Signature peak

It is often difficult to determine the degree to which the chemistry proceeded on the entire library population and whether peaks in a mass spectrum are due to the product, side reactions, reagents, solvents, or impurities. Diversity Sciences developed mass-spectral methods to distinguish all components that are cleaved from a solid support and implemented the method into the analytical construct. While early studies demonstrated promising results for fragmentation methods with tandem mass spectrometry (MS/MS), stable isotopes were routinely implemented as signature peaks for the identification of compounds that are produced from solid-phase reactions [27]. 

FIGURE 8.7 A mass spectrum that displays the use of the peak-split signature. All products that originate from the resin bead demonstrate the goalpost-like signature peaks, while the normal single peaks are not related to the synthetic chemistry on the solid phase. 

The integrated intensity of the Raman signature peak Jp has to be considered in the data treatment instead of the intensity at the maximum. In principle, this last quantity could be used but the maximum position is very sensitive to any wavenumber shift which could be caused by the spectrometer drift or a change in the experimental conditions. Considering the intensity Jp integrated over a relatively wide range - at about of 30 cm -allows one to remove these difficulties. Moreover accuracy and precision in concentration, as derived from the calculation of J are better than if the intensity of peak maximum is used (see 5). 

In principle, all of the aforementioned species may be measured by MIMS. As shown in Figure 27.5 [3], gaseous CI2, HOCl, NH2CI, NHCI2, and NCI3 can be measured directly with distinctive m/z signatures on MIMS spectra. The relative abundances of the signature peaks, as analyzed in Table 27.1, obey the isotopic ratios of [ Cl] p Cl] 3 1 [27]. The OCF concentration may... 

Figure 15 Raman spectral analysis used in the characterization of the fractionation of humic substances. The bottom FT-Raman spectrum was taken at 1064-nm excitation and shows the 1326-cm" signature peak for disordered carbon networks. The top spectrum shows that after fractionation the 1326-cm peak has disappeared (or at least has been significantly reduced), and thus the disruption of the humic substance backbone is detected. (Reprinted with permission from YH Yang, HA Chase. Applications of Raman and surface enhanced Raman scattering techniques to humic substances. Spectr Lett 31 821-848, 1998. Copyright 1998 Marcel Dekker, Inc.)...

FIGURE 51.4. CP-MAS NMR spectra of cimetidine 2 in a commercially available tablet formulation (a) an unedited spectrum. The signature peaks of cimetidine form A are denoted by the asterisks, (b) Two proton Ti relaxation edited spectra (obtained at different t delays) separating out the signals for the cimetidine API and the excipients. 

It is appropriate at this point to comment on the quantitative aspects of CP-MAS NMR. It would, of course, be useful to be able to quantify the relative proportions of an API, excipient, or impurity in situ (and nondestmctively) in a formulation or other complex mixture in which the individual components cannot be easily separated. In general, the area under a given NMR peak is proportional to the number of nuclei resonating in that frequency range and so quantification can in principle be achieved by integration of appropriate signature peaks for each component. In a CP-MAS NMR spectrum, however, an analysis of peak areas for the... 

SERS spectra of RSV-infected cell lysate and purified RSV both have SERS bands at lOOO-llOOcm and 500-600cm , whereas the SERS spectrum of the Vero cell lysate does not show these two signature peaks. 

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