Mass Spectrum: Interpretation

Given a known molecular ion mass, the unknown compound can be identified.

The empirical formula is determined from the molecular ion mass and the isotope peak intensities.

This spectrum has a molecular ion peak at m/z 16.

The nitrogen rule states that a molecular ion with even mass contains even or zero nitrogen atoms. Since the molecular ion peak is at 16, it contains no nitrogen.

The number of carbon atoms is deduced by comparing the M+1 intensity to the expected M+1 contribution per carbon atom.

The isotope peak intensities and patterns are also compared to the natural abundances of the isotopes of other likely atoms.

Since this compound contains one carbon and no nitrogen, the empirical formula is CH₄.

However, larger compounds need further examination for other atoms, the degree of unsaturation, and fragmentation patterns.

The degree-of-unsaturation formula indicates the presence of multiple bonds or rings, and signature fragments at characteristic m/z values like 15 suggest certain structural features.