Interpreting MS

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.

• This peak (if it appears) will be the highest mass peak in the spectrum,
  except for isotope peaks.
  • Nominal MW (meaning=rounded off) will be an even number for
    compounds containing only C, H, 0, S, Si.
  • Nominal MW will be an odd number if the compound also contains an odd
    number of N (1,3, ... ).

2. Try to calculate the molecular formula:

• The isotope peaks can be very useful, and are best explained with an
  example.
  • Carbon 12 has an isotope, carbon 13. Their abundances are
    12C=100, 13C=1.1 . This means that for every 100 (12)C atoms
    there are 1.1 (13)C atoms.
  • If a compound contains 6 carbons, then each atom has a 1.1 
    abundance of (13)C.
  • Therefore, if the molecular ion peak is 100, then the isotope peak
    (1 mass unit higher) would be 6x1.1 =6.6.
  • If the molecular ion peak is not 100 then you can calculate the
    relative abundance of the isotope peak to the ion peak. For
    example, if the molecular ion peak were 34 and the isotope peak
    2.3: (2.3/34)x100 = 6.8. 6.8 is the relative abundance of the
    isotope peak to the ion peak. Next, divide the relative abundance
    by the isotope abundance: 6.8/1.1 =6 carbons.
  • Follow this order when looking for information provided by isotopes: (A
    simplified table of isotopes is provided in the introduction, more detailed
    tables can be found in chemistry texts.)
    • Look for A+2 elements: 0, Si, S, CI, Br
    • Look for A+1 elements: C, N
    • "A" elements: H, F, P, I

3. Calculate the degree of unsaturation (i.e. total number of rings plus double bonds):

• For the molecular formula: C_xH_yN_zO_n

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.

Methanol

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.

methylbromide

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.