Previous efforts to understand how sulfur metabolism and biosynthetic pathways relied on expensive labeling experiments using radioactive S. By leveraging natural assimilatory processes, stable isotope ratios can be used to track the sources of sulfur for plants, plant organs used in sulfur acquisition, the movement of sulfur through plants.
Sulfur (S) stable isotope composition measurements are often performed using an Elemental Analysis-Isotope Ratio Mass Spectrometer, (EA-IRMS) in which organic sulfur from biological samples is oxidized to sulfur dioxide (SO) and analyzed on a mass spectrometer. The mass spectrometer is used to quantify the ratio of the lighter (SO) to the heavier (SO) isotopologue of SO, and this ratio is then compared to sulfur isotope standards in order to standardize data to the VSMOW scale. In biological materials, sulfur is scarce relative to other organic elements like carbon and oxygen, introducing some additional difficulty in measuring its stable isotope composition. The elemental S composition of plant matter is ≈0.2%, accounting for approximately 2 mmol/m in most leaf tissue. In order to reach detectable levels of 30 ng to 3 µg of elemental S to calculate reliable δS values, leaf tissue samples need to be between 2–5 mg.
Improvements in detection have been made in recent years in the utilization of gas chromatography coupled with multicollector ICP-MS (GC/MC-ICP-MS) to be able to measure pmol quantities of organic S. Additionally, ICP-MS has been used to measure nanomolar quantities of dissolved sulfate. Most studies have focused on measuring the bulk δS value of plant tissues and few studies have been performed on measuring the δS values of individual S-containing compounds. The coupling of high-performance liquid chromatography (HPLC) with ICP-MS has been proposed as a way to test individual S-containing compounds.