Flame atomic fluorescence spectroscopy

The section on Spectroscopy has been retained but with some revisions and expansion. The section includes ultraviolet-visible spectroscopy, fluorescence, infrared and Raman spectroscopy, and X-ray spectrometry. Detection limits are listed for the elements when using flame emission, flame atomic absorption, electrothermal atomic absorption, argon induction coupled plasma, and flame atomic fluorescence. Nuclear magnetic resonance embraces tables for the nuclear properties of the elements, proton chemical shifts and coupling constants, and similar material for carbon-13, boron-11, nitrogen-15, fluorine-19, silicon-19, and phosphoms-31. 

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. 

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

In EMEP, ICP-MS is dehned as the reference technique. The exception is mercury, where cold vapor atomic fluorescence spectroscopy (CV-AFS) is chosen. Other techniques may be used, if they are shown to yield results of a quality equivalent to that obtainable with the recommended method. These other methods include graphite furnace atomic absorption spectroscopy (GF-AAS), flame-atomic absorption spectroscopy (F-AAS), and CV-AFS. The choice of technique depends on the detection limits desired. ICP-MS has the lowest detection limit for most elements and is therefore suitable for remote areas. The techniques described in this manual are presented with minimum detection limits. Table 17.2 lists the detection limits for the different methods. 

Kingsley, G. R., Clinical chemistry. Anal. Chem. 43, 15R-41R (1971). Kirkbright, G. F., The application of non-flame atom cells in atomic absorption and atomic fluorescence spectroscopy. Analyst London) 96, 609-623 (1971). Kirkbright, G. F., Saw, C. G., and West, T. S., Determination of trace amounts of tellurium by inorganic spectrofluorimetry at liquid nitrogen temperature. Analyst London) 94, 457-460 (1969). 

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]. 

Example 5.2 Total Metal Analysis of Soil Using X-Ray Fluorescence Spectroscopy - Comparison with Acid Digestion (Method 3050B), followed by Flame Atomic Absorption Spectroscopy... 

Figure 5.28 Comparison of the methods used in the analysis of (a) Soil A and (b) Soil CONTEST 32.3A , flame atomic absorption spectroscopy 0 X-ray fluorescence spectroscopy [32] (cf. DQ 5.10).

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. 

The basic instrumentation for atomic-fluorescence spectroscopy is shown in Figure 10.13. The source is placed at right angles to the monochromator so that its radiation (except for scattered radiation) does not enter the monochromator. The source is chopped to produce an AC signal and minimize flame-emission interference. As in molecular fluorescence (Chap. 9), the intensity of atomic fluorescence is directly proportional to the intensity of the light impinging on the sample from the source. 

Non-flame atomizers have also been used in atomic-fluorescence spectroscopy. 

Monitoring items include the measurement of the 7 heavy metal indexes Cu, Pb, Zn, Cd, Cr, Hg and As. Sediments decomposition and monitoring methods can be found in Part 5 of The Specification for Marine Monitoring—Sediments Analysis (GB17378.5-2007). For Cu and Zn, the flame atomic absorption spectrophotometry is adopted for Pb, Cd and Cr, the nonflame atomic absorption spectrophotometry is used For Hg and As, the atomic fluorescence spectroscopy is applied. The accuracy of standard substances is also measured according to national offshore sediment analysis. The analysis results meet the requirement. During the analysis, reagent blank and parallel samples are casually measured, the results of which show that the analysis process is not polluted and the relative standard deviation for the parallel samples are all lower than 10%. 

It should be obvious that no flame will meet all the above requirements in practice, therefore, an attempt is made to approach this ideal situation as closely as possible. Present practice is to adapt flame cells in use for flame emission and atomic absorption for use in atomic fluorescence spectroscopy. 

Atomic fluorescence spectroscopy is a popular technique for those analytes that readily form vapours, and specialized instrumentation is now available for individual elements. Such instruments are simple to operate, easily automated, and offer good sensitivity and freedom from interferences. The use of other flame-based fluorescence techniques has waned considerably over the years, but research continues... 

G. F. Kirkbright, The application of non-flame atom cells in atom-absorption and atomic-fluorescence spectroscopy, a review. The Analyst 96, 609-623 (1971). 

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