Fragmentation high-resolution mass spectrometry

Further evidence consistent with the polar radical pair mechanism was provided by a crossover experiment (Scheme 6.26). A 1 1 mixture of labeled 8Z /8 and unlabeled 8Z/8E was heated in xylene at 125 °C for 2h and at 135 °C for 4h to afford hydroxypyrimidinones 3 and 3. Analysis of the products by high resolution mass spectrometry showed no crossover between the labeled and unlabeled fragments. This result reinforces the computational results discussed previously wherein PRP recombines to give product within the solvent cage (Scheme 6.24). 

If you are lucky, the ion with the highest mass to charge value will be the molecular ion. However, this is often not the case, as textbooks on mass spectrometry make clear. If it is possible to carry out high resolution mass spectrometry on the molecules in question, and the molecular ion is indeed observed, the exact mass can be used in combination with tables to obtain the molecular formula directly. Alternatively, you can use the internet (http //www.sisweb. com/cgi-bin/masslO.pl) to calculate and plot mass distributions for any molecular fragment you think may be present. 

Quite often a normal electron ionization mass spectrum appears insufficient for reliable analyte identification. In this case additional mass spectral possibilities may be engaged. For example, the absence of the molecular ion peak in the electron ionization spectrum may require recording another type of mass spectrum of this analyte by means of soft ionization (chemical ionization, field ionization). The problem of impurities interfering with the spectra recorded via a direct inlet system may be resolved using GC/MS techniques. The value of high resolution mass spectrometry is obvious as the information on the elemental composition of the molecular and fragment ions is of primary importance. 

The oxygen transfer from an ortho nitro group to a carbon-carbon triple bond has been studied by high-resolution mass spectrometry, linked scan techniques and chemical substitution81. Oxygen transfers to both acetylenic carbons were detected as parallel fragmentation pathways81. 

As mentioned above, gas-phase coordination of unsaturated C—C bonds to metal cations constitutes another means for double-bond localization. Peake and Gross260 determined the fragmentation of Fe+/olefin complexes by CID spectrometry and reported highly characteristic differences for several constitutional octadiene isomers. High-resolution mass spectrometry proved to be necessary in certain cases due to the isobaric masses of Fe and even two units of C2H4 and/or CO261. Although not based on gas-phase ion/molecule... 

Structure elucidation of semiochemicals by modern NMR-techniques (including HPLC/NMR) is often hampered by the very small amounts of available material and problems in the isolation of pure compounds from the complex mixtures they are embedded in. Thus, the combination of gas chromatography and mass spectrometry, GC/MS, is frequently the method of choice. Determination of the molecular mass of the target compound (by chemical ionisation) and its atomic composition (by high resolution mass spectrometry) as well as a careful use of MS-Ubraries (mass spectra of beetle pheromones and their fragmentation pattern have been described [27]) and gas chromatographic retention indices will certainly facihtate the identification procedure. In addition, the combination of gas chromatography with Fourier-transform infrared spec-... 

A unique molecular formula (or fragment formula) can often be derived from a sufficiently accurate mass measurement alone (high-resolution mass spectrometry). This is possible because the nuclide masses are not integers (see Table 2.2). For example, we can distinguish at a unit mass of 28 among CO, N2, CH2N, and C2H4. 

The extract was analyzed by low-voltage, high-resolution mass spectrometry (LVHRMS). Low ionizing voltages were used to minimize fragmentation. 

Biemann 16 in 1964 examined the methyl esters of both penicillin G and penicillin V by high-resolution mass spectrometry. This enabled him to determine the exact mass and thereby the elemental composition of all the fragment ions formed. 

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