Particulate probes

Extensive results have been obtained on the electrophoretic mobility of larger rigid probes, notably polystyrene spheres, in polymer solutions(35-37). Limited measurements find that electrophoresis with a polymer solution as a support medium gives excellent separations of intact bacteria(38). The following are representative results, ordered approximately by increasing matrix molecular weight. 

It is sometimes proposed for small spheres that the microviscosity 

At fixed polymer concentration, p/fio decreases with increasing matrix molecular weight. Rodbard and Chrambach found that p is not independent of M, consistent with many other results on probe electrophoresis and sedimentation. Nonlinear (in E) mobility behavior was observed, namely the probe mobility increased at larger applied fields. The nonlinearity was more pronounced at larger polymer concentrations. The dependence of p upon E could be said to be shear thinning, but if so the relevant shear rate (for example, involving a thin layer around each probe) must be quite large, because direct measurement at lower shear rates found no dependence of the macroscopic on /c. 

By increasing the applied field, probe motion is readily taken into a nonlinear motion regime in which p, a, and v depend on the particle electrophoretic velocity. The value of a decreases by 50% to twofold, depending on R, with a tenfold increase in E. The obvious artifact is not responsible for this nonlinear behavior Radko and Chrambach show that Joule heating is highly unlikely to have led to a nonlinear artifact(7). The dependence of the nonlinearity in on M was studied separately. 

Advancing to larger matrix chains, Radko and Chrambach compare the concentration dependences of p and rj for human serum albumin and for carboxylate and sulfate-modified polystyrene latex spheres, radii between 7 and 1085 nm, in solutions of 5 MDa polyacrylamide(36). Polyacrylamide chains had a nominal c  

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HOC Solubilization. Carbon-14 labeled DDT (2,2-bis(4-chlorophenyl)-l,l/l-trichloroethane) or pyrene were used as HOC probes to study the micelle formation phenomena. The probe dissolved in toluene was added to a culture tube, and the solvent evaporated under nitrogen. Ten milliliters of each HA or FA solution were added to the tube which was then capped with a teflon-lined lid. The tube was transferred to a water bath (25.0 °C 0.5 °C) where the solutions were equilibrated for 24 hours with periodic shaking. After equilibration the tubes where centrifuged to minimize the amount of suspended or particulate probe molecule. An aliquot of the supernatant was then withdrawn and transferred to a scintillation vial. Scintillation fluid was added to the vial and the p-emission counted at a 0.5% counting error. Additional methodological details can be found elsewhere (13). 

We begin with two experimental methods, sedimentation and electrophoresis, that measure the driven motion of polymer chains and colloidal particles. In each method, an external force is applied directly to particular molecules in solution, and particle motion is observed. The forces are buoyancy and the Coulomb force. Light pressure ( optical tweezers ) has also been used to move particles this method appears in Chapter 9. Chapter 2 presents phenomenology associated with sedimentation by polymers and sedimentation of particulates through polymer solutions. The sedimentation rate of polymers in homogeneous solution, and the sedimentation of particulate probes through polymer solutions, both depend on the polymer concentration and molecular weight and the size of the particulates. 

For sampling particulate matter, one is dealing with pollutants that have very different inertial and other characteristics from the carrying gas stream. It becomes important, therefore, to sample so that the same velocity is maintained in the probe tip as exists in the adjacent gas stream. Such sampling is called isokinetic. Isokinetic sampling, as well as anisokinetic sampling, is illustrated in Fig. 13-3. 

If the probe velocity is less than the stack velocity, particles will be picked up by the probe, which should have been carried past it by the gas streamlines. The inertia of the particles allows them to continue on their path and be intercepted. If the probe velocity exceeds the stack velocity, the inertia of the particles carries them around the probe tip even though the carrying gases are collected. Adjustment of particulate samples taken anisokinetically to the correct stack values is possible if all of the variables of the stack gas and particulate can be accounted for in the appropriate mathematical equations. 

Several separating systems are used for particulate sampling. All rely on some principle of separating the aerosol from the gas stream. Many of the actual systems use more than one type of particulate collection device in series. If a size analysis is to be made on the collected material, it must be remembered that multiple collection devices in series will collect different size fractions. Therefore, size analyses must be made at each device and mathematically combined to obtain the size of the actual particulate in the effluent stream. In any system the probe itself removes some particulate before the carrying gas reaches the first separating device, so the probe must be cleaned and the weight of material added to that collected in the remainder of the train. 

TTie system can be used for continuous measurement of the mass concentration at a single point for up to 12 hours, for traverse measurements of stack particulate mass concentrations using sample probe extensions, with the mass transducer up to 6 m in the stack, or for intermittendy measuring particulate mass concentrations of emission gases for long-term readings (e.g.. 30-sec samples every 60 minutes. ... 

The density p(r) might also be described as the fractional probability of finding the (entire) electron at point r. However, chemical experiments generally do not probe the system in this manner, so it is preferable to picture p(r) as a continuous distribution of fractional electric charge. This change from a countable to a continuous picture of electron distribution is one of the most paradoxical (but necessary) conceptual steps to take in visualizing chemical phenomena in orbital terms. Bohr s orbits and the associated particulate picture of the electron can serve as a temporary conceptual crutch, but they are ultimately impediments to proper wave-mechanical visualization of chemical phenomena. 

Since Avnir and Pfeifer s pioneer works83"86 regarding the characterization of the surface irregularity at the molecular level by applying the fractal theory of surface science, the molecular probe method using gas adsorption has played an important role in the determination of surface fractal dimension of the porous and particulate materials. 

Selection of visible light reflectance as a candidate nondestructive test method was based on results from probing experiments. It was observed that as cotton was mechanically cleaned, its visible light reflectance increased. Conversely, addition of trace amounts of particulate (trash and dust) to extensively cleaned cotton resulted in a decrease in visible light reflectance. Finally, it was noted that off-colored cotton was rendered whiter with repetitive mechanical cleaning. 

To prevent the formation of reaction products from the interaction of the ozone-air sample with Alters, they arc intentionally not used at probe inlets (see Table 6-4). Some of the newer instruments, however, require Alters at the inlet of their sampling ports to prevent the particulate matter in the ambient air from fouling reaction-chamber cells or from clogging the gas-flow controllers. When the same type of Alter also precedes the calibration and zero gas sampling ports (which has not always been the practice), the problem is minimized to the extent that similar events occur during the calibration and sampling. 

As differentiated from diffuse reflection, we use the term backscatter to mean a probe that detects both specular and diffuse light. The most common designs of these probes incorporate a number of illumination fibers (usually six) in a ring around a central detection fiber. This probe is useful in a slurry or solution high in particulates (e.g. crystallization). 

Raman spectroscopy s sensitivity to the local molecular enviromnent means that it can be correlated to other material properties besides concentration, such as polymorph form, particle size, or polymer crystallinity. This is a powerful advantage, but it can complicate the development and interpretation of calibration models. For example, if a model is built to predict composition, it can appear to fail if the sample particle size distribution does not match what was used in the calibration set. Some models that appear to fail in the field may actually reflect a change in some aspect of the sample that was not sufficiently varied or represented in the calibration set. It is important to identify any differences between laboratory and plant conditions and perform a series of experiments to test the impact of those factors on the spectra and thus the field robustness of any models. This applies not only to physical parameters like flow rate, turbulence, particulates, temperature, crystal size and shape, and pressure, but also to the presence and concentration of minor constituents and expected contaminants. The significance of some of these parameters may be related to the volume of material probed, so factors that are significant in a microspectroscopy mode may not be when using a WAl probe or transmission mode. Regardless, the large calibration data sets required to address these variables can be burdensome. 

All samples should be collected using isokinetic sampling when determining mass emissions. This means the gas sample should be pulled through the sample probe at the same velocity as the velocity of the process gas. If not, the total particulate catch will not be representative of the gas stream. 

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