X
🔍

Sulfate or Nitrate - Δ17O

Overview

We use a TC/EA to thermally degrade silver nitrate or silver sulfate to O2 and byproducts. The O2 is then passed to a Finnigan MAT253 for δ18O and δ17O analysis. We have adapted and automated previously developed techniques (Savarino et al., 2001; Michalski et al., 2002). For each sample, isolation of nitrate or sulfate and conversion to silver nitrate or silver sulfate is performed using an automated system including ion chromatographic separation, ion exchange, and a fraction collector. A Dionex Ion Chromatograph with an IonPac AS19 column (4x 250 mm) and a multistep eluent concentration gradient of 7 mM/10 mM/15 mM KOH is used to separate nitrate and sulfate fractions. Fractions then flow directly through an AMMS1 III ion exchange membrane (4 mm) with 2.5 mM Ag2SO4 regenerant for conversion to silver salts (e.g., AgNO3) prior to separate collection by the fraction collector. In multiple successive steps by a miVac Duo centrifuging concentrator at 45°C, silver nitrate fractions are transferred to and dried in  silver capsules while silver sulfate samples are transfered to and dried in quartz or gold capsules. Analysis of oxygen isotopes of nitrate is performed using a Finnigan Temperature Conversion Elemental Analyzer (TC/EA) in-line with a continuous flow Finnigan MAT 253 isotope ratio mass spectrometer with helium as the carrier gas. The TC/EA autosampler drops samples into a crimped quartz pyrolysis tube at 585°C for nitrate and 1000 °C for sulfate. In the pyrolysis tube, pyrolysis of AgNO3 samples proceeds via 2AgNO3 --> O2 + 2NO2 + 2Ag(s) + (N2, NO in trace amounts). Ag2SO4 samples proceeds via Ag2SO4 --> O2 + SO2 + 2Ag + SO3 (trace). NO2 or SO2 and other condensible by-products are removed by a liquid N2 trap before the pyrolysis products flow through a molecular sieve 5A gas chromatograph column and into the IRMS.

Evolved O2 is measured for 16O16O, 16O17O, and 16O18O, from which Δ17O is calculated (Δ17O = δ17O − 0.528 × δ18O). Our measurements of the international standard USGS-35 reference material (NaNO3) result in Δ17O(NO3) = 21.5 ± 0.4 permil (1s error) for samples of 2–5 mmol O2 (4–10 mmol nitrate), which shows excellent agreement with accepted standard values (Δ17O(NO3) = 21.6 ± 0.2 permil) [Böhlke et al., 2003; Michalski et al., 2003]. We use the nitrate USGS35 as well as a suite of in-house designer sulfate standards to calibrate.

Standard Operating Procedures

Suggested reading