Most elements can exist as more than one isotope. Usually, one isotope accounts for the vast majority (perhaps >99%) of the atoms of an element. But for some elements, atoms of several isotopes make up a significant proportion of the total in a sample. Chlorine, for example, is normally a 3:1 mixture of 35Cl and 37Cl (hence the averaged relative atomic mass of 35.5 for chlorine), while bromine is an almost 1:1 mixture of ⁷⁹Br and ⁸¹Br (hence the average mass of 80 for bromine). Because mass spectrometry weighs individual molecules, there is no averaging: instead, it detects the true weight of each molecule, whatever isotope it contains. For example, the molecular ion in the EI mass spectrum of this aryl bromide has two peaks at 186 and 188 of roughly equal intensity. Having two molecular ions of equal intensity separated by 2 mass units is indicative of bromine in a molecule.

The mass spectrum of a chlorine-containing molecule is likewise easy to identify from two peaks separated by two mass units, but this time in a ratio of 3:1, arising from the 3:1 isotopic ratio of 35Cl and 37Cl. What happens with more than one Br or Cl? Here’s an example: the painkiller diclofenac. This spectrum was obtained from commercial tablets, which contain the potassium salt of the active ingredient (it becomes protonated in the acidic environment of the stomach). The ES spectrum shows the mass of the carboxylate anion as three peaks, at 294, 296, and 298. The relative size of the peaks can be worked out from the 75% probability that each Cl atom will be 35Cl and the 25% probability it will be 37Cl. The ratios are therefore 3/4 × 3/4: 2 × 3/4 × 1/4: 1/4 × 1/4 or 9: 6: 1

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Some examples of 1H NMR spectra

A guided tour of the 13C NMR spectra of some simple molecules

Different ways of describing chemical shift

Regions of the 13C NMR spectrum

The reference sample—tetramethyl silane, TMS

The chemical shift scale

Why do chemically distinct nuclei absorb energy at different frequencies?

NMR also uses radio waves

NMR uses a strong magnetic field

Nuclear magnetic resonance What does it d

Atomic composition can be determined by high-resolution mass spectrometry

Mass spectrometry by chemical ionization, electrospray, or other methods