Optically dense samples

Most of the applications of FDCD that have been reported have been concerned with the use of this technique as a probe of specific aspects of the chiral environment of biochemical systems. Although, as indicated above, this technique is basically a probe of the molecular ground state, it uses the sensitivity and selectivity of luminescence measurements. FDCD has also been applied to highly scattering and optically dense samples for which polarized absorption measurements are not possible [58,59]. Some of the more recent applications of this technique include its use for on-column detection of chiral molecules in capillary electrophoresis [60], and in a modified phase-modulation spectrofluoremeter [61,62]. The purpose of the latter application is to develop a procedure to determine the distribution of chiral molecules in multicomponent samples [62],... 

Merck Co. [224] have a patent on a visible and IR spectroscopy method to monitor particle size distribution of on-line monitoring of optically dense samples. 

Matveeva, E.G., et al. (2005) Plastic versus glass support for an immunoassay on metal-coated surfaces in optically dense samples utilizing directional surface plasmon-coupled emissioa JFluoresc 15 865-71. 

The effective path length for the case where K is limited by spectrometer depth of field ranges from less than 100 pm to several millimeters for typical conditions. If the sample is clear, it is generally possible to make the sample depth greater than the spectrometer depth of field. However, for thin samples (or optically dense samples, as discussed below), the opposite limit may apply in which the sample is thin compared to Sa, and K is determined solely by the sample thickness. For example, a biological membrane or a sheet of plastic wrap will be thinner than the depth of field of most conventional spectrometers. In this case, the integral of dz is simply the sample thickness (b), and K = b, yielding... 

Insertion of the KQd value from Table 6.2 for an optically dense sample such as silicon into Eq, (3.8) yields... 

With all else equal, Eq. (11.3) predicts that the signal is proportional to if / ) for an optically dense or thin sample, such as silicon. This prediction is supported by the data in Table 11.2, with the signal increasing rapidly for the higher NA (lower // ) objectives. For thin or optically dense samples. 

Chemiluminescence in the liquid phase Chemiluminescent markers may be used to detect radical molecules like superoxide or NO. Luminol in particular reacts with NO to give a metastable intermediate, which relaxes by light emission at 427 nm. The method is sensitive, but unspecific, as the same emission is also observed with a range of other reactive oxygen and nitrogen species. In addition, the method cannot be used with optically dense samples like tissue. 

Figure 6 A scan of the spectral output of a 450 W xenon arc lamp measured by placing an optically dense sample of rhodamine B in the sample holder. The fluorescence passed through a 610 nm cut-off filter, and EmM was set at 620 nm. Excitation bandpass, 2 nm emission bandpass 2 nm.

Irradiations were conducted at 366 nm (Corning 7-83 filter combination) in a merry-go-round apparatus in which the samples rotated about a stationary 450-Watt Hanovia medium pressure lamp for constant light exposure. The samples consisted of degassed 5 ml solutions of the photoinitiators in neat MMA contained in Pyrex tubes. Two initiator concentrations were utilized 1.05 x 10and 4.11 x 10 which corresponded to 16 and > 99% light absorption, respectively. Samples were irradiated to about 7% monomer conversions which required 15 min for the optically dense solutions and 30 min for the tubes with low initiator concentration. The resulting poly-... 

The use of an enzyme thermistor as a specific detector for monitoring different enzymes [17,63,45] in the eluents from chromatographic procedures had the advantage of being applicable in optically dense solutions, where spectro-photometric methods fail, and of being able to operate on-line for discrete samples. 

Attenuated total reflectance infrared (ATR-IR) is used to study films, coatings, threads, powders, interfaces, and solutions. (It also serves as the basis for much of the communication systems based on fiber optics.) ATR occurs when radiation enters from a more-dense material (i.e., a material with a higher refractive index) into a material that is less dense (i.e., with a lower refractive index). The fraction of the incident radiation reflected increases when the angle of incidence increases. The incident radiation is reflected at the interface when the angle of incidence is greater than the critical angle. The radiation penetrates a short depth into the interface before complete reflection occurs. This penetration is called the evanescent wave. Its intensity is reduced by the sample which absorbs. 

In internal reflection spectroscopy (IRS), the spectrum is obtained with the sample in optical contact with another material (e.g. a prism) and the beam is passed through the prism onto the sample. The prism is optically denser than the sample, the incoming light forms a standing wave pattern at the interface within the dense prism medium whereas in the sample (with the lower refractive index), the amplitude of the electric field falls off exponentially with the distance from the phase boundary. When the sample exhibits absorbance, the reflectance measured is given by ... 

The consolidated titanate waste pellets are similar in appearance to their glass counterparts, i.e., both are dense, black and apparently homogeneous. Microscopic analyses, however, reveal important differences between these two waste forms. While little definitive work has been done with glassy waste forms, it is apparent that several readily soluble oxide particulates of various nuclides are simply encapsulated in the glass matrix. The titanate waste form has undergone extensive analyses which includes optical microscopy, x-ray, scanning electron microscopy, microprobe, and transmission electron microscopy (l ) The samples of titanate examined were prepared by pressure sintering and consisted of material from a fully loaded titanate column. Zeolite and silicon additions were also present in the samples. 

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