Monochromator, fluorescence spectroscopy

Wakeham [14] has discussed the application of synchronous fluorescence spectroscopy to the characterization of indigenous and petroleum derived hydrocarbons in lacustrine sediments. The author reports a comparison, using standard oils, of conventional fluorescence emission spectra and spectra produced by synchronously scanning both excitation and emission monochromators. 

Experimental Setup. The instrumentation (both optics and electronics) for studying saturated laser induced fluorescence spectroscopy is much less conplicated than for CARS. The experimental setup shown in Figure 18, as used in our laboratory, is typical for these studies. In some experiments it is advantageous to use a monochromator rather than band pass filters to isolate the laser induced fluorescence signal. The lasers used are either flash lamp pumped systems or NdsYAG pumped dye lasers. 

The bandpass of a typical AOTF ranges from several nanometers to tens of nanometers for the visible and NIR spectral regions. This resolution is suitable for fluorescence spectroscopy where bands are very broad, and is just adequate for Raman hyperspectral imaging, albeit with lower resolution than may be achieved with a monochromator (see Section 1.5). A NIR spectrometer based on an AOTF has also been sold commercially. However, the transmission of these devices for... 

The monochromator for x-ray fluorescence spectroscopy is called the analyzing crystal. It differs from all the monochromators described earlier for all the other optical analytical instruments. The effect used in this type of monochromator is not diffraction, but interference. The wavelength of the analyzing light is changed by rotation of the analyzing crystal by certain angle. 

Most of the unpleasant effects of a grating are avoided in prism monochromators. In principle, a prism can achieve almost 100% efficiency. The dispersion of a prism is nonlinear, but approximately proportional to the wave number. This is often considered a drawback of the prism monochromator. More important it is certainly that the low dispersion of a prism causes geometric constraints and precludes the use of f numbers much faster than f 8. In fluorescence spectroscopy, prism monochromators have fallen almost entirely out of use. 

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. 

Fluorescence spectrometers for in vivo diagnostics are commonly based on fibre optic systems [30-33], The excitation light of a lamp or a laser is guided to the tissue (e.g. some specific organ) via glass fibre using appropriate optical filters (instead of an excitation monochromator). Fluorescence spectra are usually measured either via the same fibre or via a second fibre or fibre bundle in close proximity to the excitation fibre. Scanning monochromators or OMA systems as reported above are used for emission spectroscopy. 

Both instrument design and capabilities of fluorescence spectroscopy have greatly advanced over the last several decades. Advancements include solid-state excitation sources, integration of fiber optic technology, highly sensitive multichannel detectors, rapid-scan monochromators, sensitive spectral correction techniques, and improved data manipulation software (Christian et al., 1981 Lochmuller and Saavedra, 1986 Cabaniss and Shuman, 1987 Lakowicz, 2006 Hudson et al., 2(X)7). The cumulative effect of these improvements have pushed the limits and expanded the application of fluorescence techniques to numerous scientific research fields. One of the more powerful advancements is the ability to obtain in situ fluorescence measurements of natural waters (Moore, 1994). 

The experimental arrangement for Raman spectroscopy is similar to that used for fluorescence experiments (see Figure 1.8), although excitation is always performed by laser sources and the detection system is more sophisticated in regard to both the spectral resolution (lager monochromators) and the detection limits (using photon counting techniques see Section 3.5). 

Time-Resolved Fluorescence. Emission and excitation monochromators are maintained in a specific wavelength, but the excitation is chopped off and fluorescence decay is measured as a function of time. This kind of spectroscopy is interesting for studying structural changes or different complexation sites. 

The presence of two monochromators, and the fact that not all molecules with a chromophore fluoresce, means that fluorimetry is more specific than ordinary ultraviolet spectroscopy. This allows drugs that fluoresce to be assayed in the presence of other compounds that would interfere in an ultraviolet assay. 

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