NMR Spectroscopy

A Versatile Tool for Environmental Research

Myrna J. Simpson, Andre J. Simpson, Myrna J. Simpson, Andre J. Simpson

• Publisher: Wiley
• Year: 2014
• ISBN:
  9781118616437
• Edition: 1
• Topic: Ciencias físicas
• Subtopic: Espectroscopia y análisis de espectro

The challenges faced by environmental scientists today are vast, complex, and multi-faceted. For instance, predicting the fate of an environmental pollutant or understanding ecosystem responses to climate change, necessitate a firm understanding of molecular structure and dynamics of environmental media as well as the components that exist and interact within this media. Furthermore, linking information obtained at the molecular-scale to ecosystem-level processes is a major pursuit of modern environmental research. As such, NMR spectroscopy and its scalability from the molecular-scale to the macroscopic-scale, is facilitating rapid growth in environmental science. In addition, the versatility of NMR spectroscopy has resulted in the development and implementation of different types of NMR techniques to examine the structure of various types of environmental samples, living and non-living, as well as the study of critical environmental processes.

This comprehensive handbook is a collection of chapters that span from methods to how NMR is used in environmental research to gain insight into various ecosystem properties.
It is organized into three parts:

• Part A focuses on methods used in environmental NMR which span from solution-state to magnetic resonance imaging.
• Part B emphasizes how NMR spectroscopy plays an essential role in understanding various types of environmental components and related processes, including different forms of organic matter found in soil, water, and air as well as how NMR is used to probe the fate of water, organic pollutants, and metals in the environment.
• Part C focuses on the growing field of environmental metabolomics which uses NMR as its main discovery platform.

This volume highlights the immense potential of NMR spectroscopy to expand our fundamental understanding of environmental processes and how it will continue to do so well into the future.

About eMagRes Handbooks eMagRes (formerly the Encyclopedia of Magnetic Resonance ) publishes a wide range of online articles on all aspects of magnetic resonance in physics, chemistry, biology and medicine. The existence of this large number of articles, written by experts in various fields, is enabling the publication of a series of eMagRes Handbooks on specific areas of NMR and MRI. The chapters of each of these handbooks will comprise a carefully chosen selection of eMagRes articles. In consultation with the eMagRes Editorial Board, the eMagRes handbooks are coherently planned in advance by specially-selected Editors, and new articles are written to give appropriate complete coverage. The handbooks are intended to be of value and interest to research students, postdoctoral fellows and other researchers learning about the scientific area in question and undertaking relevant experiments, whether in academia or industry.

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Part A

Fundamentals of Environmental NMR

Chapter 1
Environmental NMR: Solution-State Methods

Norbert Hertkorn

Department of Environmental Sciences (DES), Helmholtz Zentrum Muenchen (HMGU), Neuherberg 85758, Germany

1.1 Introduction

1.2 General NMR Characteristics of Nuclei across the Periodic Table

1.3 Classical NMR Observables

1.4 Practical Aspects Related to Sensitivity and Resolution in Complex Mixtures

1.5 General Characteristics of Higher Dimensional NMR Spectra of Environmental Mixtures

1.6 The Nature of Environmental Samples

1.7 NMR Analysis of Functional Groups in Environmental Organic Mixtures

1.8 Examples

1.9 Conclusion and Future Trends

References

1.1 Introduction

Magnetic resonance spectroscopy is concerned with the splitting of magnetic spins of electrons (ESR) and that of atomic nuclei (NMR) in an external magnetic field B₀.^1–7 The splitting of the NMR transition is not solely an intrinsic atomic and molecular property but also depends on the magnitude of an external magnetic field B₀. Here, an increase in B₀ results in NMR sensitivity enhancement and improved spectral resolution.^3–5 The special role of NMR spectroscopy in the molecular-level characterization of complex mixtures and amorphous materials resides in its ability to provide unsurpassed in-depth, isotope-specific information about short-range molecular order.⁷

NMR offers the capability for quantitative and nondestructive determination of chemical environments across the periodic table with a very few exceptions: only the elements Ar and Ce lack any stable, magnetically active isotope with nuclear spin >0. Quantitative relationships between number of spins and area of NMR resonances operate in the absence of differential NMR relaxation.^{1, 2, 5, 6} This key feature of NMR in the de novo analysis of complex systems implies the use of NMR spectroscopy as a quantitative reference for other, complementary analytical methods, in particular, when complex unknowns are to be characterized with molecular precision.^{7, 8} However, low intrinsic overall NMR sensitivity compared with other analytical methods restricts the accessible signal-to-noise (S/N) ratio in NMR spectra and accuracy of signal definition.

When performed properly on any environmental sample, NMR spectroscopy will provide isotope-specific information in unsurpassed detail on the arrangements of chemical bonds, including connectivities, stereochemistry, and spatial proximity as well as meaningful clues about their dynamics and reactivity.^{1–3, 9–11} However, elaborate sample preparation with attentive consideration of the physical processes initiated by the NMR pulse sequence might become essential to obtain meaningful data from polydisperse mixtures. Common environmental samples are mostly complex mixtures of small and large molecules, related by a continuous range of weak to strong interactions. Typically, formation history is poorly constrained, whereas polydispersity and molecular heterogeneity across various size scales is the norm rather than the exception. NMR spectroscopy offers uniquely versatile options to study liquids, gels (see Chapter 5), solids (see Chapter 4), gases, and any combination thereof, a very beneficial prerequisite to study environmental samples in their native state^10–13 (see Chapter 6).

The impressive contributions of, e.g., solid-state (see Chapter 4) and comprehensive multiphase NMR spectroscopy (see Chapter 6) have been addressed in excellent reviews^{10, 11} and are not considered in this account, which is concerned with solution-state NMR spectroscopy in environmental sciences. This focus implies a stronger emphasis on studies of extracts and, hence, environmental sample preparation. The often unavoidable extraction selectivity will become an asset when purposeful decrease in heterogeneity and impurities will improve sensitivity and S/N ratio in NMR spectra, which is of perpetual concern in NMR spectroscopy.

Current scientific exploration of biochemical organic molecular complexity in which clearly resolved patterns (and their alterations) are readily observed appears more attractive to many than investigation of the vastly more complex biogeochemical mixtures. Here, analytical data are subject to far more extensive intrinsic averaging and necessarily produce less resolved signatures.^{7, 14} Here, NMR spectroscopy shows the most unambiguous relationship between NMR observable (NMR with chemical shift, line shape, and couplings) and atomic process (reorientation of nuclei spinning with individual precession frequencies in an external magnetic field B₀). It cannot be overemphasized that NMR spectroscopy alone will provide the most direct evidence on molecular structure of any unknown (amorphous) organic substance and mixture; this degree of immediacy of NMR–structure relationships is not available by any other analytical technique.⁸ This allows one to define the relative quantities and remarkable structural detail of fundamental building blocks.^{15, 16} Here, multinuclear quantitative one-dimensional (1D) NMR spectroscopy provides the key margin for any structural model of a complex unknown environmental sample. The unique capability to generate and analyze data from multiple higher dimensional and multinuclear NMR experiments obtained from a single sample^{1, 2, 8, 15–19}, serves to enhance the reliability of NMR assignments and allows definition of rather extended substructures in environmental organic mixtures.^{8, 18}

The most prominent obstacles to implement the potential intrinsic to NMR in environment...