Introduction to Mass Spectrometry Interpretation.
Here is a list of steps to follow when interpreting a mass spectrum. This simplified list will help you to interpret many spectra; however, keep in mind that there are other mechanisms of fragmentation which have not been included.
1. Look for the molecular ion peak.
2. Try to calculate the molecular formula:
3. Calculate the degree of unsaturation (i.e. total number of rings plus double bonds):
degree of unsaturation = x - (1/2)y + (1/2)z + 1
4. Postulate the molecular structure consistent with abundance and m/z of
fragments.
The following example is the mass spectrum for methanol.
Ions |
m/z |
CH3OH+ |
32 |
H2C=OH+ |
31 |
HC=O+ |
29 |
CH3+ |
15 |
General Information to assist you in EI mass spectra interpretation
Molecular ion (M+): If the molecular ion appears, it will be the highest mass in an EI spectrum (except for isotope peaks discussed below). This peak will represent the molecular weight of the compound. Its appearance depends on the stability of the compound. Double bonds, cyclic structures and aromatic rings stabilize the molecular ion and increase the probability of its appearance.
Reference Spectra: Mass spectral patterns are reproducible. The mass spectra
of many compounds have been published and may be used to identify
unknowns. Instrument computers generally contain spectral libraries which can
be searched for matches.
Fragmentation: General rules of fragmentation exist and are helpful to predict or
interpret the fragmentation pattern produced by a compound. Functional groups
and overall structure determine how some portions of molecules will resist
fragmenting, while other portions will fragment easily.
Isotopes: Isotopes occur in compounds analyzed by mass spectrometry in the
same abundances that they occur in nature. A few of the isotopes commonly
encountered in the analyses of organic compounds are below along with an
example of how they can aid in peak identification.
The ratio of peaks containing 79Br and its isotope 81Br (100/98) confirms the presence of bromine in the compound.
Different Types of Ionization Methods and Sample Introduction Techniques
An array of ionization methods and mass analyzers are available to meet the needs of many types of chemical analysis. A few are listed here with a highlight of their usefulness.