Laser-excited atomic fluorescence spectroscopy

Graphite furnace AAS Atomic fluorescence spectroscopy Inductively-coupled-plasma optical-emission spectroscopy Glow-discharge optical-emission spectroscopy Laser-excited resonance ionization spectroscopy Laser-excited atomic-fluorescence spectroscopy Laser-induced-breakdown spectroscopy Laser-induced photocoustic spectroscopy Resonance-ionization spectroscopy... 

Laser-excited atomic-fluorescence spectroscopy LEAFS... 

Hanselman D. S., Withnell R. and Hieetje G. M. (1991) Side-on photomultiplier gating system for Thomson scattering and laser-excited atomic fluorescence spectroscopy,... 

An example of the use of lasers in optimizing analyte detection is provided by the technique of laser excited atomic fluorescence spectroscopy (leafs). The detection of subfemtogram amounts of cadmium, thaUium, and lead has been reported (40). In this experiment, the sample of interest is first volatilized in a plasma (see Plasma technology) and then tuned photons from one or two dye lasers excite the analyte. When these atoms or ions relax, the resulting fluorescence signal is shunted into a photomultipHer for detection. Attomole detection levels are achievable using this technique. Continued advances in complex, multilaser spectroscopic determinations are expected to result in even lower levels of detection. 

The comparison of detection limits Is a fundamental part of many decision-making processes for the analytical chemist. Despite numerous efforts to standardize methodology for the calculation and reporting of detection limits, there is still a wide divergence In the way they appear in the literature. This paper discusses valid and invalid methods to calculate, report, and compare detection limits using atomic spectroscopic techniques. Noises which limit detection are discussed for analytical methods such as plasma emission spectroscopy, atomic absorption spectroscopy and laser excited atomic fluorescence spectroscopy. 

LEAFS laser-excited atomic fluorescence spectroscopy... 

Several techniques have been developed that use an electrothermal atomizer as an atom cell. Laser-induced fluorescence (LIF) has been discussed previously, but other techniques such as laser-excited atomic fluorescence spectroscopy (LEAFS) also exist, although not used routinely. 

Electron diffraction spectroscopy ETA LEAFS Electrothermal atomisation laser-excited atomic fluorescence... 

Bolshov, M.A., Zybin, A.V. and Smirenkim, I.I., Atomic fluorescence spectroscopy with laser excitation. Spectrochim. Acta, 36B, (1988) 1143-1152. 

Butcher DJ (1995) Laser-excited atomic and molecular fluorescence in a graphite furnace. In Sneddon J (ed.) Advances in Atomic Spectroscopy, vol. 2. San Diego JAI Press. 

Fluorescence spectroscopy is often used for detection of atoms and molecules. Laser light is increasingly used for excitation (LIF Laser-induced fluorescence). 

The rapid development of lasers has led to the publication of increasing numbers of papers concerned this year with such subjects as superfluorescence and co-operative radiation processes,451 the thermodynamics of co-operative luminescence,452 saturation, collisional dephasing, and quenching of fluorescence of organic vapours in intense laser excitation studies,453 a theoretical model for fluorescence in gases subjected to continuous i.r. excitation,454 a quantum treatment of spontaneous emission from strongly driven two-level atoms,455 the development of site-selection spectroscopy,45 and measurements of relaxation times 457 using laser excitation. 

Because of the unique characteristics of their emitted energies, lasers have been used for sample vaporization and excitation sources in atomic emission spectroscopy. They also have been used as sources for atomic absorption and atomic fluorescence analysis. Their application in these areas will no doubt increase as lasers become cheaper and more readily available. 

The technique of reducing the Doppler width by the collimation of mo lecular beams was employed before the invention of lasers to produce light sources with narrow emission lines [389]. Atoms in a collimated beam were excited by electron impact. The fluorescence lines emitted by the excited atoms showed a reduced Doppler width if observed in a direction perpendicular to the atomic beam. However, the intensity of these atomic beam light sources was very weak and only the application of intense monochromatic, tunable lasers has allowed one to take full advantage of this method of Doppler-free spectroscopy. 

In atomic laser spectroscopy, the laser radiation, which is tuned to a strong dipole transition of the atoms under investigation, penetrates the volume of species evaporated from the sample. The presence of analyte atoms can be measmed by means of the specific interaction between atoms and laser photons, such as by absorption techniques (laser atomic absorption spectrometry, LAAS), by fluorescence detection (laser-induced fluorescence spectroscopy, LIFS), or by means of ionization products (electrons or ions) of the selectively excited analyte atoms after an appropriate ionization process (Figures lA and IB). Ionization can be achieved in different ways (1) by interaction with an additional photon of the exciting laser or of a second laser (resonance ionization spectroscopy, RIS, or resonance ionization mass spectrometry, RIMS, respectively, if combined with a mass detection system) (2) by an electric field applied to the atomization volume (field-ionization laser spectroscopy, FILS) or (3) by collisional ionization by surrounding atoms (laser-enhanced ionization spectroscopy, LEIS). 

The different sensitive techniques of Doppler-limited laser spectroscopy discussed in the previous sections supplement each other in an ideal way. In the visible and ultraviolet range, where electronic states of atoms or molecules are excited by absorption of laser photons, excitation spectroscopy is generally the most suitable technique, particularly at low molecular densities. Because of the short spontaneous lifetimes of most excited electronic states the quantum efficiency rjk reaches 100% in many cases. For the detection of the laser-excited fluorescence, sensitive photomultipliers or intensified CCD cameras are available that allow, together with photon-counting electronics (Sect. 4.5), the detection of single fluorescence photons with an overall efficiency of 10 —10 including the collection efficiency 5 0.01—0.3 (Sect.6.3.1). 

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