Solid artifacts

Schemes for combinatorial syntheses of compounds, particularly candidate pharmaceuticals, now attract great interest because of the potential for greatly speeding up the discovery of new drug molecules. They are based on carrying out chemical reactions on solid artifacts, the starting material, intermediates and final product being attached to the solid phase on completion the product is cleaved away into solution. Typically the solid substrate is a mass of small (of the order of tens or 100 pm or so) porous beads made of polymer or silica resin. They are suspended, by agitation, in reactant solution, which is removed on completion by filtration. The beads are washed with solvent between stages of synthesis.

Interdiffusion of bilayered thin films also can be measured with XRD. The diffraction pattern initially consists of two peaks from the pure layers and after annealing, the diffracted intensity between these peaks grows because of interdiffusion of the layers. An analysis of this intensity yields the concentration profile, which enables a calculation of diffusion coefficients, and diffusion coefficients cm /s are readily measured. With the use of multilayered specimens, extremely small diffusion coefficients (-10 cm /s) can be measured with XRD. Alternative methods of measuring concentration profiles and diffusion coefficients include depth profiling (which suffers from artifacts), RBS (which can not resolve adjacent elements in the periodic table), and radiotracer methods (which are difficult). For XRD (except for multilayered specimens), there must be a unique relationship between composition and the d-spacings in the initial films and any solid solutions or compounds that form this permits calculation of the compo-... 

Fig. 6.7. The predicted, one-dimensional, mean-bulk temperatures versus location at various times are shown for a typical powder compact subjected to the same loading as in Fig. 6.5. It should be observed that the early, low pressure causes the largest increase in temperature due to the crush-up of the powder to densities approaching solid density. The "spike in the temperature shown on the profiles at the interfaces of the powder and copper is an artifact due to numerical instabilities (after Graham [87G03]).

Wideline NMR spectra were collected using a Bruker CXP 200 NMR spectrometer, operating at u>o/2n ( H) = 30.7 MHz. To obtain spectra void of spectrometer artifacts, the solid spin echo pulse sequence, n/2) -T-n/2) -x-echo, was used. Unless otherwise noted, the delay between pulses, X, was set at 30 Js. 

The current efficiencies for the different reaction products CO2, formaldehyde, and formic acid obtained upon potential-step methanol oxidation are plotted in Fig. 13.7d. The CO2 current efficiency (solid line) is characterized by an initial spike of up to about 70% directly after the potential step, followed by a rapid decay to about 54%, where it remains for the rest of the measurement. The initial spike appearing in the calculated current efficiency for CO2 formation can be at least partly explained by a similar artifact as discussed for formaldehyde oxidation before, caused by the fact that oxidation of the pre-formed COacurrent efficiency. The current efficiency for formic acid oxidation steps to a value of about 10% at the initial period of the measurement, and then decreases gradually to about 5% at the end of the measurement. Finally, the current efficiency for formaldehyde formation, which was not measured directly, but calculated from the difference between total faradaic current and partial reaction currents for CO2 and formic acid formation, shows an apparently slower increase during the initial phase and then remains about constant (final value about 40%). The imitial increase is at least partly caused by the same artifact as discussed above for CO2 formation, only in the opposite sense. 

Hot, molten glass is thick and cohesive it can be shaped, and, as it cools down, it hardens while keeping its shape. Solid glass is extremely tough, withstands compression better than steel, is impervious to liquids, and is resistant to chemical attack. All this makes glass useful for making utilitarian artifacts, such as containers for solids and liquids, as well as ornamental and decorative objects (Tite et al. 2002 Tait 1991). 

The relative skin (5 - S0) evolution with respect to the cumulative acid volume is given in Figure 3 (solid line). Peaks at about 0.5 and 3.5 m3 are artifacts due to hammering effects following rapid variations of the injection rate. According to Equations 6 and 13, the skin evolution, if the formation were a primary porosity one, should be ... 

Figure 15 Annotated 60 Hz HSQC-1,1-ADEQUATE spectrum of strychnine (45) showing the, 3C- 3C connectivity network that defines the central "core" of the molecule as shown in grey by 46. Correlations from methylene carbons are inverted and plotted in grey correlations from methine carbons have positive intensity and are plotted in black. There are no methyl correlations, which would also have positive intensity. Adjacent carbons are defined by solid black lines perpendicular to the diagonal. Dashed horizontal black lines define correlations to adjacent protonated carbon resonances, for example, C15 (26.4 ppm) to C16 (60.3 ppm). Dashed curved lines define correlations between adjacent protonated and non-protonated carbons. These responses, in contrast to those between adjacent protonated carbons, are not diagonally symmetric. Responses enclosed in black boxes are artifacts that arise from the overlap of H14 and one of the anisochronous Hll methylene proton resonances.

The bands due to Fe(CO)4 are shown in Fig. 8. This spectrum (68) was particularly important because it showed that in the gas phase Fe(CO)4 had at least two vq—o vibrations. Although metal carbonyls have broad vC—o absorptions in the gas phase, much more overlapped than in solution or in a matrix, the presence of the two Vc—o bands of Fe(CO)4 was clear. These two bands show that in the gas phase Fe(CO)4 has a distorted non-tetrahedral structure. The frequencies of these bands were close to those of Fe(CO)4 isolated in a Ne matrix at 4 K (86). Previous matrix, isolation experiments (15) (see Section I,A) has shown that Fe(CO)4 in the matrix had a distorted C2v structure (Scheme 1) and a paramagnetic ground state. This conclusion has since been supported by both approximate (17,18) and ab initio (19) molecular orbital calculations for Fe(CO)4 with a 3B2 ground state. The observation of a distorted structure for Fe(CO)4 in the gas phase proved that the distortion of matrix-isolated Fe(CO)4 was not an artifact introduced by the solid state. 

The emissive counterpart to CD is circularly polarized photoluminescence (CPPL). Where the fluorophore is chiral, then the photo-excited state can return to the ground state with emission of circularly polarized light, the direction of polarization of which depends on the relative intensities of the right-handed and left-handed emissions (/r and /L, respectively), which in turn depends on the chirality of the material, or more accurately, the chirality of the photo-excited state of the material. CPPL studies on poly silanes are extremely rare, however, due to the low CPPL intensity and rapid sample degradation in solution, and problems due to artifacts in the solid state. 

Genuine (true) and apparent hysteresis may be considered to explain contaminant release from the subsurface solid phase. Genuine hysteresis assumes that observed data are real and the equilibrium results can be explained on the basis of well-identified phenomena. Apparent hysteresis results from an experimental artifact due, for example, to a failure to reach retention or release equilibrium. 

A commonly used method of sampling and analysis for volatile organic compounds In ambient air Is by concentration of the compounds on a solid sorbent such as Tenax and subsequent thermal desorption and GC/MS analysis of the collected compounds. The analysis phase, although not trivial, can be done well If proper care Is taken. However, the sampling phase of this process apparently Introduces artifacts and unusual results due to, as yet, unknown factors. A method to detect some sampling problems has been proposed and tested (7 ). This distributed air volume method requires a set of samples of different air volumes to be collected at different flow rates over the same time period at the sampling location. Each pollutant concentration for the samples should be equal within experimental error since the same parcel of air Is sampled In each case. Differences In results for the same pollutant In the various samples Indicates sampling problems. 

Transmission electron microscopy (TEM) can provide valuable information on particle size, shape, and structure, as well as on the presence of different types of colloidal structures within the dispersion. As a complication, however, all electron microscopic techniques applicable for solid lipid nanoparticles require more or less sophisticated specimen preparation procedures that may lead to artifacts. Considerable experience is often necessary to distinguish these artifacts from real structures and to decide whether the structures observed are representative of the sample. Moreover, most TEM techniques can give only a two-dimensional projection of the three-dimensional objects under investigation. Because it may be difficult to conclude the shape of the original object from electron micrographs, additional information derived from complementary characterization methods is often very helpful for the interpretation of electron microscopic data. 

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