A NEW METHOD OF ISOTOPIC ANALYSIS OF PERCHLORATE USING ELECTROSPRAY-ORBITRAP MASS SPECTROMETRY

Perchlorate is a well-documented groundwater, drinking water, and food contaminant with numerous sources including industrial and military use in rocketry, airbags and explosives, import of Chilean saltpeter fertilizers, and a natural background source that appears to be pervasive across the Earth. As concerns pertaining to perchlorate contamination have grown over the past two decades, so too have attempts to conclusively identify the source(s) of perchlorate, especially via isotopic fingerprinting by tracing the perchlorate’s δ³⁷Cl, δ¹⁸O, δ¹⁷O, and Δ¹⁷O signatures. These techniques require extensive and complex perchlorate purification to avoid interferences from other chlorine-bearing compounds, followed by pyrolysis for isotopic measurements on O₂, and subsequent conversion of the resulting chloride salts to methylchloride for chlorine isotope analysis.

Here we report a new method of rapidly measuring the isotopic fingerprint of unfragmented perchlorate ion (ClO₄^-) at high precision using electrospray orbitrap mass spectrometry (±1‰ δ³⁷Cl_SMOC is achieved in ~3 min; ±0.1‰ in ~30 min). This method allows the isotopes of perchlorate to be measured with minimal sample preparation and at high resolution, allowing assessment of the previously mentioned isotope signatures (δ³⁷Cl, δ¹⁸O, δ¹⁷O, and Δ¹⁷O), and adds the ability to measure clumped ³⁷Cl¹⁸O¹⁶O_3. With further work, the technique would also likely be amenable to clumped ³⁷Cl¹⁷O¹⁶O₃ measurements. This technique allows measurement of perchlorate samples with minimal sample preparation and low concentrations of <0.1 ppm ClO₄^- (potentially lower; work is ongoing). The high resolution of the analysis permits multiple dimensions of isotope fingerprinting to be explored, while preventing any isobaric interference. This technique shows promise as a method of rapidly and conclusively demonstrating the source of any given perchlorate sample, and has the potential to yield new insights to the formation processes of perchlorate on Earth and elsewhere through analysis of its isotopic systematics.