Environmental Nuclear Magnetic Resonance Spectroscopy: An

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Environmental Nuclear Magnetic Resonance Spectroscopy: An Overview and A Primer Andre J Simpson, Myrna J Simpson, and Ronald Soong Anal. Chem., Just Accepted Manuscript • DOI: 10.1021/acs.analchem.7b03241 • Publication Date (Web): 13 Nov 2017 Downloaded from http://pubs.acs.org on November 14, 2017

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Analytical Chemistry

Environmental nuclear magnetic resonance spectroscopy: An overview and a primer

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André J. Simpson*Ṫ Myrna J. Simpson*Ṫ and Ronald Soong

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Environmental NMR Centre and Department of Physical & Environmental sciences, University of

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Toronto Scarborough, Toronto, ON, Canada, M1C 1A4

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Ṫ

These authors contributed equally to the manuscript

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*Corresponding Authors: André J. Simpson, Email: [email protected], Telephone: 416-287-7547, Fax: 416-287-7279 Myrna J. Simpson, Email: [email protected], Telephone: 416-287-7234, Fax: 416-287-7279

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ABSTRACT

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NMR spectroscopy is a versatile tool for the study of structure and interactions in environmental

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media such as air, soil, and water as well as monitoring the metabolic responses of living organisms to

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an ever changing environment. Part review, part perspective and part tutorial, this feature is aimed at

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non-specialists who are interested in learning more about the potential and impact of NMR spectroscopy

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in environmental research.

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Nuclear magnetic resonance (NMR) spectroscopy is a widely recognized tool for its unparalleled

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ability to ascertain the molecular structure of matter in various states (solutions, solids, and gels)1. NMR

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spectroscopy is commonly applied in diverse fields due to its versatility in studying structure and

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interactions across a range of molecular systems. NMR experiments can range from simple one-

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dimensional (1-D) to more complex multi-dimensional experiments that identify bond connectivities and

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spin-spin interactions2. Consequently, there are potentially thousands of experiments that can be

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employed to access specific, molecular-level information often inaccessible by other analytical

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techniques.

In addition, NMR is also highly reproducible even across different laboratories, at

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difference magnetic field strengths, and with different operators3 as well as providing accurate

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quantification without the need for external standards 4,5. As such NMR is a non-selective detector (for

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example detects any molecules containing an NMR active nucleus such as 1H or 13C) making it an ideal

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tool for non-targeted analyses. Using two-dimensional (2-D) NMR experiments, unknowns can be

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identified without prior knowledge of the compounds present. This is a major benefit given that other

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analytical techniques, such as mass spectrometry (MS), require some basic prior information about the

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analytes of interest to optimize the analysis (e.g.: chromatographic separation, ionization, etc).

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Environmental systems are highly complex and variable. Ecosystems are dynamic and a number

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of complex biological, chemical, and physical processes occur within them. Furthermore, climate

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change, urbanization, agriculture and industrial activity all threaten to change ecosystem function.

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Therefore, it is imperative to understand molecular-level processes that occur within these ecosystems or

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environmental compartments (air, soil and water) to increase the fundamental knowledge of ecosystem

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processes. Environmental chemists are tasked with trying to resolve these complex processes and NMR

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spectroscopy is playing a central role in this endeavor. The search for clean energy and understanding

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the fundamental chemistry of biofuels is another important research area that necessitates the use of

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advanced analytical techniques such as NMR spectroscopy 6. In addition, NMR spectroscopy is used for

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assessing wastewater treatment efficiency7,8.

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In many ways understanding larger scale environment phenomena involves working through a

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continuum of interconnected questions as illustrated in Figure 1. In this case the ultimate question being

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asked is “how does contaminant X impact aquatic organisms”. The first line of thought may be to

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expose the organism to a contaminant and using the non-invasive nature of NMR to understand the

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biological pathways impacted, which helps explain “why” the chemical is toxic. However, related

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questions such as; “Is the chemical bioavailable in sediment?; what transformation products are ACS Paragon Plus Environment

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formed?; Does the chemical become sequestered in sediment?”; all complicate the scenario. Here

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NMR’s ability to be applied to solution, gel and solid phases is essential to study processes in situ and

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help tackle these complex and interconnected questions. Arguably however, it is nearly impossible to

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understand how a contaminant binds to sedimentary organic matter without first understanding the

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structure of the sediment, ideally in its natural fully swollen state. Here NMR’s ability to study both

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structure and molecular interactions (often determined by mapping spatial proximities while solving

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structure and conformation de novo is paramount. In summary many environmental concerns can be

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broken down into 3 basic categories: structure, interactions and impact. Fundamentally structural

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information is required to understand interactions with environmentally relevant contaminants and

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agrochemicals, these interactions in turn determine bioavailability and toxicity, which then regulate their

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impact on living organisms. As such the following feature is organized into three complementary topics,

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first the role of NMR to study structure in soil, air and water, followed by molecular interactions and

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finally the response of living systems to environmental change and stress.

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The number of experiments that can be implemented in NMR spectroscopy are vast and

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numerous. An overview of the most common forms of experiments and techniques, used in

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environmental applications are summarized in Tables 1 and 2. However, a more comprehensive

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overview is found in Simpson et al.9 or Simpson and Simpson10 with more specific literature focusing on

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various sub-areas namely solution2, High Resolution-Magic Angle Spinning (HR-MAS) (sometimes

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termed gel-state NMR)11,12, solid-state NMR13-15, low field NMR16,17 and imaging18. Table 1

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summarizes some of the main NMR techniques used in environmental research and provides some basic

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information that may be useful to non-experts wishing to perform environmental NMR studies. Readers

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should note that most of these experiments are applicable to any NMR active nuclei with 1H, 2H, 7Li,

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environmentally relevant nuclei.

C, 14N 15N, 19F, 27Al, 29Si, 31P, 111Cd, 113Cd, 195Pt, 199Hg, 207Hg, being amongst the most sensitive and


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Similarly there are a multitude of choices for experiments to use in combination with the

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techniques listed in Table 1. Due to the diverse range of media and interactions in environmental

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research nearly all well-known NMR experiments have some application. Table 2 summarizes ten of

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the most useful experiments for the characterization of materials in the solution- and solid-state. For HR-

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MAS NMR studies solution-state experiments are commonly employed. Additional review articles

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related to these topics include: Simpson et al.9, and Cardoza et al.19, along with several guides focusing

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on practical applications 11,20,21.

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ENVIRONMENTAL STRUCTURE

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As NMR emerged as an informative technique for structural information, the first applications to

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environmental chemistry focused on 1-D NMR acquisition of soil extracts22. Soil organic matter (SOM)

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stores more carbon than that in the atmosphere but with climate change, the fate of this carbon,

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especially in sensitive ecosystems such as the Arctic and peatlands, is highly uncertain 23. Furthermore,

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SOM binds with metals, organic pollutants, and soil minerals. Therefore, SOM chemistry was of

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immense interest and some of the early environmental applications targeted the structure of SOM using

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NMR spectroscopy. With respect to structural elucidation studies, SOM has been amongst the most

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commonly studied materials using NMR24-26.

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scientific endeavor that spanned decades and scientists working on this topic did not hesitate to employ

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NMR techniques to help with this important question22. SOM studies also later employed techniques

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such as cross polarization magic angle spinning (CP-MAS) solid-state 13C NMR because of its ability to

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provide an overview of all the components present in dried SOM15,22,27. Paramagnetic species such as

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iron and manganese can lead to spectral broadening in NMR and for environmental materials rich is

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these species pre-treatment using HF is recommended28. HF dissolves mineral components both

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reducing the paramagnetic content while concentrating the organics often leading to improvements in

Unravelling the complex structure of SOM was a


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both intensity and resolution28. Readers interested in HF treatment and paramagnetic influences are referred to a number of excellent studies on this topic28-31 . 13

C CP-MAS remains the most widely used to measure the composition of SOM because of the

ease of sample preparation and the scientific need to study SOM in its entirety14,27. As such, this technique will likely to be the primary method used for SOM and other forms of natural organic matter (NOM) including atmospheric particles, sediments and isolated/dried dissolved organic matter (DOM)14,15,32,33. Because NOM is a complex mixture, precise structural elucidation is difficult with solid-state experiments unless advanced techniques are employed14. Consequently, solution-state methods can complement information about NOM structure2,9 with 3-D experiments such as TOCSYHSQC showing the ability to resolve individual components34. For example, the detailed study of basesoluble soil extracts, commonly referred to as humic substances, showed that the structure was consistent with a complex mixture of plant- and microbial-derived compounds and biopolymers at varying stages of oxidation35. Previously, it was hypothesized that SOM was predominantly comprised of humic substances which ranged from 10,000-100,000 daltons36,37. Prior to the onset of advanced structural tools such as multi-dimensional NMR spectroscopy, research observations were based on more macroscopic techniques which could not distinguish between large macromolecules or aggregates of smaller molecules that behaved like a macromolecules. Also in the early 2000s, researchers were making similar observations about the chemistry of SOM using a range of molecular techniques and evidence was building against the existence of humic macromolecules (see review by Schmidt et al.37). These studies culminated in a paradigm shift in the understanding of SOM chemistry36,37 and data from solution-state and diffusion ordered spectroscopy (DOSY) NMR studies played a major role in this new understanding which also emphasizes interactions between SOM and clay minerals as a means of stabilizing carbon in soil environments 9.


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Other NMR techniques, such as HR-MAS NMR were also used to better understand the structure and physical conformation of SOM. HR-MAS NMR was developed in the late 1990s38 as a means of acquiring high-resolution spectra on samples that were not fully soluble such as foods, gels, and tissues12. With HR-MAS NMR, components that are in contact with the solvent are observable whereas true solids are not detected11,39,40. For environmental samples such as soil, this showed immense promise because widely used solid-state

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C NMR methods provide a spectrum of all structures present while

HR-MAS provides information specific to the important soil-water interface41. Evidence for the important role of physical accessibility of specific SOM components was mounting from different facets of environmental chemistry research including SOM-pollutant binding studies as well as SOM degradation studies42-44. Although the consensus was obtained indirectly, it was becoming widely accepted that the compounds observed by both solution-state and solid-state NMR were indeed similar45 and present in soil but not necessarily participating in a range of soil biogeochemical processes and interactions with organic pollutants or metals45. Many specific SOM structures were hypothesized to be protected through interactions with inorganic and organic soil components but these conclusions were made through indirect deduction42-44. Studies using 1H HR-MAS NMR showed that with varying degrees of solvation (D2O versus DMSO-d6), exposed different types of SOM structures and was attributed not only to the polarity of these compounds but their physical location within soil particles41,46. Interestingly, these results provided direct molecular-level evidence that chemical structure as well as the accessibility of these structures is paramount to understand the fate of SOM and reactivity in the environment.

The important need to also elucidate the structure of organic matter found in sediments, water and atmospheric particles have also necessitated the use of a range of NMR techniques47-51. Collectively, these studies have been influential in improving the understanding of organic matter structure, its sources and subsequent reactivity in various facets within the environment. NMR is also an important


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tool in related areas of research including chemical ecology, biofuels, and assessment of wastewater composition including detection of pollutants6-8,52,53. Its unparalleled ability to provide detailed structural information on a range of environmental samples makes it one of the most versatile analytical techniques in modern science6-8,53-55.

Another important aspect of environmental chemistry where NMR has been influential is in the identification of pollutants and their respective degradation products8,52,56. In 1976, Taves et al. used 19F NMR to identify perfluorinated organic fluorine compounds in human plasma57, decades before they were rediscovered by MS58. The hyphenation of liquid chromatography (LC) with NMR has also enabled the identification of components of more complex pollutant mixtures56. Collectively, NMR played a critical role in uncovering the structures and transformations in carbon pools in air, soil and water, as well playing a role in understanding the fate of anthropogenic chemicals. NMR also holds great potential for studying bonding within the mineral phase and probing interactions between minerals and organic matter59-64 in addition to unravelling the structures of air particles32 and monitoring atmospheric reactions65. In many of the areas the application of NMR is still in its infancy, holding great potential for significant progress in these fields. However, NMR has many more facets beyond providing structural information, in particular, its ability to probe non-covalent interactions is of great important to environmental research.

ASSOCIATIONS AND INTERACTIONS

When studying the interactions of an environmental matrix such as soil, sediment or air particles with, for example, an agrochemical or contaminant it is important to perform the studies under conditions as close to the native state as possible. A swollen soil for example may have DOM in the pore water, swollen material at the soil water interface, and true solid materials within domains that water ACS Paragon Plus Environment

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cannot penetrate. All of these sub-compartments are critical for soil processes, the pore water is essential for nutrient transfer to roots, the soil-water interface for the rate and mechanism of contaminant transfer between soil and water, while the solid domains are key domains where hydrophobic contaminants become sequestered 66. To truly understand “what soil is” environmental researchers need molecular tool that describe both the physical and chemical organization of components and under different hydration levels. Only with this knowledge it is possible to truly understand the fate, transformation, sequestration of contaminants in the environment. With this in mind it is clear that both understanding the chemical and physical organization of the matrices themselves (soils, plants etc.) as well as the interaction of xenobiotic chemicals (agrochemicals, drugs, contaminants) with these matrices is essential. As such these topics will be introduced and discussed sequentially.

Associations, Conformation and Physical Organization in Environmental Matrices While DOM is arguably one of the most complex mixtures known67 it is by definition dissolved in the aquatic environment. As such studies using solution-state NMR have a high degree of environmental relevance. Traditionally, DOM studies have been performed using isolates that are redissolved at higher concentration for NMR after pre-concentration67. Studies using DOSY NMR have shown that even at relatively low concentration DOM is in an aggregated state68-70, however, there is still concern that any sort of pre-concentration could change the natural aggregated state of the material. As such novel water suppression approaches have been developed that permit the study of DOM at natural abundance in altered samples71. These approaches have been applied to lake, ocean, river71 ground water47 as well as Antarctic glacial ice72. Arguably the most impressive application was the study of hydrodynamic size of DOM in unconcentrated pond, river, and sea waters by Zheng and Price73. They concluded the aggregation of components in pond water changed with concentration which highlighted


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the importance of hydrodynamics of DOM in unconcentrated natural waters. They further pointed out the potential to their approach to study interactions between DOM and persistent organic pollutants at natural abundance73.

Unfortunately, unlike DOM which due to its highly soluble nature in water can be analysed in its entirety in the solution-state, most environmental matrices often contain a continuum of materials ranging from liquids to true solids. While extraction will invariably change properties, even less invasive approaches such as drying (for solid-state NMR studies) will remove key information on the solidaqueous interface, may change the conformation and eliminate water mediated structure. For example in atmospheric research air particulates contain partially soluble organics that are key cloud condensation nuclei74 and understanding the surface species at different hydration levels is critical. Similar arguments can be made in regard to the fate of contaminants in soils and sediments which changes significantly with water content75. Luckily due to the versatility of NMR it can still be applied to fully swollen samples in their natural state. The simplest approach would be using two different NMR probes. 1H HR-MAS techniques to study the liquids and gels along with

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C solid-state based approaches to study the components that

remain as true solids in the present of water. Indeed if the system is relatively stable (i.e. the phases are not dynamically changing) the use of two probes to study a multiphase sample is certainly feasible76-78. However, if the study involves a dynamic process, for example swelling, flocculation, penetration of a contaminant and the goal is to monitor all phases at the same time using separate probes becomes challenging. In this case a relatively new approach that termed Comprehensive Multiphase NMR Spectroscopy (CMP NMR) may be ideal.

CMP NMR probes, combine a lock (stability over time), gradient (coherence selection and diffusion), MAS (line narrowing), susceptibility matched stator (good 1H lineshape) with high power


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circuitry (permits full solid-state NMR). When combined with a series of editing experiments, solutions, gels and solids can be differentiated in situ and in unaltered natural samples39. So far the approach has been applied to follow biofuel penetration into car components79, the germination of plants80 and seed structure81, and oil contaminated soils82. Masoom et al.83 demonstrated that when combined solution-, gel- and solid-state NMR approaches could provide a detailed insight into the organization of components within soil and how the domains and associations change with pH and solvent. Such studies provide a unique insight into soil structure in its natural swollen state which is largely inaccessible to any other analytical approach.

Interactions of Anthropogenic Organic Chemicals and Heavy Metals

NMR spectroscopy is sensitive to any perturbation of the chemical environment around nuclei. As such it is not just a powerful tool for the study of chemical structure (i.e. covalent bonds) but also non-covalent interactions

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. Non-covalent interactions are critical in environmental research and are

central to contaminant sequestration, transport and fate. For example, the association of a herbicide with soil determines it efficacy (how much reaches the target plant vs binds to the soil), it bioavailability (does it bind irreversibly to soil becoming non bioavailable), its transport (does it stick to soil or move into the aquatic environment) and its reactivity (for example bound chemicals can be protected from photochemistry). Thus for both heavy metals and organic contaminants, understanding non-covalent interactions with environmental media is arguably one of the most important questions in the field.

Example studies have researched non-covalent pollutant interactions with soil by solid-state NMR and 1H HR-MAS NMR85-91 as well as covalent binding using

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C or

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F

N NMR92,93. These

studies provide insight into the molecular environment of the pollutant which helps elucidate binding mechanisms as well as the soil components that the pollutant is binding to. Consequently, these studies are critically important as most pollutant-soil studies involve indirect methods, such as measuring ACS Paragon Plus Environment

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binding coefficients, that do not provide any molecular-level information about the mechanisms of binding43,44. For example, Sachleben et al.89 showed that pyrene was binding strongly to mobile aliphatic domains in plant cuticle by

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C NMR.

hexafluorobenzene using solid-state

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Kohl et al.87 probed the molecular environment of

F solid-state NMR and found that after sorption to soil,

hexafluorobenzene exhibited a high degree of mobility which suggested that the pollutant was binding to specific SOM components. Using 1H HR-MAS NMR, Shirzadi et al.90 monitored the sorption kinetics of several pesticides and reported that transitioning from swellable to pure solid domains was not only a function of contact time but also the moisture level within the soil.

Collectively, these types of

mechanistic studies provide complementary information that greatly improves the interpretation from more traditional studies that measure binding coefficients.

Experiments for Studying Interactions

A wide range of experiments exist to study molecular interactions and readers are referred to general reviews by Cardoza et al.19, by Simpson et al.9, Mazzei and Piccolo84 as well as specific reviews on metal interactions94, organic pollutants84 and clays95.

In general experiments that measure

interactions can be split into two main categories: 1) Experiments that detect both free and bound species, 2) Experiments that specifically select the bound component.

Experiments that Detect Free and Bound Components: In simple chemical systems, the chemical shift of the free form and bound form can be different enough that they become resolved in a 1-D NMR spectrum. This is the ideal case and other informative measurements such as relaxation (T1, T2), NOE (interactions through space and exchange) and self-diffusion of the different forms are easy to measure. Unfortunately, environmental systems such as DOM and soil contain 10,000 or 100,0000’s of chemical components51. As such chemicals can bind or partition to many different components and to a varying extents. In turn measurements such as T1 and T2 relaxation times and diffusion represent averages or


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distributions rather than discrete values68. While these averages can be very useful especially when comparing between different environmental samples, this complexity can hinder interpretation and make exact information regarding binding sites, exchange, etc. more difficult to obtain. Experiments that specifically select the bound component: Experiments can be designed that select specifically for the bound signal while other signals cancel. Such experiments tend to be more technical to implement but provide information that is simpler and easier to interpret in complex systems. A key example is saturation transfer difference (STD) NMR96. In the STD experiment 1H nuclei in the soil (receptor) are selectively saturated. This saturation is then passed from the soil onto the positions of the contaminant (ligand) in close proximity to the soil90. Other positions in the contaminant, as well as contaminant molecules not binding to the soil, do not receive saturation and are not detected using the difference approach employed. The result can be converted into an epitope map (see Figure 2) that where relative interaction strength at each position is reported allowing easy visualization of the binding orientation of the xenobiotic. STD has been used to monitor in the interactions of pesticides at the soil water interface using HR-MAS NMR90 and in SOM solutions using solution-state NMR91 97.

These types of experiments can be even more informative if a heteronuclear, such as

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F is

present in the contaminant that is not in abundance in the environmental matrix. In this case both heteronuclear saturation transfer difference (HSTD), and reverse heteronuclear saturation difference (RHSTD) can be employed. Both have been employed to study the interaction of per fluorinated chemicals with extracted SOM98 and whole soil98. HSTD provides similar information as STD (but is easier to implement due to no overlap between the ligand and receptor) while RHSTD identifies the subcomponents in the soil that bind to the xenobiotic. In RHSTD the

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F nuclei in the PFC are saturated,

which then propagates onto the 1H nuclei in the soil touching the contaminant. Due the effective spin diffusion within the soil, the components touching the contaminant become coated in saturation and the


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result (acquired by difference) is an NMR spectrum of only the components in the soil touching the contaminant (as depicted in Figure 2).

These concepts can be easily extended to the true solid-state, where CP rather than saturation transfer can be employed. Intermolecular CP is only effective in rigid solids and does not occur in solutions39. As such CP can be used as a filter to observe only the most rigid interactions which correlate to the sequestered state of the contaminant fully entrapped in the soil matrix99. By combining CP and saturation filters, it was recently shown that the fate of a molecule could be tracked as it penetrated into a whole soil66. The kinetics of sorption along with binding orientation and binding sites (i.e. protein, lignin etc.) was measured and differentiated in the solution, gel and solid-phase of the water swollen soil in situ. The result was an unprecedented insight into the molecular fate and binding of molecules within soil under environmentally relevant conditions66.

Moving forward novel methods need to be developed that continue to target interactions in complex environmental systems. Heteronuclear approaches based around 19F are unfortunately restricted to the study of interactions in perfluorinated chemicals. However, analogous experiments using 2H as the heteronuclear have huge potential albeit with less sensitivity than

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F. 2H can replace 1H in practically

any organic structure with minimal changes to the chemistry of the molecule and 2H enriched molecules tend to be generally more economical and more available than for example

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C enriched analogues.

Additionally, a wide range of experiments can be constructed around diffusion, CP and/or relaxation filters to target sub-components in a mixture66. For example a diffusion filter selects molecules that do not exhibit self-diffusion and thus can be used to select the fraction of contaminant bounds to a larger molecules or surfaces. This is a clear area within environmental NMR that will require researchers not just to use the techniques available but where possible actively develop novel techniques to address the complex questions specific to environmental research.


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METABOLOMICS

Metabolomics focuses on the detection and sensitive measurement of metabolite fluxes that can be related to changes in basic metabolic function that can occur due to disease or some external stressor100-102. Applications in both ecology and ecotoxicity have increased considerably over the past decades with these studies focusing on environment-organism interactions such as pollutant exposure, changes in nutrient or temperature, pH or salinity103,104. Environmental metabolomic studies are diverse due the high number of organisms that reside in soil and water54,104-107. As such, several different types of organisms have been targeted for environmental metabolomic studies. Furthermore, many of the keystone organisms do not have a mapped genome or a characterized proteome. This, coupled with the rapid changes that can be detected in metabolites within hours of stress, has resulted in widespread applications of NMR in environmental metabolomics.

Consequently, metabolomics has immense

potential to reshape the understanding of how environmental change impacts ecosystem health and also hold the potential as an early warning system for using in environmental monitoring103,104. NMR spectroscopy has been central to environmental metabolomic applications102,108. In most studies, the metabolites of interest where unknown and as such, NMR analysis was preferred because of its ability to provide a holistic fingerprint of all metabolites present. In addition, minimal sample preparation for NMR is conducive to high-throughput analysis, which yields large numbers of NMR spectra that can be analyzed using multi-variate statistical methods such as principal component analysis (PCA). In addition, the analytical reproducibility of 1H NMR spectra is excellent and studies have reported 100 million. Interestingly 6 and 7 dimensional NMR experiments are now available and by combining sparse random sampling and projection spectroscopy can now be collected in a day137. Such experiments could potentially reach peak capacities ACS Paragon Plus Environment

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of >1015 offering potential individual bond discrimination even in the most complex mixtures such as a DOM which have been reported to contain 1012 molecules per mL.138 The versatility of NMR to study molecules in the solid, gel and solution forms allows analysis samples in situ and provides key partitioning and sequestration information that is challenging to obtain by other analytical approaches. The non-destructive nature of NMR makes it ideal for studying in vivo samples. This will be especially important for understanding the impact of environmental factors on both human and environmental health. For example 25% of neurological diseases are now thought to be linked to environmental factors139. The combination of metabolomics and in vivo NMR hold the potential to explain why chemical are toxic, identify synergistic effects and to potentially act as an early warning indicator for environmental stress and change. It is clear that NMR has a central and essential role to play in the future of environmental research. However, despite its potential its use in environmental research is not widespread. Various factors include, cost of instrumentation, cryogenic maintenance, perceived knowledge and technical barriers as well as relatively low sensitivity. NMR operation is certainly not trivial but the instrument manufacturers have worked hard to simplify the acquisition of very complex data and provided easier access to use tools to aid with interpretation. There is no doubt that the Achilles heel of NMR is its relatively low sensitivity especially when compared to MS (most common tool in environmental research). However, there continue to be major leaps in sensitivity with modern NMR microcoils offering 5pmol detection limits making possible the analysis of individual eggs of 100pL volume, and with other technologies such as cryoprobes140, dynamic nuclear polarization, hyperpolarization141 offering further sensitivity gains. Given the potential of NMR as a non-selective detector in the discovery of novel contaminant classes (for example perfluorinated chemicals were discovered by

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NMR decades before mass spectroscopy58 ) the ever increasing sensitivity of NMR should make useful tool for discovery of new contaminants and transformation products especially when interfaced with concentration methods such as solid phase extraction. Furthermore, one of largest problems faced in ACS Paragon Plus Environment

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environmental research is that 100,000’s of molecular formulae can be generated by MS but these are difficult to assign to specific chemicals structures142. Essentially MS provides the elements in an unknown sample whereas NMR tells us how these are connected to form the structure. As such if chemicals can be fraction collected after chromatographic separation, for example 2-D gas chromatography, and analyzed by microcoil NMR combined the NMR and MS data should be able to identify even the most challenging unknowns. In summary NMR spectroscopy has a continuum of applications from structure determination (contaminants, SOM, atmospheric particles) to molecular interactions (agrochemical, drug and contaminant fate) to biological impact (which pathways are perturbed, i.e. detection and explanation of environmental stress). NMR is central to future of environmental research but to fully realize its potential more researchers actively developing techniques and approaches are needed. Environmental research with its heterogeneous and complex matrices is arguably the most challenging area for NMR. As such researchers need to be actively exploring the limits of the technology in turn encouraging the manufacturers and research field to develop more powerful solutions. Often NMR provides a wealth of information that cannot be obtained by any other approach yet is essential to progress. The widespread application of development of NMR in these directions will be critical to the future of environmental research.

Author Biographies

Andre J. Simpson is a Professor of Chemistry at the University of Toronto. He spent his career on the development of NMR spectroscopy for environmental applications. He co-founded the Environmental NMR Centre in 2004 at the University of Toronto Scarborough and currently acts as the Centre’s Director. His research interests focus on understanding structure and interactions in complex environmental samples including the development of novel NMR technology to achieve this goal. ACS Paragon Plus Environment

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Myrna J. Simpson is a Professor of Environmental Chemistry and Associate Director of the Environmental NMR Centre at the University of Toronto Scarborough. Her research interests include environmental and analytical chemistry with the specific focus on elucidation of environmental processes at the molecular-level and how these processes are impacted by anthropogenic activities.

Dr. Ronald Soong graduated from the University of Toronto with a PhD in Physical Chemistry. Later, he did 2 years of postdoctoral studies at the University of Michigan where he developed solid-state NMR pulse sequences for membrane protein structure determination under the supervision of Prof. Ramamoorthy. In 2010, he joined the Environmental NMR Centre as Senior Scientist & NMR Manager. Currently, Dr. Ronald Soong is developing NMR techniques and applications for environmental research.

Acknowledgements

A.J.S. and M.J.S thanks NSERC for support via the Strategic and Discovery Grants programs. The Canadian Foundation for Innovation (CFI), and the Ministry of Research and Innovation (MRI) and Krembil Foundation are thanked for funding in support of the Environmental NMR Centre. M.J.S. also thanks NSERC for support via the Discovery Accelerator Supplement program. A.J.S. and M.J.S would like to thank Bruker BioSpin, especially Dr. Henry Stronks, Dr. Manfred Spraul and Dr. Werner Maas for their continued support and collaborations in the development of environmental NMR spectroscopy.

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(43) Chefetz, B.; Xing, B. Environ. Sci. Technol. 2009, 43, 1680-1688. (44) Luthy, R. G.; Aiken, G. R.; Brusseau, M. L.; Cunningham, S. D.; Gschwend, P. M.; Pignatello, J. J.; Reinhard, M.; Traina, S. J.; Weber, W. J.; Westall, J. C. Environ. Sci. Technol. 1997, 31, 3341-3347. (45) Clemente, J. S.; Gregorich, E. G.; Simpson, A. J.; Kumar, R.; Courtier-Murias, D.; Simpson, M. J. Environ. Chem. 2012, 9, 97-107. (46) Genest, S. C.; Simpson, M. J.; Simpson, A. J.; Soong, R.; McNally, D. J. Environ. Chem. 2014, 11, 472-482. (47) Bliumkin, L.; Majumdar, R. D.; Soong, R.; Adamo, A.; Abbatt, J. P. D.; Zhao, R.; Reiner, E.; Simpson, A. J. Environ. Sci. Technol. 2016, 50, 5506-5516. (48) Dutta Majumdar, R.; Bliumkin, L.; Lane, D.; Soong, R.; Simpson, M.; Simpson, A. J. Water Res. 2017, 120, 64-76. (49) Pautler, B. G.; Woods, G. C.; Dubnick, A.; Simpson, A. J.; Sharp, M. J.; Fitzsimons, S. J.; Simpson, M. J. Environ. Sci. Technol. 2012, 46, 3753-3761. (50) Schmidt-Rohr, K.; Mao, J. D.; Olk, D. C. PNAS 2004, 101, 6351-6354. (51) Woods, G. C.; Simpson, M. J.; Simpson, A. J. Water Res. 2012, 46, 3398-3408. (52) Chilom, G.; Rice, J. A. eMagRes 2015, 2, 587–596. (53) Ocampos, F. M. M.; Menezes, L. R. A.; Dutra, L. M.; Santos, M. F. C.; Ali, S.; Barison, A. eMagRes 2017, 6, 325-341. (54) Charlton, A. J.; Donarski, J. A.; Jones, S. A.; May, B. D.; Clive Thompson, K. J. Environ. Monit. 2006, 8, 1106-1110. (55) Schneider, B. eMagRes 2013, 2, 451–466. (56) Godejohann, M. eMagRes 2013, 2, 477–492. (57) Taves, D. R.; Grey, W. S.; Brey Jr, W. S. Toxicol. Appl. Pharmacol. 1976, 37. (58) Renner, R. Anal. Chem. 2005, 77, 15a-16a. (59) Grey, C. P.; Nielsen, U. G.; Paik, Y.; Phillips, B.; Reeder, R. J.; Schoonen, M. Abstr Pap Am Chem S 2005, 229, U737-U737. (60) Nielsen, U. G.; Paik, Y.; Julmis, K.; Schoonen, M. A. A.; Reeder, R. J.; Grey, C. P. J. Phys. Chem. B 2005, 109, 18310-18315. (61) Williams, R. J. P.; Giles, R. G. F.; Posner, A. M. J Chem Soc Chem Comm 1981, 1051-1052. (62) Bowers, G. M.; Lipton, A. S.; Mueller, K. T. Solid State Nucl. Magn. Reson. 2006, 29, 95-103. (63) Sanders, R. L.; Washton, N. M.; Mueller, K. T. J Phys Chem C 2010, 114, 5491-5498. (64) Strepka, C.; Choi, S.; O'Day, P.; Chorover, J.; Mueller, K. Geochim. Cosmochim. Acta 2008, 72, A907-A907. (65) Zhao, R.; Lee, A. K. Y.; Soong, R.; Simpson, A. J.; Abbatt, J. P. D. Atmos Chem Phys 2013, 13, 5857-5872. (66) Masoom, H.; Courtier-Murias, D.; Soong, R.; Maas, W. E.; Fey, M.; Kumar, R.; Monette, M.; Stronks, H. J.; Simpson, M. J.; Simpson, A. J. Environ. Sci. Technol. 2015, 49, 13983-13991. (67) Li, Y.; Harir, M.; Uhl, J.; Kanawati, B.; Lucio, M.; Smirnov, K. S.; Koch, B. P.; Schmitt-Kopplin, P.; Hertkorn, N. Water Res. 2017, 116, 316-323. (68) Lam, B.; Simpson, A. J. Environ. Toxicol. Chem. 2009, 28, 931-939. (69) Simpson, A. J. Magn. Reson. Chem. 2002, 40, S72-S82. (70) Smejkalova, D.; Piccolo, A. Environ. Sci. Technol. 2008, 42, 699-706. (71) Lam, B.; Simpson, A. J. Analyst 2008, 133, 263-269. (72) Pautler, B. G.; Simpson, A. J.; Simpson, M. J.; Tseng, L. H.; Spraul, M.; Dubnick, A.; Sharp, M. J.; Fitzsimons, S. J. Environ. Sci. Technol. 2011, 45, 4710-4717. (73) Zheng, G.; Price, W. S. Environ. Sci. Technol. 2012, 46, 1675-1680. (74) Broekhuizen, K.; Kumar, P. P.; Abbatt, J. P. D. Geophys.l Res. Let. 2004, 31. (75) Schneckenburger, T.; Schaumann, G. E.; Woche, S. K.; Thiele-Bruhn, S. J. Soil Sed. 2012, 12, 1269-1279. ACS Paragon Plus Environment

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(76) Hughes, C. E.; Williams, P. A.; Harris, K. D. M. Angew.Chem. Int. Ed. 2014, 53, 8939-8943. (77) Lam, B.; Diamond, M. L.; Simpson, A. J.; Makar, P. A.; Truong, J.; Hernandez-Martinez, N. A. Atmos. Environ. 2005, 39, 6578-6586. (78) Simpson, A. J.; Lam, B.; Diamond, M. L.; Donaldson, D. J.; Lefebvre, B. A.; Moser, A. Q.; Williams, A. J.; Larin, N. I.; Kvasha, M. P. Chemosphere 2006, 63, 142-152. (79) Silva, L. M. A.; Filho, E. G. A.; Simpson, A. J.; Monteiro, M. R.; Venancio, T. Fuel 2016, 166, 436-445. (80) Wheeler, H. L.; Soong, R.; Courtier-Murias, D.; Botana, A.; Fortier-Mcgill, B.; Maas, W. E.; Fey, M.; Hutchins, H.; Krishnamurthy, S.; Kumar, R.; Monette, M.; Stronks, H. J.; Campbell, M. M.; Simpson, A. Magn. Reson. Chem. 2015, 53, 735-744. (81) Lam, L.; Soong, R.; Sutrisno, A.; de Visser, R.; Simpson, M. J.; Wheeler, H. L.; Campbell, M.; Maas, W. E.; Fey, M.; Gorissen, A.; Hutchins, H.; Andrew, B.; Struppe, J.; Krishnamurthy, S.; Kumar, R.; Monette, M.; Stronks, H. J.; Hume, A.; Simpson, A. J. J. Agric. Food. Chem. 2014, 62, 107-115. (82) Farooq, H.; Courtier-Murias, D.; Simspon, M. J.; Maas, W. E.; Fey, M.; Andrew, B.; Struppe, J.; Hutchins, H.; Krishnamurthy, S.; Kumar, R.; Monette, M.; Stronks, H. J.; Simpson, A. J. Environ. Chem. 2015, 12, 227-235. (83) Masoom, H.; Courtier-Murias, D.; Farooq, H.; Soong, R.; Kelleher, B. P.; Zhang, C.; Maas, W. E.; Fey, M.; Kumar, R.; Monette, M.; Stronks, H. J.; Simpson, M. J.; Simpson, A. J. Environ. Sci. Technol. 2016, 50, 1670-1680. (84) Mazzei, P.; Piccolo, A. Magn. Reson. Chem. 2015, 53, 667-678. (85) Guthrie, E. A.; Bortiatynski, J. M.; Van Heemst, J. D. H.; Richman, J. E.; Hardy, K. S.; Kovach, E. M.; Hatcher, P. G. Environ. Sci. Technol. 1999, 33, 119-125. (86) Hatcher, P. G.; Bortiatynski, J. M.; Minard, R. D.; Dec, J.; Bollag, J. M. Environ. Sci. Technol. 1993, 27, 2098-2103. (87) Kohl, S. D.; Toscano, P. J.; Hou, W.; Rice, J. A. Environ. Sci. Technol. 2000, 34, 204-210. (88) Nanny, M. A.; Bortiatynski, J. M.; Hatcher, P. G. Environ. Sci. Technol. 1997, 31, 530-534. (89) Sachleben, J. R.; Chefetz, B.; Deshmukh, A.; Hatcher, P. G. Environ. Sci. Technol. 2004, 38, 43694376. (90) Shirzadi, A.; Simpson, M. J.; Kumar, R.; Baer, A. J.; Xu, Y. P.; Simpson, A. J. Environ. Sci. Technol. 2008, 42, 5514-5520. (91) Shirzadi, A.; Simpson, M. J.; Xu, Y.; Simpson, A. J. Environ. Sci. Technol. 2008, 42, 1084-1090. (92) Thorn, K. A.; Goldenberg, W. S.; Younger, S. J.; Weber, E. J. Acs Sym Ser 1996, 651, 299-326. (93) Thorn, K. A.; Kennedy, K. R. Environ. Sci. Technol. 2002, 36, 3787-3796. (94) Sutrisno, A.; Simpson, A. J. eMagRes 2015, 2, 467–476. (95) Mueller, K. T.; Sanders, R. L.; Washton, N. M. eMagRes 2015, 3, 13–28. (96) Mayer, M.; Meyer, B. Angew. Chem. Int. Ed. 1999, 38, 1784-1788. (97) Mazzei, P.; Piccolo, A. Environ. Sci. Technol. 2012, 46, 5939-5946. (98) Longstaffe, J. G.; Courtier-Murias, D.; Soong, R.; Simpson, M. J.; Maas, W. E.; Fey, M.; Hutchins, H.; Krishnamurthy, S.; Struppe, J.; Alaee, M.; Kumar, R.; Monette, M.; Stronks, H. J.; Simpson, A. J. Environ. Sci. Technol. 2012, 46, 10508-10513. (99) Simpson, A. J.; Courtier-Murias, D.; Longstaffe, J. G.; Masoom, H.; Soong, R.; Lam, L.; Sutrisno, A.; Farooq, H.; Simpson, M. J.; Maas, W. E.; Fey, M.; Andrew, B.; Struppe, J.; Hutchins, H.; Krishnamurthy, S.; Kumar, R.; Monette, M.; Stronks, H. J. eMagRes 2015, 2, 399–414. (100) Nicholson, J. K.; Lindon, J. C.; Holmes, E. Xenobiotica FIELD Full Journal Title:Xenobiotica 1999, 29, 1181-1189. (101) Viant, M. Metabolomics 2009, 5, 1. (102) Viant, M. R. Methods in molecular biology (Clifton, N.J.) 2008, 410, 137. (103) Bundy, J.; Davey, M.; Viant, M. Metabolomics 2009, 5, 3. (104) Lankadurai, B. P.; Nagato, E. G.; Simpson, M. J. Environ. Rev. 2013, 21, 180-205. ACS Paragon Plus Environment

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Page 33 Table271.ofCommonly used

Description

Comments

Provides highest resolution but only for soluble components2,9. The approach is fully quantitative if collected under appropriate conditions9. For samples with low salt a cryogenically cooled probe can increase signal to noise. Both helium and nitrogen cooled versions are available. For samples with high salt content using small a diameter room temperature probe (1 mm or 1.7 mm) instead of a cryoprobe is a good option, as smaller coils are more salt tolerant. Important for environmental analysis as it provides an overview of the types of carbon in a sample13-15. This sample is spun at magic angle (54.70 to the external field) to narrow line shape and high power 1 H decoupling is required to improve carbon lineshape. The technique can be quantitative but considerable care must be taken if absolute quantification is required.

Probes range from 1 mm (5 µL) to 10 mm (4 mL) in size with 5mm (600 µL) the most common. For 5 mm cryoprobe, detection limit for individual compounds is the in ng range, but as environmental mixtures can contain a large number of components 50-100 mg is recommended (1-D and 2-D NMR). Smaller probes are ideal when sample is mass limited. For masslimited sample 1-3 mg is possible with a 1.7 mm probe. Only use 1 mm probe when there is

Environmental Nuclear Magnetic Resonance Spectroscopy: An

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