Stable isotope analyses of oxygen ( O: O: O) and chlorine ( Cl: Cl) in perchlorate: reference materials, calibrations, methods, and interferences.

Rationale: Perchlorate (ClO ) is a common trace constituent of water, soils, and plants; it has both natural and synthetic sources and is subject to biodegradation. The stable isotope ratios of Cl and O provide three independent quantities for ClO source attribution and natural attenuation studies: δ Cl, δ O, and δ O (or Δ O or Δ) values. Documented reference materials, calibration schemes, methods, and interferences will improve the reliability of such studies.

Methods: Three large batches of KClO with contrasting isotopic compositions were synthesized and analyzed against VSMOW-SLAP, atmospheric O , and international nitrate and chloride reference materials. Three analytical methods were tested for O isotopes: conversion of ClO to CO for continuous-flow IRMS (CO-CFIRMS), decomposition to O for dual-inlet IRMS (O2-DIIRMS), and decomposition to O with molecular-sieve trap (O2-DIIRMS+T). For Cl isotopes, KCl produced by thermal decomposition of KClO was reprecipitated as AgCl and converted into CH Cl for DIIRMS.

Results: KClO isotopic reference materials (USGS37, USGS38, USGS39) represent a wide range of Cl and O isotopic compositions, including non-mass-dependent O isotopic variation. Isotopic fractionation and exchange can affect O isotope analyses of ClO depending on the decomposition method. Routine analyses can be adjusted for such effects by normalization, using reference materials prepared and analyzed as samples. Analytical errors caused by SO , NO , ReO , and C-bearing contaminants include isotope mixing and fractionation effects on CO and O , plus direct interference from CO in the mass spectrometer. The results highlight the importance of effective purification of ClO from environmental samples.

Conclusions: KClO reference materials are available for testing methods and calibrating isotopic data for ClO and other substances with widely varying Cl or O isotopic compositions. Current ClO extraction, purification, and analysis techniques provide relative isotope-ratio measurements with uncertainties much smaller than the range of values in environmental ClO , permitting isotopic evaluation of environmental ClO sources and natural attenuation. Copyright © 2016 John Wiley & Sons, Ltd.