Read the molecular fingerprint.
In a simple mass spectrum with singly charged ions, a molecular-ion peak gives the relative molecular mass. The base peak is merely the strongest peak and is not necessarily the molecular ion. Fragments help narrow the structure.
An IR spectrum shows absorptions associated with bond vibrations. An O–H stretch can be broad; a carbonyl C=O stretch is usually strong near 1700 cm⁻¹. Exact ranges depend on the functional group and should be checked against the data book.
Use both types of evidence: mass helps constrain the formula, while IR narrows functional groups. A carboxylic acid combines C=O absorption with a very broad O–H region. A ketone has C=O but no O–H stretch.
Combine the clues
Schematic IR spectra showing selected diagnostic bands only, not measured reference spectra. Wavenumber decreases left to right. Check exact ranges in the official data book.
Assumed knowledge
For singly charged ions, m/z corresponds numerically to relative ion mass.
Learning checkpoints & source
YOUR SYLLABUS CHECKPOINT- Use a molecular-ion peak to infer molecular mass.
- Interpret IR absorptions to identify functional groups.
- Combine evidence to distinguish possible structures.