Augmenting the usefulness of NMR with a probe tailor-made for liquid-gel-solid samples
Environmental spills are costly, not just for finances, but also for the environment and health. Still, not enough is understood about what happens in the immediate and longer-term aftermath of a spill. That’s because complex interdependent dynamics and interactions that follow a spill make it difficult to determine what is happening at a molecular level.
While solution-state and solid-state NMR are useful in analyzing particular components of spills, researchers from the University of Toronto wanted to study the way an entire spill unfolds, from the initial liquid spill through the gel phase that results when liquid and soil mix and on through the sequestration of the contaminant. According to the research group, the synergism between liquid-gel-solid phases determines the environmental and biological activity. Studying each phase separately can perturb the sample, removing important structural information, such as the chemical interactions at the gel-solid interface, the kinetics across boundaries and conformation in the natural state. To understand and address environmental problems at the molecular level required advancing the capabilities of traditional NMR.
Necessity is the mother of invention
To investigate and develop new ways of using NMR to study spills, researchers from the University of Toronto collaborated with experts from Bruker BioSpin to develop a multiphase NMR approach that could analyze the molecular interactions and structures of liquids, gels and solids without altering natural samples.
Through a year-long collaboration, the Toronto and Bruker BioSpin teams designed two probes to study comprehensive multiphase (CMP)-NMR situations. Then they studied the molecular bonds and interactions throughout all phases of contamination.
The results appeared in, “From Spill to Sequestration: The Molecular Journey of Contamination via Comprehensive Multiphase NMR,” published online November 18, 2015, in Environmental Science & Technology. DOI: 10.1021/acs.est.5b03251
All NMR experiments were conducted on a 500 MHz Bruker Avance III Spectrometer using a prototype MAS 4 mm 1H−19F-13C−2H CMP-NMR probe fitted with an actively shielded magic angle gradient.
Introducing PFC contaminants
To illustrate how CMP-NMR could work in a real-world situation, the researchers introduced two perfluorinated chemicals (PFCs) into a soil and water sample. PFCs are common in humans and throughout the environment. Perfluorooctanoic acid (PFOA) is often used in agricultural settings and is known to bind strongly with soil. Pentafluorophenol (PFP) is used in wood preservatives, pesticides and herbicides.
PFOA quickly moved from the solution phase to the solid phase. PFP stayed in the solution and gel phases for a longer time before moving into the solid phase. Analyzing the interactions, researchers discovered that PFOA enters soil via hydrophobic tails and preferentially binds to microbial protein in the soil. PFP interacts with a number of gel and solid soil components with a preference for aromatics, specifically lignin.
The results show that traditional approaches alone cannot accurately explain contaminant behavior. The ability to predict how a contaminant will act requires considering the complex array of information gathered using the specialized probe and advanced techniques of CMS-NMR. Important factors include soil chemistry and composition, physical accessibility/swellability of organic soil components, the likelihood that a contaminant will biologically interact, and the possibility that the contaminant will alter the soil-water interface.
The researchers suggest that the CMP-NMR approaches used in this study could prove helpful in assessing the environmental relevance of mixing soil surfactants, a common agricultural practice.
For more information
Panagos, P., et. al. Contaminated sites in Europe: Review of the current situation based on data collected through a European network. J. Environ. Public Health 2013, 2013.2013110.1155/2013/158764
Denis Courtier-Murias, Hashim Farooq, Hussain Masoom, et.al., Comprehensive multiphase NMR spectroscopy: Basic experimental approaches to differentiate phases in heterogeneous samples, Journal of Magnetic Resonance, Volume 217, April 2012, Pages 61-76, ISSN 1090-7807, http://dx.doi.org/10.1016/j.jmr.2012.02.009.
The Madrid Statement on Poly- and Perfluoroalkyl Substances (PFASs) Environmental Health Perspectives, May 2015; http://dx.doi.org/10.1289/ehp.1509934
Perfluorinated Chemical (PFC) Research, U.S. Environmental Protection Agency