Isotope effects[ edit ] Isotope peaks within a spectrum can help in structure elucidation. On addition of heteroatoms, the molecular formula is adjusted by the equivalent mass of carbon and hydrogen.
Fragmentation mass spectrometry The fragmentation pattern of the spectra beside the determination of the molar weight of an unknown compound also suitable to give structural information, especially in combination with the calculation of the degree of unsaturation from the molecular formula when available.
Alkene fragmentation peaks are often most significant mode. There are, however, complications which arise because of the possibility of different isotopes either of carbon or of chlorine or bromine in the molecular ion.
The relative formula mass of the compound is therefore Finding the relative formula mass relative molecular mass from a mass spectrum is therefore trivial. You can use that more accurate information about the mass of the molecular ion to work out the molecular formula of the compound.
After the ionization, double bonds can migrate easily, resulting in almost impossible determination of isomers. Inductive cleavage[ edit ] This reaction results from the inductive effect of the radical sites, as depicted below. For example, hexane fragmentation patterns.
For alcohols, this heterolytic cleavage releases a water molecule. Again, bond migration is possible.
Even when compounds only contain elements with less intense isotope peaks carbon or oxygenthe distribution of these peaks can be used to assign the spectrum to the correct compound. This reaction is defined as a heterolytic cleavage since a pair of electrons is transferred.
These electrons have a high enough energy to knock an electron off an organic molecule to form a positive ion. It also shows how high resolution mass spectra can be used to find the molecular formula for a compound. There are two common cleavage modes.
Such rearrangement initiates charge-site reaction, resulting in the formation of an odd electron ion and a small neutral molecule water, or acid and so on. In this depiction, single-electron movements are indicated by a single-headed arrow.
Finding the relative formula mass relative molecular mass from a mass spectrum is therefore trivial. And they follow similar fragmentation pathways: Compounds containing halogens especially chlorine and bromine can produce very distinct isotope peaks.
These fragments produce the familiar stick diagram. The driving forces for such reaction is the electron donating abilities of the radical sites:Note parent peak corresponding to molecular mass M = 92 (C 7 H 8 +) and highest peak at M-1 = 91 (C 7 H 7 +, is the change in relative intensity of mass spectral peaks due to the changes in concentration of the analyte in the ion source as the mass spectrum is scanned.
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Without a molecular ion peak as a reference, the difficulty of interpreting a mass spectrum increases markedly. Fortunately, most organic compounds give mass spectra that include a molecular ion, and those that do not often respond successfully to the use of milder ionization conditions.
Molecular ion Ion formed by the removal of one or more electrons from a molecule to form a positive ion or the addition of one or more electrons to a molecule to form a negative ion.
Note: Pseudo-molecular ion and quasi-molecular ion are deprecated; molecular ion is reserved for the intact ionized molecule with no component added or removed. Aug 31, · Organic chemistry: Mass spectrometry. Molecular/parent ion; base peak.
Carbon; bromine and chlorine isotopes. Fragmentation and substitution This is a rec. Base peak: The most intense (tallest) peak in a mass spectrum, due to the ion with the greatest relative abundance (relative intensity; height of peak along the spectrum's y-axis).
Not to be confused with molecular ion: base peaks are not always molecular ions, and molecular ions are not always base peaks.Download