Zero- to ultralow-field (ZULF) NMR is the acquisition of nuclear magnetic resonance (NMR) spectra of chemicals with magnetically active nuclei (spins 1/2 and greater) in an environment carefully screened from magnetic fields (including from the Earth's field). ZULF NMR experiments typically involve the use of passive or active shielding to attenuate Earth’s magnetic field. This is in contrast to the majority of NMR experiments which are performed in high magnetic fields provided by superconducting magnets. In ZULF experiments the dominant interactions are nuclear spin-spin couplings, and the coupling between spins and the external magnetic field is a perturbation to this. There are a number of advantages to operating in this regime: magnetic-susceptibility-induced line broadening is attenuated which reduces inhomogeneous broadening of the spectral lines for samples in heterogeneous environments. Another advantage is that the low frequency signals readily pass through conductive materials such as metals due to the increased skin depth; this is not the case for high-field NMR for which the sample containers are usually made of glass, quartz or ceramic.
High-field NMR employs inductive detectors to pick up the radiofrequency signals, but this would be inefficient in ZULF NMR experiments since the signal frequencies are typically much lower (on the order of hertz to kilohertz). The development of highly sensitive magnetic sensors in the early 2000s including SQUIDs, magnetoresistive sensors, and SERF atomic magnetometers made it possible to detect NMR signals directly in the ZULF regime. Previous ZULF NMR experiments relied on indirect detection where the sample had to be shuttled from the shielded ZULF environment into a high magnetic field for detection with a conventional inductive pick-up coil. One successful implementation was using atomic magnetometers at zero magnetic field working with rubidium vapor cells to detect zero-field NMR.[2][3]
Without a large magnetic field to induce nuclear spin polarization, the nuclear spins must be polarized externally using hyperpolarization techniques. This can be as simple as polarizing the spins in a magnetic field followed by shuttling to the ZULF region for signal acquisition, and alternative chemistry-based hyperpolarization techniques can also be used.
It is sometimes but inaccurately referred to as nuclear quadrupole resonance (NQR).[4]
A sample being investigated using NMR spectroscopy in a zero-field NMR setup.[1]
https://en.wikipedia.org/wiki/Zero_field_NMR
In ZULF experiments, constant magnetic field pulses are used to induce non-stationary states of the spin system. The two main strategies consist of (1) switching of the magnetic field from pseudo-high field to zero (or ultra-low) field, or (2) of ramping down the magnetic field experienced by the spins to zero field in order to convert the Zeeman populations into zero-field eigenstates adiabatically and subsequently in applying a constant magnetic field pulse to induce a coherence between the zero-field eigenstates. In the simple case of a heteronuclear pair of J-coupled spins, both these excitation schemes induce a transition between the singlet and triplet-0 states, which generates a detectable oscillatory magnetic field. More sophisticated pulse sequences have been reported including selective pulses,[5] two-dimensional experiments and decoupling schemes.[6]
https://en.wikipedia.org/wiki/Zero_field_NMR
https://en.wikipedia.org/wiki/Magnetoresistance
https://en.wikipedia.org/wiki/Superconducting_magnet
https://en.wikipedia.org/wiki/Superconducting_wire
https://en.wikipedia.org/wiki/Electromagnetic_shielding#Magnetic_shielding
https://en.wikipedia.org/wiki/Electromagnetic_shielding#Magnetic_shielding
https://en.wikipedia.org/wiki/Permeability_(electromagnetism)
https://en.wikipedia.org/wiki/Saturation_(magnetic)
https://en.wikipedia.org/wiki/Solenoid
https://en.wikipedia.org/wiki/Helmholtz_coil
https://en.wikipedia.org/wiki/Hooke
https://en.wikipedia.org/wiki/Magnetic_trap_(atoms)
https://en.wikipedia.org/wiki/Spin-1/2
https://en.wikipedia.org/wiki/Rubidium
https://en.wikipedia.org/wiki/Nuclear_quadrupole_resonance
https://en.wikipedia.org/wiki/J-coupling
https://en.wikipedia.org/wiki/Selection_rule
https://en.wikipedia.org/wiki/Selection_rule#anchor_forbidden_trans
https://en.wikipedia.org/wiki/Chemical_shift
https://en.wikipedia.org/wiki/Photoemission_spectroscopy
https://en.wikipedia.org/wiki/Angle-resolved_photoemission_spectroscopy
https://en.wikipedia.org/wiki/Ultra-high_vacuum
https://en.wikipedia.org/wiki/Free_molecular_flow
https://en.wikipedia.org/wiki/Adsorption
https://en.wikipedia.org/wiki/Chemisorption
https://en.wikipedia.org/wiki/Heterogeneous_catalysis
https://en.wikipedia.org/wiki/Physisorption
The exact nature of the bonding depends on the details of the species involved, but the adsorption process is generally classified as physisorption (characteristic of weak van der Waals forces) or chemisorption (characteristic of covalent bonding). It may also occur due to electrostatic attraction.[5][6]
https://en.wikipedia.org/wiki/Adsorption
https://en.wikipedia.org/wiki/Surface_tension
https://en.wikipedia.org/wiki/Capillary_action
https://en.wikipedia.org/wiki/Carbon-fiber-reinforced_polymers
https://en.wikipedia.org/wiki/Polyester
https://en.wikipedia.org/wiki/Parahydrogen-induced_polarization
https://en.wikipedia.org/wiki/Hyperpolarization_(physics)#Noble_Gases_and_Alkali_Metals
https://en.wikipedia.org/wiki/Spin_isomers_of_hydrogen
Metastability exchange optical pumping
3He can also be hyperpolarized using metastability exchange optical pumping (MEOP).[citation needed] This process is able to polarize 3He nuclei in the ground state with optically pumped 3He nuclei in the metastable state. MEOP only involves 3He nuclei at room temperature and at low pressure (≈a few mbars). The process of MEOP is very efficient (high polarization rate), however, compression of the gas up to atmospheric pressure is needed.
Dynamic nuclear polarizationCompounds containing NMR-sensitive nuclei, such as 1H, 13C or 15N, can be hyperpolarized using Dynamic nuclear polarization (DNP). DNP is typically performed at low temperature (≈1 K) and high magnetic field (≈3 T). The compound is subsequently thawed and dissolved to yield a room temperature solution containing hyperpolarized nuclei.[41] This liquid can be used in in vivo metabolic imaging[42] for oncology[43] and other applications. The 13C polarization levels in solid compounds can reach up to ≈64% and the losses during dissolution and transfer of the sample for NMR measurements can be minimized to a few percent.[44] Compounds containing NMR-active nuclei can also be hyperpolarized using chemical reactions with para-hydrogen, see Para-Hydrogen Induced Polarization (PHIP).
https://en.wikipedia.org/wiki/Spin_isomers_of_hydrogen
Imaging Applications of SEOP
Steps are also being taken in academia and industry to use this hyperpolarized gas for lung imaging. Once the gas (129Xe) is hyperpolarized through the SEOP process and the alkali metal is removed, a patient (either healthy or suffering from a lung disease), can breathe in the gas and an MRI can be taken.[35] This results in an image of the spaces in the lungs filled with the gas. While the process to get to the point of imaging the patient may require knowledge from scientists very familiar with this technique and the equipment, steps are being taken to eliminate the need for this knowledge so that a hospital technician would be able to produce the hyperpolarized gas using a polarizer.[22][23]
Hyperpolarization machines are currently being used to develop hyperpolarized xenon gas that is used as a visualization agent for the lungs. Xenon-129 is a safe inert noble gas that can be used to quantify lung function. With a single 10-second breath hold, hyperpolarized Xenon-129 is used with MRI to enable 3-dimensional lung imaging.[36] Xenon MRI is being used to monitor patients with pulmonary-vascular, obstructive, or fibrotic lung disease.[37]
Temperature-ramped 129Xe SEOP in an automated high-output batch model hyperpolarized 129Xe can utilize three prime temperature range to put certain conditions: First, 129Xe hyperpolarization rate is superlative high at hot condition. Second, in warm condition the hyperpolarization of 129Xe is unity. Third, at cold condition, the level of hyperpolarization of 129Xe gas at least can get the (at human body's temperature) imaging although during the transferring into the Tedlar bag having poor percentage of 87Rb (less than 5 ng/L dose).[38]
Multiparameter analysis of 87Rb/129Xe SEOP at high xenon pressure and photon flux could be used as 3D-printing and stopped flow contrasting agent in clinical scale.[39] In situ technique, the NMR machine was run for tracking the dynamics of 129Xe polarization as a function of SEOP-cell conditioning with different operating parameters such as data collecting temperature, photon flux, and 129Xe partial pressure to enhance the 129Xe polarization (PXe).[39]
Table 3. 129Xe polarization values for different partial pressures.[39] PXe 95±9% 73±4% 60±2% 41±1% 31±1%
Partial pressure of Xe (torr) 275 515 1000 1500 2000
All of those polarization values of 129Xe has been approved by pushing the hyperpolarized 129Xe gas and all MRI experiment also done at lower magnetic field 47.5 mT.[39] Finally demonstrations indicated that such a high pressure region, polarization of 129Xe gases could be increment even more that the limit that already has been shown. Better SEOP thermal management and optimizing the polarizing kinetics has been further improved with good efficacy.[39]
SEOP on Solids
Not only can SEOP be used to hyperpolarize noble gases, but a more recent development is SEOP on solids. It was first performed in 2007[21] and was used to polarize nuclei in a solid, allowing for nuclei that cannot be polarized by other methods to become hyperpolarized.[21] For example, nuclear polarization of 133Cs in the form of a solid film of CsH can be increased above the Boltzmann limit.[21] This is done by first optically pumping cesium vapor, then transferring the spin polarization to CsH salt, yielding an enhancement of 4.0.[21]
The cells are made as previously described using distillation, then filled with hydrogen gas and heated to allow for the Cs metal to react with the gaseous hydrogen to form the CsH salt.[21] Unreacted hydrogen was removed, and the process was repeated several times to increase the thickness of the CsH film, then pressurized with nitrogen gas.[21] Usually, SEOP experiments are done with the cell centered in Helmholtz or electromagnetic coils, as previously described, but these experiments were done in a superconducting 9.4 T magnet by shining the laser through the magnet and electrically heating the cell.[21] In the future, it may be possible to use this technique to transfer polarization to 6Li or 7Li, leading to even more applications since the T1 is expected to be longer.[21] Since the discovery of this technique that allows solids to be characterized, it has been improved in such a way where polarized light is not necessary to polarize the solid; instead, unpolarized light in a magnetic field can be used.[40] In this method, glass wool is coated with CsH salt, increasing the surface area of the CsH and therefore increasing the chances of spin transfer, yielding 80-fold enhancements at low field (0.56 T).[40] Like in hyperpolarizing CsH film, the cesium metal in this glass wool method was allowed to react with hydrogen gas, but in this case the CsH formed on the glass fibers instead of the glass cell.[40]
https://en.wikipedia.org/wiki/Hyperpolarization_(physics)#Noble_Gases_and_Alkali_Metals
Parahydrogen induced polarization
Molecular hydrogen, H2, contains two different spin isomers, para-hydrogen and ortho-hydrogen, with a ratio of 25:75 at room temperature. Creating para-hydrogen induced polarization (PHIP)[45] means that this ratio is increased, in other words that para-hydrogen is enriched. This can be accomplished by cooling hydrogen gas and then inducing ortho-to-para conversion via an iron-oxide or charcoal catalyst. When performing this procedure at ≈70 K (i.e. with liquid nitrogen), para-hydrogen is enriched from 25% to ca. 50%. When cooling to below 20 K and then inducing the ortho-to-para conversion, close to 100% parahydrogen can be obtained.[citation needed]
For practical applications, the PHIP is most commonly transferred to organic molecules by reacting the hyperpolarized hydrogen with precursor molecules in the presence of a transition metal catalyst. Proton NMR signals with ca. 10,000-fold increased intensity[46] can be obtained compared to NMR signals of the same organic molecule without PHIP and thus only "thermal" polarization at room temperature.
Signal amplification by reversible exchange (SABRE)
Signal amplification by reversible exchange (SABRE) is a technique to hyperpolarize samples without chemically modifying them. Compared to orthohydrogen or organic molecules, a much greater fraction of the hydrogen nuclei in parahydrogen align with an applied magnetic field. In SABRE, a metal center reversibly binds to both the test molecule and a parahydrogen molecule facilitating the target molecule to pick up the polarization of the parahydrogen.[47] This technique can be improved and utilized for a wide range of organic molecules by using an intermediate "relay" molecule like ammonia. The ammonia efficiently binds to the metal center and picks up the polarization from the parahydrogen. The ammonia then transfers it other molecules that don't bind as well to the metal catalyst.[48] This enhanced NMR signal allows the rapid analysis of very small amounts of material.
https://en.wikipedia.org/wiki/Hyperpolarization_(physics)#Noble_Gases_and_Alkali_Metals
https://en.wikipedia.org/wiki/Hyperpolarized_carbon-13_MRI
Hyperpolarization is the nuclear spin polarization of a material in a magnetic field far beyond thermal equilibrium conditions determined by the Boltzmann distribution.[1] It can be applied to gases such as 129Xe and 3He, and small molecules where the polarization levels can be enhanced by a factor of 104-105 above thermal equilibrium levels. Hyperpolarized noble gases are typically used in magnetic resonance imaging (MRI) of the lungs.[2] Hyperpolarized small molecules are typically used for in vivo metabolic imaging. For example, a hyperpolarized metabolite can be injected into animals or patients and the metabolic conversion can be tracked in real-time. Other applications include determining the function of the neutron spin-structures by scattering polarized electrons from a very polarized target (3He), surface interaction studies, and neutron polarizing experiments.[3]
Spin-exchange optical pumping
Introduction
Spin exchange optical pumping (SEOP)[3] is one of several hyperpolarization techniques discussed on this page. This technique specializes in creating hyperpolarized (HP) noble gases, such as 3He, 129Xe, and quadrupolar 131Xe, 83Kr, and 21Ne.[4] Noble gases are required because SEOP is performed in the gas phase, they are chemically inert, non-reactive, chemically stable with respect to alkali metals, and their T1 is long enough to build up polarization. Spin 1/2 noble gases meet all these requirements, and spin 3/2 noble gases do to an extent, although some spin 3/2 do not have a sufficient T1. Each of these noble gases has their own specific application, such as characterizing lung space and tissue via in vivo molecular imaging and functional imaging of lungs, to study changes in metabolism of healthy versus cancer cells,[4] or use as targets for nuclear physics experiments.[5] During this process, circularly polarized infrared laser light, tuned to the appropriate wavelength, is used to excite electrons in an alkali metal, such as caesium or rubidium inside a sealed glass vessel. Infrared light is necessary because it contains the wavelengths necessary to excite the alkali metal electrons, although the wavelength necessary to excite sodium electrons is below this region (Table 1).
Table 1. Wavelengths Required to Excite Alkali Metal Electrons. Alkali Metal Wavelength (nm)
Sodium[6] 590.0
Rubidium[7] 794.7
Cesium[8] 894.0
The angular momentum is transferred from the alkali metal electrons to the noble gas nuclei through collisions. Nitrogen is used as a quenching gas, which prevents the fluorescence of the polarized alkali metal, which would lead to de-polarization of the noble gas. If fluorescence was not quenched, the light emitted during relaxation would be randomly polarized, working against the circularly polarized laser light. While different sizes of glass vessels (also called cells), and therefore different pressures, are used depending on the application, one amagat of total pressure of noble gas and nitrogen is sufficient for SEOP and 0.1 amagat of nitrogen density is needed to quench fluorescence.[3] Great improvements in 129Xe hyperpolarization technology have achieved > 50% level at flow rates of 1–2 L/min, which enables human clinical applications.[9]
History
The discovery of SEOP took decades for all the pieces to fall into place to create a complete technique. First, in 1897, Zeeman's studies of sodium vapor led to the first result of optical pumping.[4][10] The next piece was found in 1950 when Kastler determined a method to electronically spin-polarize rubidium alkali metal vapor using an applied magnetic field and illuminating the vapor with resonant circularly polarized light.[4] Ten years later, Marie-Anne Bouchiat, T. M. Carver, and C. M. Varnum performed spin exchange, in which the electronic spin polarization was transferred to nuclear spins of a noble gas (3He and 129Xe) through gas-phased collisions.[4] Since then, this method has been greatly improved and expanded to use with other noble gases and alkali metals.
Theory
Figure 1. Excitation transitions of a rubidium electron.
To explain the processes of excitation, optical pumping, and spin exchange easier, the most common alkali metal used for this process, rubidium, will be used as an example. Rubidium has an odd number of electrons, with only one in the outermost shell that can be excited under the right conditions. There are two transitions that can occur, one referred to as the D1 line where the transition occurs from the 52S1/2 state to the 52P3/2 state and another referred to the D2 line where the transition occurs from the 52S1/2 to the 52P1/2 state.[7][11] The D1 and D2 transitions can occur if the rubidium atoms are illuminated with light at a wavelength of 794.7 nm and 780 nm, respectively (Figure 1).[7] While it is possible to cause either excitation, laser technology is well-developed for causing the D1 transition to occur. Those lasers are said to be tuned to the D1 wavelength (794.7 nm) of rubidium. 
Figure 2. Effect of applied magnetic field on spin where there is energy splitting in the presence of a magnetic field, B0.
In order to increase the polarization level above thermal equilibrium, the populations of the spin states must be altered. In the absence of magnetic field, the two spin states of a spin I = ½ nuclei are in the same energy level, but in the presence of a magnetic field, the energy levels split into ms = ±1/2 energy levels (Figure 2).[12] Here, ms is the spin angular momentum with possible values of +1/2 (spin up) or -1/2 (spin down), often drawn as vectors pointing up or down, respectively. The difference in population between these two energy levels is what produces an NMR signal. For example, the two electrons in the spin down state cancel two of the electrons in the spin up state, leaving only one spin up nucleus to be detected with NMR. However, the populations of these states can be altered via hyperpolarization, allowing the spin up energy level to be more populated and therefore increase the NMR signal. This is done by first optically pumping alkali metal, then transferring the polarization to a noble gas nucleus to increase the population of the spin up state. 
Figure 3. Transitions that occur when circularly polarized light interacts with the alkali metal atoms.
The absorption of laser light by the alkali metal is the first process in SEOP.[3] Left-circularly polarized light tuned to the D1 wavelength of the alkali metal excites the electrons from the spin down 2S1/2 (ms=-1/2) state into the spin up 2P1/2 (ms=+1/2) state, where collisional mixing then occurs as the noble gas atoms collide with the alkali metal atoms and the ms=-1/2 state is partially populated (Figure 3).[3] Circularly polarized light is necessary at low magnetic fields because it allows only one type of angular momentum to be absorbed, allowing the spins to be polarized.[3] Relaxation then occurs from the excited states (ms=±1/2) to the ground states (ms=±1/2) as the atoms collide with nitrogen, thus quenching any chance of fluorescence and causing the electrons to return to the two ground states in equal populations.[3] Once the spins are depolarized (return to the ms=-1/2 state), they are excited again by the continuous wave laser light and the process repeats itself. In this way, a larger population of electron spins in the ms=+1/2 state accumulates. The polarization of the rubidium, PRb, can be calculated by using the formula below:
P R b = n ↑ − n ↓ n ↑ + n ↓ 
Where n↑ and n↓ and are the number of atoms in the spin up (mS=+1/2) and spin down (mS=-1/2) 2S1/2 states.[13] 
Figure 4. Transfer of polarization via A) binary collisions and B) van der Waals forces.
Next, the optically pumped alkali metal collides with the noble gas, allowing for spin exchange to occur where the alkali metal electron polarization is transferred to the noble gas nuclei (Figure 4). There are two mechanisms in which this can occur. The angular momentum can be transferred via binary collisions (Figure 4A, also called two-body collisions) or while the noble gas, N2 buffer gas, and vapor phase alkali metal are held in close proximity via van der Waals forces (Figure 4B, also called three body collisions).[3] In cases where van der Waals forces are very small compared to binary collisions (such is the case for 3He), the noble gas and alkali metal collide and polarization is transferred from the AM to the noble gas.[3] Binary collisions are also possible for 129Xe. At high pressures, van der Waals forces dominate, but at low pressures binary collisions dominate.[3]
Build Up of Polarization
This cycle of excitation, polarization, depolarization, and re-polarization, etc. takes time before a net polarization is achieved. The buildup of nuclear polarization, PN(t), is given by:
P N ( t ) = ⟨ P A ⟩ ( γ S E γ S E + Γ ) [ 1 − e ( γ S E + Γ ) t ] 
Where ⟨PA⟩ is the alkali metal polarization, γSE is the spin exchange rate, and Γ is the longitudinal relaxation rate of the noble gas.[14] Relaxation of the nuclear polarization can occur via several mechanisms and is written as a sum of these contributions:
Γ = Γ t + Γ p + Γ g + Γ w 
Where Γt, Γp, Γg, and Γw represent the relaxation from the transient Xe2 dimer, the persistent Xe2 dimer, diffusion through gradients in the applied magnetic field, and wall relaxation, respectively.[14] In most cases, the largest contributors to the total relaxation are persistent dimers and wall relaxations.[14] A Xe2 dimer can occur when two Xe atoms collide and are held together via van der Waals forces, and it can be broken when a third atom collides with it.[15] It is similar to Xe-Rb during spin exchange (spin transfer) where they are held in close proximity to each other via van der Waals forces.[15] Wall relaxation is when the hyperpolarized Xe collides with the walls of the cell and is de-polarized due to paramagnetic impurities in the glass.
The buildup time constant, ΓB, can be measured by collecting NMR spectra at time intervals falling within the time it takes to reach steady-state polarization (i.e. the maximum polarization that can be achieved, seen by the maximum signal output). The signal integrals are then plotted over time and can be fit to obtain the buildup time constant. Collecting a buildup curve at several different temperatures and plotting the values as a function of alkali metal vapor density (since vapor density increases with an increase in cell temperature) can be used to determine the spin destruction rate and the per-atom spin exchange rate using:
Γ B = γ ′ × [ A M ] + Γ S D 
Where γ' is the per-atom spin exchange rate, [AM] is the alkali metal vapor density, and ΓSD is the spin destruction rate.[16] This plot should be linear, where γ' is the slope and ΓSD is the y-intercept.
Relaxation: T1
Spin exchange optical pumping can continue indefinitely with continuous illumination, but there are several factors that cause relaxation of polarization and thus a return to the thermal equilibrium populations when illumination is stopped. In order to use hyperpolarized noble gases in applications such as lung imaging, the gas must be transferred from the experimental setup to a patient. As soon as the gas is no longer actively being optically pumped, the degree of hyperpolarization begins to decrease until thermal equilibrium is reached. However, the hyperpolarization must last long enough to transfer the gas to the patient and obtain an image. The longitudinal spin relaxation time, denoted as T1, can be measured easily by collecting NMR spectra as the polarization decreases over time once illumination is stopped. This relaxation rate is governed by several depolarization mechanisms and is written as:
1 T 1 = ( 1 T 1 ) C R + ( 1 T 1 ) M F I + ( 1 T 1 ) O 2 
Where the three contributing terms are for collisional relaxation (CR), magnetic field inhomogeneity (MFI) relaxation, and relaxation caused by the presence of paramagnetic oxygen (O2).[17] The T1 duration could be anywhere from minutes to several hours, depending on how much care is put into lessening the effects of CR, MFI, and O2. The last term has been quantified to be 0.360 s−1 amagat−1,[18] but the first and second terms are hard to quantify since the degree of their contribution to the overall T1 is dependent on how well the experimental setup and cell are optimized and prepared.[18]
Experimental Setup in SEOP
Figure 5. Photo of 2” diameter 10” length optical cells.
In order to perform SEOP, it is first necessary to prepare the optical cell. Optical cells (Figure 5) are designed for the particular system in mind and glass blown using a transparent material, typically pyrex glass (borosilicate). This cell must then be cleaned to eliminate all contaminants, particularly paramagnetic materials which decrease polarization and the T1. The inner surface of the cell is then coated to (a) serve as a protective layer for the glass in order to lessen the chance of corrosion by the alkali metal, and (b) minimize depolarization caused by the collisions of polarized gas molecules with the walls of the cell.[19] Decreasing wall relaxation leads to longer and higher polarization of the noble gas.[19] 
Figure 6. Structure of SurfaSil.
While several coatings have been tested over the years, SurfaSil (Figure 6, now referred to as hydrocarbon soluble siliconizing fluid) is the most common coating used in a ratio of 1:10 SurfaSil: hexane because it provides long T1 values.[19] The thickness of the SurfaSil layer is about 0.3-0.4 μm.[19] Once evenly coated and dried, the cell is then placed in an inert environment and a droplet of alkali metal (≈200 mg) is placed in the cell, which is then dispersed to create an even coating on the walls of the cells. One method for transferring the alkali metal into the cell is by distillation.[20] In the distillation method, the cell is connected to a glass manifold equipped to hold both pressurized gas and vacuum, where an ampoule of alkali metal is connected.[21] The manifold and cell are vacuumed, then the ampoule seal is broken and the alkali metal is moved into the cell using the flame of a gas torch.[21] The cell is then filled with the desired gas mixture of nitrogen and noble gas.[5] Care must be taken not to poison the cell at any stage of cell preparation (expose the cell to atmospheric air).
Several cell sizes and designs have been used over the years. The application desired is what governs the design of the optical pumping cell and is dependent on laser diameter, optimization needs, and clinical use considerations. The specific alkali metal(s) and gases are also chosen based on the desired applications. 
Figure 7. Experimental setup that involves illuminating an optical cell containing alkali metal, a noble gas, and nitrogen gas.
Once the cell is complete, a surface coil (or coils, depending on the desired coil type) is taped to the outside of the cell, which a) allows RF pulses to be produced in order to tip the polarized spins into the detection field (x,y plane) and b) detects the signal produced by the polarized nuclear spins. The cell is placed in an oven which allows for the cell and its contents to be heated so the alkali metal enters the vapor phase, and the cell is centered in a coil system which generates an applied magnetic field (along the z-axis). A laser, tuned to the D1 line (electric-dipole transition[14]) of the alkali metal and with a beam diameter matching the diameter of the optical cell, is then aligned with the optical flats of the cell in such a way where the entirety of the cell is illuminated by laser light to provide the largest polarization possible (Figure 7). The laser can be anywhere between tens of watts to hundreds of watts,[3] where higher the power yields larger polarization but is more costly. In order to further increase polarization, a retro-reflective mirror is placed behind the cell in order to pass the laser light through the cell twice. Additionally, an IR iris is placed behind the mirror, providing information of laser light absorption by the alkali metal atoms. When the laser is illuminating the cell, but the cell is at room temperature, the IR iris is used to measure the percent transmittance of laser light through the cell. As the cell is heated, the rubidium enters the vapor phase and starts to absorb laser light, causing the percent transmittance to decrease. The difference in the IR spectrum between a room temperature spectrum and a spectrum taken while the cell is heated can be used to calculate an estimated rubidium polarization value, PRb.
As SEOP continues to develop and improve, there are several types of NMR coils, ovens, magnetic field generating coils, and lasers that have been and are being used to generate hyperpolarized gases. Generally, the NMR coils are hand made for the specific purpose, either by turning copper wire by hand in the desired shape,[22] or by 3D printing the coil.[23] Commonly, the oven is a forced-air oven, with two faces made of glass for the laser light to pass through the cell, a removable lid, and a hole through which a hot air line is connected, which allows the cell to be heated via conduction.[24] The magnetic field generating coils can be a pair of Helmholtz coils, used to generate the desired magnetic field strength,[24] whose desired field is governed by:
ω = γ B 0 
Where ω is the Larmour frequency, or desired detection frequency, γ is the gyromagnetic ratio of the nuclei of interest, and B0 is the magnetic field required to detect the nuclei at the desired frequency.[25] A set of four electromagnetic coils can also be used (i.e. from Acutran)[22] and other coil designs are being tested.
In the past, laser technology was a limiting factor for SEOP, where only a couple alkali metals could be used due to the lack of, for example, cesium lasers. However, there have been several new developments, including better cesium lasers, higher power, narrower spectral width, etc. which are allowing the reaches of SEOP to increase. Nevertheless, there are several key features required. Ideally, the laser should be continuous wave to ensure the alkali metal and noble gas remains polarized at all times. In order to induce this polarization, the laser light must be circularly polarized in the direction which allows the electrons to become spin polarized. This is done by passing the laser light through a polarizing beam splitter to separate the s and p components, then through a quarter wave plate, which converts the linearly polarized light into circularly polarized light.[17]
Noble Gases and Alkali Metals
SEOP has successfully been used and is fairly well developed for 3He, 129Xe, and 83Kr for biomedical applications.[4] Additionally, several improvements are under way to get enhanced and interpretable imaging of cancer cells in biomedical science.[26] Studies involving hyperpolarization of 131Xe are underway, piquing the interest of physicists. There are also improvements being made to allow not only rubidium to be utilized in the spin transfer, but also cesium. In principle, any alkali metal can be used for SEOP, but rubidium is usually preferred due to its high vapor pressure, allowing experiments to be carried out at relatively low temperatures (80 °C-130 °C), decreasing the chance of damaging the glass cell.[3] Additionally, laser technology for the alkali metal of choice has to exist and be developed enough get substantial polarization. Previously, the lasers available to excite the D1 cesium transition were not well-developed, but they are now becoming more powerful and less expensive. Preliminary studies even show that cesium may provide better results than rubidium, even though rubidium has been the go-to alkali metal of choice for SEOP.
The hyperpolarization method called spin-exchange optical pumping (SEOP) is being used to hyperpolarize noble gases such as Xenon-129 and Helium-3. When an inhaled hyperpolarized gas like 3He or 129Xe is imaged, there is a higher magnetization density of NMR-active molecules in the lung compared to traditional 1H imaging, which improves the MRI images that can be obtained. Unlike proton MRI which reports on anatomical features of lung tissues, XenonMRI reports lung function including gas ventilation, diffusion, and perfusion.[27]
Rationale
Our target is to identify the infection or disease (cancer, for example) anywhere in our body like cerebral, brain, blood, and fluid, and tissues. This infectious cell is called collectively biomarker.[28] According to the World Health Organization (WHO) and collaborating with United Nations and International Labor organization have convincingly defined the Biomarker as “any substance, structure, or process that can be measured in the body or its products and influence or predict the incidence of outcome or disease”. Biomarker has to be quantifiable up-to certain level in biological process in well-being.[28]
One specific example of biomarker is blood cholesterol that is commonly acquainted with us reliable for coronary heart disease; another biomarker is PSA (Prostate-Specific Antigen) and has been contributing to prostate cancer.[28] There are a lot of biomarkers are considering as being cancer: Hepatitis C virus ribonucleic acid (HCV-RNA), International Normalized Ratio (INR), Prothrombin Time (PT), Monoclonal Protein (M protein), Cancer Antigen-125 (CA-125), Human Immunodeficiency Virus -Ribonucleic Acid (HIV RNA), B-type Natriuretic Peptide (BNP).27and Lymphoma cell (Ramos cell lines and Jurkat cell lines) a form of cancer.[29]
Other common biomarkers are breast cancer, Ovarian cancer, Colorectal cancer, Lung cancer and brain tumor.[30]
This disease-causing verdict agent is the biomarker is existing extremely trace amount especially initial state of the disease. Therefore, identifying or getting images of biomarker is tricky and, in few circumstances, uncertain by NMR tech. Hence, we must use the contrasting agent to enhance the images at least to visualize level to Physicians. As molecules of biomarker is less abundant in vivo system. The NMR or MRI experiment provides a very small signal even in some cases, the analyzer can miss the signal peak in data due to the lack in abundance of biomarkers. Therefore, to make sure, to reach the true conclusion about the existence of trouble-causing biomarkers, we need to enhance the probe (contrasting mechanisms) to get the clear peak at the most visible level of peak height as well as the position of the peak in data. If it is possible to gather the acceptable and clearly interpretable data from NMR or MRI experiment by using the contrasting agent, then experts can take a right initial step to recover the patients who already have been suffering from cancer.[28] Among the various technique to get the enhanced data in MRI experiment, SEOP is one of them.
Researchers in SEOP are interested to use the 129Xe.[citation needed] Because 129Xe has a number of favorable facts in NMR Tech. for working as a contrasting agent even over the other novel gases: Inert xenon does not show chemical reaction like other metals and non-metals because Xenon's electronic configuration is fully occupied as well as it is not radioactive also.[citation needed]
To get the solid, liquid state from naturally occurring gaseous state is easy going (figure-8). The solid and liquid state of 129Xe are existing experimentally doable temperature and pressure ranges.[citation needed]
Figure 8. Diagram above shows the highest temperature and pressure at which xenon gas can exist in liquid and gaseous states simultaneously.30
Xenon possesses highly polarizable electron cloud surrounding the nucleus. Therefore, easily prone to be soluble with lipid or organic compounds especially in vivo environment in biological respect.[citation needed] (table-2)
Xenon does not alter the structurally or chemically (similarly other noble gases) when interacting with other molecules.
According to the scientist Ostwald, solubility is defined as the partition coefficient of the gas absorbed to the volume of the absorbing liquid. The solubility of Xenon, SXe(g) = V absorbed amount of Xe(g) /V absorbing liquid at standard temperature and pressure (STP).
Solubility of Xenon in water medium 11% means at 25 °C 11 mL Xenon gas could be absorbed by 100 mL of water.
Table 2. Solubility values for 129Xe in different media according to the Ostwald Law for Solubility of a component.[citation needed] Name of Solvent Compound Temperature (°C) Ostwald Solubility in (v/v)%
Water 25 0.11
Hexane 25 4.8
Benzene 25 3.1
Fluorobenzene 25 3.3
Carbon Disulfide 25 4.2
Water 37 0.08
Saline 37 0.09
Plasma 37 0.10
Erythorcytes (98%) 37 0.20
Human albumin (100% extrapolated) 37 0.15
Blood 37 0.14
Oil 37 1.90
Fat tissue 37 1.30
DMSO 37 0.66
Intralipid (20%) 37 0.40
PFOB (perflubron) 37 1.20
PFOB (90% w/v, estimated) 37 0.62
Xenon atomic size is large and outer shell electrons are far from the nuclei, outermost electron is highly prone to be polarized specially lipid environment. Table 2 shows Xenon solubility in water medium at 37 °C is 8% but fat tissue in vivo environment the solubility value is 130%. Solubility leads the Xenon using in biological system as a contrasting agent.[citation needed]
Solvent effect of xenon is very large for of 129Xenon by the fact of solubility (table 2).[citation needed] Chemical shift value range for Xenon is more than 7500 ppm. However solvent effect is limited range for 1H & 13C (MRI active nuclei) because of low chemical shift value range for 1H is 20 ppm and for 13C is 300 ppm.[citation needed] Therefore, using the 129Xe is preferred.
Figure-9 below, In NMR experimental data, there are different chemical shift values for different tissues in in vivo environment. All peaks are positioned through such a big range of chemical shift values for 129Xe is viable. Because 129Xe has long range up-to 1700ppm chemical shift value range in NMR data.[citation needed] Other important spectral information includes: 
Figure 9. NMR data for Xe-129 biosensor in in vivo biological system.[citation needed] Naturally 129Xe NMR peak has been counted as reference at 0.0ppm.[citation needed]
When 129Xe incorporated and bind with the Cryptophane-A molecule, then the chemical shift value in NMR acquisition shifted to around 70ppm.[citation needed]
If hyperpolarized 129Xe gas is dissolved into the brain, then five NMR spectral peaks can be observed.[31]
Among them top sharp peak at 194.7ppm. In addition, at 189 ppm peak come out from the non-brain tissues.[citation needed]
Another two peaks are still unknown at 191.6 ppm and 197.8 ppm. At 209.5 ppm smaller but broad peak has been found in NMR data when 129Xe was dissolved in the blood stream.[citation needed]
Hyperpolarized 129Xe is very sensitive detector of biomarker (form of cancer in living system).[citation needed]
The nuclear spin polarization of 129Xe or in generally for noble gases we can increase up to fivefold via SEOP technique.[3]
Using SEOP hyperpolarization technique, we can get images of uptake of xenon in the human brain tissue.[32] 
Figure 10. Measurements of the Polarization of 129Xe(g) in presence of low and intermediate magnetic fields. All (A-D) figures are NMR signal amplitude in μV/KHz vs Larmor Frequency in KHz. (A) Enhanced 129Xe(g) NMR signal at 62 kHz Larmor Frequency from the SEOP cell; Xenon(g) has 1545 torr and Nitrogen(g) has 455 torr pressure and NMR data was collected in presence of 5.26mT magnetic field. (B) Reference NMR signal for water Proton Spin (111M), doping with CuSO4. 5H2O(s), 5.0mM and polarization has been created thermally in presence of 1.46 mT magnetic fields (number of scans was 170,000 times). (C) NMR data for Hyperpolarized 129Xe was collected in presence of 47.5mT magnetic fields.(129Xe was 300 torr and N2 was 1700 torr).(D) Reference NMR signal for 13C was collected from 170.0mM CH3COONa(l) in presence of 47.5mT magnetic field.32
(Figure-10) 129Xe(g) shows satisfactory enhancement in polarization during SEOP compared to the thermal enhancement in polarization. This is demonstrated by the experimental data values when NMR spectra are acquired at different magnetic field strengths.[22] A couple of important points from experimental data are: The 129Xe polarization has increased about 144,000-fold in SEOP tech. over thermally enhanced for 1H polarization in NMR experiment. Both experiments that showed this have been done in identical conditions and using same radio frequency during NMR experiment.[22]
A similar value of 140,000-fold signal enhancement for 129Xe hyperpolarization in SEOP compare to the reference thermally enhanced 13C NMR signal is also seen in experimental NMR data. Both data have been collected in identical Larmor frequency and other experimental conditions and same radio frequency during NMR data collection.[22] 
Figure 11. 129Xe(g) MRI studying in presence of high field vs T1(longitudinal Spin Relaxation Time) during the decaying of hyperpolarization of 129Xe(g) in presence of magnetic field different strengths; 3.0 T for blue triangle, approximately 1.5 mT for red circles and approximately 0.0 mT for white squares. Hyperpolarized 129Xe(g) has transferred to kids bags then counted the decay time T1 in presence of different magnetic fields separately. Increasing the magnetic field strength (1.5mT to 3000mT) causing the decay time approximately up to eight-fold increments.
(Figure 11) Longitudinal spin relaxation time (T1) is very sensitive with an increase of magnetic field and hence enhance the NMR signals is noticeable in SEOP in case of 129Xe.[22] As T1 is higher for blue marking conditioning NMR experiment shows more enhanced peak compare to other.[22] For hyperpolarized 129Xe in tedlar bags, the T1 is 38±12 minutes when data collected in presence of 1.5 mT magnetic field. However, satisfactory increment in T1delay time (354±24 minutes) when data was collected in presence of 3000 mT magnetic field.[22]
Use of Rb vs. Cs in SEOP NMR Experiments
In general, we can use the either 87Rb or 133Cs alkali metal atoms with inert nitrogen gas. However, we are using 133Cs atoms with nitrogen to make the spin exchange with 129Xe for number of advantages: 133Cs has natural perfect abundance while rubidium has two (85Rb and 87Rb) isotopes. Abstraction of one isotope separately from these two (85Rb and 87Rb) is difficult compare to collect the 133Cs isotope. Abstraction of 133Cs is convenient.[citation needed]
Optical pumping cell normally is operated at lower temperature to avoid chemically breakdown issue. SEOP is using 133Cs at low temperature and hence it has fewer chemical corrosion with SEOP cell wall glass.[citation needed]
133Cs-129Xe couple have spin exchange rate about 10% that is more compare to the 87Rb-129Xe couple have.[citation needed]
Although 129Xe has a bunch of preferable characteristic applications in NMR technique, 83Kr can also be used since it has a lot of advantages in NMR techniques in different ways than 129Xe. 83Kr stable isotope has spin I=9/2 and has larger Van der Waals size 2.02A0 .[33] It has quadrupolar effect can be diffuse to nearby environment shortly & distinctively (polar to nonpolar media in vivo system).[34]
Chemical composition of materials can influence the longitudinal relaxation of hyperpolarized 83Kr.[34]
The relaxation can distinguish among the hydrophobic and hydrophilic substrate. Although 3He and 129Xe have spin half but they are not quadrupolar active.[34]
However, the 21Ne (I=3/2), 83Kr(I=9/2) and 131Xe (I=3/2) have Quadrupolar moment.34 Quadrupolar interactions make these isotopes having spin relaxation.[34]
Due to this spin relaxation and evolution, these isotopes can be used as contrasting agents to say about the probe can determine the structural feature and chemical compositions of the surfaces for a permeable media.[34]
SEOP can calculate the relaxation of spin T1 by using the equation nonlinear least-squares fitting for 83Kr signal as a function of time as well as experimental number of media flip angle (≈12°) for NMR experimenting radio frequency pulses.[34]
Hyperpolarized 83Kr is being separated from 87Rb gases after spin exchanging in the optical pumping process and then used in variety of in vivo system to get MRI signal. This is the first isotope showed lots of applicability for MRI technique even though has the spin is 9½.[34]
During experiment of canine lung tissue, the using magnet was 9.4 T, media was porous and similar porosity to alveolar dimensions which is disseminated at atmospheric pressure. Spin lattice relaxation was reasonably long enough so it is applicable in vivo system although the oxygen level could be 20%.[34]
As 83Kr contrasting agent is promising to develop pristine in vivo MRI methodology to identify the lung diseases epically those effect have been caused in parenchyma surface due to the surfactant concentration.[34]
Boyed the boundary this particular contrasting agent can work to figure out the size of pour of porous media in materials science.[34]
In addition, this technique can take us about to prepare the surface coating, spatial fluctuations of surfaces. Eventually, never ending the good sign of this contrasting agent like natural abundance (11.5% of 83Kr) makes it easy to get with reasonable price $5/L.[34]
Imaging Applications of SEOP
Steps are also being taken in academia and industry to use this hyperpolarized gas for lung imaging. Once the gas (129Xe) is hyperpolarized through the SEOP process and the alkali metal is removed, a patient (either healthy or suffering from a lung disease), can breathe in the gas and an MRI can be taken.[35] This results in an image of the spaces in the lungs filled with the gas. While the process to get to the point of imaging the patient may require knowledge from scientists very familiar with this technique and the equipment, steps are being taken to eliminate the need for this knowledge so that a hospital technician would be able to produce the hyperpolarized gas using a polarizer.[22][23]
Hyperpolarization machines are currently being used to develop hyperpolarized xenon gas that is used as a visualization agent for the lungs. Xenon-129 is a safe inert noble gas that can be used to quantify lung function. With a single 10-second breath hold, hyperpolarized Xenon-129 is used with MRI to enable 3-dimensional lung imaging.[36] Xenon MRI is being used to monitor patients with pulmonary-vascular, obstructive, or fibrotic lung disease.[37]
Temperature-ramped 129Xe SEOP in an automated high-output batch model hyperpolarized 129Xe can utilize three prime temperature range to put certain conditions: First, 129Xe hyperpolarization rate is superlative high at hot condition. Second, in warm condition the hyperpolarization of 129Xe is unity. Third, at cold condition, the level of hyperpolarization of 129Xe gas at least can get the (at human body's temperature) imaging although during the transferring into the Tedlar bag having poor percentage of 87Rb (less than 5 ng/L dose).[38]
Multiparameter analysis of 87Rb/129Xe SEOP at high xenon pressure and photon flux could be used as 3D-printing and stopped flow contrasting agent in clinical scale.[39] In situ technique, the NMR machine was run for tracking the dynamics of 129Xe polarization as a function of SEOP-cell conditioning with different operating parameters such as data collecting temperature, photon flux, and 129Xe partial pressure to enhance the 129Xe polarization (PXe).[39]
Table 3. 129Xe polarization values for different partial pressures.[39] PXe 95±9% 73±4% 60±2% 41±1% 31±1%
Partial pressure of Xe (torr) 275 515 1000 1500 2000
All of those polarization values of 129Xe has been approved by pushing the hyperpolarized 129Xe gas and all MRI experiment also done at lower magnetic field 47.5 mT.[39] Finally demonstrations indicated that such a high pressure region, polarization of 129Xe gases could be increment even more that the limit that already has been shown. Better SEOP thermal management and optimizing the polarizing kinetics has been further improved with good efficacy.[39]
SEOP on Solids
Not only can SEOP be used to hyperpolarize noble gases, but a more recent development is SEOP on solids. It was first performed in 2007[21] and was used to polarize nuclei in a solid, allowing for nuclei that cannot be polarized by other methods to become hyperpolarized.[21] For example, nuclear polarization of 133Cs in the form of a solid film of CsH can be increased above the Boltzmann limit.[21] This is done by first optically pumping cesium vapor, then transferring the spin polarization to CsH salt, yielding an enhancement of 4.0.[21]
The cells are made as previously described using distillation, then filled with hydrogen gas and heated to allow for the Cs metal to react with the gaseous hydrogen to form the CsH salt.[21] Unreacted hydrogen was removed, and the process was repeated several times to increase the thickness of the CsH film, then pressurized with nitrogen gas.[21] Usually, SEOP experiments are done with the cell centered in Helmholtz or electromagnetic coils, as previously described, but these experiments were done in a superconducting 9.4 T magnet by shining the laser through the magnet and electrically heating the cell.[21] In the future, it may be possible to use this technique to transfer polarization to 6Li or 7Li, leading to even more applications since the T1 is expected to be longer.[21] Since the discovery of this technique that allows solids to be characterized, it has been improved in such a way where polarized light is not necessary to polarize the solid; instead, unpolarized light in a magnetic field can be used.[40] In this method, glass wool is coated with CsH salt, increasing the surface area of the CsH and therefore increasing the chances of spin transfer, yielding 80-fold enhancements at low field (0.56 T).[40] Like in hyperpolarizing CsH film, the cesium metal in this glass wool method was allowed to react with hydrogen gas, but in this case the CsH formed on the glass fibers instead of the glass cell.[40]
Metastability exchange optical pumping
3He can also be hyperpolarized using metastability exchange optical pumping (MEOP).[citation needed] This process is able to polarize 3He nuclei in the ground state with optically pumped 3He nuclei in the metastable state. MEOP only involves 3He nuclei at room temperature and at low pressure (≈a few mbars). The process of MEOP is very efficient (high polarization rate), however, compression of the gas up to atmospheric pressure is needed.
Dynamic nuclear polarization
Compounds containing NMR-sensitive nuclei, such as 1H, 13C or 15N, can be hyperpolarized using Dynamic nuclear polarization (DNP). DNP is typically performed at low temperature (≈1 K) and high magnetic field (≈3 T). The compound is subsequently thawed and dissolved to yield a room temperature solution containing hyperpolarized nuclei.[41] This liquid can be used in in vivo metabolic imaging[42] for oncology[43] and other applications. The 13C polarization levels in solid compounds can reach up to ≈64% and the losses during dissolution and transfer of the sample for NMR measurements can be minimized to a few percent.[44] Compounds containing NMR-active nuclei can also be hyperpolarized using chemical reactions with para-hydrogen, see Para-Hydrogen Induced Polarization (PHIP).
Parahydrogen induced polarization
Molecular hydrogen, H2, contains two different spin isomers, para-hydrogen and ortho-hydrogen, with a ratio of 25:75 at room temperature. Creating para-hydrogen induced polarization (PHIP)[45] means that this ratio is increased, in other words that para-hydrogen is enriched. This can be accomplished by cooling hydrogen gas and then inducing ortho-to-para conversion via an iron-oxide or charcoal catalyst. When performing this procedure at ≈70 K (i.e. with liquid nitrogen), para-hydrogen is enriched from 25% to ca. 50%. When cooling to below 20 K and then inducing the ortho-to-para conversion, close to 100% parahydrogen can be obtained.[citation needed]
For practical applications, the PHIP is most commonly transferred to organic molecules by reacting the hyperpolarized hydrogen with precursor molecules in the presence of a transition metal catalyst. Proton NMR signals with ca. 10,000-fold increased intensity[46] can be obtained compared to NMR signals of the same organic molecule without PHIP and thus only "thermal" polarization at room temperature.
Signal amplification by reversible exchange (SABRE)
Signal amplification by reversible exchange (SABRE) is a technique to hyperpolarize samples without chemically modifying them. Compared to orthohydrogen or organic molecules, a much greater fraction of the hydrogen nuclei in parahydrogen align with an applied magnetic field. In SABRE, a metal center reversibly binds to both the test molecule and a parahydrogen molecule facilitating the target molecule to pick up the polarization of the parahydrogen.[47] This technique can be improved and utilized for a wide range of organic molecules by using an intermediate "relay" molecule like ammonia. The ammonia efficiently binds to the metal center and picks up the polarization from the parahydrogen. The ammonia then transfers it other molecules that don't bind as well to the metal catalyst.[48] This enhanced NMR signal allows the rapid analysis of very small amounts of material.
See alsoDynamic nuclear polarization
Electron paramagnetic resonance
Hyperpolarized carbon-13 MRI
References
Leawoods, Jason C.; Yablonskiy, Dmitriy A.; Saam, Brian; Gierada, David S.; Conradi, Mark S. (2001). "Hyperpolarized 3He Gas Production and MR Imaging of the Lung". Concepts in Magnetic Resonance. 13 (5): 277–293. CiteSeerX 10.1.1.492.8128. doi:10.1002/cmr.1014.
Altes, Talissa; Salerno, Michael (2004). "Hyperpolarized Gas Imaging of the Lung". J Thorac Imaging. 19 (4): 250–258. doi:10.1097/01.rti.0000142837.52729.38. PMID 15502612.
Walker, Thad G.; Happer, William (1997-04-01). "Spin-exchange optical pumping of noble-gas nuclei". Reviews of Modern Physics. 69 (2): 629–642. Bibcode:1997RvMP...69..629W. doi:10.1103/revmodphys.69.629. ISSN 0034-6861.
Nikolaou, Panayiotis; Goodson, Boyd M.; Chekmenev, Eduard Y. (2015-02-06). "Inside Cover: NMR Hyperpolarization Techniques for Biomedicine (Chem. Eur. J. 8/2015)". Chemistry - A European Journal. 21 (8): 3134. doi:10.1002/chem.201590031. ISSN 0947-6539.
Chupp, T. E.; Coulter, K. P. (1985-09-02). "Polarization ofNe21by Spin Exchange with Optically Pumped Rb Vapor". Physical Review Letters. 55 (10): 1074–1077. doi:10.1103/physrevlett.55.1074. ISSN 0031-9007. PMID 10031721.
Steck, D. A., Sodium D Line Data. Oregon Center for Optics and Department of Physics, University of Oregon, 2000.
Steck, D. A., Rubidium 85 D Line Data. Oregon Center for Optics and Department of Physics, University of Oregon, 2013.
Steck, D. A., Cesium D Line Data. Oregon Center for Optics and Department of Physics, University of Oregon, 2010.
F. William Hersman; et al. (2008). "Large Production System for Hyperpolarized 129Xe for Human Lung Imaging Studies". Acad. Radiol. 15 (6): 683–692. doi:10.1016/j.acra.2007.09.020. PMC 2475596. PMID 18486005.
ZEEMAN, P. (1897). "The Effect of Magnetisation on the Nature of Light Emitted by a Substance". Nature. 55 (1424): 347. Bibcode:1897Natur..55..347Z. doi:10.1038/055347a0. ISSN 0028-0836.
Steck, D. A., Rubidium 87 D Line Data. Oregon Center for Optics and Department of Physics, University of Oregon, 2015.
Levitt, M. H., Spin Dynamics. John Wiley & Sons, Ltd.: 2003.
Dreiling, J. M.; Norrgard, E. B.; Tupa, D.; Gay, T. J. (2012-11-26). "Transverse measurements of polarization in optically pumped Rb vapor cells". Physical Review A. 86 (5): 053416. Bibcode:2012PhRvA..86e3416D. doi:10.1103/physreva.86.053416. ISSN 1050-2947.
Anger, B. C.; Schrank, G.; Schoeck, A.; Butler, K. A.; Solum, M. S.; Pugmire, R. J.; Saam, B. (2008-10-08). "Gas-phase spin relaxation ofXe129". Physical Review A. 78 (4): 043406. Bibcode:2008PhRvA..78d3406A. doi:10.1103/physreva.78.043406. ISSN 1050-2947.
Chann, B.; Nelson, I. A.; Anderson, L. W.; Driehuys, B.; Walker, T. G. (2002-02-28). "129Xe−Xe Molecular Spin Relaxation". Physical Review Letters. 88 (11): 113201. Bibcode:2002PhRvL..88k3201C. doi:10.1103/physrevlett.88.113201. ISSN 0031-9007. PMID 11909399.
Whiting, Nicholas; Eschmann, Neil A.; Goodson, Boyd M.; Barlow, Michael J. (2011-05-26). "Xe129-Cs (D1,D2) versusXe129-Rb (D1) spin-exchange optical pumping at high xenon densities using high-power laser diode arrays". Physical Review A. 83 (5): 053428. Bibcode:2011PhRvA..83e3428W. doi:10.1103/physreva.83.053428. ISSN 1050-2947.
Burant, A. Characterizing Hyperpolarized 129Xe Depolarization Mechanisms during Continuous-Flow Spin Exchange Optical Pumping and as a Source of Image Contrast. University of North Carolina, Chapel Hill, 2018.
Hughes-Riley, Theodore; Six, Joseph S.; Lilburn, David M.L.; Stupic, Karl F.; Dorkes, Alan C.; Shaw, Dominick E.; Pavlovskaya, Galina E.; Meersmann, Thomas (2013). "Cryogenics free production of hyperpolarized 129Xe and 83Kr for biomedical MRI applications". Journal of Magnetic Resonance. 237: 23–33. Bibcode:2013JMagR.237...23H. doi:10.1016/j.jmr.2013.09.008. ISSN 1090-7807. PMC 3863958. PMID 24135800.
Breeze, Steven R.; Lang, Stephen; Moudrakovski, Igor; Ratcliffe, Chris I.; Ripmeester, John A.; Santyr, Giles; Simard, Benoit; Zuger, Irene (2000). "Coatings for optical pumping cells and short-term storage of hyperpolarized xenon". Journal of Applied Physics. 87 (11): 8013–8017. Bibcode:2000JAP....87.8013B. doi:10.1063/1.373489. ISSN 0021-8979.
Sharma, M.; Babcock, E.; Andersen, K. H.; Barrón-Palos, L.; Becker, M.; Boag, S.; Chen, W. C.; Chupp, T. E.; Danagoulian, A. (2008-08-20). "Neutron Beam Effects on Spin-Exchange-PolarizedHe3". Physical Review Letters. 101 (8): 083002. arXiv:0802.3169. Bibcode:2008PhRvL.101h3002S. doi:10.1103/physrevlett.101.083002. ISSN 0031-9007. PMID 18764610. S2CID 29364905.
Ishikawa, K.; Patton, B.; Jau, Y. -Y.; Happer, W. (2007-05-04). "Spin Transfer from an Optically Pumped Alkali Vapor to a Solid". Physical Review Letters. 98 (18): 183004. Bibcode:2007PhRvL..98r3004I. doi:10.1103/physrevlett.98.183004. ISSN 0031-9007. PMID 17501572.
Nikolaou, P.; Coffey, A. M.; Walkup, L. L.; Gust, B. M.; Whiting, N.; Newton, H.; Barcus, S.; Muradyan, I.; Dabaghyan, M. (2013-08-14). "Near-unity nuclear polarization with an open-source 129Xe hyperpolarizer for NMR and MRI". Proceedings of the National Academy of Sciences. 110 (35): 14150–14155. Bibcode:2013PNAS..11014150N. doi:10.1073/pnas.1306586110. ISSN 0027-8424. PMC 3761567. PMID 23946420.
Nikolaou, Panayiotis; Coffey, Aaron M.; Walkup, Laura L.; Gust, Brogan M.; LaPierre, Cristen D.; Koehnemann, Edward; Barlow, Michael J.; Rosen, Matthew S.; Goodson, Boyd M. (2014-01-21). "A 3D-Printed High Power Nuclear Spin Polarizer". Journal of the American Chemical Society. 136 (4): 1636–1642. doi:10.1021/ja412093d. ISSN 0002-7863. PMC 4287367. PMID 24400919.
Ghosh, Rajat K.; Romalis, Michael V. (2010-04-26). "Measurement of spin-exchange and relaxation parameters for polarizingNe21with K and Rb". Physical Review A. 81 (4): 043415. Bibcode:2010PhRvA..81d3415G. doi:10.1103/physreva.81.043415. ISSN 1050-2947. S2CID 4288279.
Garg, A., Classical Electromagnetism in a Nutshell. Princeton University Press: 2012.
Chen, W. C.; Gentile, T. R.; Ye, Q.; Walker, T. G.; Babcock, E. (2014-07-07). "On the limits of spin-exchange optical pumping of 3He". Journal of Applied Physics. 116 (1): 014903. Bibcode:2014JAP...116a4903C. doi:10.1063/1.4886583. ISSN 0021-8979. S2CID 119764802.
Khan; Harvey; Birchall; Irwin; Nikolaou; Schrank; Emami; Dummer; Barlow; Goodson; Chekmenev (2021-10-04). "Enabling Clinical Technologies for Hyperpolarized 129 Xenon Magnetic Resonance Imaging and Spectroscopy". Angewandte Chemie International Edition in English. 60 (41): 22126–22147. doi:10.1002/anie.202015200. ISSN 1521-3773. PMC 8478785. PMID 34018297.
Kyle, S.; A, T. J., What is Biomarkers. US National Library of Medicine National Institutes of Health 2011, 1.
Jeong, Keunhong; Netirojjanakul, Chawita; Munch, Henrik K.; Sun, Jinny; Finbloom, Joel A.; Wemmer, David E.; Pines, Alexander; Francis, Matthew B. (2016). "Targeted Molecular Imaging of Cancer Cells Using MS2-Based 129Xe NMR". Bioconjugate Chemistry. 27 (8): 1796–1801. doi:10.1021/acs.bioconjchem.6b00275. ISSN 1043-1802. PMID 27454679.
Chatterjee, Sabarni K; Zetter, Bruce R (2005). "Cancer biomarkers: knowing the present and predicting the future". Future Oncology. 1 (1): 37–50. doi:10.1517/14796694.1.1.37. ISSN 1479-6694. PMID 16555974.
Rao, Madhwesha; Stewart, Neil J.; Norquay, Graham; Griffiths, Paul D.; Wild, Jim M. (2016). "High resolution spectroscopy and chemical shift imaging of hyperpolarized 129Xe dissolved in the human brain in vivo at 1.5 tesla". Magnetic Resonance in Medicine. 75 (6): 2227–2234. doi:10.1002/mrm.26241. ISSN 1522-2594. PMC 4950000. PMID 27080441.
Rao, Madhwesha R.; Stewart, Neil J.; Griffiths, Paul D.; Norquay, Graham; Wild, Jim M. (2017-08-31). "Imaging Human Brain Perfusion with Inhaled Hyperpolarized 129Xe MR Imaging". Radiology. 286 (2): 659–665. doi:10.1148/radiol.2017162881. ISSN 0033-8419. PMID 28858563.
en:Van_der_Waals_radius, oldid 890239244[circular reference]
Pavlovskaya, G. E.; Cleveland, Z. I.; Stupic, K. F.; Basaraba, R. J.; Meersmann, T. (2005-12-12). "Hyperpolarized krypton-83 as a contrast agent for magnetic resonance imaging". Proceedings of the National Academy of Sciences. 102 (51): 18275–18279. Bibcode:2005PNAS..10218275P. doi:10.1073/pnas.0509419102. ISSN 0027-8424. PMC 1317982. PMID 16344474.
Barskiy, Danila A.; Coffey, Aaron M.; Nikolaou, Panayiotis; Mikhaylov, Dmitry M.; Goodson, Boyd M.; Branca, Rosa T.; Lu, George J.; Shapiro, Mikhail G.; Telkki, Ville-Veikko (2016-12-05). "NMR Hyperpolarization Techniques of Gases". Chemistry - A European Journal. 23 (4): 725–751. doi:10.1002/chem.201603884. ISSN 0947-6539. PMC 5462469. PMID 27711999.
Qing, Kun; Tustison, Nicholas J.; Mugler, John P; Mata, Jaime F.; Lin, Zixuan; Zhao, Li; Wang, Da; Feng, Xue; Shin, Ji Young; Callahan, Sean J.; Bergman, Michael P. (March 2019). "Probing changes in lung physiology in COPD using CT, perfusion MRI and hyperpolarized xenon-129 MRI". Academic Radiology. 26 (3): 326–334. doi:10.1016/j.acra.2018.05.025. ISSN 1076-6332. PMC 6361721. PMID 30087065.
Marshall, Helen; Stewart, Neil J.; Chan, Ho-Fung; Rao, Madhwesha; Norquay, Graham; Wild, Jim M. (2021-02-01). "In vivo methods and applications of xenon-129 magnetic resonance". Progress in Nuclear Magnetic Resonance Spectroscopy. 122: 42–62. doi:10.1016/j.pnmrs.2020.11.002. ISSN 0079-6565. PMC 7933823. PMID 33632417.
Nikolaou, Panayiotis; Coffey, Aaron M.; Barlow, Michael J.; Rosen, Matthew S.; Goodson, Boyd M.; Chekmenev, Eduard Y. (2014-07-10). "Temperature-Ramped 129Xe Spin-Exchange Optical Pumping". Analytical Chemistry. 86 (16): 8206–8212. doi:10.1021/ac501537w. ISSN 0003-2700. PMC 4139178. PMID 25008290.
Nikolaou, Panayiotis; Coffey, Aaron M.; Ranta, Kaili; Walkup, Laura L.; Gust, Brogan M.; Barlow, Michael J.; Rosen, Matthew S.; Goodson, Boyd M.; Chekmenev, Eduard Y. (2014-04-25). "Multidimensional Mapping of Spin-Exchange Optical Pumping in Clinical-Scale Batch-Mode 129Xe Hyperpolarizers". The Journal of Physical Chemistry B. 118 (18): 4809–4816. doi:10.1021/jp501493k. ISSN 1520-6106. PMC 4055050. PMID 24731261.
Ishikawa, Kiyoshi (2011-07-07). "Glass-wool study of laser-induced spin currents en route to hyperpolarized Cs salt". Physical Review A. 84 (1): 013403. Bibcode:2011PhRvA..84a3403I. doi:10.1103/physreva.84.013403. ISSN 1050-2947.
Jan H. Ardenkjær-Larsen; Björn Fridlund; Andreas Gram; Georg Hansson; Lennart Hansson; Mathilde H. Lerche; Rolf Servin; Mikkel Thaning; Klaes Golman (2003). "Increase in signal-to-noise ratio of > 10,000 times in liquid-state NMR". Proc. Natl. Acad. Sci. U.S.A. 100 (18): 10158–10163. Bibcode:2003PNAS..10010158A. doi:10.1073/pnas.1733835100. PMC 193532. PMID 12930897.
Klaes Golman; Jan H. Ardenkjær-Larsen; J. Stefan Petersson; Sven MÃ¥nsson; Ib Leunbach (2003). "Molecular imaging with endogenous substances". Proc. Natl. Acad. Sci. U.S.A. 100 (18): 10435–10439. Bibcode:2003PNAS..10010435G. doi:10.1073/pnas.1733836100. PMC 193579. PMID 12930896.
Day SE, Kettunen MI, Gallagher FA, Hu DE, Lerche M, Wolber J, Golman K, Ardenkjaer-Larsen JH, Brindle KM (2007). "Detecting tumor response to treatment using hyperpolarized 13C magnetic resonance imaging and spectroscopy". Nat. Med. 13 (11): 1382–1387. doi:10.1038/nm1650. PMID 17965722. S2CID 11576068.
Haukur Jóhannesson; Sven Macholl; Jan H. Ardenkjær-Larsen (2009). "Dynamic Nuclear Polarization of [1-13C]pyruvic acid at 4.6 tesla". J. Magn. Reson. 197 (2): 167–175. Bibcode:2009JMagR.197..167J. doi:10.1016/j.jmr.2008.12.016. PMID 19162518.
Natterer, Johannes; Bargon, Joachim (1997). "Parahydrogen induced polarization". Progress in Nuclear Magnetic Resonance Spectroscopy. 31 (4): 293–315. doi:10.1016/s0079-6565(97)00007-1.
Duckett, S. B.; Mewis, R. E. (2012). "Application of Parahydrogen Induced Polarization Techniques in NMR Spectroscopy and Imaging". Acc. Chem. Res. 45 (8): 1247–57. doi:10.1021/ar2003094. PMID 22452702.
Eshuis, Nan; Aspers, Ruud L.E.G.; van Weerdenburg, Bram J.A.; Feiters, Martin C.; Rutjes, Floris P.J.T.; Wijmenga, Sybren S.; Tessari, Marco (2016). "Determination of long-range scalar 1 H– 1 H coupling constants responsible for polarization transfer in SABRE". Journal of Magnetic Resonance. 265: 59–66. Bibcode:2016JMagR.265...59E. doi:10.1016/j.jmr.2016.01.012. hdl:2066/161984. ISSN 1090-7807. PMID 26859865.
Iali, Wissam; Rayner, Peter J.; Duckett, Simon B. (2018). "Using para hydrogen to hyperpolarize amines, amides, carboxylic acids, alcohols, phosphates, and carbonates". Science Advances. 4 (1): eaao6250. Bibcode:2018SciA....4O6250I. doi:10.1126/sciadv.aao6250. ISSN 2375-2548. PMC 5756661. PMID 29326984.
External links
Europhysics - "Take a breath of polarized noble gas"
University of Virginia - "Hyperpolarized Gas MR Imaging"
Swiss DNP Initiative - "DNP-NMR 13C Hyperpolarization"
Categories: Nuclear physics
Magnetic resonance imaging
Nuclear magnetic resonance
https://en.wikipedia.org/wiki/Hyperpolarization_(physics)#Spin-Exchange_Optical_Pumping
https://en.wikipedia.org/wiki/Spin_isomers_of_hydrogen
Molecular hydrogen occurs in two isomeric forms, one with its two proton nuclear spins aligned parallel (orthohydrogen), the other with its two proton spins aligned antiparallel (parahydrogen).[1] These two forms are often referred to as spin isomers[2] or as nuclear spin isomers.[3]
Parahydrogen is in a lower energy state than is orthohydrogen. At room temperature and thermal equilibrium, thermal excitation causes hydrogen to consist of approximately 75% orthohydrogen and 25% parahydrogen. When hydrogen is liquified at low temperature, there is a slow spontaneous transition to a predominantly para ratio, with the released energy having implications for storage. Essentially pure parahydrogen form can be obtained at very low temperatures, but it is not possible to obtain a sample containing more than 75% orthohydrogen by heating.
A mixture or 50:50 mixture of ortho- and parahydrogen can be made in the laboratory by passing it over an iron(III) oxide catalyst at liquid nitrogen temperature (77 K)[4] or by storing hydrogen at 77 K for 2–3 hours in the presence of activated charcoal.[5] In the absence of a catalyst, gas phase parahydrogen takes days to relax to normal hydrogen at room temperature while it takes hours to do so in organic solvents.[5]
https://en.wikipedia.org/wiki/Spin_isomers_of_hydrogen
Dynamic nuclear polarization (DNP) results from transferring spin polarization from electrons to nuclei, thereby aligning the nuclear spins to the extent that electron spins are aligned. Note that the alignment of electron spins at a given magnetic field and temperature is described by the Boltzmann distribution under the thermal equilibrium.[1][2][3] It is also possible that those electrons are aligned to a higher degree of order by other preparations of electron spin order such as: chemical reactions (leading to Chemical-Induced DNP, CIDNP), optical pumping and spin injection. DNP is considered one of several techniques for hyperpolarization. DNP can also be induced using unpaired electrons produced by radiation damage in solids.[4][5]
When electron spin polarization deviates from its thermal equilibrium value, polarization transfers between electrons and nuclei can occur spontaneously through electron-nuclear cross relaxation and/or spin-state mixing among electrons and nuclei. For example, the polarization transfer is spontaneous after a homolysis chemical reaction. On the other hand, when the electron spin system is in a thermal equilibrium, the polarization transfer requires continuous microwave irradiation at a frequency close to the corresponding electron paramagnetic resonance (EPR) frequency. In particular, mechanisms for the microwave-driven DNP processes are categorized into the Overhauser effect (OE), the solid-effect (SE), the cross-effect (CE) and thermal-mixing (TM).
The first DNP experiments were performed in the early 1950s at low magnetic fields[6][7] but until recently the technique was of limited applicability for high-frequency, high-field NMR spectroscopy, because of the lack of microwave (or terahertz) sources operating at the appropriate frequency. Today such sources are available as turn-key instruments, making DNP a valuable and indispensable method especially in the field of structure determination by high-resolution solid-state NMR spectroscopy.[8][9][10]
https://en.wikipedia.org/wiki/Dynamic_nuclear_polarization
CIDNP (chemically induced dynamic nuclear polarization), often pronounced like "kidnip", is a nuclear magnetic resonance (NMR) technique that is used to study chemical reactions that involve radicals. It detects the non-Boltzmann (non-thermal) nuclear spin state distribution produced in these reactions as enhanced absorption or emission signals.
CIDNP was discovered in 1967 by Bargon and Fischer, and, independently, by Ward and Lawler.[1][2] Early theories were based on dynamic nuclear polarisation (hence the name) using the Overhauser Effect. The subsequent experiments, however, have found that in many cases DNP fails to explain CIDNP polarization phase. In 1969 an alternative explanation which relies on the nuclear spins affecting the probability of a radical pair recombining or separating.
It is related to chemically induced dynamic electron polarization (CIDEP) insofar as the radical-pair mechanism explains both phenomena.[3]
Concept and experimental set-up
The effect is detected by NMR spectroscopy, usually using 1H NMR spectrum, as enhanced absorption or emission signals ("negative peaks"). The effect arises when unpaired electrons (radicals) are generated during a chemical reaction involving heat or light within the NMR tube. The magnetic field in the spectrometer interacts with the magnetic fields that are caused by the spins of the protons. The two spins of protons produce two slightly different energy levels. In normal conditions, slightly more nuclei, about 10 parts in a million are found in the lower energy level. In contrast, CIDNP produces greatly imbalanced populations, with far greater numbers of spins in upper energy level in some products of the reaction and greater numbers in the lower energy level in other products. The spectrometer uses radio frequencies to detect these differences.
Radical pair mechanism
The radical pair mechanism is currently accepted as the most common cause of CIDNP. This theory was proposed by Closs,[4] and, independently, by Kaptein and Oosterhoff.[5] There are, however, exceptions, and the DNP mechanism was found to be operational, for example, in many fluorine-containing radicals.
The chemical bond is a pair of electrons with opposite spins. Photochemical reactions or heat can cause an electron in the bond to change its spin. The electrons are now unpaired, in what is known as a triplet state, and the bond is broken. The orientation of some of the nuclear spins will favour some unpaired electrons changing their spins and so revert to the normal pairs as chemical bonds. This quantum interaction is known as spin-orbit coupling. Other nuclear spins will exert a different influence on the triplet pairs, giving the radical pairs more time to separate and react with other molecules. Consequently, the products of recombination will have different distributions of nuclear spins from the products produced by separated radicals.
Typical photochemical reaction
The generation of CIDNP in a typical photochemical system (target + photosensitizer, flavin in this example) is a cyclic photochemical process shown schematically in Figure 1. The chain of reactions is initiated by a blue light photon, which excites the flavin mononucleotide (FMN) photosensitizer to the singlet excited state. The fluorescence quantum yield of this state is rather low, and approximately half of the molecules undergo intersystem crossing into the long-lived triplet state. Triplet FMN has a remarkable electron affinity. If a molecule with a low ionization potential (e.g. phenols, polyaromatics) is present in the system, the diffusion-limited electron transfer reaction forms a spin-correlated triplet electron transfer state – a radical pair. The kinetics are complicated and may involve multiple protonations and deprotonations, and hence exhibit pH dependence. 
An example of Radical Pair Mechanism
The radical pair may either cross over to a singlet electron state and then recombine, or separate and perish in side reactions. The relative probability of these two pathways for a given radical pair depends on the nuclear spin state and leads to the nuclear spin state sorting and observable nuclear polarization.
Applications
Detected as enhanced absorptive or emissive signals in the NMR spectra of the reaction products, CIDNP has been exploited for the last 30 years to characterise transient free radicals and their reaction mechanisms. In certain cases, CIDNP also offers the possibility of large improvements in NMR sensitivity. The principal application of this photo-CIDNP technique, as devised by Kaptein in 1978, has been to proteins in which the aromatic amino acid residues histidine, tryptophan and tyrosine can be polarized using flavins or other aza-aromatics as photosensitisers. The key feature of the method is that only solvent accessible histidine, tryptophan and tyrosine residues can undergo the radical pair reactions that result in nuclear polarization. Photo-CIDNP has thus been used to probe the surface structure of proteins, both in native and partially folded states, and their interactions with molecules that modify the accessibility of the reactive side chains.
Although usually observed in liquids, the photo-CIDNP effect has also been detected in solid state, for example on 13C and 15N nuclei in photosynthetic reaction centres, where significant nuclear polarization can accumulate as a result of spin selection processes in the electron transfer reactions.
See alsoDynamic nuclear polarisation
Electron paramagnetic resonance
References
Bargon, J.; Fischer, H.; Johnsen, U. (1967). "Kernresonanz-Emissionslinien während rascher Radikalreaktionen". Zeitschrift für Naturforschung A. 22 (10): 1551. doi:10.1515/zna-1967-1014. S2CID 201828719.
"Nuclear magnetic resonance emission and enhanced absorption in rapid organometallic reactions". Journal of the American Chemical Society. 89: 5517. 1967.
Vyushkova, Maria (April 2011). "Basic principles and applications of spin chemistry" (PDF). nd.edu. University of Notre Dame. Retrieved November 21, 2016.
Closs, G. L. (1974). "Chemically Induced Dynamic Nuclear Polarization". Advances in Magnetic and Optical Resonance. Vol. 7. pp. 157–229. doi:10.1016/B978-0-12-025507-8.50009-7. ISBN 978-0120255078.
Kaptein, R.; Oosterhoff, J. L. (1969). "Chemically induced dynamic nuclear polarization II: (Relation with anomalous ESR spectra)". Chemical Physics Letters. 4 (4): 195. Bibcode:1969CPL.....4..195K. doi:10.1016/0009-2614(69)80098-9.
Further reading
Muus, L. T.; Atkins, P.W.; McLauchlan, K.A.; Pedersen, J. B., eds. (1977). Chemically Induced Magnetic Polarisation. Dordrecht: D. Reidel.
Goez, Martin (2007). "Photochemically Induced Dynamic Nuclear Polarization". Advances in Photochemistry. pp. 63–163. doi:10.1002/9780470133545.ch2. ISBN 9780470133545.
Kaptein, Robert (1982). "Photo-CIDNP Studies of Proteins". Biological Magnetic Resonance. pp. 145–191. doi:10.1007/978-1-4615-6540-6_3. ISBN 978-1-4615-6542-0.
Kaptein, R.; Dijkstra, K.; Nicolay, K. (1978). "Laser photo-CIDNP as a surface probe for proteins in solution". Nature. 274 (5668): 293–294. Bibcode:1978Natur.274..293K. doi:10.1038/274293a0. PMID 683312. S2CID 4162279.
Hore, J.; Broadhurst, R.W. (1993). "Photo-CIDNP of biopolymers". Progress in Nuclear Magnetic Resonance Spectroscopy. 25 (4): 345–402. doi:10.1016/0079-6565(93)80002-B.
Kuprov, I.; Hore, P.J. (2004). "Chemically amplified 19F–1H nuclear Overhauser effects". Journal of Magnetic Resonance. 168 (1): 1–7. Bibcode:2004JMagR.168....1K. doi:10.1016/j.jmr.2004.01.011. PMID 15082243.
Prakash, Shipra; Alia; Gast, Peter; De Groot, Huub J. M.; Matysik, Jörg; Jeschke, Gunnar (2006). "Photo-CIDNP MAS NMR in Intact Cells ofRhodobactersphaeroidesR26: Molecular and Atomic Resolution at Nanomolar Concentration". Journal of the American Chemical Society. 128 (39): 12794–12799. doi:10.1021/ja0623616. hdl:1887/3455644. PMID 17002374.
Categories: Physical chemistry
Nuclear magnetic resonance
https://en.wikipedia.org/wiki/CIDNP
The fluorescence quantum yield of this state is rather low, and approximately half of the molecules undergo intersystem crossing into the long-lived triplet state.
https://en.wikipedia.org/wiki/CIDNP
https://en.wikipedia.org/wiki/CIDNP
https://en.wikipedia.org/wiki/Photosensitizer
Photodegradation is the alteration of materials by light. Commonly, the term is used loosely to refer to the combined action of sunlight and air, which cause oxidation and hydrolysis.
Often photodegradation is intentionally avoided, since it destroys
paintings and other artifacts. It is, however, partly responsible for
remineralization of biomass and is used intentionally in some
disinfection technologies. Photodegradation does not apply to how
materials may be aged or degraded via infrared light or heat, but does include degradation in all of the ultraviolet light wavebands.
https://en.wikipedia.org/wiki/Photodegradation
https://en.wikipedia.org/wiki/Photosensitizer
https://en.wikipedia.org/wiki/Polymerization#Photopolymerization
https://en.wikipedia.org/wiki/Photosensitizer
https://en.wikipedia.org/wiki/Photon_upconversion
https://en.wikipedia.org/wiki/Photoexcitation
https://en.wikipedia.org/wiki/Photoacid
https://en.wikipedia.org/wiki/Photocatalysis
https://en.wikipedia.org/wiki/Photochemistry
https://en.wikipedia.org/wiki/Radical_(chemistry)
https://en.wikipedia.org/wiki/Triplet_oxygen
https://en.wikipedia.org/wiki/Radio_wave
https://en.wikipedia.org/wiki/Photoinitiator
https://en.wikipedia.org/wiki/Ray_(optics)
https://en.wikipedia.org/wiki/Intersystem_crossing
https://en.wikipedia.org/wiki/Singlet_state
https://en.wikipedia.org/wiki/Triplet_state
https://en.wikipedia.org/wiki/Quantum_state#Pure_states
https://en.wikipedia.org/wiki/Introduction_to_eigenstates
https://en.wikipedia.org/wiki/Probability_distribution
https://en.wikipedia.org/wiki/SQUID
https://en.wikipedia.org/wiki/Superconducting_magnet
https://en.wikipedia.org/wiki/Cryogenics
https://en.wikipedia.org/wiki/SCMaglev
https://en.wikipedia.org/wiki/Zero_field_NMR
https://en.wikipedia.org/wiki/Larmor_precession
https://en.wikipedia.org/wiki/Earth%27s_field_NMR
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 time-varying (e.g., pulsed or alternating) magnetic field, NMR active nuclei resonate at characteristic frequencies. Examples of such NMR active nuclei are the isotopes carbon-13 and hydrogen-1 (which in NMR is conventionally known as proton NMR). 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 laboratory 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 shifts 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.
Applications
Applications of EFNMR include:
- Proton precession magnetometers (PPM) or proton magnetometers, 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 spectroscopy 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 imaging.
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.
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 Decay (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 ratios. 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:
- Electron-mediated heteronuclear J-couplings (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."[1]
- Chemical shifts of several parts per million (ppm) are clearly separated in high field NMR spectra, but have separations of only a few milliherz at proton EFNMR frequencies, and so are undetectable in an experiment that takes place on a timescale of tenths of a second.
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.
Proton EFNMR frequencies
The geomagnetic field strength and hence precession frequency varies with location and time.
- Larmor precession frequency = magnetogyric ratio 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 (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, and the audio-magnetotelluric (AMT) frequencies of geophysics.
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.
See also
References
- Robinson J. N.; et al. (2006). "Two-dimensional NMR spectroscopy in Earth's magnetic field" (PDF). Journal of Magnetic Resonance. 182 (2): 343–347. Bibcode:2006JMagR.182..343R. doi:10.1016/j.jmr.2006.06.027. PMID 16860581.
External links
- TeachSpin EFNMR web site
- Magritek EFNMR web site
- Two dimensional EFNMR imaging
- Earth's field NMR/MRI practical course, SS24 October 2009. Department of Physics, University of Oxford
- NMR Using Earth’s Magnetic Field
- Open source Earth's Field NMR Spectrometer
- Magnetic Resonance Imaging System Based on Earth’s Magnetic Field
- Applications of Earth’s Field NMR to porous systems and polymer gels
https://en.wikipedia.org/wiki/Earth%27s_field_NMR
Low field NMR spans a range of different nuclear magnetic resonance (NMR) modalities, going from NMR conducted in permanent magnets, supporting magnetic fields of a few tesla (T), all the way down to zero field NMR, where the Earth's field is carefully shielded such that magnetic fields of nanotesla (nT) are achieved where nuclear spin precession is close to zero. In a broad sense, Low-field NMR is the branch of NMR that is not conducted in superconducting high-field magnets. Low field NMR also includes Earth's field NMR where simply the Earth's magnetic field is exploited to cause nuclear spin-precession which is detected. With magnetic fields on the order of μT and below magnetometers such as SQUIDs or atomic magnetometers (among others) are used as detectors. "Normal" high field NMR relies on the detection of spin-precession with inductive detection with a simple coil. However, this detection modality becomes less sensitive as the magnetic field and the associated frequencies decrease. Hence the push toward alternative detection methods at very low fields.
Readings
- Blümich, Bernhard; Casanova, Federico; Appelt, Stephan (2009). "NMR at low magnetic fields". Chemical Physics Letters. Elsevier BV. 477 (4–6): 231–240. doi:10.1016/j.cplett.2009.06.096. ISSN 0009-2614.
- Volegov, P.L.; Matlachov, A.N.; Kraus, R.H. (2006). "Ultra-low field NMR measurements of liquids and gases with short relaxation times". Journal of Magnetic Resonance. Elsevier BV. 183 (1): 134–141. doi:10.1016/j.jmr.2006.07.021. ISSN 1090-7807.
- Theis, T.; Ganssle, P.; Kervern, G.; Knappe, S.; Kitching, J.; Ledbetter, M. P.; Budker, D.; Pines, A. (2011-05-01). "Parahydrogen-enhanced zero-field nuclear magnetic resonance". Nature Physics. Springer Science and Business Media LLC. 7 (7): 571–575. arXiv:1102.5378. doi:10.1038/nphys1986. ISSN 1745-2473.
- Vesanen, Panu T.; Nieminen, Jaakko O.; Zevenhoven, Koos C. J.; Dabek, Juhani; Parkkonen, Lauri T.; et al. (2012-07-17). "Hybrid ultra-low-field MRI and magnetoencephalography system based on a commercial whole-head neuromagnetometer". Magnetic Resonance in Medicine. Wiley. 69 (6): 1795–1804. doi:10.1002/mrm.24413. ISSN 0740-3194.
 | This nuclear physics or atomic physics–related article is a stub. You can help Wikipedia by expanding it. |
https://en.wikipedia.org/wiki/Low_field_nuclear_magnetic_resonance
https://en.wikipedia.org/wiki/Magnetometer
https://en.wikipedia.org/wiki/Centimetre%E2%80%93gram%E2%80%93second_system_of_units
https://en.wikipedia.org/wiki/MKS_system_of_units
https://en.wikipedia.org/wiki/Base_unit_(measurement)
https://en.wikipedia.org/wiki/Physical_quantity
https://en.wikipedia.org/wiki/Physical_property
https://en.wikipedia.org/wiki/Radiance
https://en.wikipedia.org/wiki/Laser_science
https://en.wikipedia.org/wiki/Stimulated_emission
https://en.wikipedia.org/wiki/Spontaneous_emission
https://en.wikipedia.org/wiki/Emulation
https://en.wikipedia.org/wiki/Brain%E2%80%93computer_interface
No comments:
Post a Comment