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University of Kurdistan Food Quality Evaluation Methods (FQEM) Lecturer: Kaveh Mollazade, Ph.D. Department of Biosystems Engineering, Faculty of Agriculture,

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Presentation on theme: "University of Kurdistan Food Quality Evaluation Methods (FQEM) Lecturer: Kaveh Mollazade, Ph.D. Department of Biosystems Engineering, Faculty of Agriculture,"— Presentation transcript:

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2 University of Kurdistan Food Quality Evaluation Methods (FQEM) Lecturer: Kaveh Mollazade, Ph.D. Department of Biosystems Engineering, Faculty of Agriculture, University of Kurdistan, Sanandaj, IRAN. Lecture 4: Nuclear Magnetic Resonance Spectroscopy / Magnetic Resonance Imaging

3 Food Quality Evaluation Methods– Department of Biosystems Engineering – University of Kurdistan http://agri.uok.ac.ir/k.mollazade 1 Contents This lecture will cover: –An introduction to nuclear magnetic resonance (NMR) spectroscopy and magnetic resonance imaging (MRI) –Principles of NMR –NMR spectrum interpretation –Apparatus and devices Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

4 Food Quality Evaluation Methods– Department of Biosystems Engineering – University of Kurdistan http://agri.uok.ac.ir/k.mollazade 2 Introduction  Magnetic resonance is a phenomenon that occurs between atomic particles and an external magnetic field.  The atomic particles responsible for this interaction are the electrons and the nucleus.  The interaction between the atomic particles and an external magnetic field is similar to what happens when iron filings are placed near a bar magnet. The filings become oriented and a magnetic field is induced in the metal. However, unlike the filings, the physical orientation of the atomic particles is not altered.  At most common magnetic field strengths only the magnetic moment of the atomic particles is induced.

5 Food Quality Evaluation Methods– Department of Biosystems Engineering – University of Kurdistan http://agri.uok.ac.ir/k.mollazade 3 Introduction  The phenomenon of resonance is observed in these systems because they absorb and emit energy at specific frequencies.  The specific frequency depends on the individual atomic particle and the strength of the applied magnetic fields.  Work discussed in this chapter will focus on using the nucleus as the atomic particle, and in this case the phenomenon is referred to as nuclear magnetic resonance (NMR).

6 Food Quality Evaluation Methods– Department of Biosystems Engineering – University of Kurdistan http://agri.uok.ac.ir/k.mollazade 4 Introduction  Common nuclei with magnetic moments include 1 H, 13 C, 31 P, 15 N, and 23 Na.  The most commonly studied nucleus in food systems is the 1 H.  In both medical and food applications of NMR, the technique is referred to simply as magnetic resonance (MR). The term nuclear is omitted so that patients/consumers will not confuse this technique with nuclear procedures that use radioactive materials. MR is a safe experimental procedure and does not harm or alter the sample, operator, or environment

7 Food Quality Evaluation Methods– Department of Biosystems Engineering – University of Kurdistan http://agri.uok.ac.ir/k.mollazade 5 Introduction  MR is a very useful spectroscopy because the signal emitted from a sample is sensitive to the number of nuclei, to the chemical and electronic surroundings of the nuclei at the molecular level, and to the diffusion or flow of the nuclei.  This sensitivity to a range of sample characteristics is responsible for the wide use of MR in chemistry, biochemistry, biotechnology, petrophysics, plastics, engineering, building materials, consumer products, medicine, and food technology.  When MR is used to make internal images of objects it is commonly called magnetic resonance imaging (MRI). MRI is a spectroscopic technique based on the magnetic properties of nuclei. MRI is the extension of MR spectroscopy to obtain the signal as a function of spatial coordinates within the sample.

8 Food Quality Evaluation Methods– Department of Biosystems Engineering – University of Kurdistan http://agri.uok.ac.ir/k.mollazade 6 Principles of NMR  Two common types of NMR spectroscopy are used to characterize organic structure: 1 H NMR is used to determine the type and number of H atoms in a molecule; 13 C NMR is used to determine the type of carbon atoms in the molecule.  The source of energy in NMR is radio waves which have long wavelengths, and thus low energy and frequency.  When low-energy radio waves interact with a molecule, they can change the nuclear spins of some elements, including 1 H and 13 C.

9 Food Quality Evaluation Methods– Department of Biosystems Engineering – University of Kurdistan http://agri.uok.ac.ir/k.mollazade 7 Principles of NMR  When a charged particle such as a proton spins on its axis, it creates a magnetic field. Thus, the nucleus can be considered to be a tiny bar magnet.  Normally, these tiny bar magnets are randomly oriented in space. However, in the presence of a magnetic field B 0, they are oriented with or against this applied field. More nuclei are oriented with the applied field because this arrangement is lower in energy.  The energy difference between these two states is very small (<0.1 cal).

10 Food Quality Evaluation Methods– Department of Biosystems Engineering – University of Kurdistan http://agri.uok.ac.ir/k.mollazade 8 Principles of NMR  In a magnetic field, there are now two energy states for a proton: a lower energy state with the nucleus aligned in the same direction as B 0, and a higher energy state in which the nucleus aligned against B 0.  When an external energy source (h ) that matches the energy difference (  E) between these two states is applied, energy is absorbed, causing the nucleus to “spin flip” from one orientation to another.  The energy difference between these two nuclear spin states corresponds to the low frequency RF region of the electromagnetic spectrum.

11 Food Quality Evaluation Methods– Department of Biosystems Engineering – University of Kurdistan http://agri.uok.ac.ir/k.mollazade 9 Principles of NMR  Thus, two variables characterize NMR: - An applied magnetic field B 0, the strength of which is measured in tesla (T) - The frequency of radiation used for resonance, measured in hertz (Hz), or megahertz (MHz).

12 Food Quality Evaluation Methods– Department of Biosystems Engineering – University of Kurdistan http://agri.uok.ac.ir/k.mollazade 10 Principles of NMR  The frequency needed for resonance and the applied magnetic field strength are proportionally related:  NMR spectrometers are referred to as 300 MHz instruments, 500 MHz instruments, and so forth, depending on the frequency of the RF radiation used for resonance.  These spectrometers use very powerful magnets to create a small but measurable energy difference between two possible spin states.

13 Food Quality Evaluation Methods– Department of Biosystems Engineering – University of Kurdistan http://agri.uok.ac.ir/k.mollazade 11 Principles of NMR  Schematic of an NMR spectrometer:

14 Food Quality Evaluation Methods– Department of Biosystems Engineering – University of Kurdistan http://agri.uok.ac.ir/k.mollazade 12 Principles of NMR  Protons in different environments absorb at slightly different frequencies, so they are distinguishable by NMR.  The frequency at which a particular proton absorbs is determined by its electronic environment.  The size of the magnetic field generated by the electrons around a proton determines where it absorbs.

15 Food Quality Evaluation Methods– Department of Biosystems Engineering – University of Kurdistan http://agri.uok.ac.ir/k.mollazade 13 Principles of NMR  Only nuclei that contain odd mass numbers (such as 1 H, 13 C, 19 F and 31 P) or odd atomic numbers (such as 1 H and 7 N) give rise to NMR signals. This means they have angular momentum.  Modern NMR spectrometers use a constant magnetic field strength B 0, and then a narrow range of frequencies is applied to achieve the resonance of all protons.

16 Food Quality Evaluation Methods– Department of Biosystems Engineering – University of Kurdistan http://agri.uok.ac.ir/k.mollazade 14 1 H NMR—The spectrum  An NMR spectrum is a plot of the intensity of a peak against its chemical shift, measured in parts per million (ppm).

17 Food Quality Evaluation Methods– Department of Biosystems Engineering – University of Kurdistan http://agri.uok.ac.ir/k.mollazade 15 1 H NMR—The spectrum  NMR absorptions generally appear as sharp peaks.  Increasing chemical shift is plotted from right to left.  Most protons absorb between 0-10 ppm.  The terms “upfield” and “downfield” describe the relative location of peaks. Upfield means to the right. Downfield means to the left.  NMR absorptions are measured relative to the position of a reference peak at 0 ppm on the  scale due to tetramethylsilane (TMS). TMS [Si(CH ₃ ) ₄ ] is a volatile inert compound that gives a single peak upfield from typical NMR absorptions.

18 Food Quality Evaluation Methods– Department of Biosystems Engineering – University of Kurdistan http://agri.uok.ac.ir/k.mollazade 16 1 H NMR—The spectrum  The chemical shift of the x axis gives the position of an NMR signal, measured in ppm, according to the following equation:  By reporting the NMR absorption as a fraction of the NMR operating frequency, we get units, ppm, that are independent of the spectrometer.

19 Food Quality Evaluation Methods– Department of Biosystems Engineering – University of Kurdistan http://agri.uok.ac.ir/k.mollazade 17 1 H NMR—The spectrum  Four different features of a 1 H NMR spectrum provide information about a compound’s structure: - Number of signals - Position of signals - Intensity of signals - Spin-spin splitting of signals

20 Food Quality Evaluation Methods– Department of Biosystems Engineering – University of Kurdistan http://agri.uok.ac.ir/k.mollazade 18 1 H NMR—Number of signals  The number of NMR signals equals the number of different types of protons in a compound.  Protons in different environments give different NMR signals. Equivalent protons give the same NMR signal.  To determine equivalent protons in cycloalkanes and alkenes, always draw all bonds to hydrogen.

21 Food Quality Evaluation Methods– Department of Biosystems Engineering – University of Kurdistan http://agri.uok.ac.ir/k.mollazade 19 1 H NMR—Number of signals  The number of 1 H NMR signals of some representative organic compounds:

22 Food Quality Evaluation Methods– Department of Biosystems Engineering – University of Kurdistan http://agri.uok.ac.ir/k.mollazade 20 1 H NMR—Position of signals  In the vicinity of the nucleus, the magnetic field generated by the circulating electron decreases the external magnetic field that the proton “feels”.  Since the proton experiences a lower magnetic field strength, it needs a lower frequency to achieve resonance. Lower frequency is to the right in an NMR spectrum, toward a lower chemical shift, so shielding shifts the absorption upfield.

23 Food Quality Evaluation Methods– Department of Biosystems Engineering – University of Kurdistan http://agri.uok.ac.ir/k.mollazade 21 1 H NMR—Position of signals  The less shielded the nucleus becomes, the more of the applied magnetic field (B 0 ) it feels.  This deshielded nucleus experiences a higher magnetic field strength, to it needs a higher frequency to achieve resonance.  Higher frequency is to the left in an NMR spectrum, toward higher chemical shift— so deshielding shifts an absorption downfield.  Protons near electronegative atoms are deshielded, so they absorb downfield.

24 Food Quality Evaluation Methods– Department of Biosystems Engineering – University of Kurdistan http://agri.uok.ac.ir/k.mollazade 22 1 H NMR—Position of signals  How chemical shift is affected by electron density around a nucleus:

25 Food Quality Evaluation Methods– Department of Biosystems Engineering – University of Kurdistan http://agri.uok.ac.ir/k.mollazade 23 1 H NMR—Position of signals  Shielding and deshielding effects:

26 Food Quality Evaluation Methods– Department of Biosystems Engineering – University of Kurdistan http://agri.uok.ac.ir/k.mollazade 24 1 H NMR—Position of signals

27 Food Quality Evaluation Methods– Department of Biosystems Engineering – University of Kurdistan http://agri.uok.ac.ir/k.mollazade 25 1 H NMR—Chemical shift values  Protons in a given environment absorb in a predictable region in an NMR spectrum.

28 Food Quality Evaluation Methods– Department of Biosystems Engineering – University of Kurdistan http://agri.uok.ac.ir/k.mollazade 26  The chemical shift of a C—H bond increases with increasing alkyl substitution. 1 H NMR—Chemical shift values

29 Food Quality Evaluation Methods– Department of Biosystems Engineering – University of Kurdistan http://agri.uok.ac.ir/k.mollazade 27  In a magnetic field, the loosely held  electrons of the double bond create a magnetic field that reinforces the applied field in the vicinity of the protons.  The protons now feel a stronger magnetic field, and require a higher frequency for resonance. Thus the protons are deshielded and the absorption is downfield. 1 H NMR—Chemical shift values

30 Food Quality Evaluation Methods– Department of Biosystems Engineering – University of Kurdistan http://agri.uok.ac.ir/k.mollazade 28  In a magnetic field, the  electrons of a carbon-carbon triple bond are induced to circulate, but in this case the induced magnetic field opposes the applied magnetic field (B 0 ).  Thus, the proton feels a weaker magnetic field, so a lower frequency is needed for resonance. The nucleus is shielded and the absorption is upfield. 1 H NMR—Chemical shift values

31 Food Quality Evaluation Methods– Department of Biosystems Engineering – University of Kurdistan http://agri.uok.ac.ir/k.mollazade 29  Regions in the1H NMR spectrum: 1 H NMR—Chemical shift values

32 Food Quality Evaluation Methods– Department of Biosystems Engineering – University of Kurdistan http://agri.uok.ac.ir/k.mollazade 30 1 H NMR—Intensity of signals  The area under an NMR signal is proportional to the number of absorbing protons.  An NMR spectrometer automatically integrates the area under the peaks, and prints out a stepped curve (integral) on the spectrum.  The height of each step is proportional to the area under the peak, which in turn is proportional to the number of absorbing protons.  Modern NMR spectrometers automatically calculate and plot the value of each integral in arbitrary units.

33 Food Quality Evaluation Methods– Department of Biosystems Engineering – University of Kurdistan http://agri.uok.ac.ir/k.mollazade 31 1 H NMR—Intensity of signals

34 Food Quality Evaluation Methods– Department of Biosystems Engineering – University of Kurdistan http://agri.uok.ac.ir/k.mollazade 32  Consider the spectrum below: 1 H NMR—Spin-spin splitting

35 Food Quality Evaluation Methods– Department of Biosystems Engineering – University of Kurdistan http://agri.uok.ac.ir/k.mollazade 33 1 H NMR—Spin-spin splitting  Spin-spin splitting occurs only between nonequivalent protons on the same carbon or adjacent carbons.  Let to consider how the doublet due to the CH2 group on BrCH2CHBr2 occurs: o When placed in an applied electric field, (B 0 ), the adjacent proton (CHBr2) can be aligned with (  ) or against (  ) B 0. o Thus, the absorbing CH2 protons feel two slightly different magnetic fields—one slightly larger than B 0, and one slightly smaller than B 0. o Since the absorbing protons feel two different magnetic fields, they absorb at two different frequencies in the NMR spectrum, thus splitting a single absorption into a doublet.

36 Food Quality Evaluation Methods– Department of Biosystems Engineering – University of Kurdistan http://agri.uok.ac.ir/k.mollazade 34  The frequency difference, measured in Hz between two peaks of the doublet is called the coupling constant, J. 1 H NMR—Spin-spin splitting

37 Food Quality Evaluation Methods– Department of Biosystems Engineering – University of Kurdistan http://agri.uok.ac.ir/k.mollazade 35  Let now to consider how a triplet arises:  When placed in an applied magnetic field (B 0 ), the adjacent protons H a and H b can each be aligned with (  ) or against (  ) B 0.  Thus, the absorbing proton feels three slightly different magnetic fields—one slightly larger than B 0, one slightly smaller than B 0, and one the same strength as B 0. 1 H NMR—Spin-spin splitting

38 Food Quality Evaluation Methods– Department of Biosystems Engineering – University of Kurdistan http://agri.uok.ac.ir/k.mollazade 36 1 H NMR—Spin-spin splitting

39 Food Quality Evaluation Methods– Department of Biosystems Engineering – University of Kurdistan http://agri.uok.ac.ir/k.mollazade Kurdistan Nature Zrebar Lake, Marivan


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