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Phosphorus-32 is a radioactive isotope of phosphorus with 15 protons and 17 neutrons. It undergoes beta-minus decay to sulfur-32, preserving the mass number while increasing the atomic number by one. The cited laboratory evaluation gives a half-life of $14.273(7)\mathrm{d}$.^[ref]

Its radioactivity makes phosphorus-containing material experimentally traceable. In the 1952 Hershey-Chase experiment, phosphorus-32 labelled the DNA component of Bacteriophage T2. Mechanical agitation removed much less phosphorus label than sulfur label from infected bacteria, helping distinguish the material entering the cells from material remaining at the surface.^[ref] The interpretation relies on the labelling and separation in this system; it is not a rule that all biological phosphorus belongs to DNA.

For a closed sample changing only by radioactive decay, $A(t)/A(0)=2^{-t/T_{1/2}}$. After two quoted half-lives, about 28.546 days, the radioactivity is one quarter of its initial value. A falling signal can therefore reflect decay even without loss of labelled material from a vessel. Movement of material, chemical processing and detector response require additional measurements when interpreting a real tracer experiment.

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.