33S

33S, or sulfur-33, is a stable isotope of sulfur with mass number 33. It contains 16 protons and 17 neutrons. Its natural abundance is about 0.75% of all sulfur atoms found in nature, with the remaining sulfur consisting mainly of 32S, 34S, and 36S. Like other sulfur isotopes, 33S is non-radioactive and persists in environmental and geological samples over long timescales.

In nature, 33S arises from stellar nucleosynthesis and is incorporated into sulfur-containing minerals and compounds, including sulfates and sulfides. The relative abundances of sulfur isotopes are measured to study chemical and physical processes that affect sulfur-bearing species. Because 33S is one of several isotopes used in multi-isotope analyses, researchers often report its ratio to 32S as δ33S, expressed in per mil (permil) relative to a standard. A related metric, Δ33S, captures deviations from the expected mass-dependent fractionation pattern and is used to identify mass-independent fractionation effects.

Analytical methods for 33S measurement rely on high-precision isotope ratio mass spectrometry, typically after chemical isolation of sulfur and careful sample preparation. Advances in instrumentation allow accurate determination of δ33S and Δ33S at trace levels in geological, environmental, and biological samples.

Applications of 33S analysis include tracing the sulfur cycle in soils and oceans, studying microbial sulfate reduction, and reconstructing past environmental and climatic conditions. The isotope serves as a complementary tool alongside other sulfur isotopes to understand geochemical and biogeochemical processes.