Stable Sulfur Isotope Ratio Measurement in Plant Tissues, Fertilizer Granules, and Leachates Containing Enriched 34 S.

The use of stable isotopes, especially sulfur, as a tracer in routine plant nutrient uptake studies requires an appropriate technique for determination. One of the proven techniques for the rapid measurement of stable S isotopes is the continuous flow isotope ratio mass spectrometry (CF-IRMS), which involves a coupling of an elemental analyzer to an MS through a gas chromatography (GC) column.  For solid samples such as plant tissues and soils, this technique has been modified to involve oxidative-reduction of the pulverized (< 0.25 mm) sample in a combustion cell followed by drying of H₂O and separation of SO₂ formed from CO₂ and N₂ in the GC column before delivery into the mass spectrometer (MS). Direct measurement of SO₂⁺ in the MS presents some analytical challenges; instead SO⁺ was used as a proxy for SO₂⁺. The other available technique is the ICP-MS fitted with a hexapole or octopole reaction cell. In this technique, a fraction or total S in the sample is extracted and converted into sulfate through an acid digestion. The digest is nebulized into the plasma cone and carried into the reaction gas where Xe and H₂ are introduced as reactive gases to induce a charge transfer from the O⁺. This is necessary to reduce O₂⁺ and NO⁺ that may interfere with the mass to charge (m/z) peaks at 32 and 34 for O₂⁺ and 32 for NO⁺. In this study, fertilizer granules, plant tissues, and leachates (all containing enriched ³⁴S isotope) were analyzed by both CF-IRMS and octopole ICP-MS.  This presentation will highlight the suitability of these two methods for different samples used in nutrient tracing studies and inherent analytical difficulties with ammonium based fertilizer.