Transcript Mass Spectrometry - George Mason University

Mass Spectrometry
• The substance being analyzed (solid or liquid) is injected into the mass
spectrometer and vaporized at elevated temperature and reduced pressure.
• The gaseous molecules are bombarded with high-energy electrons, which
convert some of the molecules into positively charged ions.
• Magnetic and electric fields in the spectrometer separate the ions according
to their mass/charge ratios (m/z).
• The vast majority of the ions generated carry only a single positive charge,
i.e. z = 1, so m/Z represents the molecular mass of the ions detected.
• The spectrometer displays a spectrum of peak intensity (y-axis) versus
m/z (x-axis).
• The peak intensity is shown as a % (relative to the abundance of the
most abundant peak (the base peak, which is set as 100%).
Diagram of a Simple Mass Spectrometer,
Showing the Separation of Neon Isotopes
Interpretation of Mass Spectra.
• The most important information obtained is the integral molecular weight
of the compound. i.e. the mass in the largest fragment.
• The integral molecular weight is obtained from the molecular ion peak
of the compound, designated as m+. This will appear at the high end
(to the right hand side) of the m/z scale.
• Two molecular ion peaks may be found when the parent compound
contains an element having two abundant isotopes (e.g. Cl and Br).
• In some instances a molecule will not show a molecular ion peak
(because it has decomposed into smaller fragments) or its intensity will be too
low to be recorded.
Determination of Molecular Structure from Fragmentation Pattern.
• The molecular structure can often be deduced from the masses of the fragments
obtained from the breakdown of the compound into smaller fragments,
i.e. from the fragmentation pattern.
• Higher mass fragments are usually more important than smaller ones for structure
determination.
• Each major peak identifies a particular mass fragment.
• Intense peaks correspond to high probability fragments.
• Rearrangements of ionized fragments, to structures not obviously related to the
parent compound, can sometimes complicate the analysis.
• The fragmentation pattern obtained depends on the activation energy for bond
cleavage ( bond energy) and on the stability of the resulting positive ion.
• The stability of the positive ion is generally of greatest importance. It will depend
on the effectiveness with which the positive ion fragment can delocalize its charge.
Base Peak
100
90
80
70
60
50
40
30
20
10
0
CH2
M - (H2O and CH2=CH2)
OH+
M - (H2O and CH3)
M - H2 O
Molecular Ion Peak
M+ - 1
20
30
40
50
60
70
m/z
80
90
Isotope Peak Rules
• When the parent compound contains an element that has more than one
stable abundant isotope, more than one peak will be found for each fragment
containing this element.
• Although nearly all elements occur naturally as a mixture of isotopes, for the
lighter elements of interest in organic chemistry (H, C, N, O), one isotope,
the lighter one, predominates.
• Since the abundances of 79Br and 81Br are 50.5% and 49.5%, two peaks of
nearly equal intensity, separated by two mass units, will occur for all
bromine-containing fragments. e.g. in the spectrum of CH3Br, two peaks of
nearly equal intensity will occur at m/e values of 94 and 96, corresponding
to (CH379Br)+ and (CH381Br)+.
• Since the abundances of 37Cl and 35Cl are 24.6% and 75.4%, two peaks
with an intensity ratio of 1:3, separated by two mass units will occur in a
mass spectrum of a compound containing a single Cl atom.
The Nitrogen Rule
• The integral molecular weight of the majority of organic compounds
(containing the lighter elements H, C, N, O, P, S, Cl, Br, I) will be an
even number, except for those containing an odd number of N atoms.
• The appearance of an odd number for the integral molecular weight of the
molecular ion indicates that the compound contains an odd number of N atoms.