Molecular weight chemical-ionization mass spectra

Ozonolysis of 5 yielded a compound which provided an El mass spectrum possessing many ions in common with 5. Chemical ionization mass spectral analysis of the ozonolysis product gave a molecular weight of 296, thus locating the side chain double bond between the carbons 10 and 11 These data collectively indicated a tentative structure of 3,6-dihydroxy-2-[l-oxo-10(E)-tetradecenyl]-cyclohex-2-en-l-one for the major component. Other than the report by Mudd, the only other reported occurrence of this class of compound is tom the fruits of the Brazilian trees Virola elongata and V. sebifera (2). 

Polymers that are amenable to MALDI must have some degree of polarity -polyethene and polypropene are still essentially incompatible with the technique. Another essential requirement is that the polymers be narrowly distributed in terms of molecular weight, chemical composition, and functionality. If the molecular weight distribution is not very narrow, then the smallest molecules will be over-represented in the resulting mass spectrum ( discrimination ). If one type of molecule is abundant, it may dominate the entire mass spectrum, completely suppressing the ionization of polymeric components present in lower concentrations ( ion suppression ). Selective ionization may also be observed in the case of variations in functional groups or end groups. 

Molecular Identification. In the identification of a compound, the most important information is the molecular weight. The mass spectrometer is able to provide this information, often to four decimal places. One assumes that no ions heavier than the molecular ion form when using electron-impact ionization. The chemical ionization spectrum will often show a cluster around the nominal molecular weight. 

For ionization of the analytes, electron-impact is the most common type of ion formation, but chemical ionization is also often used. For electron impact ionization, a collimated beam of electrons makes an impact on the sample molecules, causing loss of an electron from the molecule. When the resulting peak from this molecular ion is seen in a mass spectrum, it gives the molecular weight of the compound. 

The following discussion will be concerned primarily with applications of the ms/ms technique in the synfuel area. Attempts will be made to illustrate the unique capabilities of the ms/ms analysis with examples taken from our work on coal liquefaction products. Figure 5 shows the positive ion chemical ionization (PCI) mass spectrum of the coal liquid in question (SRC II mid heavy distillate, total bottoms). This spectrum is actually the normalized sum of approximately 500 individual mass spectra taken while the SRC II was thermally vaporized from a solids probe into the source of a mass spectrometer, and represents the molecular weight profile of this distillate fraction. Since isobutane Cl gives to a first approximation only protonated molecular ions (and no fragment ions), the peaks represent the individual components in the SRC II arranged incrementally by molecular weight. 

Mass spectrum interpretation is essential to solve one or more of the following problems establishment of molecular weight and of empirical formula detection of functional groups and other substituents determination of molecular skeleton (atom connectivity) elucidation of precise structure and, even in favorable cases, certain stereochemical features. As discussed in the previous chapters, electrospray (ESI) and atmospheric pressure chemical ionization (APCI) are two of the most effective and successful interfaces for the liquid chromatography—mass spectrometry (LC—MS) that have been developed. Thus, we will focus on how to interpret the mass spectral data generated by either ESI or APCI in this section. 

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