Fluorescence, types direct line

Nonresonance Fluorescence. Nonresonance fluorescence occurs when the exciting wavelength and the wavelength of the emitted fluorescence line are different. There are two basic types direct-line fluorescence and stepwise-line fluorescence. 

All types of nonresonance fluorescence, particularly direct-line fluorescence, can be analytically useful sometimes it is more intense than resonance fluorescence, and it offers the advantage that scattering of the exciting radiation can be eliminated from the fluorescence spectrum by removing it with a filter or a monochromator. Self-absorption problems (absorption of the emitted radiation by the sample atoms) can also be avoided by measuring fluorescence at a nonresonance line that is not also absorbed. 

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. 

Atomic fluorescence transitions may be divided into three types a) resonance fluorescence (the excitation and fluorescence lines have the same upper and lower energy levels), b) direct line fluorescence (the excitation line and fluorescence line have the same upper energy level, but different lower energy levels), and (c) stepwise line fluorescence (the excitation line and the fluorescence line have different upper energy levels) (Figure 141). 

Fluorescence of the type shown in Figure 2-1 Ic is called direct line fluorescence. Excitation raises the electron to an excited state above the... 

X-ray fluoroscopy is used for direct on-line examination. A fluorescent screen is used to convert x-ray photons into visible light photons. A television camera receives the visible image and displays it on a television screen (see Fig. 19). This type of system is used for security screening of carry-on luggage at airports. 

Soon after Dennison had deduced from the specific-heat curve that ordinary hydrogen gas consists of a mixture of two types of molecule, the so-called ortho and para hydrogen, a similar state of affairs in the case of iodine gas was demonstrated by direct experiment by R. W. Wood and F. W. Loomis.1 In brief, these experimenters found that the iodine bands observed in fluorescence stimulated by white light differ from those in the fluorescence excited by the green mercury line X 5461, which happens to coincide with one of the iodine absorption lines. Half of the lines are missing in the latter case, only those being present which are due to transitions in which the rotational quantum number of the upper state is an even integer. In other words, in the fluorescence spectrum excited by X 5461 only those lines appear which are due to what we may provisionally call the ortho type of iodine molecule. 

To be referred to next is the most modem diamond anvU type, generating pressures of order of 10 -100 GPa. The cell illustrated in Figure 6(e) consists of two gem diamonds with optically flat surfaces, between which a sample confined in a drilled hole of a thin metal gasket is sandwiched. To attain isostatic compression an inert gas or an organic liquid, like a 4 1 volume mixture of methanol and ethanol, is contained with the sample. The generated pressure is measured directly from the pressure shift of the fluorescence line of mby powder mixed with the sample. Temperatures to 5000 K can be obtained by laser heating. The quantity of sample confined in a typically 0.1-mm-wide hole is extremely small, just a few microcrystals. At present, research has focused on in situ observations using X-ray and other optical methods, rather... 

The fluorescence studies of the interaction of cytochrome c with the anilinonaphthalene sulfonate-apoenzyme and protoporphyrin-apoenzyme complexes provide another line of evidence (39) in support of the above-mentioned conclusion. Both fluorescence steady-state and lifetime titrations of these fluorescence-labeled apoenzymes with ferro- and ferricytochrome c indicates the formation of a 1 1 complex, the affinity for ferricytochrome c being less than that for ferrocytochrome c. From the phosphorescence and fluorescence quenching, the distance between the emitter (a fluorescence label) and the quencher (the heme of cytochrome c) can be calculated by assuming that no direct electronic interaction exists between them, and that the quenching is derived from the Forster-type energy transfer (65) to the heme. The distance from the apoenzyme-bound emitter to the heme group of cytochrome c is estimated to be 19 A and 14 A for the anilinonaphthalene sulfonate and protoporphyrin... 

Another area of interest in quantum interference effects, which has been studied extensively, is the response of a V-type three-level atom to a coherent laser field directly coupled to the decaying transitions. This was studied by Cardimona et al. [36], who found that the system can be driven into a trapping state in which quantum interference prevents any fluorescence from the excited levels, regardless of the intensity of the driving laser. Similar predictions have been reported by Zhou and Swain [5], who have shown that ultrasharp spectral lines can be predicted in the fluorescence spectrum when the dipole moments of the atomic transitions are nearly parallel and the fluorescence can be completely quenched when the dipole moments are exactly parallel. 

Despite the numerous advantages the instrumental demands of microcolumn LC are considerable, and these demands are further accentuated as the requirements vary from one column type to another. A consequence of the reduced flow rates is that the detector flow-cell volume should be reduced to <10nl for OTCs, 0.1 pi for packed microcapillaries and 1 pi for microbore columns. An additional demand of the detector is that it should have a rapid response, <0.5 s. Development of suitable detectors is paramount if the potential of micro-LC is to be realised. Study of detector systems has focused in two areas firstly, the miniaturisation of ultraviolet, fluorescence and electrochemical systems, using in the former two systems LASERS as excitation sources and ultraviolet fibre optic and on-line cells to reduce band broadening and increase sensitivity [123,124] secondly, the direct interfacing with systems which previously required transport and/or concentration of the eluant. Interfacing of HPLC with mass spectroscopy has been undertaken by Barefoot et al. [125] and Lisek et al. [126] and flame systems (FPD and TSD) have been reviewed by Kientz et al. [127]. Jinno has reviewed the interfacing of micro-LC with ICP [128]. 

Lasers are another source of excitation radiation used in fluorescence detection systems. The high-directional output of a laser maximizes the fraction of total output that can be easily focused down to a spot size compatible with the dimensions of CE detection cells. The output of a laser is also typically monochromatic, or a discrete set of spectrally narrow lines. This type of output makes it relatively easy to filter out low-level incoherent plasma radiation and undesired emission lines without greatly diminishing the overall output power. In addition, many lasers provide flexibility in terms of pulse width and repetition rate, which allows one to optimize excitation with respect to analyte photostability. 

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