LC–ICP-MS to Probe Metal Speciation in the Environment

Christian Dewey, a post-doctoral researcher at the University of Minnesota, presented research at the Winter Conference on Plasma Spectrochemistry on quantitative liquid chromatography–inductively coupled plasma–mass spectrometry (LC–ICP-MS) to probe the metal speciation in environmental samples (1).

Dissolved organic matter (DOM) plays a crucial role in controlling the solubility and reactivity of trace metals in the environment. However, the mechanisms governing metal-DOM complexation have long been elusive, hindered by analytical challenges in fractionating and quantifying metal-organic species within the complex mixture of organic compounds that make up DOM. Understanding these metal-DOM interactions is crucial in environmental analysis because they affect the behavior, transport, and potential impact of trace metal ions in aquatic systems.

Dewey and his team employed LC–ICP-MS for the fractionation and element-specific detection of organic-metal complexes. The scientists used a post-column compensation gradient, which stabilizes ICP-MS elemental response across the LC solvent gradient. This addresses a major barrier to achieving quantitative accuracy with LC–ICP-MS, allowing for more precise measurements of organic-metal complex concentrations.

“This technique is also well suited for a range of metals,” Dewey said. “In many environmental samples that we look at, we see a number of characteristics in our chromatograms.”

With external calibration and internal standard correction, the method consistently yields concentrations of organic-metal complexes within 6% of their true values, irrespective of the complex's elution time (1).

The study also evaluated the impact of different stationary phases (C18, phenyl, amide, and pentafluoroylphenyl propyl) on the recovery and separation of environmentally relevant trace metals (Mn, Fe, Co, Ni, Cu, Zn, Cd, and Pb) in Suwannee River Fulvic Acid and Suwannee River Natural Organic Matter (1). The C18, amide, and phenyl phases demonstrated optimal metal recoveries, with the phenyl phase proving particularly effective in separating polar species.

Further application of the method involved the fractionation of organic-bound Fe, Cu, and Ni in oxidized and reduced soils. This revealed divergent metal-DOM speciation across soil redox environments, providing valuable insights into how these interactions vary in different soil conditions.

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Reference

Dewey, C.; Kaplan, D. I.; Fendorf, S.; et al. Quantitative Separation of Unknown Organic–Metal Complexes by Liquid Chromatography–Inductively Coupled Plasma-Mass Spectrometry. Anal. Chem. 2023, 95 (20), 7960–7967. DOI:10.1021/acs.analchem.3c00696.