Instrument line shape model

These line shapes are generally not very useful for collision-induced absorption work, because pressure broadening, Doppler effect and instrumental resolution are here of no great concern. In Chapters 5 and 6 we will consider a number of other ad hoc model functions that have acquired a certain significance in collision-induced absorption. 

From these initial results, a four- and a five-component PARAFAC model was examined in more detail. The initial results from these indicated that there were some potential nonextreme outliers and that the low-excitation wavelengths were very noisy. This can be seen, for example, from the estimated five-component excitation loadings as well as from plots of the residuals (Figure 10.10). Apparently, the low excitations are not well modeled leading to noisy line shapes in that area and to relatively high residuals compared to the remaining part of the data. The reason for this noise in the data is likely to be found in the instrument. 

The diffraction lines due to the crystalline phases in the samples are modeled using the unit cell symmetry and size, in order to determine the Bragg peak positions 0q. Peak intensities (peak areas) are calculated according to the structure factors Fo (which depend on the unit cell composition, the atomic positions and the thermal factors). Peak shapes are described by some profile functions 0(2fi—2fio) (usually pseudo-Voigt and Pearson VII). Effects due to instrumental aberrations, uniform strain and preferred orientations and anisotropic broadening can be taken into account. 

In many spectroscopic techniques, it is not unusual to encounter baseline offsets from spectrum to spectrum. If present, these kinds of effects can have a profound effect on a PCA model by causing extra factors to appear. In some cases, the baseline effect may consist of a simple offset however, it is not uncommon to encounter other kinds of baselines with a structure such as a gentle upward or downward sloping line caused by instrument drift, or even a broad curved shape. For example, in Raman emission spectroscopy a small amount of fluorescence background signals can sometimes appear as broad, weak curves. 

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