of H2S04/MeC00H in Milli-Q water (2 mL). The extract was injected in the headspace separation was by capillary gas chromatography (1 m length column with 3 mm internal diameter Chromosorb WAW 80-100 mesh loaded with 10% AT-1000 stationary phase injector temperature of 120°C, column temperature of 180 °C Ar as carrier gas at 100 mLmin ). Final detection was by atomic fluorescence spectrometry. Calibration was by standard additions, using MeHgCl in Milli-Q water as calibrant. 

Distillation was performed on a sample of 300 mg by addition of 0.5 mL H2SO4 (9 mol L ) in 20% KCl in H2O at a temperature of 145 °C distillation recovery was ca. 90%. Derivatization was by addition of 1% NaBEt4 in acetic acid. Separation was by gas liquid chromatography (column of 0.5 m length, internal diameter of 4 mm Chromosorb W AW-DMSC 60-80 mesh, loaded with 15% OV-3 stationary phase temperature of the injector of 500 °C, detector temperature at 20 °C column temperature of 100 °C He as carrier gas at 40 mL min ). Detection was by cold vapour atomic fluorescence spectrometry. Calibration was by calibration graph and standard additions using MeHgCl calibrant in Milli-Q water. 

Atomic fluorescence spectrometry is certainly an excellent detector for the determination of arsenic compounds. Necessary low detection limits can be obtained only after hydride generation. This is certainly a drawback as the nonhydride-forming arsenic compounds have to be converted into the volatile hydride-forming ones. 

Table Ib. Absolute Detection Limits Using Atomic Fluorescence Spectrometry and Several Other Methods...

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]. 

TABLE IV. Limiting Noises and Detection Limits in Laser-Excited Atomic Fluorescence Spectrometry using the 296.7/373.5 nm Transition of Iron... 

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). 

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