Peak time factors affecting

When the simple one-pulse experiment is again considered, there is only one time factor (or variable) that affects the spectrum, namely the acquisition time, f2. We now consider a multiple-pulse sequence in which the equilibration period is followed by two pulses with an intervening time interval, the final pulse being the irll acquisition pulse. Thus, we have inserted an evolution period between the pulses. If we now vary this evolution time interval (f3) over many different experiments and collect the resulting FIDs into one overall experiment, we have the basis of a 2-D experiment. Sequential Fourier transformation of these FIDs yields a set of spectra whose peak intensities vary sinusoidally. This first series of Fourier transformations result in the second frequency axis, v2, derived from the acquisition time, r2, of each FID. The data are now turned by 90°, and a second Fourier transformation is carried out at right angles to the first series of transformations. This second series of Fourier transformations result in the first frequency axis, iq, a function of the evolution time, f1 which you recall was changed (i.e., incremented) in the pulse sequence for each successive FID. 

Regarding quantitation in the CP/MAS experiment, for peak areas to accurately represent the number of nuclei resonating, one of the conditions that must be met is that the time constant for cross polarization must be significantly less than the time constant for proton spin lattice relaxation in the rotating fi ame, Tch or Tnh TipH. Other factors affecting quantitation in CP/MAS have been discussed in several reviews (28-33). Since no analyses of the spin dynamics were performed in this study, the solid state spectra presented in this manuscript will be interpreted only semiquantitatively. 

Analytical method validation has developed within the pharmaceutical industry over the years in order to produce an assurance of the capabilities of an analytical method. A recent text on validation of analytical techniques has been published by the international Conference on Harmonisation (ICH) [19]. This discusses the four most common analytical procedures (1) identification test, (2) quantitative measurements for content of impurities, (3) limit test for the control of impurities and (4) quantitative measurement of the active moiety in samples of drug substance or drug product or other selected components of the drug product. As in any analytical method, the characteristics of the assay are determined and used to provide quantitative data which demonstrate the analytical validation. The reported validation data for CE are identical to those produced by an LC or GC method [11] and are derived from the same parameters, i.e. peak time and response. Those validation parameters featured by the ICH (Table 1) are derived from the peak data generated by the method. Table 1 also indicates those aspects of a CE method (instrumentation and chemistry), peculiar to the technique, which can affect the peak data and highlights factors which can assist the user in demonstrating the validation parameters. 

Before we discuss the influence of pH, additives, and temperature in more detail, we need to understand another important factor for optimization of peak resolution. Peak resolution is affected by the difference in retention time between peaks and the peak shape. We have discussed the intrinsic effect of band dispersion in chromatography, which leads to a symmetrical broadening of peaks. This peak width is described in a standardized way by the plate number. In practical LC, however, perfectly symmetric peaks are rather the exception, and it is an important criterion of method optimization to remove root causes for peak distortion. Every increase in peak asymmetry negatively affects resolution (under otherwise constant conditions). It must be emphasized in this context that Eq. 2.1 in Section 2.2.1 is only valid for Gaussian peaks with perfect symmetry, while any peak distortion will lead to smaller effective resolution. There are cases where the... 

Furthermore, when a normal-phase HPLC column is employed for separation of different lipid classes, species are not uniformly distributed in the eluted peak (i.e., each individual species of a class may possess its own distinct retention time and peak shape due to differential interactions with the stationary phase). Dynamic ion suppression is then a major factor affecting the accuracy in quantification. 

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