Spectral analysis, collected light

Figure 2. The spectral analysis of light collected by the fiber placed in a pH=7 phosphate-buffered distilled water sample. The spectriun shows the important interferences which must be eliminated to relate the fluorescence intensity to concentration.

Conventional Raman spectroscopy involves the collection and spectral analysis of light that is incoherently scattered in many directions. Nonlinear Raman spectroscopy requires careful alignment and overlap of multiple laser beams in order to produce a coherent output beam. Phase matching is also required for CARS and some other closely related nonlinear techniques (e.g. CSRS). 

An additional attractive feature of NSOM is that it can be used to perform localized spectroscopy. For example, the NSOM probe can excite the dye aggregates in Fig. 14 at a particular location and the spectram of the fluorescence emission can be recorded by sending the collected light to a spectrophotometer. The possibility of localized spectral analysis of sample emission dramatically increases the amount of spatially resolved, detailed structural and chemical information that can be determined by NSOM. Time-resolved NSOM spectral studies have also recently... 

Other sources of error, particularly in quantitative Raman analysis, include laser self-absorption effects leading to attenuation of some spectral bands. Similarly diffuse reflectance of the laser light, which is dependent on the particle size of the formulation components, may increase or decrease the collection volume. However, normalisation techniques can be used to overcome some of these effects [35]. 

Recent developments of pulsed light sources, optical components, fast and sensitive detectors and electronic equipment for data collection and analysis have permitted the construction of numerous instruments, often commercially available, for the collection of luminescence data with excellent resolution in time, spectral distribution and space. The sensitivity has reached the ultimate level that allows the characterization of such properties for single molecules (see Section 3.13). Only an overview of some of these techniques is given here. 

Two important properties in NIRS should be considered in analysis of sediment samples. These are the influence of the particle sizes and the water content on the spectral signal. Differences in particle sizes and sample cup packaging may result in light-scattering effects that are not necessarily of interest for the specific constituent that is to be analysed. NIR analysis of samples with a high water content results in broad peaks with high absorptivity that may hide spectral information from other constituents than water. These problems have to be dealt with in sample preparation and by spectral pre-treatments (see next section). The basic rule is that the samples should be both dried and as homogenous as possible, before the spectral information is collected. 

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