Nuclear Magnetic Resonance (NMR) is a robust non-selective analytical tool that allows you to ascertain molecular structure inclusive of relative configuration, relative as well as absolute concentrations, and even intermolecular interactions without the decay of the analyte. NMR spectroscopy is utilized to study chemical makeup making use of basic one-dimensional methods. Two-dimensional techniques are being used to deduce the structure of more complex molecules. These kind of techniques are steadily replacing x-ray crystallography for the determination of protein structure. Time domain NMR spectroscopic methods are being utilized to probe molecular dynamics in solutions. Solid state NMR spectroscopy is being used to deduce the molecular constitution of solids. Scientists have also developed NMR methods for quantifying diffusion coefficients.
The flexibility of NMR causes it to be invasive in the sciences. Researchers as well as students are finding out that familiarity with the science and technology of NMR is necessary for employing, in addition to developing, innovative applications for it. NMR isÂ without a doubtÂ an imperativeÂ toolÂ for theÂ contemporaryÂ scientist. NMR techniquesÂ which were once challengingÂ and specialized have becomeÂ routine. Chemists, withÂ minimal understanding ofÂ NMR,Â at this recent timeÂ are able toÂ obtainÂ 2-Â and evenÂ 3-dimensional spectra with aÂ few clicksÂ ofÂ theÂ button. CareÂ should beÂ taken, nonetheless, when usingÂ such 'black box' methods.Â AlthoughÂ theÂ customaryÂ parametersÂ utilized inÂ the set-upÂ macros for the study may well beÂ passable forÂ oneÂ sample, they may beÂ incorrect for another.Â One single incorrectly set parameter can mean the difference between obtaining a reliable, accurate & practical spectrum and getting a meaningless spectrum.
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2.0 HistoryC:\Users\User\Desktop\McGill Logo.jpg
Isidor Rabi and co. were the first to observe the phenomenon of Nuclear Magnetic Resonance (NMR) using molecular beams in the year 1937. Isidor Rabi received the Nobel Prize in Physics in 1944 for this work.Â Edward Mills Purcell, Pound and Torrey detected proton magnetic resonance in paraffin wax. Felix Bloch and Packard independently observed the phenomenon of NMR in water. Felix Bloch and Edward Mills Purcell were jointly awarded the Nobel Prize in Physics for their discoveries. After these discoveries, NMR emerged as a tool for reliable and precise measurement of magnetic moments of the nucleus and earth's magnetic field. During the next two decades following the discovery of NMR in bulk materials, the basic concepts and theories underlying NMR parameters, such as nuclear quadrupole interactions, chemical shifts, relaxation rates, nuclear spin-spin couplings, dipolar interactions, Nuclear Overhauser Nffect (NOE) and effect of chemical exchange on line-shapes were established. The power of NMR in chemical research improved significantly during this period due to the various technical & technological developments. Some technological developments: use of signal averaging to ameliorate signal to noise (S/N) ratio; better probe designs; use of field/frequency lock to stabilize magnetic fields; electronic shims to improve field homogeneity; use of double resonance techniques & development of magnets with higher and homogeneous magnetic fields. Spin-Echo (SE) by Hahn was one of the major discoveries during this period. It plays a crucial role both in Magnetic Resonance Imaging (MRI) & Magnetic Resonance Spectroscopy (MRS). The power of NMR spectroscopy was highly enhanced with the advancements in electronics and computer technology resulting NMR becoming an essential tool in chemical and physical sciences. Ernst and Anderson, in 1966 developed the technique of Fourier Transform (FT) spectroscopy which was considered as a major breakthrough at that time. They used short and powerful radiofrequency (RF) pulse for excitation, attainment of the signal in time-domain, followed by Fourier Transformation of the acquired signal, which improved the sensitivity of Nuclear Magnetic Resonance experiment several fold. This technique (FT) soon displaced the previously employed slow passage continuous wave (CW) method which relies on changing the magnetic field to reach resonance condition. Ernst was awarded to the Nobel prize in 1991 for his work in FT spectroscopy. Advancements in computer software, hardware and magnet technologies coupled with improved design of the Radio Frequency (RF) probes opened new boundaries for NM R applications in biological sciences. Damadian in 1971 discovered that the 1H relaxation rates of water in normal and malignant cells are dissimilar. This finding uncovered the possibility of exercising this property for medical diagnosis. Paul Lauterbur utilized magnetic field gradients to locate NMR signal information in space. His work showed that it can be used to generate images of objects. This work formed the basis for the Magnetic Resonance Imaging (MR1). Peter Mansfield engineered the technique of echo-planar imaging. Lauterbur and Mansfield received the Nobel Prize for their work in 2003.
3.0 NMR Principles
3.1 Nuclear spin and the splitting of energy levels in a magnetic field
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Electrons, protons and neutrons can be conceived as spinning on their axes. In certain atoms, these spins are paired against each other resulting in the absence of overall spin of the nucleus of the atom. While in some atoms (such asÂ 1H andÂ 13C) the nucleus does comprise an overall spin. The rules which determine the net spin of a nucleus are as follows:
The nucleus has no spin, if the number of neutrons and the number of protons is odd.
The nucleus has a half-integer spin (ex. 1/2, 3/2, 5/2), if the number of neutronsÂ plusÂ the number of protons is odd.
The nucleus has an integer spin (ex. 1, 2, 3), if the number of neutronsÂ andÂ the number of protons are both odd.
The overall spin designated as 'I', is important. According to quantum mechanics a nucleus of spinÂ IÂ will have 2IÂ + 1 possible orientations. For example, a nucleus with spin 1/2 will have 2 possible orientations. These orientations are of equal energy in the absence of an external magnetic field. The energy levels split in presence of a magnetic field. Each level is assigned aÂ magnetic quantum number.http://teaching.shu.ac.uk/hwb/chemistry/tutorials/molspec/nmrlev1.gif
In a nucleus exposed to a magnetic field, thermodynamics decide the initial populations of the energy levels, as explained by the Boltzmann distribution. This is crucial, and it signifies that the lower energy level will contain slightly more nuclei than the higher level. These nuclei can be excited to the upper levels with electromagnetic radiation. The frequency of radiation required is determined by the difference in energy between the energy levels.C:\Users\User\Desktop\McGill Logo.jpg
3.2 Calculating transition energy
The nucleus has a positive charge and is spinning. This generates a small magnetic field. The nucleus therefore possesses a magnetic moment,Â Âµ, which is proportional to its spin,I.
Î“ is theÂ magnetogyric ratio and is a fundamental nuclear constant specific to a nucleus. It varies form one nucleus to the other. hÂ is the Plancks constant.
The energy of a particular energy level is given by:
Where,Â BÂ is the strength of the magnetic fieldÂ at the nucleus.
The transition energy which is the difference in energy between levels can be found from:
From the above expression we can deduce that, if the magnetic field,Â B, is increased, so isÂ âˆ†E. It also means that if a nucleus has a relatively large magnetogyric ratio, thenÂ âˆ†EÂ is correspondingly large.
ï·oÂ =Â ï§BoÂ ...(1) (the Larmor frequency, in Hz)Â
3.3 The absorption of radiation by a nucleus in a magnetic fieldC:\Users\User\Desktop\McGill Logo.jpghttp://teaching.shu.ac.uk/hwb/chemistry/tutorials/molspec/precess.gif
In quantum mechanical terms, the nuclear magnetic moment of a nucleus can align with an externally applied magnetic field of strengthÂ BÂ in only 2I+1 ways, either re-inforcing or opposingÂ B. A nucleus (say, of spin 1/2) that is rotating on its axis in the magnetic field & present in the reduced energy level (i.e. its magnetic moment won't oppose the applied field), can align with the field in TWO means. In the existence of a magnetic field, this axis of rotation will precess round the magnetic field.
The frequency of precession is known as the Larmor frequency that is similar to the transition frequency. The potential energy of this precessing nucleus is given by:
E =Â -Â mÂ B cosÂ Éµ
Where,Â ÉµÂ is the angle between the direction of the applied field and the axis of nuclear rotation.
If energy is absorbed by the nucleus, then the angle of precession,Â q, will change. For a nucleus of spin 1/2, absorption of radiation "flips" the magnetic moment so that itÂ opposesÂ the applied field (the higher energy state).
It is vital to understand that merely a tiny ratio of "target" nuclei tend to be within the lower energy state (which enables them to absorb radiation). There is likelihood that through exciting these nuclei, the inhabitants belonging to the greater and lesser energy levels will end up becoming identical. In the event that this takes place, then there won't be any additional absorption of radiation. The spin system is saturated. The likelihood of saturation implies that we need to be knowledgeable of the relaxation processes which return nuclei to the reduced energy states.http://teaching.shu.ac.uk/hwb/chemistry/tutorials/molspec/nmrflip.gif
3.4 Relaxation processesC:\Users\User\Desktop\McGill Logo.jpg
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The nuclei in the higher energy state de-excite to the lower energy state through either emission of radiation or non-radiative relaxation processes which are based on thermodynamic principles. Emission of radiation is definitely inconspicuous considering that the probability of re-emission of photon changes with the cube of frequency. At radio frequencies, re-emission is negligible. In non-radiative relaxation processes the NMR spectroscopists presumably wish the relaxation rate is quick, but not too rapid. In the event the relaxation rate is rapid, then saturation is decreased. If the relaxation rate is too rapid, libe-broadening in the resultant NMR spectrum is noticed.
The two chief relaxation processes are:
Spin - lattice (longitudinal) relaxation
Spin - spin (transverse) relaxation
3.4.1 Spin - lattice relaxation .
In an NMR experiment nuclei are in a sample. This sample is held in what is termed as the lattice. The nuclei present inside the lattice are in vibrational and rotational motion, that generates an elaborate magnetic field. This particular magnetic field attributable to movement of nuclei within the lattice is referred to as lattice field. This specific lattice field pocesses several components. A few of these components will be identical in frequency as well as phase to the Larmor frequency of your nuclei of interest. Such components of the lattice field can interact with nuclei in the upper energy state, and also cause them to lose energy resulting in deexcitation to a lower state. The energy that the nucleus loses raises the number of vibration and rotation inside the lattice which results in a little increase in the temperature of the sample).
The relaxation time, T1 which is the typical lifetime of a nuclei in the upper energy state is reliant on the magnetogyric ratio of the nucleus as well as the mobility of the lattice. As mobility improves, the vibrational and rotational frequencies increase, making it more likely for any constituent of the lattice field to have the ability to interact with excited nuclei. Nevertheless, at excessive mobility rates, the probability of a component of the lattice field being able to interact with excited nuclei reduces.
3.4.2 Spin - spin relaxation .
Spin - spin relaxatiion explains the interaction among neighbouring nuclei and similar precessional frequencies yet distinctive magnetic quantum states. With this scenario, the nuclei can swap quantum states; a nucleus from the lower energy level will be excited, while the excited nucleus relaxes to the lesser energy state. There isn't a resultant change in the populations of the energy states, however, the average lifetime of the nucleus in an excited state will reduce. This possibly can result in line-broadening.C:\Users\User\Desktop\McGill Logo.jpg
3.5 Chemical shift/ Nuclear Shielding
The magnetic field at the nucleus will not be equal to the administrated magnetic field; electrons around this particular nucleus shield the nucleus from the applied field. The difference between all of the applied magnetic field and the field at the nucleus is termed the nuclear shielding.
S-orbital electrons have spherical symmetry and rotate inside the administrated field, which aproduces a magnetic field that will opposes the applied field. In other words the administrated field strength ought to be elevated for the nucleus to absorb at its transition frequency. This kind of upfield shift is usually termed diamagnetic shift.
Electrons in p-orbitals do not have any spherical symmetry. They generate resonably significant magnetic fields at the nucleus, which give a diminished field shift. This "deshielding" is termed paramagnetic shift.In proton (1H) NMR, p-orbitals are absent, resulting in merely a small range of chemical shift (10 ppm). the effect of s-electrons on the chemical shift can be determined by considering substituted methanes, CH3X. As X becomes more and more electronegative and so the electron density surrounding the protons reduces, and they resonate at decreased field strengths (increasing dH values).http://teaching.shu.ac.uk/hwb/chemistry/tutorials/molspec/espin.gif
Nuclear shielding is a function of the nucleus and its particular environment. Its determined relative to a reference compound. For 1H NMR, the reference is usually tetramethylsilane, Si (CH3)4.
3.6 Spin - spin couplingC:\Users\User\Desktop\McGill Logo.jpg
Protons have a magnetic field associated with them since they have a nuclear spin. When placed in a magnetic field approximately half of the protons become aligned with the field and half become aligned against the field. It is the transition between these two states can be obderved in NMR.Â .
In the simplest of cases a single peak for each type of proton is expected in a molecule. If a prton (HA) near another nonequivalent proton (HB) is considered, in half of the molecules the HAÂ proton will be adjacent to an HBÂ aligned with the field and in the other half the HAÂ proton will be adjacent to an HBÂ aligned against the field. Thus, half the HA's in the sample will feel a slightly larger magnetic field than they would in the absence of HBÂ and half will feel a slightly smaller magnetic field. Thus, two absorptions for the HAÂ proton are observed and vice versa. This splitting of the HAÂ resonance into two peaks is termed "spin-spin coupling" or "spin-spin splitting" and the distance between the two peaks (in Hz) is called the "coupling constant" (represented as J). The spin-spin coupling is transmitted through the electrons in the bonds and so depends on the bonding relationship between the two hydrogens.
The study of the interaction of electromagnetic radiation with matter is termed as spectroscopy. NMR spectroscopy is the use of the phenomenon, Nuclear Magnetic Resonance to understand physical, chemical, as well as biological properties associated with matter. In truth, NMR spectroscopy finds purposes in several areas of science. NMR spectroscopy is time and again utilized by chemists in order to study chemical makeup making use of basic one-dimensional methods. Two-dimensional techniques are being used to deduce the structure of more complex molecules. These kind of techniques are steadily replacing x-ray crystallography for the determination of protein structure.
4.0 NMR Spectroscopy: InstrumentationC:\Users\User\Desktop\McGill Logo.jpg
The standard set up of an NMR spectrometer is displayed in the figure below. The sample is placed in the magnetic field and excited via pulsations in the radio frequency input circuit. The realigned magnetic fields generate a radio signal in the output circuit which can be utilised to produce the output signal. Fourier analysis of the sophisticated output provides the specific spectrum. The pulse is replicated numerous times to enable the signals to be recognized within the background noise.Description: Basic arrangement of an NMR spectrometer
Two varities of NMR spectrometers are widely used, pulsed or Fourier-Transform (FT-NMR) spectrophotometers & continuous-wave (cw-NMR). Cw-NMR spectrometers have mostly been replaced with pulsed FT-NMR instruments. Even so, as a result of lesser operating and up keeping costs of cw
instruments, they are still being utilized for the 1H NMR spectroscopy. Water can be used for cooling purposes for electromagnets in low-resolution cw devices, whereas in FT-NMR spectrometers magnets are to be cooled using liquid helium.Description: Contineous wave NMR
4.1 NMR - Continuous-wave Nuclear Magnetic Resonance
A continuous-wave NMR instrument comprises of the listed units: a magnet which to separates all the nuclear spin energy states; a minimum of 2 radiofrequency channels, one of which furnishes the RF irradiation energy & the other for field/frequency stabilization and a sample probe containing coils which couple the sample with the RF field; a detector which processes the NMR signals; a sweep generator which sweeps either the magnetic or RF field via the resonance frequencies of the sample; and a recorder which displays the spectrum.
The spectrum is usually scanned using the actual field-sweep approach or the frequency-sweep method. While in the frequency-sweep procedure, the actual magnetic field is maintained constant, which in turn maintains the nuclear spin energy amounts consistent, following of which the RF signal is swept in order to work out the frequencies where energy is absorbed.While in field sweep procedure, the RF signal is kept consistent, then the magnetic field is swept, which often alters the energy levels, to find out the magnetic field strengths which will produce resonance at a fixed resonance frequency.C:\Users\User\Desktop\McGill Logo.jpg
Description: Fourier transform NMR
4.2 NMR - Fourier-Transform Nuclear Magnetic Resonance
Fourier-Transform NMR spectrometers make use of a pulse of radiofrequency radiation to trigger nucleiinside a magnetic field to help flip directly into the greater-energy alignment. RF pulse of length 1-10 Âµs is usually broad enough to concurrently excite nucle in all of the local environments. The period of time amongst pulses T is generally one to quite a few mere seconds. In the course of T, a time-domain RF signal referred to as the free induction decay (FID) signal is released as nuclei go back to their primary state. Description: C:\Users\User\Desktop\1.jpg
FID could be discovered using a radio-receiver coil which is perpendicular towards the static magnetic field. The FID transmissions can be digitized and saved in the computer system for data analysis. Time-domain decay signals coming from many successive pulses can be summed plus averaged to enhance the signal-to-noise percentage. This outcome is then altered to a frequency-domain signal by the way of Fourier transformation. The resultant frequency-domain output is comparable to the spectrum generated by a scanning continuous-wave experiment.
5.0 NMR Spectroscopy: Applications in Food IndustryC:\Users\User\Desktop\McGill Logo.jpg
High resolution NMR spectroscopy is applied for the analysis of food samples, biological tissues, as well as biofluids because it supplies complete and comprehensive information on all of the wide range of components found in food matrix in a single experiment. It presents added advantages of being non-destructive, simplifying the sample preparation and swiftness of analysis. On top of that, very little time (few minutes) is required to obtain the NMR spectra, which in concert with automation enables analysis of numerous samples with minimal operator input.
The application of NMR to the food industry for analysis is basically two types: (1) identification of distinct resonances and, therefore, specific compounds, and (2) use of chemometric profile analysis, in which the whole spectral profile is used without assigning particular resonances.
5.1 Oils & Fats
5.1.1 Fatty Acid Profile
Fatty acid profiles influence the physical and chemical properties of oils, fats, and their derivatives. Gas chromatography (CC) is commonly employed for determining the fatty acid profile. Although GC provides reliable information about complete fatty acid profile, it doesn't provide any data on fatty acid distribution on the glycerol anchors, which is vital to determine the functionality of the ingredient in processing. For example, for quality pie crusts or croissants, the correct type of fat is a requisite. The common unsaturated fatty acids, such as oleic, linoleic, and Iinolenic acids in an oil or fat sample, can be quantified using 1H-NMR, by integration of select signals in the spectra. 13C-NMR analysis can be used to obtain the fatty acid distribution on the glycerol anchors. There are two groups of resonances in the carbonyl region of the spectrum; the first resonance is due to fatty acids in positions 1,3, and the second is from fatty acids in position 2 of the glycerol moiety.
5.1.2 Verification of Vegetable Oil Identity C:\Users\User\Desktop\McGill Logo.jpg
The adulteration of oils which are of high value with those of lesser value is an issue of commercial and economic importance. This is more prevalent with olive oil as it is expensive and has a high nutritional value. Many studies from Greece, Italy, and Spain which are the major olive oil-producing Mediterranean countries, deal with recognizing lower-value oils, such as hazelnut oil, used for adulterating olive oil. This problem is complicated by the fact that the lower-value oils usually have fatty acid profiles similar to olive oil. '3C-NMR and 'I-I-NMR spectrometry are the methods used for examining potentially adulterated olive oil. For example, NMR is used in conjunction with multivariate statistical analyses of specific resonances in NMR spectra of olive oil diluted with hazelnut or sunflower oil. This method can be used to identify the geographical origin and variety of the oil.
5.1.3 Monitoring of Oxidation
A significant quality problem in the food industry is oxidation of vegetable oils and it often leads to further deterioration of the oil. The fatty acids, with bis-allylic methylene groups which are highly unsaturated, are very susceptible to oxidation. Hydroperoxides and aldehydes which are the primary and secondary oxidation products can be easily detected by 'H-NMR analyses. 1H-NMR is useful for such analyses because the samples do not require any additional treatments, such as derivatization, that could cause degradation.
5.1.4 Solid Fat Content C:\Users\User\Desktop\McGill Logo.jpg
A low-resolution pulsed NMR spectrometer can be used to detect the solid fat content (SFC) of a given sample. For example, the amount of triacylglycerols in an oil/fat at various temperatures can be determined using NMR methodology. This method utilizes the difference in relaxation times between solids and liquids. The NMR signal of the liquid fat is measured after a delay. The solid content is then estimated. Using Solid Fat Content (SFC) measurements, crystallization mechanisms of fat blends also can be studied.
Glass transitionÂ is the reversible change inÂ amorphousÂ materials from a hard and relatively brittle state into a molten orÂ rubber-like state. It is a notable property of foods, and the glass transition temperature (Tg), which relies primarily on water content, effects both the processing and the storage of food products. Tg can be obtained with an NMR state diagram. It is a curve which relates NMR relaxation time to glass transition temperature at various different moisture contents. This information is crucial since processing and storage temperatures above Tg at any stage during production, distribution and consumption of the product is associated with swift deterioration. Spin-spin relaxation time (TZ) is frequently employed as an indication of proton mobility. The value of TZ differs above and below the Tg of a given product. Though Differential Scanning Calorimeter (DSC) can be used for simple Tg analyses, NMR, with its ability to generate NMR state diagrams increases its value of for many applications.
5.3 Ingredient Assays
The adulteration of fruit juice is widespread and is not easy to detect by color or taste. For example, grapefruit juice which is relatively inexpensive can be blended with orange juice, but the presence of grapefruit juice in a commercially sold orange juice product presents some potentially dangerous health risks for consumers with medical conditions. Juice form grape fruit has many coumarin-like flavonoids and other CYP450 inhibitors which are powerful and can negatively impact the metabolism of numerous prescribed drugs. Hence, the detection and prevention of this kind of adulteration is very crucial. Chemometric approaches based on NMR along with Independent Component Analysis, are now applied for the detection purposes. Specific selective regions of the 1H NMR spectra, which are recognized to comprise characteristic flavonoid glycoside signals, are precisely analyzed in a relatively short time.
Distinguishing between freshly squeezed juices and those manufactured from pulp washes and later added to fresh-squeezed orange juice to reduce manufacture costs is another issue of concern with regards to juice preparation. 'H NMR, in conjunction with Principal Component Analyses, can simply and precisely discriminate the fresh-squeezed and pulp-wash orange juice.C:\Users\User\Desktop\McGill Logo.jpg
Large multinational breweries use NMR methods to monitor batch-to-batch quality and production site differences in beer as they prepare their beers at various different geographic locations and require methods for quality control at an elaborated molecular level. NMR in combination with principal component analysis can be used to differentiate beer from different production sites based on lactic acid, dextran, adenosine, pyruvic acid, inosine, tyrosine, and uridine, 2-phenylethanol content. Producers can identify the production sites with greater variability (i.e. poorer quality control) by quantifying these compounds.
Other producers use NMR methods to improve quality control in juice production, soft drink production, and vegetable oil manufacturing. NMR methods also are employed to monitor the quality of functional foods and neutraceuticals that are harvested from diverse geographic locations.
* Instumental Methods of Food Analysis 6th EditionC:\Users\User\Desktop\McGill Logo.jpg
6.0 ReferencesC:\Users\User\Desktop\McGill Logo.jpg
C. P. Slichter, "Principles of Magnetic Resonance" (3rd edn), Springer-Verlag, Berlin (1990).
J. R. J. Paré,Â J. M. R. Bélanger, "Instrumental methods in food analysis" (4th edn), Elsevier (1997).
Â N.S. Suzanne, "Food Analysis" (4th ed), Illus., (2010)
Ray Freeman, "A short history of NMR" (Vol. 31), No. 9, Khimiya Geterotsiklicheskikh Soedinenii (1995)
R. K. Harris, "Nuclear Magnetic Resonance Spectroscopy chemical View", Longman, London (1986).