Mass profile library

Rourick, R. A. Fink, S. W. Volk, K. J. Whitney, J. L. Klohr, S. E. Kerns, E. H. Lee, M. S. 1998. The impact of LC/MS based structure profile libraries on the drug discovery-development continuum. Proc. 46th ASMS Conf. Mass Spectrom. and Allied Topics (Orlando, Florida), 17. 

The Fiehn Library is a mass spectral and retention index library for comprehensive metabolic profiling. The library comprises over 1000 identified metabohtes that are currently screened by the Fiehn laboratory. The compound list is continually extended. The Fiehn mass spectral library is commercially available for GC-quadrupole mass spectrometers from Agilent Technologies, and for GC-TOF mass spectrometers from Leco Corporation, see http //fiehnlab.ucdavis.edu/Metabolite-Library-2007/... 

Obviously, use of such databases often fails in case of interaction between additives. As an example we mention additive/antistat interaction in PP, as observed by Dieckmann et al. [166], In this case analysis and performance data demonstrate chemical interaction between glycerol esters and acid neutralisers. This phenomenon is pronounced when the additive is a strong base, like synthetic hydrotalcite, or a metal carboxylate. Similar problems may arise after ageing of a polymer. A common request in a technical support analytical laboratory is to analyse the additives in a sample that has prematurely failed in an exposure test, when at best an unexposed control sample is available. Under some circumstances, heat or light exposure may have transformed the additive into other products. Reaction product identification then usually requires a general library of their spectroscopic or mass spectrometric profiles. For example, Bell et al. [167] have focused attention on the degradation of light stabilisers and antioxidants... 

Fig. 21.11. Mass spectra of the unknown off-flavor compound after spectral subtraction from the co-eluting peak and the matching spectrum from the NIST library. (Redrawn/redrawn from J. Chromatogr., 351, R.A. Sanders, and T.R. Morsch, Ion profiling approach to detailed mixture comparison. Application to a polypropylene off-odor problem, 525-531, Copyright (1986) with permission from Elsevier.)...

The conventional approach is to extract from the TIC profile the mass spectra of all peaks above a predetermined intensity and to perform either manual or computer-assisted [39] identification of each mass spectrum. Analysis of mass spectra should be carried out only by properly trained technologists, under the supervision of a qualified laboratory director. Libraries of mass spectra should be available for identification of peaks that are not readily recognized. This library should be user-created, indexed by retention time and molecular weight, and have the capacity to be expanded and edited. 

Figure 2.13—Detection by mass spectrometry. TIC chromatogram obtained with a mass spectrometer as a detection system. The instrument is capable of obtaining hundreds of spectra per minute. The above chromatogram corresponds to the total ion current at each instant of the elution profile. It is possible to identify each of the components using its mass spectrum. In many instances, compounds can be identified with the use of a library of mass spectra. (Chromatogram of a mixture of 71 volatile organic compounds (VOCs), reproduced by permission ofTekmarand Restek, USA.)...

Volatile profiles of raw and cooked-beef flavor samples, prepared by the procedures of Figure 1, were obtained after capillary GC and FPD. Although the identification of these sulfur containing compounds is as yet incomplete, the chromatograms demonstrated that there were a number of new sulfur compounds produced on cooking that were not present in the raw beef. Three prominent sulfur compounds were identified as markers in subsequent meat flavor deterioration experiments, namely, methional (13.2 min), methyl sulfone (13.8 min), and benzothiazole (25.3 min). Each compound produced an adequate mass spectrum for spectral library search and positive identification. 

The advantages of pulsed ultrafiltration-mass spectrometry include the variety of different applications that may be carried out, the convenience of on-line screening, solution-phase screening, the ability to screen either combinatorial libraries or natural product extracts, the diversity of receptors that may be screened, and the freedom to use either volatile or non-volatile binding buffers. For metabolic and toxicity screening, flow injection analyses have the additional advantages that product feedback inhibition is prevented so that the metabolic profile more closely approximates the in vivo system ( 70). Finally, the... 

If combinatorial libraries consist of compound mixtures (e.g., after cleavage from the resin beads), a reliable analytical characterization is difficult. In this case the utilization of mass spectrometry for analysis is based on the prediction of mass distribution of the library. Computer-generated distribution profiles can be compared with the actual profile obtained from the compound library [93,94]. Evaluation of mass distribution detects synthetic problems based on incomplete coupling (shift toward lower molecular masses), incomplete deprotection, or unwanted library modification, such as oxidation, acylation, or alkylation (shift toward higher molecular masses). However, for a larger library of compound mixtures many different compounds will have the same molecular mass. This greatly complicates structural determination and even makes it impossible at a certain mixture complexity. 

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