Nuclear Magnetic Resonance for Environmental Monitoring

Nuclear magnetic resonance (NMR) technology is based on discoveries made in 1946, which have provided the basis for many applications in chemical identification, characterization, and imaging. The earliest use was for NMR spectroscopic studies of the molecular distribution of hydrogen, the nucleus detected by NMR with the greatest sensitivity. The use of NMR imaging was first explored in early 1970s, using fruit and vegetable specimens which provided good models for engineering. These early experiments clearly demonstrated the potential utility of NMR for nondestructive visualization of the internal components of intact specimens. Nuclei which can be detected and studied by NMR include those with unpaired electrons, such as 1H, 2H, 31P, 14N, 15N, 17O, 19F, 27Al, 23Na and 39K. Contemporary NMR units used for either imaging or spectroscopy have three major components: a large magnet, a radiofrequency (rf) coil, and a computer-driven console to control the pulse sequence and collect data. Samples are placed into the center of the magnet, allowing nuclei to become aligned either parallel or antiparallel to the magnetic field while precessing at a defined frequency which is proportional to the magnetic field strength. The nuclear precession is then realigned to a temporary magnetic field which is perpendicular to the primary magnetic field and is induced by the rf coil. Finally the rf coil is turned off, allowing spins to realign with the primary magnetic field and give off energy which is detected as an alternating current in the process. In NMR spectroscopic experiments, the molecular environment of each nucleus induces slight alterations in the magnetic field, which are detected as slight changes in the frequency of precession. This information is used to create spectra showing details of the molecular environment of the nuclei being detected, and information describing the structure of the molecule. With imaging experiments, electromagnetic coils are placed in the bore of the magnet, to create linearly defined alterations in the magnetic field. As with spectroscopic experiments, the specimen is placed in the center of the stationary magnet. Nuclei are realigned by pulses from an rf coil, and the energy given off is detected as an alternating current. With imaging, changes in the frequency of precession provide information describing the spatial location of the specimen within the bore, and allow images of the specimen volume to be acquired without disruption to the specimen. Use of electromagnets which create a linear magnetic field allows the precise position of the specimen and its morphology to be determined. Contrast within the images is determined both by the content of detected nuclei (usually 1H within water), and by the interactions between nuclei and their surroundings. The advantage of analysis by NMR spectroscopy and imaging is that the samples can be studied without disruption to the specimen. Chemical analysis can even be done in vivo or in an undisturbed setting. Similarly, imaging can be done on live, intact specimens, the only requirement being that they remain immobile for some period of time. The limitation of these techniques is that only specific nuclei can be detected, with most imaging experiments primarily limited to water distribution and binding. Specimens must be sufficiently small to fit within the bore of the stationary magnet. The magnet bore size may range from a few millimeters for some spectroscopic experiments to up 100 cm with some imaging units. Additionally, the technique is relatively insensitive compared with other analytical techniques such as immunoassays or fluorescent assays. Nevertheless, NMR spectroscopy and imaging provide a unique and powerful set of tools for environmental assessment. Applications include examination of humic substances in soil, soil chemistry, chemistry of surface waters, aluminum chemistry in situ, pesticide fate when introduced into the environment, water uptake and translocation by plants, and structural studies of living specimens such as plant roots within a soil volume. New applications are constantly appearing as magnet technology improves, new pulse sequences are developed, and coil design progresses.