Stable isotope analyses of oxygen (18 O:17 O:16 O) and chlorine (37 Cl:35 Cl) in perchlorate: reference materials, calibrations, methods, and interferences.

Abstract

Perchlorate (ClO4- ) 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 ClO4- source attribution and natural attenuation studies: δ37 Cl, δ18 O, and δ17 O (or Δ17 O or 17 Δ) values. Documented reference materials, calibration schemes, methods, and interferences will improve the reliability of such studies. Three large batches of KClO4 with contrasting isotopic compositions were synthesized and analyzed against VSMOW-SLAP, atmospheric O2 , and international nitrate and chloride reference materials. Three analytical methods were tested for O isotopes: conversion of ClO4- to CO for continuous-flow IRMS (CO-CFIRMS), decomposition to O2 for dual-inlet IRMS (O2-DIIRMS), and decomposition to O2 with molecular-sieve trap (O2-DIIRMS+T). For Cl isotopes, KCl produced by thermal decomposition of KClO4 was reprecipitated as AgCl and converted into CH3 Cl for DIIRMS. KClO4 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 ClO4- 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 SO42- , NO3- , ReO42- , and C-bearing contaminants include isotope mixing and fractionation effects on CO and O2 , plus direct interference from CO2 in the mass spectrometer. The results highlight the importance of effective purification of ClO4- from environmental samples. KClO4 reference materials are available for testing methods and calibrating isotopic data for ClO4- and other substances with widely varying Cl or O isotopic compositions. Current ClO4- extraction, purification, and analysis techniques provide relative isotope-ratio measurements with uncertainties much smaller than the range of values in environmental ClO4- , permitting isotopic evaluation of environmental ClO4- sources and natural attenuation. Copyright © 2016 John Wiley & Sons, Ltd.