The nature of the fragments often provides a clue to the molecular structure, but if the molecular ion has a lifetime of less than a few microseconds it will not survive long enough to be observed. Copyright © 1991-2020 John Wiley & Sons, Inc. All Rights Reserved. Extremely small samples of an unknown substance (a microgram or less) are sufficient for such analysis. The following diagram displays the mass spectra of three simple gaseous compounds, carbon dioxide, propane and cyclopropane. Cleavage of a carbon-carbon bond gives methyl and ethyl fragments, one of which is a carbocation and the other a radical. The calculator on the right may be used to calculate the isotope contributions to ion abundances 1 and 2 amu greater than the molecular ion (M). When a high energy electron collides with a molecule it often ionizes it by knocking away one of the molecular electrons (either bonding or non-bonding). HTML 5 canvas support. Go To: Top, Mass spectrum (electron ionization), Notes, Go To: Top, Mass spectrum (electron ionization), References. Select two adjacent atoms in the molecular graphics display to observe the corresponding mass fragment or on a peak in the mass spectrum … A less common fragmentation, in which an even-electron neutral fragment is lost, produces an odd-electron radical cation fragment ion. NIST subscription sites provide data under the The separated ions are then measured, and the results displayed on a chart. The mass spectrum of dodecane on the right illustrates the behavior of an unbranched alkane. This site uses cookies. shall not be liable for any damage that may result from Lighter ions are deflected more than heavier ions. intended to imply recommendation or endorsement by the National 1. This page is the property of William Reusch. Of the three cleavage reactions described here, the alpha-cleavage is generally favored for nitrogen, oxygen and sulfur compounds. Among simple organic compounds, the most stable molecular ions are those from aromatic rings, other conjugated pi-electron systems and cycloalkanes. Since a molecule of carbon dioxide is composed of only three atoms, its mass spectrum is very simple. The following problems require an understanding of the principles of mass spectrometry. For example, the small m/z=99 amu peak in the spectrum of 4-methyl-3-pentene-2-one (above) is due to the presence of a single 13C atom in the molecular ion. All rights reserved. Toggle navigation Toggle search bar. It should be noted that the presence of halogen atoms in a molecule or fragment ion does not change the odd-even mass rules given above. The weak even -electron ions at m/z=15 and 29 are due to methyl and ethyl cations (no nitrogen atoms). Plausible assignments may be seen by clicking on the spectrum, and it should be noted that all are even-electron ions. Diethylmethylamine, on the other hand, has one nitrogen and its molecular mass (m/z = 87) is an odd number. The complexity of fragmentation patterns has led to mass spectra being used as "fingerprints" for identifying compounds. and HTML 5 enabled browser. A similar bond cleavage in cyclopropane does not give two fragments, so the molecular ion is stronger than in propane, and is in fact responsible for the the base peak. Chem., Univ. The charge distributions shown above are common, but for each cleavage process the charge may sometimes be carried by the other (neutral) species, and both fragment ions are observed. The Ion Source
2. High Resolution Mass Spectrometry
In assigning mass values to atoms and molecules, we have assumed integral values for isotopic masses.