TANDEM MASS SPECTROMETRY: ANALYSIS OF COMPLEX MIXTURES
Abstract
Applications of tandem mass spectrometry for the analysis of complex mixtures are described. Increased specificity and selectivity were achieved by using a variety of reagent gases in both negative and positive ion modes. Natural isotopic abundance ratios were examined in both simple and complex mixtures using parent, daughter and neutral loss scans. Expressions for the isotopic abundances ratios were derived for the various types of MS/MS scans, comparisons being made within a given spectrum and between spectra. Although different from the characteristic ion clusters evident in simple mass spectra themselves, these ratios are characteristic of the elemental compositions of the selected ions. An additional degree of selectivity results because not only must specific ions be present in a spectrum (or spectra), they must be present in the correct isotopic abundance ratios. MS/MS was also used to discover new compounds. Daughter scans were used to identify seven new alkaloids in a cactus species. Three of these alkaloids were novel compounds, and included the first simple, fully aromatic isoquinoline alkaloids reported in Cactaceae. MS/MS was used to characterize the chemical reaction products of coal in studies designed to probe its macromolecular structure. Negative ion chemical ionization was utilized to study reaction products resulting from the oxidation of coal. Possible structural units in the precursor coal were predicted based on the reaction products identified, aliphatic and aromatic acids and their anhydrides. Some of the structural units postulated include C(,0) and C(,2)-C(,6) aliphatic bridges between aromatic units, fused ring aromatic structures and acenaphthene-, indan- and tetralin-like structures. In a separate study, parent scans were used to characterize series of alkylaromatic compounds, including alkylated acenaphthenes, naphthalenes and anthracenes and/or phenanthrenes, in coal liquefaction and/or extraction products. The various reaction products were examined for differences in nature and in hexane-soluble yields which could be related to the structure and processing of coal, respectively. The similarity of the product distributions was considered to be evidence for a two-phase structure of coal in which a mobile phase (characterized by the alkylaromatics) is released from an insoluble, macromolecular network. These studies illustrate the types of problems for which MS/MS is useful. Emphasis has been placed on characterization of complex mixtures by selecting experimental parameters which enhance the information obtained. The value of using MS/MS in conjunction with other analytical techniques as well as with chemical pretreatment is demonstrated.
Degree
Ph.D.
Subject Area
Analytical chemistry
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