Mass spectra, correlation with molecular

Fig. 11.3. Electron ionization and methane Cl mass spectra of toluene. The key features of the respective mass spectra are labeled. Spectral interpretation is based on recognition and understanding of these key features and how they correlate with structural elements of the analyte molecule of interest. The signal representing the most abundant ion in a mass spectrum is referred to as the base peak, and may or may not be the molecular ion peak (which carries the molecular mass information). Cl spectra provide confirmation of molecular mass in situations where the El signal for the molecular ion (M+ ) is weak or absent. The Cl mass spectrum provides reliable molecular mass information, but relatively little structural information (low abundance of the fragment ions). Compare with Fig. 11.4.

FIGURE 8.6 (a) Molecular-ion region mass spectrum of a mixture of tyrocidin A, B, and C, obtained in the linear mode with the reflectron voltage turned off, (b) molecular-ion region obtained in the reflectron mode, (c) neutral mass spectrum of the molecular-ion region with the reflectron voltage turned on, (d) correlated product-ion mass spectrum of tyrocidin A, and (e) correlated product-ion mass spectrum of tyrocidin B. (Reprinted with permission from reference 7). 

Electron ionization occurs when an electron beam crosses an ion source (box) and interacts with sample molecules that have been vaporized into the source. Where the electrons and sample molecules interact, ions are formed, representing intact sample molecular ions and also fragments produced from them. These molecular and fragment ions compose the mass spectrum, which is a correlation of ion mass and its abundance. El spectra of tens of thousands of substances have been recorded and form the basis of spectral libraries, available either in book form or stored in computer memory banks. 

Mass spectrum of compound 12 indicated a molecular ion peak at miz 662.3200 for NMR findings correlated the given formula with 1 methyl, 2... 

Smith et al. studied the binding of biotin and some biotin derivatives to avidin and strepavidin.38 In the initial experiment, they measured the ESI mass spectrum of avidin (Fig. 12A) observing four charge states (15 + to 18 +) corresponding to a molecular mass of 63,915 Da which correlates well with the expected mass of 63,870 Da. The spectrum obtained for a mixture of avidin-biotin (lower plot) shows that peaks are shifted to higher m/z values that correspond to a mass increase of 973 u. This indicated that four biotin molecules are indeed bound to avidin. The mass increase is accompanied by a reduction of one unit in the charge states, which now range from 14+ to 17 +. This study demonstrated that the ESI process transfers the solution complex to the gas phase. 

It is possible to derive structural information from the fragmentation pattern in a spectrum. The appearance of prominent peaks at certain mass numbers is empirically correlated with certain structural features. For example, the mass spectrum of an aromatic compound is usually dominated by a peak at m/z 91, corresponding to the tropylium ion. Structural information can also be obtained from the differences between the masses of two peaks in a spectrum. For instance, a fragment ion occurring 20 mass numbers below the molecular ion strongly suggests a loss of a HF moiety. Thus, a fluorine atom is likely to be present in the substance analyzed. 

The mass spectrum of a molecule is unique and can be stored in a computer. A match of the spectrum with those in the computer library is made in terms of molecular weight and the 10 most abundant peaks and a selection of possibilities will be presented. At this point you need to correlate all the information obtained from the spectroscopic techniques described in Chapters 26, 28, 29 and 30 together with the chemistry of the molecule to attempt to identify the structure of the molecule. 

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