More recently Thiemens's group used beamline 9.0.2 to perform the first VUV experiments on sulfur, using the results to build a model of chemical evolution in the primitive solar nebula that could yield the isotopic ratios of sulfur seen in meteorites. They report their findings in Proceedings of the National Academy of Sciences.
Mass versus chemistry
Oxygen is the most abundant element on Earth, present in air, water, and rocks; 99.762 percent of it is the isotope oxygen-16, with eight protons and eight neutrons. Oxygen-18 has two additional neutrons and accounts for another two-tenths of a percent; oxygen-17, with one extra neutron, provides the last smidgen, less than four-hundredths of a percent.
Sulfur, with four stable isotopes, is less abundant but essential to life. Sulfur-32 accounts for 95.02 percent, sulfur 34 4.21 percent, sulfur-33 0.75 percent, and sulfur-36's mere 0.02 percent brings up the rear.
Ahmed explains the two basic kinds of processes that account for these ratios. "One depends on the mass of the isotopes themselves," he says. "Oxygen-18 is two neutrons heavier than oxygen-16. One effect of this, although not the only one, is that when the temperature rises, oxygen-16 evaporates faster. And when the temperature falls, oxygen-18 condenses faster."
Changes in temperature and other physical factors can thus produce different isotope ratios that's why there's a greater proportion of oxygen-18 in raindrops than in the clouds they fall from, for example.
Isotope-ratio researchers commonly graph these processes by plotting samples with increasing proportions of oxygen-18 relative to oxygen-16 along
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DOE/Lawrence Berkeley National Laboratory