Light scattering phosphorescence

In these sensors, the intrinsic absorption of the analyte is measured directly. No indicator chemistry is involved. Thus, it is more a kind of remote spectroscopy, except that the instrument comes to the sample (rather than the sample to the instrument or cuvette). Numerous geometries have been designed for plain fiber chemical sensors, all kinds of spectroscopies (from IR to mid-IR and visible to the UV from Raman to light scatter, and from fluorescence and phosphorescence intensity to the respective decay times) have been exploited, and more sophisticated methods including evanescent wave spectroscopy and surface plasmon resonance have been applied. 

Flow cytometers developed from simpler cell counters, but now they are used to record cell size (from light scattering), optical absorbance, fluorescence, and phosphorescence. The optical properties are often enhanced by staining. The use of two dyes that fluoresce at different wavelengths permits the construction of two-dimensional plots as in the accompanying figure. 

The reflecting surface can be a mirror or a membrane with a light-scattering surface. In any case, the sensor has the appearance of a monolithic probe (i.e., a dip-stick probe). Optical sensors based on absorption, fluorescence, phosphorescence, and luminescence can employ such a configuration. Various highly optimized fiberoptic probes for UV-Vis, NIR, and IR ranges are now commercially available, and their designs are shown in Fig. 9.23. 

Secondly, one should also be very careful about using data from model systems (such as light scattering and phosphorescence quenching) to explain termination kinetics. 

X-ray Photoelectron Spectroscopy XPSj Circular Dichroism Spectroscopy Nuclear Magnetic Resonance NMR Imaging Fluorescence Spectroscopy Phosphorescence Spectroscopy Luminescence Spectroscopy Light Scattering X-ray Diffraction Electron Diffraction Microscopy Thermal - M hanical Methods... 

The physical basis of spectroscopy is the interaction of light with matter. The main types of interaction of electromagnetic radiation with matter are absorption, reflection, excitation-emission (fluorescence, phosphorescence, luminescence), scattering, diffraction, and photochemical reaction (absorbance and bond breaking). Radiation damage may occur. Traditionally, spectroscopy is the measurement of light intensity... 

Carbon particles in the size range we are considering can both absorb and scatter light. Light absorption and conversion to heat (consumptive absorption) are considered below. A further possibility is absorption and re-emission (fluorescence or phosphorescence) which are not explicitly considered here and seem unlikely. Scattering can also result in dispersion of the scattered light, and this is the possibility under consideration. 

The Phosphoroscope. This is a simple mechanical device which allows the separation of long-lived emissions (phosphorescence) from short-lived emissions which consist of scattered light and fluorescence. It is a disc or drum in which there are holes or slots placed in such a way that the excitation and emission beams reach the sample and the detector respectively at different times. With the fastest practicable rotation velocities of the phosphoroscope, the cut-off time is of the order of 1 ms. 

This has important consequences concerning the spatial resolution of the detector system. Due to scattering and total reflection at the edges of the polycrystalline particles, more phosphorescent light is lost in a thick layer than in a thinner one. In addition, the visible photons are more spread-out laterally in thicker screens. 

Condition 1. vs in (47) and vs in (48) should differ by significant amounts so that the experimental distinction between sensitized fluorescence and sensitized phosphorescence in the laboratory is feasible. Color filters with photo and photomultiplier tubes are most convenient, but the results are subject to serious error unless care is used. Spec-trophotometric methods also may be difficult if scattered light for one type of radiation may be confused with the other. ... 

When the sample is stimulated hy application of an external electromagnetic radiation source, several processes are possible. For example, the radiation can be scattered or reflected. What is important to us is that some of the incident radiation can be absorbed and thus promote some of the analyte species to an excited state, as shown in Figure 24-5. In absorption spectroscopy, we measure the amount of light absorbed as a function of wavelength. This can give both qualitative and quantitative information about the sample. In photoluminescence spectroscopy (Figure 24-6), the emission of photons is measured after absorption. The most important forms of photoluminescence for analytical purposes are fluorescence and phosphorescence spectroscopy. 

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