chapter12

2 interpreting mass spectra molecular weight from the

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Unformatted text preview: om the mass of the molecular ion Double-focusing instruments provide high-resolution "exact mass" 0.0001 atomic mass units distinguishing specific atoms C5H12 72.0939 amu exact mass C4H8O 72.0575 amu exact mass Result from fractional mass differences of atoms 16O = 15.99491, 12C = 12.0000, 1H = 1.00783 Example MW "72" is ambiguous: C5H12 and C4H8O but: Instruments include computation of formulas for each peak 9 Other Mass Spectral Features If parent ion not present due to electron bombardment causing breakdown, "softer" methods such as chemical ionization are used Peaks above the molecular weight appear as a result of naturally occurring heavier isotopes in the sample (M+1) from 13C that is randomly present 10 Interpreting Mass-Spectral Fragmentation Patterns Molecular Ion m/z = 72 Fragment Ion Fragment Radical mass = 15 m/z = 57 loss of mass 15 m/z = 72 No molecular ion, it fragments too quickly. This is common for alkanes, i.e M+ is missing or small. 11 Even mass Molecular ion is an even mass Fragment ions are odd masses m/z = 29 m/z = 43 m/z = 57 m/z = 71 Loss of mass = 14 Loss of mass = 15 Problem 12.27 C3 H C 3C2C2CC3 H H H HH 2-Methylpentane (C6H14) has the mass spectrum shown below. Which peak is M+? Which is the base peak B+? Propose structures for the fragment ions of m/z 71, 57, 43 and 29. Why does the base peak have the mass it does? p. 435...
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