Earth's field NMR
Encyclopedia
Nuclear magnetic resonance
(NMR) in the geomagnetic field is conventionally referred to as Earth's field NMR (EFNMR). EFNMR is a special case of low field NMR
.
When a sample is placed in a constant magnetic field and stimulated (perturbed) by a pulsed or alternating magnetic field, NMR active nuclei resonate at characteristic frequencies. Examples of such nuclei are the isotope
s carbon-13
, and hydrogen-1 also referred to as protons. The resonant frequency of each isotope is directly proportional to the strength of the applied magnetic field, and the magnetogyric or gyromagnetic ratio of that isotope. The signal strength is proportional both to the stimulating magnetic field and the number of nuclei of that isotope in the sample. Thus in the 21 tesla
magnetic field that may be found in high resolution laboratory NMR spectrometers
, protons resonate at 900 MHz. However in the Earth's magnetic field the same nuclei resonate at audio frequencies of around 2 kHz and generate very weak signals.
The location of a nucleus within a complex molecule affects the 'chemical environment' (i.e. the rotating magnetic fields generated by the other nuclei) experienced by the nucleus. Thus different hydrocarbon
molecules containing NMR active nuclei in different positions within the molecules produce slightly different patterns of resonant frequencies.
EFNMR signals can be affected by both magnetically noisy laborotory environments and natural variations in the Earth's field, which originally compromised its usefulness. However this disadvantage has been overcome by the introduction of electronic equipment which compensates changes in ambient magnetic fields.
Whereas chemical shift
s are important in NMR, they are insignificant in the Earth's field. The absence of chemical shifts causes features such as spin-spin multiplets (that are separated by high fields) to be superimposed in EFNMR. Instead, EFNMR spectra are dominated by spin-spin coupling (J-coupling
) effects. Software optimised for analysing these spectra can provide useful information about the structure of the molecules in the sample.
The advantages of the Earth's field instruments over conventional (high field strength) instruments include the portability of the equipment giving the ability to analyse substances on-site, and their lower cost. The much lower geomagnetic field strength, that would otherwise result in poor signal-to-noise ratios, is compensated by homogeneity of the Earth's field giving the ability to use much larger samples. Their relatively low cost and simplicity make them good educational tools.
Examples (illustrated) are the TeachSpin and Terranova MRI instruments.
Although those commercial EFNMR spectrometers and MRI instruments aimed at universities etc. are necessarily sophisticated and are too costly for most hobbyists, internet search engines find data and designs for basic proton precession magnetometers which claim to be within the capability of reasonably competent electronic hobbyists or undergraduate students to build from readily available components costing no more than a few tens of US dollars.
(FID) is the magnetic resonance due to Larmor precession
that results from the stimulation of nuclei by means of either a pulsed dc magnetic field or a pulsed resonant frequency (rf) magnetic field, somewhat analogous respectively to the effects of plucking or bowing a stringed instrument. Whereas a pulsed rf field is usual in conventional (high field) NMR spectrometers, the pulsed dc polarising field method of stimulating FID is usual in EFNMR spectrometers and PPMs.
EFNMR equipment typically incorporates several coils, for stimulating the samples and for sensing the resulting NMR signals. Signal levels are very low, and specialised electronic amplifiers are required to amplify the EFNMR signals to usable levels. The stronger the polarising magnetic field, the stronger the EFNMR signals and the better the signal-to-noise ratio
s. The main trade-offs are performance versus portability and cost.
Since the FID resonant frequencies of NMR active nuclei are directly proportional to the magnetic field affecting those nuclei, we can use widely available NMR spectroscopy data to analyse suitable substances in the Earth's magnetic field
.
An important feature of EFNMR compared with high-field NMR is that some aspects of molecular structure can be observed more clearly at low fields and low frequencies, whereas other features observable at high fields may not be observable at low fields. This is because:
For more context and explanation of NMR principles, please refer to the main articles on NMR
and NMR spectroscopy
. For more detail see proton NMR
and carbon-13 NMR
.
Thus proton
(hydrogen nucleus) EFNMR frequencies are audio frequencies of about 1.3 kHz near the Equator to 2.5 kHz near the Poles, around 2 kHz being typical of mid-latitudes. In terms of the electromagnetic spectrum
EFNMR frequencies are in the VLF
and ULF
radio frequency
bands.
Examples of molecules containing hydrogen nuclei useful in proton EFNMR are water
, hydrocarbons such as natural gas
and petroleum
, and carbohydrates such as occur in plants and animals.
) circuits (see for example Scientific American's, Amateur Scientist, by C. L. Stong April, 1959). Sir Peter Mansfield
's first acquaintance with NMR was an undergraduate project to develop a transistorized EFNMR spectrometer in the late 1950s http://nobelprize.org/nobel_prizes/medicine/laureates/2003/mansfield-autobio.html. Following that introduction to NMR, he went on to invent an MRI scanner, for which he shared a Nobel prize.
(NMR) in the geomagnetic field is conventionally referred to as Earth's field NMR (EFNMR). EFNMR is a special case of low field NMR
.
When a sample is placed in a constant magnetic field and stimulated (perturbed) by a pulsed or alternating magnetic field, NMR active nuclei resonate at characteristic frequencies. Examples of such nuclei are the isotope
s carbon-13
, and hydrogen-1 also referred to as protons. The resonant frequency of each isotope is directly proportional to the strength of the applied magnetic field, and the magnetogyric or gyromagnetic ratio of that isotope. The signal strength is proportional both to the stimulating magnetic field and the number of nuclei of that isotope in the sample. Thus in the 21 tesla
magnetic field that may be found in high resolution laboratory NMR spectrometers
, protons resonate at 900 MHz. However in the Earth's magnetic field the same nuclei resonate at audio frequencies of around 2 kHz and generate very weak signals.
The location of a nucleus within a complex molecule affects the 'chemical environment' (i.e. the rotating magnetic fields generated by the other nuclei) experienced by the nucleus. Thus different hydrocarbon
molecules containing NMR active nuclei in different positions within the molecules produce slightly different patterns of resonant frequencies.
EFNMR signals can be affected by both magnetically noisy laborotory environments and natural variations in the Earth's field, which originally compromised its usefulness. However this disadvantage has been overcome by the introduction of electronic equipment which compensates changes in ambient magnetic fields.
Whereas chemical shift
s are important in NMR, they are insignificant in the Earth's field. The absence of chemical shifts causes features such as spin-spin multiplets (that are separated by high fields) to be superimposed in EFNMR. Instead, EFNMR spectra are dominated by spin-spin coupling (J-coupling
) effects. Software optimised for analysing these spectra can provide useful information about the structure of the molecules in the sample.
The advantages of the Earth's field instruments over conventional (high field strength) instruments include the portability of the equipment giving the ability to analyse substances on-site, and their lower cost. The much lower geomagnetic field strength, that would otherwise result in poor signal-to-noise ratios, is compensated by homogeneity of the Earth's field giving the ability to use much larger samples. Their relatively low cost and simplicity make them good educational tools.
Examples (illustrated) are the TeachSpin and Terranova MRI instruments.
Although those commercial EFNMR spectrometers and MRI instruments aimed at universities etc. are necessarily sophisticated and are too costly for most hobbyists, internet search engines find data and designs for basic proton precession magnetometers which claim to be within the capability of reasonably competent electronic hobbyists or undergraduate students to build from readily available components costing no more than a few tens of US dollars.
(FID) is the magnetic resonance due to Larmor precession
that results from the stimulation of nuclei by means of either a pulsed dc magnetic field or a pulsed resonant frequency (rf) magnetic field, somewhat analogous respectively to the effects of plucking or bowing a stringed instrument. Whereas a pulsed rf field is usual in conventional (high field) NMR spectrometers, the pulsed dc polarising field method of stimulating FID is usual in EFNMR spectrometers and PPMs.
EFNMR equipment typically incorporates several coils, for stimulating the samples and for sensing the resulting NMR signals. Signal levels are very low, and specialised electronic amplifiers are required to amplify the EFNMR signals to usable levels. The stronger the polarising magnetic field, the stronger the EFNMR signals and the better the signal-to-noise ratio
s. The main trade-offs are performance versus portability and cost.
Since the FID resonant frequencies of NMR active nuclei are directly proportional to the magnetic field affecting those nuclei, we can use widely available NMR spectroscopy data to analyse suitable substances in the Earth's magnetic field
.
An important feature of EFNMR compared with high-field NMR is that some aspects of molecular structure can be observed more clearly at low fields and low frequencies, whereas other features observable at high fields may not be observable at low fields. This is because:
For more context and explanation of NMR principles, please refer to the main articles on NMR
and NMR spectroscopy
. For more detail see proton NMR
and carbon-13 NMR
.
Thus proton
(hydrogen nucleus) EFNMR frequencies are audio frequencies of about 1.3 kHz near the Equator to 2.5 kHz near the Poles, around 2 kHz being typical of mid-latitudes. In terms of the electromagnetic spectrum
EFNMR frequencies are in the VLF
and ULF
radio frequency
bands.
Examples of molecules containing hydrogen nuclei useful in proton EFNMR are water
, hydrocarbons such as natural gas
and petroleum
, and carbohydrates such as occur in plants and animals.
) circuits (see for example Scientific American's, Amateur Scientist, by C. L. Stong April, 1959). Sir Peter Mansfield
's first acquaintance with NMR was an undergraduate project to develop a transistorized EFNMR spectrometer in the late 1950s http://nobelprize.org/nobel_prizes/medicine/laureates/2003/mansfield-autobio.html. Following that introduction to NMR, he went on to invent an MRI scanner, for which he shared a Nobel prize.
(NMR) in the geomagnetic field is conventionally referred to as Earth's field NMR (EFNMR). EFNMR is a special case of low field NMR
.
When a sample is placed in a constant magnetic field and stimulated (perturbed) by a pulsed or alternating magnetic field, NMR active nuclei resonate at characteristic frequencies. Examples of such nuclei are the isotope
s carbon-13
, and hydrogen-1 also referred to as protons. The resonant frequency of each isotope is directly proportional to the strength of the applied magnetic field, and the magnetogyric or gyromagnetic ratio of that isotope. The signal strength is proportional both to the stimulating magnetic field and the number of nuclei of that isotope in the sample. Thus in the 21 tesla
magnetic field that may be found in high resolution laboratory NMR spectrometers
, protons resonate at 900 MHz. However in the Earth's magnetic field the same nuclei resonate at audio frequencies of around 2 kHz and generate very weak signals.
The location of a nucleus within a complex molecule affects the 'chemical environment' (i.e. the rotating magnetic fields generated by the other nuclei) experienced by the nucleus. Thus different hydrocarbon
molecules containing NMR active nuclei in different positions within the molecules produce slightly different patterns of resonant frequencies.
EFNMR signals can be affected by both magnetically noisy laborotory environments and natural variations in the Earth's field, which originally compromised its usefulness. However this disadvantage has been overcome by the introduction of electronic equipment which compensates changes in ambient magnetic fields.
Whereas chemical shift
s are important in NMR, they are insignificant in the Earth's field. The absence of chemical shifts causes features such as spin-spin multiplets (that are separated by high fields) to be superimposed in EFNMR. Instead, EFNMR spectra are dominated by spin-spin coupling (J-coupling
) effects. Software optimised for analysing these spectra can provide useful information about the structure of the molecules in the sample.
The advantages of the Earth's field instruments over conventional (high field strength) instruments include the portability of the equipment giving the ability to analyse substances on-site, and their lower cost. The much lower geomagnetic field strength, that would otherwise result in poor signal-to-noise ratios, is compensated by homogeneity of the Earth's field giving the ability to use much larger samples. Their relatively low cost and simplicity make them good educational tools.
Examples (illustrated) are the TeachSpin and Terranova MRI instruments.
Although those commercial EFNMR spectrometers and MRI instruments aimed at universities etc. are necessarily sophisticated and are too costly for most hobbyists, internet search engines find data and designs for basic proton precession magnetometers which claim to be within the capability of reasonably competent electronic hobbyists or undergraduate students to build from readily available components costing no more than a few tens of US dollars.
(FID) is the magnetic resonance due to Larmor precession
that results from the stimulation of nuclei by means of either a pulsed dc magnetic field or a pulsed resonant frequency (rf) magnetic field, somewhat analogous respectively to the effects of plucking or bowing a stringed instrument. Whereas a pulsed rf field is usual in conventional (high field) NMR spectrometers, the pulsed dc polarising field method of stimulating FID is usual in EFNMR spectrometers and PPMs.
EFNMR equipment typically incorporates several coils, for stimulating the samples and for sensing the resulting NMR signals. Signal levels are very low, and specialised electronic amplifiers are required to amplify the EFNMR signals to usable levels. The stronger the polarising magnetic field, the stronger the EFNMR signals and the better the signal-to-noise ratio
s. The main trade-offs are performance versus portability and cost.
Since the FID resonant frequencies of NMR active nuclei are directly proportional to the magnetic field affecting those nuclei, we can use widely available NMR spectroscopy data to analyse suitable substances in the Earth's magnetic field
.
An important feature of EFNMR compared with high-field NMR is that some aspects of molecular structure can be observed more clearly at low fields and low frequencies, whereas other features observable at high fields may not be observable at low fields. This is because:
For more context and explanation of NMR principles, please refer to the main articles on NMR
and NMR spectroscopy
. For more detail see proton NMR
and carbon-13 NMR
.
Thus proton
(hydrogen nucleus) EFNMR frequencies are audio frequencies of about 1.3 kHz near the Equator to 2.5 kHz near the Poles, around 2 kHz being typical of mid-latitudes. In terms of the electromagnetic spectrum
EFNMR frequencies are in the VLF
and ULF
radio frequency
bands.
Examples of molecules containing hydrogen nuclei useful in proton EFNMR are water
, hydrocarbons such as natural gas
and petroleum
, and carbohydrates such as occur in plants and animals.
) circuits (see for example Scientific American's, Amateur Scientist, by C. L. Stong April, 1959). Sir Peter Mansfield
's first acquaintance with NMR was an undergraduate project to develop a transistorized EFNMR spectrometer in the late 1950s http://nobelprize.org/nobel_prizes/medicine/laureates/2003/mansfield-autobio.html. Following that introduction to NMR, he went on to invent an MRI scanner, for which he shared a Nobel prize.
Nuclear magnetic resonance
Nuclear magnetic resonance is a physical phenomenon in which magnetic nuclei in a magnetic field absorb and re-emit electromagnetic radiation...
(NMR) in the geomagnetic field is conventionally referred to as Earth's field NMR (EFNMR). EFNMR is a special case of low field NMR
Low field NMR
Low field NMR is a branch of nuclear magnetic resonance that is either related to Earth's field NMR, or to NMR at a man-made very low magnetic field and shielding from the Earth's magnetic field. With magnetic fields on the order of μT or nT, SQUIDs are typically used as...
.
When a sample is placed in a constant magnetic field and stimulated (perturbed) by a pulsed or alternating magnetic field, NMR active nuclei resonate at characteristic frequencies. Examples of such nuclei are the isotope
Isotope
Isotopes are variants of atoms of a particular chemical element, which have differing numbers of neutrons. Atoms of a particular element by definition must contain the same number of protons but may have a distinct number of neutrons which differs from atom to atom, without changing the designation...
s carbon-13
Carbon-13
Carbon-13 is a natural, stable isotope of carbon and one of the environmental isotopes. It makes up about 1.1% of all natural carbon on Earth.- Detection by mass spectrometry :...
, and hydrogen-1 also referred to as protons. The resonant frequency of each isotope is directly proportional to the strength of the applied magnetic field, and the magnetogyric or gyromagnetic ratio of that isotope. The signal strength is proportional both to the stimulating magnetic field and the number of nuclei of that isotope in the sample. Thus in the 21 tesla
Tesla (unit)
The tesla is the SI derived unit of magnetic field B . One tesla is equal to one weber per square meter, and it was defined in 1960 in honour of the inventor, physicist, and electrical engineer Nikola Tesla...
magnetic field that may be found in high resolution laboratory NMR spectrometers
NMR spectroscopy
Nuclear magnetic resonance spectroscopy, most commonly known as NMR spectroscopy, is a research technique that exploits the magnetic properties of certain atomic nuclei to determine physical and chemical properties of atoms or the molecules in which they are contained...
, protons resonate at 900 MHz. However in the Earth's magnetic field the same nuclei resonate at audio frequencies of around 2 kHz and generate very weak signals.
The location of a nucleus within a complex molecule affects the 'chemical environment' (i.e. the rotating magnetic fields generated by the other nuclei) experienced by the nucleus. Thus different hydrocarbon
Hydrocarbon
In organic chemistry, a hydrocarbon is an organic compound consisting entirely of hydrogen and carbon. Hydrocarbons from which one hydrogen atom has been removed are functional groups, called hydrocarbyls....
molecules containing NMR active nuclei in different positions within the molecules produce slightly different patterns of resonant frequencies.
EFNMR signals can be affected by both magnetically noisy laborotory environments and natural variations in the Earth's field, which originally compromised its usefulness. However this disadvantage has been overcome by the introduction of electronic equipment which compensates changes in ambient magnetic fields.
Whereas chemical shift
Chemical shift
In nuclear magnetic resonance spectroscopy, the chemical shift is the resonant frequency of a nucleus relative to a standard. Often the position and number of chemical shifts are diagnostic of the structure of a molecule...
s are important in NMR, they are insignificant in the Earth's field. The absence of chemical shifts causes features such as spin-spin multiplets (that are separated by high fields) to be superimposed in EFNMR. Instead, EFNMR spectra are dominated by spin-spin coupling (J-coupling
J-coupling
J-coupling is the coupling between two nuclear spins due to the influence of bonding electrons on the magnetic field running between the two nuclei. J-coupling contains information about dihedral angles, which can be estimated using the Karplus equation...
) effects. Software optimised for analysing these spectra can provide useful information about the structure of the molecules in the sample.
Applications
Applications of EFNMR include:- Proton precession magnetometers (PPM) or proton magnetometerProton magnetometerThe proton magnetometer, also known as the proton precession magnetometer , uses the principle of Earth's field nuclear magnetic resonance to measure very small variations in the Earth's magnetic field, allowing ferrous objects on land and at sea to be detected.It is used in land-based archaeology...
s, which produce magnetic resonance in a known sample in the magnetic field to be measured, measure the sample's resonant frequency, then calculate and display the field strength. - EFNMR spectrometers, which use the principle of NMR spectroscopyNMR spectroscopyNuclear magnetic resonance spectroscopy, most commonly known as NMR spectroscopy, is a research technique that exploits the magnetic properties of certain atomic nuclei to determine physical and chemical properties of atoms or the molecules in which they are contained...
to analyse molecular structures in a variety of applications, from investigating the structure of ice crystals in polar ice-fields, to rocks and hydrocarbons on-site. - Earth's field MRI scanners, which use the principle of magnetic resonance imagingMagnetic resonance imagingMagnetic resonance imaging , nuclear magnetic resonance imaging , or magnetic resonance tomography is a medical imaging technique used in radiology to visualize detailed internal structures...
.
The advantages of the Earth's field instruments over conventional (high field strength) instruments include the portability of the equipment giving the ability to analyse substances on-site, and their lower cost. The much lower geomagnetic field strength, that would otherwise result in poor signal-to-noise ratios, is compensated by homogeneity of the Earth's field giving the ability to use much larger samples. Their relatively low cost and simplicity make them good educational tools.
Examples (illustrated) are the TeachSpin and Terranova MRI instruments.
Although those commercial EFNMR spectrometers and MRI instruments aimed at universities etc. are necessarily sophisticated and are too costly for most hobbyists, internet search engines find data and designs for basic proton precession magnetometers which claim to be within the capability of reasonably competent electronic hobbyists or undergraduate students to build from readily available components costing no more than a few tens of US dollars.
Mode of operation
Free Induction DecayFree induction decay
In Fourier Transform NMR, free induction decay is the observable NMR signal generated by non-equilibrium nuclear spin magnetisation precessing about the magnetic field ....
(FID) is the magnetic resonance due to Larmor precession
Larmor precession
In physics, Larmor precession is the precession of the magnetic moments of electrons, atomic nuclei, and atoms about an external magnetic field...
that results from the stimulation of nuclei by means of either a pulsed dc magnetic field or a pulsed resonant frequency (rf) magnetic field, somewhat analogous respectively to the effects of plucking or bowing a stringed instrument. Whereas a pulsed rf field is usual in conventional (high field) NMR spectrometers, the pulsed dc polarising field method of stimulating FID is usual in EFNMR spectrometers and PPMs.
EFNMR equipment typically incorporates several coils, for stimulating the samples and for sensing the resulting NMR signals. Signal levels are very low, and specialised electronic amplifiers are required to amplify the EFNMR signals to usable levels. The stronger the polarising magnetic field, the stronger the EFNMR signals and the better the signal-to-noise ratio
Signal-to-noise ratio
Signal-to-noise ratio is a measure used in science and engineering that compares the level of a desired signal to the level of background noise. It is defined as the ratio of signal power to the noise power. A ratio higher than 1:1 indicates more signal than noise...
s. The main trade-offs are performance versus portability and cost.
Since the FID resonant frequencies of NMR active nuclei are directly proportional to the magnetic field affecting those nuclei, we can use widely available NMR spectroscopy data to analyse suitable substances in the Earth's magnetic field
Earth's magnetic field
Earth's magnetic field is the magnetic field that extends from the Earth's inner core to where it meets the solar wind, a stream of energetic particles emanating from the Sun...
.
An important feature of EFNMR compared with high-field NMR is that some aspects of molecular structure can be observed more clearly at low fields and low frequencies, whereas other features observable at high fields may not be observable at low fields. This is because:
- Electron-mediated heteronuclear J-couplingJ-couplingJ-coupling is the coupling between two nuclear spins due to the influence of bonding electrons on the magnetic field running between the two nuclei. J-coupling contains information about dihedral angles, which can be estimated using the Karplus equation...
s (spin-spin couplings) are field independent, producing clusters of two or more frequencies separated by several Hz, which are more easily observed in a fundamental resonance of about 2 kHz. "Indeed it appears that enhanced resolution is possible due to the long spin relaxation times and high field homogeneity which prevail in EFNMR." - Chemical shiftChemical shiftIn nuclear magnetic resonance spectroscopy, the chemical shift is the resonant frequency of a nucleus relative to a standard. Often the position and number of chemical shifts are diagnostic of the structure of a molecule...
s of several ppmPPM- Culture :*Peter, Paul and Mary, a 1960s folk music trio*Picture Postcard Monthly, a magazine for collectors of postcards*Please Please Me, the first album by The Beatles- Health :*Permanent pacemaker or artificial pacemaker...
are clearly separated in high field NMR spectra, but have separations of only a few milliherz at proton EFNMR frequencies, so are lost in noise due to interfering magnetic fields etc.
For more context and explanation of NMR principles, please refer to the main articles on NMR
NMR
NMR may refer to:Applications of Nuclear Magnetic Resonance:* Nuclear magnetic resonance* NMR spectroscopy* Solid-state nuclear magnetic resonance* Protein nuclear magnetic resonance spectroscopy* Proton NMR* Carbon-13 NMR...
and NMR spectroscopy
NMR spectroscopy
Nuclear magnetic resonance spectroscopy, most commonly known as NMR spectroscopy, is a research technique that exploits the magnetic properties of certain atomic nuclei to determine physical and chemical properties of atoms or the molecules in which they are contained...
. For more detail see proton NMR
Proton NMR
Proton NMR is the application of nuclear magnetic resonance in NMR spectroscopy with respect to hydrogen-1 nuclei within the molecules of a substance, in order to determine the structure of its molecules. In samples where natural hydrogen is used, practically all of the hydrogen consists of the...
and carbon-13 NMR
Carbon-13 NMR
Carbon-13 NMR is the application of nuclear magnetic resonance spectroscopy to carbon. It is analogous to proton NMR and allows the identification of carbon atoms in an organic molecule just as proton NMR identifies hydrogen atoms...
.
Proton EFNMR frequencies
The geomagnetic field strength and hence precession frequency varies with location and time.- Larmor precession frequency = magnetogyric ratioMagnetogyric ratioIn physics, the gyromagnetic ratio of a particle or system is the ratio of its magnetic dipole moment to its angular momentum, and it is often denoted by the symbol γ, gamma...
x magnetic field - Proton magnetogyric ratio = 42.576 Hz/μT (also written 42.576 MHz/T or 0.042576 Hz/nT)
- Earth's magnetic field: 30 μT near Equator to 60 μT near Poles, around 50 μT at mid-latitudes.
Thus proton
Proton
The proton is a subatomic particle with the symbol or and a positive electric charge of 1 elementary charge. One or more protons are present in the nucleus of each atom, along with neutrons. The number of protons in each atom is its atomic number....
(hydrogen nucleus) EFNMR frequencies are audio frequencies of about 1.3 kHz near the Equator to 2.5 kHz near the Poles, around 2 kHz being typical of mid-latitudes. In terms of the electromagnetic spectrum
Electromagnetic spectrum
The electromagnetic spectrum is the range of all possible frequencies of electromagnetic radiation. The "electromagnetic spectrum" of an object is the characteristic distribution of electromagnetic radiation emitted or absorbed by that particular object....
EFNMR frequencies are in the VLF
Very low frequency
225px|thumb|right|A VLF receiving antenna at [[Palmer Station]], Antarctica, operated by Stanford UniversityVery low frequency or VLF refers to radio frequencies in the range of 3 kHz to 30 kHz. Since there is not much bandwidth in this band of the radio spectrum, only the very simplest signals...
and ULF
Ultra low frequency
Ultra-low frequency is the frequency range of electromagnetic waves between 300 hertz and 3 kilohertz. In magnetosphere science and seismology, alternative definitions are usually given, including ranges from 1 mHz to 100 Hz, 1 mHz to 1 Hz, 10 mHz to 10 Hz...
radio frequency
Radio frequency
Radio frequency is a rate of oscillation in the range of about 3 kHz to 300 GHz, which corresponds to the frequency of radio waves, and the alternating currents which carry radio signals...
bands.
Examples of molecules containing hydrogen nuclei useful in proton EFNMR are water
Water
Water is a chemical substance with the chemical formula H2O. A water molecule contains one oxygen and two hydrogen atoms connected by covalent bonds. Water is a liquid at ambient conditions, but it often co-exists on Earth with its solid state, ice, and gaseous state . Water also exists in a...
, hydrocarbons such as natural gas
Natural gas
Natural gas is a naturally occurring gas mixture consisting primarily of methane, typically with 0–20% higher hydrocarbons . It is found associated with other hydrocarbon fuel, in coal beds, as methane clathrates, and is an important fuel source and a major feedstock for fertilizers.Most natural...
and petroleum
Petroleum
Petroleum or crude oil is a naturally occurring, flammable liquid consisting of a complex mixture of hydrocarbons of various molecular weights and other liquid organic compounds, that are found in geologic formations beneath the Earth's surface. Petroleum is recovered mostly through oil drilling...
, and carbohydrates such as occur in plants and animals.
History
Early NMR instruments were developed in the 1950s using thermionic valve (vacuum tubeVacuum tube
In electronics, a vacuum tube, electron tube , or thermionic valve , reduced to simply "tube" or "valve" in everyday parlance, is a device that relies on the flow of electric current through a vacuum...
) circuits (see for example Scientific American's, Amateur Scientist, by C. L. Stong April, 1959). Sir Peter Mansfield
Peter Mansfield
Sir Peter Mansfield, FRS, , is a British physicist who was awarded the 2003 Nobel Prize in Physiology or Medicine for his discoveries concerning magnetic resonance imaging . The Nobel Prize was shared with Paul Lauterbur, who also contributed to the development of MRI...
's first acquaintance with NMR was an undergraduate project to develop a transistorized EFNMR spectrometer in the late 1950s http://nobelprize.org/nobel_prizes/medicine/laureates/2003/mansfield-autobio.html. Following that introduction to NMR, he went on to invent an MRI scanner, for which he shared a Nobel prize.
See also
Nuclear magnetic resonanceNuclear magnetic resonance
Nuclear magnetic resonance is a physical phenomenon in which magnetic nuclei in a magnetic field absorb and re-emit electromagnetic radiation...
(NMR) in the geomagnetic field is conventionally referred to as Earth's field NMR (EFNMR). EFNMR is a special case of low field NMR
Low field NMR
Low field NMR is a branch of nuclear magnetic resonance that is either related to Earth's field NMR, or to NMR at a man-made very low magnetic field and shielding from the Earth's magnetic field. With magnetic fields on the order of μT or nT, SQUIDs are typically used as...
.
When a sample is placed in a constant magnetic field and stimulated (perturbed) by a pulsed or alternating magnetic field, NMR active nuclei resonate at characteristic frequencies. Examples of such nuclei are the isotope
Isotope
Isotopes are variants of atoms of a particular chemical element, which have differing numbers of neutrons. Atoms of a particular element by definition must contain the same number of protons but may have a distinct number of neutrons which differs from atom to atom, without changing the designation...
s carbon-13
Carbon-13
Carbon-13 is a natural, stable isotope of carbon and one of the environmental isotopes. It makes up about 1.1% of all natural carbon on Earth.- Detection by mass spectrometry :...
, and hydrogen-1 also referred to as protons. The resonant frequency of each isotope is directly proportional to the strength of the applied magnetic field, and the magnetogyric or gyromagnetic ratio of that isotope. The signal strength is proportional both to the stimulating magnetic field and the number of nuclei of that isotope in the sample. Thus in the 21 tesla
Tesla (unit)
The tesla is the SI derived unit of magnetic field B . One tesla is equal to one weber per square meter, and it was defined in 1960 in honour of the inventor, physicist, and electrical engineer Nikola Tesla...
magnetic field that may be found in high resolution laboratory NMR spectrometers
NMR spectroscopy
Nuclear magnetic resonance spectroscopy, most commonly known as NMR spectroscopy, is a research technique that exploits the magnetic properties of certain atomic nuclei to determine physical and chemical properties of atoms or the molecules in which they are contained...
, protons resonate at 900 MHz. However in the Earth's magnetic field the same nuclei resonate at audio frequencies of around 2 kHz and generate very weak signals.
The location of a nucleus within a complex molecule affects the 'chemical environment' (i.e. the rotating magnetic fields generated by the other nuclei) experienced by the nucleus. Thus different hydrocarbon
Hydrocarbon
In organic chemistry, a hydrocarbon is an organic compound consisting entirely of hydrogen and carbon. Hydrocarbons from which one hydrogen atom has been removed are functional groups, called hydrocarbyls....
molecules containing NMR active nuclei in different positions within the molecules produce slightly different patterns of resonant frequencies.
EFNMR signals can be affected by both magnetically noisy laborotory environments and natural variations in the Earth's field, which originally compromised its usefulness. However this disadvantage has been overcome by the introduction of electronic equipment which compensates changes in ambient magnetic fields.
Whereas chemical shift
Chemical shift
In nuclear magnetic resonance spectroscopy, the chemical shift is the resonant frequency of a nucleus relative to a standard. Often the position and number of chemical shifts are diagnostic of the structure of a molecule...
s are important in NMR, they are insignificant in the Earth's field. The absence of chemical shifts causes features such as spin-spin multiplets (that are separated by high fields) to be superimposed in EFNMR. Instead, EFNMR spectra are dominated by spin-spin coupling (J-coupling
J-coupling
J-coupling is the coupling between two nuclear spins due to the influence of bonding electrons on the magnetic field running between the two nuclei. J-coupling contains information about dihedral angles, which can be estimated using the Karplus equation...
) effects. Software optimised for analysing these spectra can provide useful information about the structure of the molecules in the sample.
Applications
Applications of EFNMR include:- Proton precession magnetometers (PPM) or proton magnetometerProton magnetometerThe proton magnetometer, also known as the proton precession magnetometer , uses the principle of Earth's field nuclear magnetic resonance to measure very small variations in the Earth's magnetic field, allowing ferrous objects on land and at sea to be detected.It is used in land-based archaeology...
s, which produce magnetic resonance in a known sample in the magnetic field to be measured, measure the sample's resonant frequency, then calculate and display the field strength. - EFNMR spectrometers, which use the principle of NMR spectroscopyNMR spectroscopyNuclear magnetic resonance spectroscopy, most commonly known as NMR spectroscopy, is a research technique that exploits the magnetic properties of certain atomic nuclei to determine physical and chemical properties of atoms or the molecules in which they are contained...
to analyse molecular structures in a variety of applications, from investigating the structure of ice crystals in polar ice-fields, to rocks and hydrocarbons on-site. - Earth's field MRI scanners, which use the principle of magnetic resonance imagingMagnetic resonance imagingMagnetic resonance imaging , nuclear magnetic resonance imaging , or magnetic resonance tomography is a medical imaging technique used in radiology to visualize detailed internal structures...
.
The advantages of the Earth's field instruments over conventional (high field strength) instruments include the portability of the equipment giving the ability to analyse substances on-site, and their lower cost. The much lower geomagnetic field strength, that would otherwise result in poor signal-to-noise ratios, is compensated by homogeneity of the Earth's field giving the ability to use much larger samples. Their relatively low cost and simplicity make them good educational tools.
Examples (illustrated) are the TeachSpin and Terranova MRI instruments.
Although those commercial EFNMR spectrometers and MRI instruments aimed at universities etc. are necessarily sophisticated and are too costly for most hobbyists, internet search engines find data and designs for basic proton precession magnetometers which claim to be within the capability of reasonably competent electronic hobbyists or undergraduate students to build from readily available components costing no more than a few tens of US dollars.
Mode of operation
Free Induction DecayFree induction decay
In Fourier Transform NMR, free induction decay is the observable NMR signal generated by non-equilibrium nuclear spin magnetisation precessing about the magnetic field ....
(FID) is the magnetic resonance due to Larmor precession
Larmor precession
In physics, Larmor precession is the precession of the magnetic moments of electrons, atomic nuclei, and atoms about an external magnetic field...
that results from the stimulation of nuclei by means of either a pulsed dc magnetic field or a pulsed resonant frequency (rf) magnetic field, somewhat analogous respectively to the effects of plucking or bowing a stringed instrument. Whereas a pulsed rf field is usual in conventional (high field) NMR spectrometers, the pulsed dc polarising field method of stimulating FID is usual in EFNMR spectrometers and PPMs.
EFNMR equipment typically incorporates several coils, for stimulating the samples and for sensing the resulting NMR signals. Signal levels are very low, and specialised electronic amplifiers are required to amplify the EFNMR signals to usable levels. The stronger the polarising magnetic field, the stronger the EFNMR signals and the better the signal-to-noise ratio
Signal-to-noise ratio
Signal-to-noise ratio is a measure used in science and engineering that compares the level of a desired signal to the level of background noise. It is defined as the ratio of signal power to the noise power. A ratio higher than 1:1 indicates more signal than noise...
s. The main trade-offs are performance versus portability and cost.
Since the FID resonant frequencies of NMR active nuclei are directly proportional to the magnetic field affecting those nuclei, we can use widely available NMR spectroscopy data to analyse suitable substances in the Earth's magnetic field
Earth's magnetic field
Earth's magnetic field is the magnetic field that extends from the Earth's inner core to where it meets the solar wind, a stream of energetic particles emanating from the Sun...
.
An important feature of EFNMR compared with high-field NMR is that some aspects of molecular structure can be observed more clearly at low fields and low frequencies, whereas other features observable at high fields may not be observable at low fields. This is because:
- Electron-mediated heteronuclear J-couplingJ-couplingJ-coupling is the coupling between two nuclear spins due to the influence of bonding electrons on the magnetic field running between the two nuclei. J-coupling contains information about dihedral angles, which can be estimated using the Karplus equation...
s (spin-spin couplings) are field independent, producing clusters of two or more frequencies separated by several Hz, which are more easily observed in a fundamental resonance of about 2 kHz. "Indeed it appears that enhanced resolution is possible due to the long spin relaxation times and high field homogeneity which prevail in EFNMR." - Chemical shiftChemical shiftIn nuclear magnetic resonance spectroscopy, the chemical shift is the resonant frequency of a nucleus relative to a standard. Often the position and number of chemical shifts are diagnostic of the structure of a molecule...
s of several ppmPPM- Culture :*Peter, Paul and Mary, a 1960s folk music trio*Picture Postcard Monthly, a magazine for collectors of postcards*Please Please Me, the first album by The Beatles- Health :*Permanent pacemaker or artificial pacemaker...
are clearly separated in high field NMR spectra, but have separations of only a few milliherz at proton EFNMR frequencies, so are lost in noise due to interfering magnetic fields etc.
For more context and explanation of NMR principles, please refer to the main articles on NMR
NMR
NMR may refer to:Applications of Nuclear Magnetic Resonance:* Nuclear magnetic resonance* NMR spectroscopy* Solid-state nuclear magnetic resonance* Protein nuclear magnetic resonance spectroscopy* Proton NMR* Carbon-13 NMR...
and NMR spectroscopy
NMR spectroscopy
Nuclear magnetic resonance spectroscopy, most commonly known as NMR spectroscopy, is a research technique that exploits the magnetic properties of certain atomic nuclei to determine physical and chemical properties of atoms or the molecules in which they are contained...
. For more detail see proton NMR
Proton NMR
Proton NMR is the application of nuclear magnetic resonance in NMR spectroscopy with respect to hydrogen-1 nuclei within the molecules of a substance, in order to determine the structure of its molecules. In samples where natural hydrogen is used, practically all of the hydrogen consists of the...
and carbon-13 NMR
Carbon-13 NMR
Carbon-13 NMR is the application of nuclear magnetic resonance spectroscopy to carbon. It is analogous to proton NMR and allows the identification of carbon atoms in an organic molecule just as proton NMR identifies hydrogen atoms...
.
Proton EFNMR frequencies
The geomagnetic field strength and hence precession frequency varies with location and time.- Larmor precession frequency = magnetogyric ratioMagnetogyric ratioIn physics, the gyromagnetic ratio of a particle or system is the ratio of its magnetic dipole moment to its angular momentum, and it is often denoted by the symbol γ, gamma...
x magnetic field - Proton magnetogyric ratio = 42.576 Hz/μT (also written 42.576 MHz/T or 0.042576 Hz/nT)
- Earth's magnetic field: 30 μT near Equator to 60 μT near Poles, around 50 μT at mid-latitudes.
Thus proton
Proton
The proton is a subatomic particle with the symbol or and a positive electric charge of 1 elementary charge. One or more protons are present in the nucleus of each atom, along with neutrons. The number of protons in each atom is its atomic number....
(hydrogen nucleus) EFNMR frequencies are audio frequencies of about 1.3 kHz near the Equator to 2.5 kHz near the Poles, around 2 kHz being typical of mid-latitudes. In terms of the electromagnetic spectrum
Electromagnetic spectrum
The electromagnetic spectrum is the range of all possible frequencies of electromagnetic radiation. The "electromagnetic spectrum" of an object is the characteristic distribution of electromagnetic radiation emitted or absorbed by that particular object....
EFNMR frequencies are in the VLF
Very low frequency
225px|thumb|right|A VLF receiving antenna at [[Palmer Station]], Antarctica, operated by Stanford UniversityVery low frequency or VLF refers to radio frequencies in the range of 3 kHz to 30 kHz. Since there is not much bandwidth in this band of the radio spectrum, only the very simplest signals...
and ULF
Ultra low frequency
Ultra-low frequency is the frequency range of electromagnetic waves between 300 hertz and 3 kilohertz. In magnetosphere science and seismology, alternative definitions are usually given, including ranges from 1 mHz to 100 Hz, 1 mHz to 1 Hz, 10 mHz to 10 Hz...
radio frequency
Radio frequency
Radio frequency is a rate of oscillation in the range of about 3 kHz to 300 GHz, which corresponds to the frequency of radio waves, and the alternating currents which carry radio signals...
bands.
Examples of molecules containing hydrogen nuclei useful in proton EFNMR are water
Water
Water is a chemical substance with the chemical formula H2O. A water molecule contains one oxygen and two hydrogen atoms connected by covalent bonds. Water is a liquid at ambient conditions, but it often co-exists on Earth with its solid state, ice, and gaseous state . Water also exists in a...
, hydrocarbons such as natural gas
Natural gas
Natural gas is a naturally occurring gas mixture consisting primarily of methane, typically with 0–20% higher hydrocarbons . It is found associated with other hydrocarbon fuel, in coal beds, as methane clathrates, and is an important fuel source and a major feedstock for fertilizers.Most natural...
and petroleum
Petroleum
Petroleum or crude oil is a naturally occurring, flammable liquid consisting of a complex mixture of hydrocarbons of various molecular weights and other liquid organic compounds, that are found in geologic formations beneath the Earth's surface. Petroleum is recovered mostly through oil drilling...
, and carbohydrates such as occur in plants and animals.
History
Early NMR instruments were developed in the 1950s using thermionic valve (vacuum tubeVacuum tube
In electronics, a vacuum tube, electron tube , or thermionic valve , reduced to simply "tube" or "valve" in everyday parlance, is a device that relies on the flow of electric current through a vacuum...
) circuits (see for example Scientific American's, Amateur Scientist, by C. L. Stong April, 1959). Sir Peter Mansfield
Peter Mansfield
Sir Peter Mansfield, FRS, , is a British physicist who was awarded the 2003 Nobel Prize in Physiology or Medicine for his discoveries concerning magnetic resonance imaging . The Nobel Prize was shared with Paul Lauterbur, who also contributed to the development of MRI...
's first acquaintance with NMR was an undergraduate project to develop a transistorized EFNMR spectrometer in the late 1950s http://nobelprize.org/nobel_prizes/medicine/laureates/2003/mansfield-autobio.html. Following that introduction to NMR, he went on to invent an MRI scanner, for which he shared a Nobel prize.
See also
Nuclear magnetic resonanceNuclear magnetic resonance
Nuclear magnetic resonance is a physical phenomenon in which magnetic nuclei in a magnetic field absorb and re-emit electromagnetic radiation...
(NMR) in the geomagnetic field is conventionally referred to as Earth's field NMR (EFNMR). EFNMR is a special case of low field NMR
Low field NMR
Low field NMR is a branch of nuclear magnetic resonance that is either related to Earth's field NMR, or to NMR at a man-made very low magnetic field and shielding from the Earth's magnetic field. With magnetic fields on the order of μT or nT, SQUIDs are typically used as...
.
When a sample is placed in a constant magnetic field and stimulated (perturbed) by a pulsed or alternating magnetic field, NMR active nuclei resonate at characteristic frequencies. Examples of such nuclei are the isotope
Isotope
Isotopes are variants of atoms of a particular chemical element, which have differing numbers of neutrons. Atoms of a particular element by definition must contain the same number of protons but may have a distinct number of neutrons which differs from atom to atom, without changing the designation...
s carbon-13
Carbon-13
Carbon-13 is a natural, stable isotope of carbon and one of the environmental isotopes. It makes up about 1.1% of all natural carbon on Earth.- Detection by mass spectrometry :...
, and hydrogen-1 also referred to as protons. The resonant frequency of each isotope is directly proportional to the strength of the applied magnetic field, and the magnetogyric or gyromagnetic ratio of that isotope. The signal strength is proportional both to the stimulating magnetic field and the number of nuclei of that isotope in the sample. Thus in the 21 tesla
Tesla (unit)
The tesla is the SI derived unit of magnetic field B . One tesla is equal to one weber per square meter, and it was defined in 1960 in honour of the inventor, physicist, and electrical engineer Nikola Tesla...
magnetic field that may be found in high resolution laboratory NMR spectrometers
NMR spectroscopy
Nuclear magnetic resonance spectroscopy, most commonly known as NMR spectroscopy, is a research technique that exploits the magnetic properties of certain atomic nuclei to determine physical and chemical properties of atoms or the molecules in which they are contained...
, protons resonate at 900 MHz. However in the Earth's magnetic field the same nuclei resonate at audio frequencies of around 2 kHz and generate very weak signals.
The location of a nucleus within a complex molecule affects the 'chemical environment' (i.e. the rotating magnetic fields generated by the other nuclei) experienced by the nucleus. Thus different hydrocarbon
Hydrocarbon
In organic chemistry, a hydrocarbon is an organic compound consisting entirely of hydrogen and carbon. Hydrocarbons from which one hydrogen atom has been removed are functional groups, called hydrocarbyls....
molecules containing NMR active nuclei in different positions within the molecules produce slightly different patterns of resonant frequencies.
EFNMR signals can be affected by both magnetically noisy laborotory environments and natural variations in the Earth's field, which originally compromised its usefulness. However this disadvantage has been overcome by the introduction of electronic equipment which compensates changes in ambient magnetic fields.
Whereas chemical shift
Chemical shift
In nuclear magnetic resonance spectroscopy, the chemical shift is the resonant frequency of a nucleus relative to a standard. Often the position and number of chemical shifts are diagnostic of the structure of a molecule...
s are important in NMR, they are insignificant in the Earth's field. The absence of chemical shifts causes features such as spin-spin multiplets (that are separated by high fields) to be superimposed in EFNMR. Instead, EFNMR spectra are dominated by spin-spin coupling (J-coupling
J-coupling
J-coupling is the coupling between two nuclear spins due to the influence of bonding electrons on the magnetic field running between the two nuclei. J-coupling contains information about dihedral angles, which can be estimated using the Karplus equation...
) effects. Software optimised for analysing these spectra can provide useful information about the structure of the molecules in the sample.
Applications
Applications of EFNMR include:- Proton precession magnetometers (PPM) or proton magnetometerProton magnetometerThe proton magnetometer, also known as the proton precession magnetometer , uses the principle of Earth's field nuclear magnetic resonance to measure very small variations in the Earth's magnetic field, allowing ferrous objects on land and at sea to be detected.It is used in land-based archaeology...
s, which produce magnetic resonance in a known sample in the magnetic field to be measured, measure the sample's resonant frequency, then calculate and display the field strength. - EFNMR spectrometers, which use the principle of NMR spectroscopyNMR spectroscopyNuclear magnetic resonance spectroscopy, most commonly known as NMR spectroscopy, is a research technique that exploits the magnetic properties of certain atomic nuclei to determine physical and chemical properties of atoms or the molecules in which they are contained...
to analyse molecular structures in a variety of applications, from investigating the structure of ice crystals in polar ice-fields, to rocks and hydrocarbons on-site. - Earth's field MRI scanners, which use the principle of magnetic resonance imagingMagnetic resonance imagingMagnetic resonance imaging , nuclear magnetic resonance imaging , or magnetic resonance tomography is a medical imaging technique used in radiology to visualize detailed internal structures...
.
The advantages of the Earth's field instruments over conventional (high field strength) instruments include the portability of the equipment giving the ability to analyse substances on-site, and their lower cost. The much lower geomagnetic field strength, that would otherwise result in poor signal-to-noise ratios, is compensated by homogeneity of the Earth's field giving the ability to use much larger samples. Their relatively low cost and simplicity make them good educational tools.
Examples (illustrated) are the TeachSpin and Terranova MRI instruments.
Although those commercial EFNMR spectrometers and MRI instruments aimed at universities etc. are necessarily sophisticated and are too costly for most hobbyists, internet search engines find data and designs for basic proton precession magnetometers which claim to be within the capability of reasonably competent electronic hobbyists or undergraduate students to build from readily available components costing no more than a few tens of US dollars.
Mode of operation
Free Induction DecayFree induction decay
In Fourier Transform NMR, free induction decay is the observable NMR signal generated by non-equilibrium nuclear spin magnetisation precessing about the magnetic field ....
(FID) is the magnetic resonance due to Larmor precession
Larmor precession
In physics, Larmor precession is the precession of the magnetic moments of electrons, atomic nuclei, and atoms about an external magnetic field...
that results from the stimulation of nuclei by means of either a pulsed dc magnetic field or a pulsed resonant frequency (rf) magnetic field, somewhat analogous respectively to the effects of plucking or bowing a stringed instrument. Whereas a pulsed rf field is usual in conventional (high field) NMR spectrometers, the pulsed dc polarising field method of stimulating FID is usual in EFNMR spectrometers and PPMs.
EFNMR equipment typically incorporates several coils, for stimulating the samples and for sensing the resulting NMR signals. Signal levels are very low, and specialised electronic amplifiers are required to amplify the EFNMR signals to usable levels. The stronger the polarising magnetic field, the stronger the EFNMR signals and the better the signal-to-noise ratio
Signal-to-noise ratio
Signal-to-noise ratio is a measure used in science and engineering that compares the level of a desired signal to the level of background noise. It is defined as the ratio of signal power to the noise power. A ratio higher than 1:1 indicates more signal than noise...
s. The main trade-offs are performance versus portability and cost.
Since the FID resonant frequencies of NMR active nuclei are directly proportional to the magnetic field affecting those nuclei, we can use widely available NMR spectroscopy data to analyse suitable substances in the Earth's magnetic field
Earth's magnetic field
Earth's magnetic field is the magnetic field that extends from the Earth's inner core to where it meets the solar wind, a stream of energetic particles emanating from the Sun...
.
An important feature of EFNMR compared with high-field NMR is that some aspects of molecular structure can be observed more clearly at low fields and low frequencies, whereas other features observable at high fields may not be observable at low fields. This is because:
- Electron-mediated heteronuclear J-couplingJ-couplingJ-coupling is the coupling between two nuclear spins due to the influence of bonding electrons on the magnetic field running between the two nuclei. J-coupling contains information about dihedral angles, which can be estimated using the Karplus equation...
s (spin-spin couplings) are field independent, producing clusters of two or more frequencies separated by several Hz, which are more easily observed in a fundamental resonance of about 2 kHz. "Indeed it appears that enhanced resolution is possible due to the long spin relaxation times and high field homogeneity which prevail in EFNMR." - Chemical shiftChemical shiftIn nuclear magnetic resonance spectroscopy, the chemical shift is the resonant frequency of a nucleus relative to a standard. Often the position and number of chemical shifts are diagnostic of the structure of a molecule...
s of several ppmPPM- Culture :*Peter, Paul and Mary, a 1960s folk music trio*Picture Postcard Monthly, a magazine for collectors of postcards*Please Please Me, the first album by The Beatles- Health :*Permanent pacemaker or artificial pacemaker...
are clearly separated in high field NMR spectra, but have separations of only a few milliherz at proton EFNMR frequencies, so are lost in noise due to interfering magnetic fields etc.
For more context and explanation of NMR principles, please refer to the main articles on NMR
NMR
NMR may refer to:Applications of Nuclear Magnetic Resonance:* Nuclear magnetic resonance* NMR spectroscopy* Solid-state nuclear magnetic resonance* Protein nuclear magnetic resonance spectroscopy* Proton NMR* Carbon-13 NMR...
and NMR spectroscopy
NMR spectroscopy
Nuclear magnetic resonance spectroscopy, most commonly known as NMR spectroscopy, is a research technique that exploits the magnetic properties of certain atomic nuclei to determine physical and chemical properties of atoms or the molecules in which they are contained...
. For more detail see proton NMR
Proton NMR
Proton NMR is the application of nuclear magnetic resonance in NMR spectroscopy with respect to hydrogen-1 nuclei within the molecules of a substance, in order to determine the structure of its molecules. In samples where natural hydrogen is used, practically all of the hydrogen consists of the...
and carbon-13 NMR
Carbon-13 NMR
Carbon-13 NMR is the application of nuclear magnetic resonance spectroscopy to carbon. It is analogous to proton NMR and allows the identification of carbon atoms in an organic molecule just as proton NMR identifies hydrogen atoms...
.
Proton EFNMR frequencies
The geomagnetic field strength and hence precession frequency varies with location and time.- Larmor precession frequency = magnetogyric ratioMagnetogyric ratioIn physics, the gyromagnetic ratio of a particle or system is the ratio of its magnetic dipole moment to its angular momentum, and it is often denoted by the symbol γ, gamma...
x magnetic field - Proton magnetogyric ratio = 42.576 Hz/μT (also written 42.576 MHz/T or 0.042576 Hz/nT)
- Earth's magnetic field: 30 μT near Equator to 60 μT near Poles, around 50 μT at mid-latitudes.
Thus proton
Proton
The proton is a subatomic particle with the symbol or and a positive electric charge of 1 elementary charge. One or more protons are present in the nucleus of each atom, along with neutrons. The number of protons in each atom is its atomic number....
(hydrogen nucleus) EFNMR frequencies are audio frequencies of about 1.3 kHz near the Equator to 2.5 kHz near the Poles, around 2 kHz being typical of mid-latitudes. In terms of the electromagnetic spectrum
Electromagnetic spectrum
The electromagnetic spectrum is the range of all possible frequencies of electromagnetic radiation. The "electromagnetic spectrum" of an object is the characteristic distribution of electromagnetic radiation emitted or absorbed by that particular object....
EFNMR frequencies are in the VLF
Very low frequency
225px|thumb|right|A VLF receiving antenna at [[Palmer Station]], Antarctica, operated by Stanford UniversityVery low frequency or VLF refers to radio frequencies in the range of 3 kHz to 30 kHz. Since there is not much bandwidth in this band of the radio spectrum, only the very simplest signals...
and ULF
Ultra low frequency
Ultra-low frequency is the frequency range of electromagnetic waves between 300 hertz and 3 kilohertz. In magnetosphere science and seismology, alternative definitions are usually given, including ranges from 1 mHz to 100 Hz, 1 mHz to 1 Hz, 10 mHz to 10 Hz...
radio frequency
Radio frequency
Radio frequency is a rate of oscillation in the range of about 3 kHz to 300 GHz, which corresponds to the frequency of radio waves, and the alternating currents which carry radio signals...
bands.
Examples of molecules containing hydrogen nuclei useful in proton EFNMR are water
Water
Water is a chemical substance with the chemical formula H2O. A water molecule contains one oxygen and two hydrogen atoms connected by covalent bonds. Water is a liquid at ambient conditions, but it often co-exists on Earth with its solid state, ice, and gaseous state . Water also exists in a...
, hydrocarbons such as natural gas
Natural gas
Natural gas is a naturally occurring gas mixture consisting primarily of methane, typically with 0–20% higher hydrocarbons . It is found associated with other hydrocarbon fuel, in coal beds, as methane clathrates, and is an important fuel source and a major feedstock for fertilizers.Most natural...
and petroleum
Petroleum
Petroleum or crude oil is a naturally occurring, flammable liquid consisting of a complex mixture of hydrocarbons of various molecular weights and other liquid organic compounds, that are found in geologic formations beneath the Earth's surface. Petroleum is recovered mostly through oil drilling...
, and carbohydrates such as occur in plants and animals.
History
Early NMR instruments were developed in the 1950s using thermionic valve (vacuum tubeVacuum tube
In electronics, a vacuum tube, electron tube , or thermionic valve , reduced to simply "tube" or "valve" in everyday parlance, is a device that relies on the flow of electric current through a vacuum...
) circuits (see for example Scientific American's, Amateur Scientist, by C. L. Stong April, 1959). Sir Peter Mansfield
Peter Mansfield
Sir Peter Mansfield, FRS, , is a British physicist who was awarded the 2003 Nobel Prize in Physiology or Medicine for his discoveries concerning magnetic resonance imaging . The Nobel Prize was shared with Paul Lauterbur, who also contributed to the development of MRI...
's first acquaintance with NMR was an undergraduate project to develop a transistorized EFNMR spectrometer in the late 1950s http://nobelprize.org/nobel_prizes/medicine/laureates/2003/mansfield-autobio.html. Following that introduction to NMR, he went on to invent an MRI scanner, for which he shared a Nobel prize.
See also
- CarbohydrateCarbohydrateA carbohydrate is an organic compound with the empirical formula ; that is, consists only of carbon, hydrogen, and oxygen, with a hydrogen:oxygen atom ratio of 2:1 . However, there are exceptions to this. One common example would be deoxyribose, a component of DNA, which has the empirical...
- CarbonCarbonCarbon is the chemical element with symbol C and atomic number 6. As a member of group 14 on the periodic table, it is nonmetallic and tetravalent—making four electrons available to form covalent chemical bonds...
- Carbon-13Carbon-13Carbon-13 is a natural, stable isotope of carbon and one of the environmental isotopes. It makes up about 1.1% of all natural carbon on Earth.- Detection by mass spectrometry :...
- Carbon-13 NMRCarbon-13 NMRCarbon-13 NMR is the application of nuclear magnetic resonance spectroscopy to carbon. It is analogous to proton NMR and allows the identification of carbon atoms in an organic molecule just as proton NMR identifies hydrogen atoms...
- Rate of change of Earth's magnetic field
- Free induction decayFree induction decayIn Fourier Transform NMR, free induction decay is the observable NMR signal generated by non-equilibrium nuclear spin magnetisation precessing about the magnetic field ....
(FID) - HydrocarbonHydrocarbonIn organic chemistry, a hydrocarbon is an organic compound consisting entirely of hydrogen and carbon. Hydrocarbons from which one hydrogen atom has been removed are functional groups, called hydrocarbyls....
- HydrogenHydrogenHydrogen is the chemical element with atomic number 1. It is represented by the symbol H. With an average atomic weight of , hydrogen is the lightest and most abundant chemical element, constituting roughly 75% of the Universe's chemical elemental mass. Stars in the main sequence are mainly...
- Hydrogen-1
- Larmor precessionLarmor precessionIn physics, Larmor precession is the precession of the magnetic moments of electrons, atomic nuclei, and atoms about an external magnetic field...
- Low field NMRLow field NMRLow field NMR is a branch of nuclear magnetic resonance that is either related to Earth's field NMR, or to NMR at a man-made very low magnetic field and shielding from the Earth's magnetic field. With magnetic fields on the order of μT or nT, SQUIDs are typically used as...
- Magnetogyric or Gyromagnetic ratio
- MagnetometerMagnetometerA magnetometer is a measuring instrument used to measure the strength or direction of a magnetic field either produced in the laboratory or existing in nature...
- MRI
- NMRNMRNMR may refer to:Applications of Nuclear Magnetic Resonance:* Nuclear magnetic resonance* NMR spectroscopy* Solid-state nuclear magnetic resonance* Protein nuclear magnetic resonance spectroscopy* Proton NMR* Carbon-13 NMR...
- NMR spectroscopyNMR spectroscopyNuclear magnetic resonance spectroscopy, most commonly known as NMR spectroscopy, is a research technique that exploits the magnetic properties of certain atomic nuclei to determine physical and chemical properties of atoms or the molecules in which they are contained...
- ProtonProtonThe proton is a subatomic particle with the symbol or and a positive electric charge of 1 elementary charge. One or more protons are present in the nucleus of each atom, along with neutrons. The number of protons in each atom is its atomic number....
- Proton NMRProton NMRProton NMR is the application of nuclear magnetic resonance in NMR spectroscopy with respect to hydrogen-1 nuclei within the molecules of a substance, in order to determine the structure of its molecules. In samples where natural hydrogen is used, practically all of the hydrogen consists of the...
- Zero field NMRZero field NMRZero field NMR is the NMR spectrum analysis of certain substances in an environment carefully screened from magnetic fields. It is useful for studying nuclei with spins greater than 1/2, and for studying molecular dynamics ....
External links
