Nontransparent samples

For nontransparent samples measured by front illumination, the effective IRF can be recorded by using seattering at a rough surfaee. To avoid multiple seatter-ing the reference body should have high absorption and no fluoreseenee. Figure 5.15 shows an IRF recorded for a pieee of blaek velvet, reeorded in the same system as Fig. 5.14, right. The width of this IRF is substantially shorter than the IRF measured in the cuvette. The FWHM is 54 ps, eompared to 97 to 103 ps obtained in the cuvette. 

To eonelude this seetion we summarize the main advantages of the NMR diffusion method as applied to emulsion droplet sizing. It is nonperturbing, requiring no sample manipulation (such as dilution with the continuous phase) and, as noted above, it is nondestructive. It is insensitive to the physical appearance of the sample, and can be applied to nontransparent samples. It requires small amounts of sample (typically of the order of a few hundred milligrams) and is normally quite rapid (of the order of 10 min per sample). 

Another effective way to cure nontransparent samples is by using electron beams (EB) that can initiate both radical and cationic-type polymerization. EB-curable laminating adhesives have been shown to exhibit good performance over a wide temperature range on a variety of... 

ATR or diffuse reflection techniques are widely used for materials which are difficult to analyze by absorption methods, such as thin layers on nontransparent substrates, substances with very high absorption which are difficult to prepare in thin layers, or substances with a special consistency. Some basic considerations concerning quantitative ATR spectroscopy have been described by Muller et al. (1981). This publication emphasizes the fact that the functional behavior of the ATR spectrum of an absorbing sample must be evaluated with regard to the refractive index as well as to the absorption index of the sample. It is shown that, as a consequence, reflection measurements can be used to determine concentrations of nonabsorbing samples. Further information on reflection spectroscopy is presented in Sec. 6.4. 

The images in Fig. 2.33a show that large zones in some of the Pb02 particles are nontransparent for the electron beams (dark zones), while others let the electron beams through. The latter zones are heterogeneous in structure comprising fully transparent and less transparent zones. When the sample is heated in the microscope, it boils and water is released, as a result of which the Pb02 phase concentrates into a compact mass. This indicates that the electron transparent zones in the particles are composed of hydrated lead dioxide (Pb(OH)4 and PbO(OH)2). 

If F is unknown, the sphere factor has to be determined as described for the Hoppler viscosimeter via calibration measurements. An exact evaluation of the state of flow is not possible. When a doubling of the traction does not lead to half the measuring time, it is very likely that the sample shows a non-Newtonian flow behavior, since the product of force and time should be constant for a Newtonian fluid. Advantages of the viscobalance are the great measuring range and the possibility to examine nontransparent and high viscous fluids of unknown density. 

In the case of secondary electrons the STEM acts as a high performance SEM. A very thin sample has no low resolution background due to secondaries produced by backscattered electrons, but secondaries come from both surfaces. This means that topographic information from both sides of the specimen is superimposed on the image, making interpretation more complicated. If the specimen is thicker, but still transmits the electrons in a spread beam, there will be a large low resolution signal from the back surface of the specimen. Then it would be better to mount the specimen on a solid substrate to make it nontransparent. 

IR reflection spectroscopy is normally used for samples which are difficult to analyze in transmission, such as bulk samples or thin layers on nontransparent substrates. In this case, the IR radiation is directed at a sample surface, usually at an angle larger than 0° off-normal, with the attenuated radiation, reflected back from that surface, being detected. Reflection techniques can be based on specular reflection (internal or external reflection), where the reflectivity R, at normal incidence, is given by... 

Fig. 8.19. Main sdieme of themial recording in a layer of a homeotropically oriented liquid crystal [66] a) transparent layer of the liquid crystal b) perturbation of orientation as a result of local heating of the sample above c) frozen transparent layer of the liquid crystal with nontransparent, strongly scattering segments (wide arrow laser beam).

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