Background scattering

Attempts to deuterate the OH group failed as the background scattering increased markedly. 

In principle, it is possible to fully automate the procedure (2) and software can be written to obtain the results to an operator spemFied precision, as the error equations are available. Unfortunately, this online procedure is sometimes difficult with catalysts because most supported metal catalysts contain only the order of one percent or less of metal, the peaks are broad due to the small size of the crystallites, and the large amount of support gives strong background scattering which has features of its own. Visual inspection of the data is often necessary prior to processing. 

Principles and Characteristics The prospects of Raman analysis for structural information depend upon many factors, including sample scattering strength, concentration, stability, fluorescence and background scattering/fluorescence from the TLC substrate. Conventional dispersive Raman spectroscopy has been considered as a tool for in situ analysis of TLC spots, since most adsorbents give weak Raman spectra and minimal interference with the spectra of the adsorbed species. Usually both silica and cellulose plates yield good-quality conventional Raman spectra, as opposed to polyamide plates. Detection limits for TLC fractions... 

Practically all classical methods of atomic spectroscopy are strongly influenced by interferences and matrix effects. Actually, very few analytical techniques are completely free of interferences. However, with atomic spectroscopy techniques, most of the common interferences have been studied and documented. Interferences are classified conveniently into four categories chemical, physical, background (scattering, absorption) and spectral. There are virtually no spectral interferences in FAAS some form of background correction is required. Matrix effects are more serious. Also GFAAS shows virtually no spectral interferences, but... 

Peaks in the spectrum, 1(E), identify the location of possible resonance states. Then, the initial state >(0) is optimized to reduce direct background scattering and the resonance state is extracted using the standard relation... 

The 28 scan and layer line scan functions are provided as an aid to obtaining better estimates of the unit cell parameters for the substance being studied and in assessing contributions from background scattering and overlapping diffraction maxima. 

The spectra of interfacial water molecules are characterized by the presence of the 0-H stretch and bond vibrational modes, the 0-H bend mode and the Fermi resonance mode, superimposed on the continuum background scattering. 

The one major difficulty with use of a flow cell is that the combination of pipe cleaner fibers and a large volume of mother liquor around the crystid greatly increases the background scatter of X-rays, particularly in the 3 A resolution region. For diffractometric data collection—and, probably, for area detector work as well—this problem is not serious, but... 

As discussed previously, the anvil seats on which the diamond anvils are mounted have traditionally been made of X-ray transparent beryllium (Fig. 6a), although other hard, low-Z materials have also been used, such as B4C and BN. The disadvantage of all of these is the large background scatter that arises once the... 

So far, only normal diffraction (with sharp maxima at the points of reciprocal lattice) has been mentioned. Investigation of background scattering can provide extended information on various kinds of crystal defects (well beyond stacking faults and the like) as has been demonstrated e.g. on metal and feldspar structures. 

Intensity of scattered neutrons is measured as a function of scattering angle 20. The measured response of the neutron detector is the sum of the coherent scattered intensity of the sample particles (Eq.2), the scattering from the solvent, the scattering from the sample cell, and the electronic noise in the detector. To obtain the scattering from the sample particles, background scattering due to solvent and sample cell, and noise counts in the detector, must be subtracted from the experimental scattered intensity. The result is normalized to an... 

Figure 2. Time course of light scattering increase from addition of NaCl to 5fiM PS vesicles at 20°C (a) scattering from 0.1 M NaCl buffer and PS vesicles (SfiM) is 1.1 units background scattering from 0.1M and IM NaCl buffer was 0.4 and 0.7 units, respectively.

Figure 10. Elastic neutron diffraction pattern from a 0.4-layer nitric oxide film adsorbed on a recompressed graphite powder Papyex (431. Background scattering from the substrate has been subtracted. The molecules are assumed to be lying down as shown in the unit cell (inset). The arrows indicate the position and relative intensities of the Bragg peaks calculated for this structure.

Problems encountered with this method include differences in spot size and shape, background scatter, and the fact that a very limited range of densities were measurable. It becomes inaccurate when applied to spots on a dense background or to very dense spots on any type of background. 

Figure 1 Example of an x-ray fiber diffraction pattern suitable for analysis using AXIS. The spots are discrete, there is no layer line streaking, and the background scatter may be seen clearly. The sample (potassium hyaluronate) was slightly tilted out of the plane perpendicular to the x-ray beam.

Because, in general, background scatter does not exhibit rapid fluctuations, we find that acceptable results are obtained using a small number of equispaced knots, although frequently the number of knots required in the x and y directions is different. When selecting the knot positions, care must be taken that panels are not inadvertently defined which are completely inside a spot boundary (and hence contain no unflagged data). The fit obtained using such panels is likely to be poor. 

Four major computational steps are necessary to separate the individual peaks and the different profile-broadening components (i) correction and normalisation of the diffraction data, (ii) resolution of the total peak scattering from the so-called background scatter, and resolution of crystallographic, para-crystalline, and amorphous peaks from each other, (iii) correction of the resolved profiles for instrumental broadening, (iv) separation of the corrected profiles into size and distortion components. In this paper we will discuss these steps in turn, but most attention will be paid to the hitherto largely neglected step of profile resolution. 

Error Analysis. The difficulties encountered in resolving peak profiles from each other, and at the same time determining the background scatter, call to question the extent of the error involved in this type of analysis, given that the final resolution appears realistic in the light of known information about the structure of the material. The problem of error in profile resolution has been considered (14) in terms of the equatorial trace of a viscose rayon fibre specimen which is similar to Fortisan, Figure 5 ... 

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