Peak, asymmetrical shape, experimental

The curing of UPR initiated with MEK peroxide and Co octoate as the promoter was studied by DSC [168]. The kinetic analysis of an asymmetrical DSC peak was performed. It was assumed that the asymmetrical shape of the DSC curves resulted from two independent reactions. When two independent reactions were considered, the fit of the experimental data was better. 

The observed cross sections for the 18s (0,0) collisional resonance with v E and v 1 E are shown in Fig. 14.12. The approximately Lorentzian shape for v 1 E and the double peaked shape for v E are quite evident. Given the existence of two experimental effects, field inhomogeneties and collision velocities not parallel to the field, both of which obscure the predicted zero in the v E cross section, the observation of a clear dip in the center of the observed v E cross section supports the theoretical description of intracollisional interference given earlier. It is also interesting to note that the observed v E cross section of Fig. 14.12(a) is clearly asymmetric, in agreement with the transition probability calculated with the permanent electric dipole moments taken into account, as shown by Fig. 14.6. 

In utilizing the Scherrer equation, care must be exercised to properly account for instrumental factors which contribute to the measured peak width at half maximum. This "intrinsic" width must be subtracted from the measured width to yield a value representative of the sample broadening. When the experimental conditions have been properly accounted for, reasonably accurate values for the average crystallite size can be determined. Peak shapes and widths, however, can also provide other information about the catalyst materials being studied. For example, combinations of broad and sharp peaks or asymmetric peak shapes in a pattern can be manifestations of structural disorder, morphology, compositional variations, or impurities. 

There is peak overlap. In this case, it is necessary to decompose the peak by modelling using software. A least squares error minimisation procedure can be employed to adjust the positions, intensities and full width half maxima of the components and provides an indication of the quality of the model in relation to the actual shape of the peak. Various peak shapes are available for modelling purposes in particular the use of experimentally obtained shapes becomes extremely useful in the case of asymmetric peaks of transition metals (Fig. 5.7). 

Barnett, D.A., Guevremont, R., Purves, R.W., Determination of parts-per-trillion levels of chlorate, hromate, and iodate hy electrospray ionization/high-field asymmetric waveform ion mohility spectrometry/mass spectrometry. Appl. Spectrosc. 1999, 53, 1367. Guevremont, R., Purves, R.W., Comparison of experimental and calculated peak shapes for three cylindrical geometry FAIMS prototypes of differing electrode diameters. J. Am. Soc. Mass Spectrom. 2005, 16, 349. 

The shape of elution peaks of injected substances can supply information regarding the rate of diffusion of vapours in polymers [229]. As the injected sample travels towards the column outlet the component zone broadens due to the scattering through diffusion in the gas and liquid phases. Given suitable experimental conditions, the diffusion coefficient of the injected substance in polymers can be determined from the elution asymmetric peak width. 

Sandorfy and his co-workers have derived an expression (see Fig. 14) in which, by means of a triangular slit function they relate the shape of the absorption curve not only to the value of the half band width Aj/ /, but also the additional values Ai /, Ai / Aj/ /g. These values represent the width of the bands at /4lnf V4ln(T o/T j ax, and /% n(To/T), respectively. The mathematical expression they arrive at after integration matches the experimental curve exactly at eight points and is very close to all other points of the curve. However, this is valid only for symmetrical absorption bands more times than not, one encounters asymmetrical absorption bands. In this case the curve is divided into components to the right and left of a line perpendicular to the peak absorbance of the band. Now, instead of measuring Ai/i/, one measures Ai/f/ and Ai f/. Of course, the sum Ai/ The Sandorfy equation in final... 

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