Mass spectrometry fragmentation helps students identify molecules by studying the ions formed when a compound breaks apart. This cheat sheet summarizes the key peaks, common fragment ions, isotope clues, and neutral losses used in high school and introductory chemistry. It is useful because mass spectra can look complicated until the main patterns become familiar.
Students can use it as a quick reference when interpreting unknown compounds or checking structural evidence.
The most important idea is that the molecular ion gives the molar mass, while fragments give clues about functional groups and carbon skeletons. The mass-to-charge ratio is written as , and most ions in basic spectra have charge , so often equals the ion mass. Isotope patterns help identify elements such as chlorine and bromine because their natural isotopes create predictable peak ratios.
Diagnostic losses, such as for loss of or for loss of , help connect peaks to likely fragmentation pathways.
Key Facts
- The mass-to-charge ratio is , and for the value of equals the ion mass in atomic mass units.
- The molecular ion peak, often written as , represents the intact molecule after losing one electron and is used to estimate molecular mass.
- The base peak is the tallest peak in the spectrum and is assigned a relative intensity of .
- The nitrogen rule states that an organic molecule with C, H, N, O, S, or halogens has an odd nominal molecular mass if it contains an odd number of nitrogen atoms.
- A chlorine-containing molecule often shows and peaks in an approximate ratio because of and .
- A bromine-containing molecule often shows and peaks in an approximate ratio because of and .
- Common neutral losses include for , for or , for or , and for .
- Hydrocarbon fragments commonly appear at for , for , for or acylium ions, and for .
Vocabulary
- Molecular ion
- The ion formed when a molecule loses one electron without breaking apart, usually written as .
- Base peak
- The tallest peak in a mass spectrum, assigned a relative intensity of .
- Fragment ion
- A charged piece of a molecule produced when the molecular ion breaks into smaller parts.
- Mass-to-charge ratio
- The measured value that compares an ion's mass to its charge.
- Isotope pattern
- A set of related peaks caused by atoms of the same element having different masses, such as and .
- Neutral loss
- An uncharged molecule or radical lost during fragmentation, shown by the difference between the molecular ion and a fragment peak.
Common Mistakes to Avoid
- Treating the base peak as the molecular ion is wrong because the tallest peak is only the most abundant fragment or ion, not necessarily the intact molecule.
- Ignoring and peaks is wrong because isotope peaks can reveal elements such as carbon, chlorine, and bromine.
- Forgetting that most simple spectra have is wrong because students may overcomplicate when the peak value usually equals the ion mass.
- Assigning every peak to a full molecule is wrong because most peaks are fragments, rearranged ions, or isotope peaks rather than separate compounds.
- Using only one diagnostic peak to identify a compound is wrong because reliable interpretation requires matching the molecular ion, fragments, isotope pattern, and functional group evidence.
Practice Questions
- 1 A mass spectrum has a molecular ion at and a strong fragment at . What is the likely molecular mass, and what common fragment might explain ?
- 2 A compound shows at and at with an approximate intensity ratio. Which halogen is likely present?
- 3 A spectrum shows a molecular ion at and a major peak at . What neutral loss does this suggest if the difference is ?
- 4 Explain why a molecule with a weak or missing molecular ion can still be identified using fragment peaks and isotope patterns.