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Sulfur-32 is a stable isotope of sulfur whose atomic nucleus contains 16 protons and 16 neutrons. Its mass number is $32$, the sum of those counts. The measured relative atomic mass is approximately $31.9720711744$; a mass number is an integer count, whereas a relative atomic mass is a mass ratio.^[ref]

The same NIST table gives a representative isotopic abundance of $0.9499$, or $94.99\%$, making sulfur-32 the predominant natural sulfur isotope. This is a representative composition, not an immutable percentage in every sample. Using the listed sulfur-34 fraction $0.0425$, the corresponding isotope ratio is $0.0425/0.9499\approx 0.04474$: about 4.47 sulfur-34 atoms per 100 sulfur-32 atoms.^[ref] An isotope ratio and a percentage of all sulfur atoms have different denominators.

Sulfur-32 is also the daughter of phosphorus-32 beta-minus decay. One nuclear neutron becomes a proton, changing $Z=15$ to $Z=16$ while retaining mass number $32$.^[ref] This connects a radioactive tracer to a stable daughter without turning their chemical identities into the same thing.

Sulfur-34 is a stable isotope of sulfur with 16 protons and 18 neutrons in its atomic nucleus. Its mass number is $34$, while its relative atomic mass is approximately $33.967867004$. NIST lists a representative isotopic abundance of $0.0425$, or $4.25\%$, alongside $0.9499$ for sulfur-32.^[ref] The representative composition does not require every natural sample to have exactly those fractions.

Comparing sulfur-34 with sulfur-32 illustrates how an isotope ratio differs from a fraction of all sulfur. The representative ratio isIn an idealized measurement that has already corrected differences in response and interferences, suppose the inferred atom counts are $N_{34}=450$ and $N_{32}=10000$. Their ratio is $0.045$, about $0.58\%$ higher than that representative ratio. This compares two isotopes; calculating the sulfur-34 fraction of all sulfur additionally needs the other isotopes in the denominator. Raw mass spectrometer signals are not automatically corrected atom counts.

Sulfur-34 and sulfur-32 have the same atomic number, so replacing one with the other preserves the element while changing nuclear mass. This makes isotope composition an additional descriptor of a sulfur-containing sample, distinct from its chemical formula. A stable isotope also provides a different kind of label from radioactive sulfur-35: its usefulness does not depend on detecting radioactive decay.