Monday, November 2, 2009: 11:00 AM
Convention Center, Room 320, Third Floor
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.