Mass spectral fragmentation identification

Mass spectral fragmentation patterns of alkyl and phenyl hydantoins have been investigated by means of labeling techniques (28—30), and similar studies have also been carried out for thiohydantoins (31,32). In all cases, breakdown of the hydantoin ring occurs by a-ftssion at C-4 with concomitant loss of carbon monoxide and an isocyanate molecule. In the case of aryl derivatives, the ease of formation of Ar—NCO is related to the electronic properties of the aryl ring substituents (33). Mass spectrometry has been used for identification of the phenylthiohydantoin derivatives formed from amino acids during peptide sequence determination by the Edman method (34). 

A common method for identification of organic compounds is mass spectrometry (MS) in combination with GC. After separation of the component by GC the mass spectrometer transform the analyte into gaseous ions in vacuum in the ion source. For electron impact ionization this results in different mass fragmentation patterns with different mass-to-charge ratios (m/z). From this fragmentation pattern it will be possible to identify the compound by comparison with commercial mass spectral libraries. Identification of unknown compounds can be facilitated by... 

The rationale used in the interpretation of the mass spectra of methylalkanes has been presented in several reports 2- vs. 4-methylalkanes (Baker et al., 1978 Scammells and Hickmott, 1976 McDaniel, 1990 Bonavita-Cougourdan et al., 1991) 2,X- and 3,X-dimethylalkanes (Nelson et al., 1980 Thompson et al., 1981) and internally branched mono-, di- and trimethylalkanes (Blomquist et al., 1987 Pomonis et al., 1980). In the majority of reports, identification is based on GC and MS data, but the conclusions are not confirmed with standards or synthesis of the proposed structures. However, there are reports of chemical ionization (Howard et al., 1980) and electron impact of synthetic methyl-branched hydrocarbons (Carlson et al., 1978, 1984 Pomonis et al., 1978, 1980) and these have been very useful in confirming mass spectral fragmentation patterns with chemical structures. 

Definitive confirmation of pesticide residues was obtained by comparison of parent and fragment ion intensities and mass numbers of eluted pesticides and reference pesticides. Table I lists the residues encountered and the mass numbers and intensities of the characteristic fragments employed for identification in the adipose tissue sample. The mass spectral fragmentation patterns for all the compounds included in Table I with the exception of -HCH have been adequately discussed by other investigators (7). 

GC-mass spectrometry (GC-MS) is most frequently and effectively used to identify the essential oil constituents by using database libraries of both retention indices and mass spectral fragmentation patterns. LC-mass spectrometry is less frequently used for the identification of the essential oil constituents due to increased experimental complexity. One of the recent technological developments is the combined use of GC-MS and FTIR spectrometries which can provide additional real time information for molecular identification without the need for macroscopic separation of mixtures [55,61-67]. 

Such fingerprints of mass spectral fragmentation have been useful in dereplication and in identification of family similarities. For instance, a family of 18 different naturally occurring taxol analogs were each identified from crude extracts of Taxus brevifolia and Taxus baccata by a procedure primarily based on analysis of their behavior under LC-MS-MS conditions (33). Fourteen different molecular ions were obtained for the eighteen compounds, but after the four or five fragmentations depicted in Fig. 4, each compound exhibited an identical template ion of 327 m/z, representing the ionized taxol core. 

Some of the databases (Table 4) are beginning to incorporate searchable fields containing other physical characteristics, such as C-NMR chemical shifts and mass spectral fragment ions. Some of these are simulated data that are less accurate for the more unusual compounds. The most readily obtained physical data is normally a UV spectrum, and several of the commercial databases are searchable by this characteristic. Each new characteristic that agrees with the literature value adds a level of confidence to the identification of an unknown compound as a literature compound. The level of confidence required depends on the questions being asked. 

Mass spectrometry techniques have been described for the analysis of rubber compounds. A GC/mass spectrometric procedure has been described [132] for the single injection separation and identification of allerogenic vulcanisation agents and antioxidants from isoprene rubber. Mass spectral fragmentation mechanisms were proposed for each of the additives studied. 

Analysis of the oil was carried out by GC/MS imder condition that allowed direct comparison with reported data on reference terpenes [12]. Both the retention times and mass spectral fragmentation were used in the identification of the individual components of the oil. Table 1 shows a list of all components and their relative percentage in the oil. For all imidentified components, the most significant mass spectral fragments and retention times (in seconds) are provided. 

The composition is determined based on relative response in the chromatograms. The identification of each component is based on direct comparison of the retention time and mass spectral fragmentation with the data published by Adams [12] and others [13-16]. The mass spectral library fit (Wiley 139 Library) for all identified components was more than 90%. It should be mentioned that the GC column used in this study (Rtx -5, 30m x 0.25 mm, 0.25pm film thickness) was similar to that reported by Adams [12]. 

A further means for ready identification of a galloyl ester is by mass spectroscopy of the methylated (diazomethane) ester. The mass spectral fragmentation pattern usually displays the ion 16 as base peak, m/e 195 (37, 112). 

High mass resolution of ToF-SIMS instruments (10 to 10 amu) enables exact mass measurements, calculation of empirical formulae and more reliable unknown peak identification [130]. Accurate mass determination by ToF-SIMS is very simple because no special calibration procedures or calibration standards are required. The accuracy is about 10 ppm for atomic species and for molecules in the low mass range. For organic molecules in the high mass range an accuracy better than 5 ppm was obtained. In cases where the mass resolution is not sufficient, mass spectral fragmentation patterns may be compared to library spectra for identification. 

Terpenoids can be analyzed by the usual methods. For the volatile members of the family, gas chromatography-mass spectrometry (gc-ms) is a particularly useful tool. In laboratories (e.g., those in the major fragrance companies), which are accustomed to analyzing mixtures of volatile terpenoids, gc-ms is the major analytical technique employed and such laboratories will have extensive libraries of mass spectra of terpenoids to assist in this. However, the mass spectral fragmentation patterns of closely related terpenoids are often so similar as to render definitive identification by ms alone, impossible. For these materials and those for which there is no reference (e.g., compounds newly isolated from nature), nuclear magnetic resonance (nmr) spectroscopy is the analytical tool of choice. Physical techniques, e.g., density, refractive index, and optical rotation, are relatively inexpensive and prove useful in quality control. 

If PBM cannot identify the unknown, which it could not if there is no corresponding reference spectrum in the database, an interpretive algorithm can be used to find at least partial structural information. This should aid in interpretation of the spectrum and in structural identification of the unknown. STIRS combines a knowledge of mass-spectral fragmentation rules with an empirical search for correlations of reference spectra. To accomplish the former, 26 classes... 

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