Nuclear Magnetic Resonance (Nmr)
The field of nuclear physics depends upon a working knowledge of atoms and atomic structure. In 1937, Isodor Isaac Rabi was the first to observe the effect known as nuclear magnetic resonance (NMR) in atoms of silver. Before this time, scientists knew that atomic nuclei spun, but no reliable process yet existed by which the magnetic strengths of atoms could be measured. Rabi's discovery paved the way for modern nuclear physics.
Rabi had taken a sample of silver and vaporized it. The vapor was shot through an alternating magnetic field; this caused the spinning nuclei within the metal's atoms to "wobble," much like a spinning top, which wobbles as gravity pulls on it. He then introduced a radio signal, varying the frequency until the silver atoms' nuclei all reversed their spin spontaneously; this reversal of spin is called nuclear magnetic resonance, since it occurs when the radio frequency and the "wobble" frequency resonate. The radio frequency at this moment, along with the spectrum derived at the point of NMR, reveals important information about the sample.
The next breakthrough in nuclear science came in 1946. Edward Purcell had spent the years during World War II researching radar technology for the United States government. Using this new expertise, Purcell applied radar theory to Rabi's process. He developed a system by which NMR could be observed without vaporizing the sample, a system both more flexible and more precise than Rabi's original process. Amazingly, another American scientist, Felix Bloch, imagined and designed an NMR system nearly identical to Purcell's. The two introduced their discoveries within weeks of each other, and they shared the 1952 Nobel Prize for Physics for their efforts.
For a while, NMR scanners were considered too accurate, since they also revealed the magnetic fields of electrons surrounding thesample nuclei. An annoyance to physicists, this effect was a great boon to chemists, who used NMR's tremendous analytical capabilities to identify the molecular compositions of unknown substances.
In 1951, Purcell found that his discovery could be used to investigate the macrocosm as well as the microcosm. Known for its ability to identify elements, NMR was used to search the sky for clouds of interstellar hydrogen, since most of this material did not give off light and so was not detectable through telescopes. By looking for hydrogen's radio-frequency "fingerprint," much of the cosmos previously invisible could be mapped out. NMR was also used to look through large clouds of dust that obscured much of space, so that astronomers could see what lay beyond the clouds.
One of the most important uses of NMR spectroscopy is in medicine. In the 1960s, American Raymond Damadian (1936-) realized that a combination of harmless radio waves and magnetic fields would be ideal for use on living tissue. Damadian's studies allowed doctors to examine the spectrum of an NMR scan and, thus, follow the path of a chemical reaction or detect the presence of abnormal cell growth. Later improvements enabled researchers to actually look for specific types of cells in the human body. By knowing the resonance frequency of a cell type, doctors can fire a radio signal of that frequency into a body of spinning nuclei; if any of the nuclei resonate, the presence of that cell type is revealed. This process, which is also called magnetic resonance imaging (MRI), is especially useful in detecting cancer.
In 1966, the Swedish physical chemist Richard R. Ernst made improvements upon existing MRI technology, making the system more sensitive and easier to interpret. Current MRI systems can show the three-dimensional structure of large molecules and is often used to examine soft tissues too thin for X-ray spectroscopy.
In 1998, researchers also announced that MRI is effective for determining the spread of breast cancer to other parts of the body. Using what is called total body echoplanar MRI, physicians can scan the entire human body in as little as 18 seconds. This approach promises to combine and consolidate into one quick process the several tests and days needed for staging cancer. Scientists are also making advances in using NMR to obtain three-dimensional structures of proteins and other large molecules. Determining these structures is an important step in developing treatments for a variety of diseases, including AIDS and cancer. For example, in 1998, researchers at the Ohio State University determined the structure of a protein produced by an important human tumor-suppressor gene. They were able to do this by using NMR to study the protein's structure in a water solution, which mimics the molecule's structure as though it were still inside the cell. Determining the structure of this protein in an environment similar to its natural solution state will further advance efforts for developing a drug that could duplicate the molecule's tumor-suppressor action.
When Ernst was awarded the 1991 Nobel Prize for Chemistry. At the award presentation, the Royal Swedish Academy of Sciences called NMR spectroscopy "perhaps the most important instrumental measuring technique within chemistry."
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