Isomer selective ionization

Oehme M, Kirschmer P (1984), Anal. Chem. 56 2754-2759. Isomer-selective determination of tetrachlorinated dibenzo-p-dioxins using hydroxyl negative ion chemical ionization mass spectrometry combined with high resolution gas chromatography"... 

Zimmermann R, Boesl U, Weickhardt C, Lenoir D, Schramm KW, Kettrup A, Schlag EW (1994), Chemosphere 29 1877-1888. Isomer-selective ionization of chlorinated aromatics with lasers for analytical time-of-flight mass spectrometry first results for polychlorinated dibenzo-p-dioxins (PCDD), biphenyls (PCB) and benzenes (PCBz) ... 

Isomer selectivity At a finer level of discrimination, vdW complexes or clusters of a given size may occur as a distribution of structural isomers. Isomers of van der Waals and H-bonded complexes have been detected by rotationally resolved UV or IR spectroscopy [29-34], as have isomers of vdW solvent clusters with aromatic molecules [34-38]. The ionization potentials of vdW isomers can differ substantially [17,18,39], allowing mass- and isomer-selective electronic spectroscopy to be performed in a mixture of clusters by selective-ionization (SI) two-color R2PI combined with mass spectrometry in a mixture of clusters [17]. This increase in selectivity is quite general and may also be applied in IR-UV and microwave-UV excitation-ionization schemes. 

In the second application of selective ionization, we select not different rigid cluster isomers but differentiate between rigid and fluxional cluster subpopulations of... 

This review will first concentrate on the unimolecular gas-phase chemistry of diene and polyene ions, mainly cationic but also anionic species, including some of their alicyclic and triply unsaturated isomers, where appropriate. Well-established methodology, such as electron ionization (El) and chemical ionization (Cl), combined with MS/MS techniques in particular cases will be discussed, but also some special techniques which offer further potential to distinguish isomers will be mentioned. On this basis, selected examples on the bimolecular gas-phase ion chemistry of dienes and polyenes will be presented in order to illustrate the great potential of this field for further fundamental and applied research. A special section of this chapter will be devoted to shed some light on the present knowledge concerning the gas-phase derivatization of dienes and polyenes. A further section compiles some selected aspects of mass spectrometry of terpenoids and carotenoids. 

Both valence isomers easily give off an electron the first vertical ionization energies amount to only 7.5 eV and 6.35 eV (29), and therefore, both compounds can be oxidized in solution using the selective one-electron oxidizing system AICI3/H2CCI2 (3). The result, however, is puzzling in both cases, the tetrakis-(tert. butyl) cyclobutadiene radical cation is identified by its ESR spectrum (30). 

Generally, the detector response will depend on the type of aromatic amine detected. Table I shows that response to N,N-di-methylaniline is about twice as high as the response to its primary isomer 2,6-dimethylaniline. Sensitivity for a selected number of aromatic amines was found to be increased by a factor 5-16 in comparison to a flame-ionization detector. 

Figure F2.4.1 Liquid chromatography/mass spectrometry (LC/MS) analysis of isomeric carotenes in a hexane extract from 0.5 ml human serum. Positive ion electrospray ionization MS was used on a quadrupole mass spectrometer with selected ion monitoring to record the molecular ions of lycopene, p-carotene, and a-carotene at m/z (mass-to-charge ratio) 536. A C30 HPLC column was used for separation with a gradient from methanol to methyl-ferf-butyl ether. The a -trans isomer of lycopene was detected at a retention time of 38.1 min and various c/ s isomers of lycopene eluted between 27 and 39 min. The all-frans isomers of a-carotene and P-carotene were detected at 17.3 and 19.3 min, respectively.

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