Rationale

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: δ³⁷Cl, δ¹⁸O, and δ¹⁷O (or Δ¹⁷O or ¹⁷Δ) values. Documented reference materials, calibration schemes, methods, and interferences will improve the reliability of such studies.

Methods

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 CH₃Cl for DIIRMS.

Results

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.

Conclusions

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.