Atomic fluorescence, detection limits intensity

The instrumentation required for atomic fluorescence measurements is simpler than that used for absorption. As the detector is placed so as to avoid receiving radiation directly from the lamp, it is not strictly necessary to use a sharp-line source or a monochromator. Furthermore, fluorescence intensities are directly related to the intensity of the primary radiation so that detection limits can be improved by employing a high-intensity discharge lamp. 

Elements such as As, Se and Te can be determined by AFS with hydride sample introduction into a flame or heated cell followed by atomization of the hydride. Mercury has been determined by cold-vapour AFS. A non-dispersive system for the determination of Hg in liquid and gas samples using AFS has been developed commercially (Fig. 6.4). Mercury ions in an aqueous solution are reduced to mercury using tin(II) chloride solution. The mercury vapour is continuously swept out of the solution by a carrier gas and fed to the fluorescence detector, where the fluorescence radiation is measured at 253.7 nm after excitation of the mercury vapour with a high-intensity mercury lamp (detection limit 0.9 ng I l). Gaseous mercury in gas samples (e.g. air) can be measured directly or after preconcentration on an absorber consisting of, for example, gold-coated sand. By heating the absorber, mercury is desorbed and transferred to the fluorescence detector. 

In atomic absorption the background continuum is usually negligible and the resonance line intense. To give the maximum discrimination against stray radiation, and hence the lowest detection limit, the slit width should be small. In atomic emission and fluorescence the analytical signal is smaller and the background due to scattered light and con-... 

Elements with low intensity fluorescence lines (e.g. Eu, Tm and Y) have been determined in aqueous solutions by depositing and drying nanoliter amounts of sample on the Ni cathode of a miniature GD source used as the atom reservoir [665], The atomic cloud thus formed was extited by a Cu-vapor laser-pumped dye laser to detect fluorescence directly. Absolute detection limits of 2 fg for Eu, 0.08 fg for Tm and 1.2 pg for Y were achieved and the total time for analysis from sample probing to data acquisition did not exceed 5 min. 

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. 

Slow Discharge Chamber. This is used for the analysis of solid metal samples. Atomization is produced by sputtering and intensive excitation radiation is focused onto the atom vapour formed. The fluorescence radiation produced is then detected at right angles to the incident radiation. The precision obtained has been about 2% for many metals and the detection limits reported have varied between 1 and 100 mgl . ... 

Although weak fluorescence in the P— P lines was observed, from a practical standpoint only the P— P lines were found to be intense enough for resonance fluorescence work at low atom concentrations. The fluorescence count rates using the whole of the fully allowed P— P transition of F were typically 2 counts s" at [F] = 1 X 10 cm . These data set a lower concentration limit of 1 X 10 cm" for the smallest detectable concentration of F P atoms. Similar lower limits for O, Br, and I atoms are appreciably less, and are continually being improved by better attention to collimation and detection. Because of the low count rates observed in the F-atom resonance fluorescence studies, it is a better approach to use resonance absorption with a non-reversed line source (see above). 

Organic solvents have been used frequently in flame emission and atomic absorption to enhance the analytical signal. It has been shown that the intensity of a fluorescence signal also is enhanced by using organic solvents with premixed laminar flow burners, although the effect has not yet been extensively studied. Several examples of enhancement by use of organic solvents include an improvement in the detection limit for silver of about 40 (see footnote 11) and an improvement by a factor of five in the detection limit for zinc (see footnote 8). 

Some of the characteristics of atomic fluorescence that will influence its analytical use include its sensitivity and its large linear calibration range. For many elements present detection limits are an order of magnitude better than atomic absorption if high-intensity sources are used The large linear calibration range provides a convenient system to use without the need for solution of samples. 

Instrumentation for atomic fluorescence is not expensive. A simple monochromator and detector are adequate. Alternating current amplification is highly desirable to aid in eliminating effects due to emission from the flame and to reduce the effects of scattered radiation. High-intensity sources are necessary to achieve low detection limits. The electrodeless discharge... 

The principles of the laser-excited atomic fluorescence (LEAF) technique are very simple. A liquid or solid sample is atomized in an appropriate device. The atomic vapor is illuminated by laser radiation tuned to a strong resonance transition of an analyte atom. The excited analyte atoms spontaneously radiate fluorescence photons and a recording. system registers the intensity of fluorescence (or total number of fluorescent photons). The extremely high spectral brightness of lasers makes it possible to saturate a resonance transition of an analyte atom. Therefore, the maximum fluorescence intensity of the free analyte atoms can be achieved while the effect of intensity fluctuations of the excitation source are minimized. Both factors provide the main advantage of LEAF— extremely high sensitivity. The best absolute detection limits achieved in direct analysis by LEAF... 

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