Nuclear magnetic resonance, as abbreviated by scientists is a phenomenon occurring when nuclei of some given atoms are immersed in a static magnetic field while at the same time being exposed to second oscillating magnetic field. While some nuclei may experience this phenomenon others do not depending on if they posses spin property. Most of the matters that one can examine with nuclear magnetic resonance are composed of molecules while molecules are composed of atoms. For example when we consider a water molecule, it is made up of one oxygen atom and two hydrogen atoms. If we get deeper in one of the hydrogen atom, we shall realize a nucleus which is composed of a single proton. The proton has a basic property known as spin which can be perceived of as a small magnetic field responsible for causing production of NMR signal by a nucleus. It should be clear that its not all nucleus that are capable of possessing the spin property (Haner & Keifer, 2009).
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Source: Hornak, 2002, The Basics of NMR, retrieved on 4th February from: http://www.cis.rit.edu/htbooks/nmr/inside.htm
All the stable nuclides containing odd numbers of proton and neutron have angular momentum and intrinsic magnetic moments. That means they have a non zero spin while on the other hand all the nuclides that have an even number of protons and neutrons have spin of 0. Some of the most commonly studied nuclei include: 1H (this is the second most sensitive NMR isotope from the radioactive 3H) (Hornak, 2002).
When the nuclear magnetic moment that is associated with a spin of nuclear is positioned in an exterior magnetic field, the diverse spin states are given dissimilar magnetic latent energies. In the existence of the inert magnetic field which generates a petite amount of spin polarization, an RF signal of the appropriate frequency can stimulate a transition linking spin states. This spin flip locates some of the spins in their elevated energy state. If the RF signal is then switched off, the relaxation of the spins rears to the lower state generating a measurable amount of radio frequency signal at the resonant frequency which is associated with the spin flip. The magnetic dipole moment, also called magnetic moments, which exist in magnetic field, have a latent energy that is related to its orientation in respect to the field (Tyszka, Fraser, & Jacobs, 2005).
One of the most key characteristic of nuclear magnetic resonance is that the frequency of the resonance of a given substance is directly proportional to strength of applied magnetic field. This special feature is the one that is utilized in imagining techniques; this means if some samples are placed in non uniform magnetic fields, the sample nuclei's resonance will depend on the location of their placement in the field. Owing to the fact that the imagining technique's resolution is dependent on the hugeness of the gradient of the field, there are many efforts that are made so as to develop some more powerful magnets which will often use superconductor. NMR's effectiveness can also be improved by use of hyper-polarization by use of two dimensional multi frequency procedures (Hornak, 2002).
All Nuclear magnetic resonance technologies rely on the spin property. During the determination of a nuclei spin in an atom, the counting of the number of protons and neutrons in the atom takes place. In a situation where the neutrons and protons number in a nucleus has a characteristic odd number, the sum of spin of nuclei are greater than zero. That nucleus is thus said to possess the characteristic of spin. Any nucleus that has spin can be scanned using Nuclear magnetic resonance technology (Nave, n.d.).
The principle of NMR involves some two steps that are occurs in sequence:
The alignment, also known as 'magnetic nuclear spins' polarization' in an applied and constant magnetic field, perturbation of the alignment of the nuclear spins by use of an electro-magnetic mostly radio frequency pulse. The nuclei observation and static magnetic field is the determinant of the required perturbing frequency (Hornak, 2002).
The two fields should be chosen such that they are perpendicular to each other since this leads to the maximization of the strength of NMR's signals. The response that results from the total magnetization of the nuclear spins is the ideology that is used in magnetic resonance imaging and nuclear magnetic resonance spectroscopy. Both of them make use of intense applied magnetic fields so as to achieve dispersion as well as a high stability ion delivering spectral resolution, whereby the details are described by the Zeeman Effect, knight shift found in metals, and the chemical shift (Hornak, 2002).
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The History of NMR
Nuclear Magnetic Resonance can be traced to the workings of Isidor Rabi in 1938 when he described and measured it in molecular beams (Rabi, Zacharias, & Kusch 1938). Some few years later (1946), utilization process on solids and liquids was redesigned by Felix Bloch and Edward Mill. This enabled or created an opportunity for them to win a Physics Noble Prize in 1952 which they shared (Filler, 2009). The previous career of Purcell facilitated to the discovery; he had been involved in development as well as in the application of radar during the 2nd world war at Massachusetts Institute of Technology's in the Radiation Laboratory. His responsibility during the project on detection and production of radio frequency energy and the absorption of the energy by matter was very important to this discovery.
During the discovery process they realized that some nuclei in a magnetic field could absorb the radio frequency energy; such an absorption process corresponds with a strength that was specific to the identity of the nuclei. After the absorption has taken place the nucleus is described as going through a resonance. Diverse atomic nuclei within a given molecule resonate at dissimilar radio frequencies for the same strength in magnetic field. The examination of such magnetic resonance frequencies of nuclei that is present in a molecule will allow any trained user to realize critical, chemical and structural information regarding the molecule (Becker, 2000).
The theory behind Electromagnetic Resonance:
The nuclear magnetic concept is based on the fact that all nucleons (protons and neutrons) that compose an atomic nucleus have intrinsic quantum characteristic of spin. The entire spin of a nucleus is established by the quantity of spin quantum S. In a situation where the protons number and neutrons number a given nuclide are even the S equals to Zero. This means that there is no overall spin, such that just as electrons pairs up in atomic orbital so do the neutrons or protons number pairs up giving a 0 overall spin. Nevertheless proton and neutron will have lower energy in a situation where their spins are not anti-parallel, are parallel, since the parallel spin alignment does not cause an infringement of Pauli principle, rather has to operate with quark fine structure of both nucleons. Hence the spin ground state for deuteron that contains only a neutron and a proton that is correspondent to a spin value of one but not of zero. The isolated (single) deuteron exhibits an NMR absorption spectrum property of quarupolar nucleus of spin one, which is in the inflexible state at quite low temperatures is a characteristic doublet. On the other hand, due to Pauli principle, the (radioactive) tritium isotope must encompass a brace of anti-parallel spin neutrons, plus a proton of spin 0.5; consequently, the character of the tritium nucleus is again magnetic dipolar, but not quadrupolar, just like its non-radioactive deuteron neighbor, as well as the tritium nucleus full spin value is again 0.5, like for the simpler and abundant hydrogen isotope, 1H nucleus (proton). The Nuclear magnetic resonance absorption radio-frequency for the tritium is nonetheless somewhat higher for tritium than that of 1H since the tritium nucleus has to some extent elevated gyromagnetic ratio than 1H. In numerous other cases of non-radioactive nuclei, the general spin is also non-zero. For instance, the 27Al nucleus contains a general spin value S = 5â„2 (Ming, 1999).
A non-zero spin is ever associated with a non-zero magnetic moment (Î¼) through a relation of Î¼ = Î³S. In this case Î³ stands as the gyromagnetic ratio. This magnetic moment allows the NMR absorption spectra's observation which is caused by the transitions that happen between nuclear spin levels. The majorities of nuclides that have even proton and neutron numbers have also a characteristic zero nuclear magnetic moments. They also have zero magnetic dipole plus quadrupole moments; consequently, such nuclides do not display any NMR absorption spectra. Therefore, 18O is an exemplar of a nuclide that does not posses NMR absorption, while elements that possess isotopes, such as Chlorine (35Cl and 37Cl) exhibit nuclides that exhibit NMR absorption spectra. Other elements that show a similar characteristic include carbon 13 (13C) and Phosphorus 31 (31P). 35Cl and 37Cl nuclei are quadrupolar nuclei while 13C and 31P nuclei are dipolar ones (Hornak, 2002).
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Spin angular Momentum Values:
The angular momentum that is associated with nuclear spin is quantized, which means that the degree of angular momentum is quantized whereby S is limited to some given range of value that it can take. At the same time the orientation of the associated angular impetus is quantized. Associated quantum number is called magnetic quantum number (m). Magnetic quantum can take values that range fromâˆ’S, to +S to in steps of integer. Consequently for any given nucleus, we will obtain a sum of 2S + 1 in an angular momentum states. Z components of angular momentum (s) vector is hence Sz = mÄ§. In this case, Ä§ is the reduced plank constant (Hornak, 2002).
The application of Nuclear magnetic resonance
Nuclear magnetic resonances (NMR) exist on the fact that once a pulse of oscillating electromagnetic is applied to nuclei contained in a magnetic field, separate nuclei absorb energy, after which they release that energy following some specific patterns. The pattern of energy assimilation and release is dependent on the potency of the magnetic field and some other given variables. Through the examination of these patterns, physicists are positioned in a way that they can investigate atomic nuclei's quantum mechanical properties. Chemists can use Nuclear magnetic resonances technology to examine samples. This practice helps to give the chemical and structural composition of the elements in the sample. NMR technology acts the basis of a regularly-used type of medical imaging equipment (Lloyd, 2003).
In NMR spectroscopy, nuclear magnetic resonance spectrometer machine is used to obtain information about arrangement of nuclei within a given sample, the type of sample provided and the number or quantity of nuclei within the sample. Analysis of a nuclear magnetic resonance spectrum by a chemist can supply information about some different sorts of chemicals that are present within a sample, and the structure of some diverse molecules that are present in that sample. Nuclear magnetic resonance spectroscopy has, been instrumental in the understanding the structure of proteins and nucleic acids; it also provides hints on the working of the molecules (Lloyd, 2003).
The foundation of NMR imaging depends largely on characteristic exhibited by dissimilar molecules in that their resonance frequency is relative to the potency of the magnetic field applied to them. When one places a sample under test within the designed area of an oscillating magnetic field, a difference in the resonance frequency of the nuclei may show depending on their location within that field. The difference can then be used in the building up a picture of that very sample (Lloyd, 2003).
In the field of medicine, NMR technique uses magnetic resonance properties and is thus referred to as magnetic resonance imaging (MRI). Medical imaging equipment makes use of magnetic fields in aligning the hydrogen atoms present in water. In view of the fact that the body of a human body encloses a large quantity of water, aligning of hydrogen atoms in such a manner enhances production of enough information so as to build up a picture of the inner body makeup. A characteristic spin in a nuclei have been found to be a useful conception in medical imaging equipment technology. This is because atoms of hydrogen, which posses a character of spin, respond in a different ways to magnetic fields depending on the other types of molecules that they are connected to, and even the nature of molecules they are placed close to (Lloyd, 2003).
Through the study of the peaks of NMR spectra, a chemist can be able to determine the structure of most of the compounds that he/ she interacts with daily. This can be an extremely selective technique that distinguishes among most of the atoms that make up a molecule or a collection of molecules that are of same type but only dissimilar in relation to their local chemical location. Through studying of information on T2* chemists are able to determine compounds identity through comparison of the observed nuclear precession frequencies to those frequencies that are known. Additional structural facts can be explicated by observing spin-spin coupling; process by which the nuclei's precession frequency can be influenced by the magnetization shift from nearby nuclei. The Spin-spin coupling is mainly observed in nuclear magnetic resonance that involves ordinary isotopes, such as Hydrogen-1 (1H NMR).
Figure 1: The T2 relaxation time
Figure 1 show the conversion from multiple pore sizes in the echo decay time plot to the T2 distribution plot. Source: http://www.petrolog.net/webhelp/Logging_Tools/tool_nmr/nmr011.gif
Because of the sensitivity of NMR signals consequently the sensitivity of the technique is dependent on the magnetic field's strength. This strength has been advanced over the years hence producing more powerful magnets. The advances that have been made on the audio visual technology have to a great extent improved the signals generation as well as the capability of processing some recent machines. The sensitivity of NMR signals also relies on the presence or the absence of nuclides that are magnetically susceptible and hence, either on the ordinary profusion of such nuclides or on the capability of the experimentalist to synthetically supplement the molecules being studied with such nuclides. Among the most copious naturally-occurring isotopes of phosphorous and hydrogen are both magnetically vulnerable and readily useful for NMR spectroscopy. In disparity, nitrogen and carbon have useful isotopes although they occur only in extremely low natural abundance. Some other limitations on sensitivity emanate from quantum mechanical trait of phenomenon. For the quantum state that is separated by energy that is equivalent to RF, thermal energy that comes from the atmosphere causing the population of the states to near an equal. Because the incoming radiation has an equal likelihood of causing a stimulated emission as absorption, the nuclear magnetic resonance effect will depend on the excesses of the nuclei in the lower states. There are several factors that can play the role of reducing sensitivity; they include (Breitmaier & Bauer, 1984).
Increasing temperature: the increasing temperature evens out the states population. On the other hand low temperature nuclear magnetic resonance can at times produce some better results than the room temperature nuclear magnetic resonance hence providing the sample remains liquid (Breitmaier, & Bauer, 1984).
Saturation of the sample with energy that is supplied at the resonant RF. This is manifested in both continuous wave (CW) and nuclear magnetic resonance. In the preceding continuous wave case, it happens by the use of too much continuous power hence keeping the upper spin levels absolutely populated. In the nuclear magnetic resonance case, the saturation occurs through too frequent pulsing without providing an allowance for the nuclei to go back to thermal equilibrium via spin lattice relaxation. Some nuclei like 29Si experiences serious practical problem since their time of relaxation is measured in seconds. The protons that are in pure ice such as 19F in high purity the spin lattice time of relaxation can be an hour or even longer time. The utilization of shorter radio frequencies pulses that is bale to tip the magnetization by less than ninety degrees can partially solve the problem through allowing special acquisition with no complete loss of nuclear magnetic resonance signal (Breitmaier, & Bauer, 1984).
The non-magnetic effects: This may include examples such as electric-quadrupole pairing of spin-1and spin-3â„2 nuclei with their confined environment broadening and weakening absorption peaks. For this reason 14N which is an abundant spin-1 nuclei's becomes difficult to study. High resolution nuclear magnetic resonance instead probes molecules by use of the rarer 15N isotope which posses a spin of -0.5 (Zekter, 1988).