Sulfur (or sulphur) (1) is element number 16 in the periodic table. Thirteen isotopes of sulfur have been studied (2), ranging in atomic mass from S (half-life = 0.12s) to S (half-life = 9s) (see
Radioactivity and Radioisotopes ). Four stable isotopes are found in nature: JZS at 95.02%, JJS at 0.75%, 34S at 4.21%, and 36S at 0.02% abundance.
The most important radioactive isotope of sulfur is S (half-life = 87.44days). It decays by beta-minus emission to chlorine-35, which is stable. Sulfur-35 yields one beta particle per decay, with an energy of 0.167 MeV maximum, 0.0488 MeV on average.
Sulfur-35 is prepared in an accelerator by bombarding a stable chlorine-37 target with deuterons according to the reaction Cl(d,a) S. Carrier-free sulfur-35 may be prepared in a reactor according 35 35 to the reaction Cl(n,p) S. Sulfur-35 may also be prepared by activation of natural sulfur according to the reaction 34S(n,g) 35S.
Sulfur-35 was first used in biological studies in the 1930s to study sulfur metabolism. Penicillin was labeled with sulfur-35 in 1940. Plasma proteins were first labeled with S-cysteine in 1943 using cyclotron-produced sulfur-35 (3). Ceccaldi and co-workers in 1953 used sulfur-35 to identify the origin of enterocytes and mucocytes and their migration along villi walls (4).
Sulfur-35-labeled cysteine and methionine are incorporated into proteins and used to monitor protein biosynthesis. One of the advantages of S-labeling is that high specific activities can be achieved (eg, about 200 times greater than that possible with 14C labels). Consequently, sulfur-35 is used in molecular biology whenever possible. Sulfur-35 is detected by beta-particle liquid scintillation counting and by autoradiography and fluorography.
