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

3. This lecture will cover:
Contents
This lecture will cover:
An introduction to NIR spectroscopy
Principles of NIR spectroscopy
Devices and apparatus
Chemometrics 1

4. Introduction: Spectroscopy definition
Spectroscopy is the study of matter using electromagnetic radiation.
Spectroscopy is based on quantum mechanics, the prevailing theory of the behavior of atoms and molecules. One of the conclusions of quantum mechanics is that the energies of the various forms of motion within atoms and molecules are limited to certain discrete values; that is, they are quantized.
When an atomic or molecular system absorbs or emits light, the system goes from one quantized energy level to another. 2

5. Electromagnetic region
Introduction: Spectroscopy definition
Energy transmission at different regions of electromagnetic spectrum
Electromagnetic region
Energy transmission
Gamma radiation
Atom’s nucleus stimulation
X radiation
Electronics' inner layers
Ultraviolet and visible
Electronics‘ outer (shell) layer
Infrared
Molecular vibration
Microwave
Molecular rotation
Radio region
Orbits 3

6. Introduction: Spectroscopy definition
The electromagnetic spectrum 4

7. Introduction: Spectroscopy definition
The Bohr frequency condition states that the difference in the energy levels must equal the energy of the light absorbed or emitted.
Spectroscopy uses this principle to probe the energy levels of the matter under study. Ultimately, spectroscopy helps us learn how matter and energy interact.
Light is not the only probe used in spectroscopy. Several types of spectroscopy use magnetic fields in conjunction with light to probe the nature of matter. 5

8. Introduction: Spectroscopy definition
In some cases, like nuclear magnetic resonance (NMR) spectroscopy, it is clear from the name of the method that magnetic fields are involved. In other cases, like Zeeman spectroscopy or Raman spectroscopy, it is not clear from the name of the technique.
Spectrometry is a more restrictive term. It refers to the measurement of the intensity of absorption or emission of light at one or more specific wavelengths, rather than a range of wavelengths. 6

9. Introduction: NIR spectroscopy
Absorption of near-infrared (NIR) radiation, particularly by CH, NH, and OH bonds, commonly present in components of food materials, has been used for determining chemical compositions and internal quality in foods and food products as a nondestructive analytical technique.
Advantages of NIR spectroscopy:
- Response time is fast
- Sample preparation is easy
- Multiple components can be analyzed
- Anyone can operate the instrumentation
- The instrumentation cost is lower than that for ultraviolet, visible, mid-infrared, Raman, and other spectral techniques. 7

10. Introduction: NIR spectroscopy
The spectral region of the NIR radiation: ,500 nm (13, ,000 cm–1).
Wavelength/Wavenumber converter:
x (nm) = 10,000,000 / x (cm–1)
y (cm–1) = 10,000,000 / y (nm)
The first application of NIR spectroscopy for agricultural and food purposes started in the 1960s with Karl Norris of the U.S. Department of Agriculture (USDA). 8

11. Principles of technique: Physical and spectroscopic principles
Light (electromagnetic radiation) from the sun appears white, but if the light passes a prism (matter), it shows colors from violet to red.
The human eye can recognize this visible light, but other types of light also dispersed by the prism are not visible to the human eye.
Spectroscopy is a useful tool by which to measure the interaction between the electromagnetic radiation and matter that is composed of molecules, atoms, or ions, and may exist in gaseous, liquid, or solid form. 9

12. Principles of technique: Physical and spectroscopic principles
The properties of electromagnetic waves can be represented as oscillating perpendicular electric and magnetic fields. These fields are at right angles to each other and to the direction of propagation of the light. The oscillation shape appears sinusoidal. 10

13. Principles of technique: Physical and spectroscopic principles
The crest-to-crest distance between two successive maxima is defined as the wavelength, λ, the maximum of the vector from the origin to a point displacement of the oscillation is defined as the amplitude, and the number of crests passing a fixed point per second is the frequency, ν, of the wave. 11

14. Principles of technique: Physical and spectroscopic principles
The speed of light, c, can be presented by the wavelength of light, λ (m), and frequency, ν (HZ):
In a vacuum, the speed of light is at a maximum, × m/s, and does not depend on the wavelength. The frequency of light is determined by the source and does not vary.
When light passes through matter, its speed is decreased. Because the frequency remains invariant, the wavelength must decrease. 12

15. Principles of technique: Physical and spectroscopic principles
The emission and absorption processes have been explained by quantum theory based on the following two important postulates.
First, atoms, ions, and molecules can exist only in certain specific discrete states, characterized by defined discrete amounts of energy. If the state is changed from one specific state to another, the amount of energy involved in the emission and absorption processes is equal to the energy difference between the two states.
Second, the frequency and wavelength of the radiation absorbed or emitted when a particle makes the transition from one energy state to another is related to the energy difference between the states. This energy is quantized. 13

16. Planck’s constant, 6.626 × 10–34 (J.s)
Principles of technique: Physical and spectroscopic principles E2
Emission
Absorption E1
Energy (J)
Planck’s constant, × 10–34 (J.s)
Frequency (Hz) 14

17. Principles of technique: Physical and spectroscopic principles
Information about the sample matter is acquired by measuring the amount of electromagnetic radiation emitted by the sample as it returns from the excited state to the ground state, or by measuring the amount of electromagnetic radiation that was absorbed or scattered when the sample was excited from the ground state.
Radiant power is produced by the emission of excess energy in the form of photons while the excited particles (atoms, ions, or molecules) return to the ground state. This can provide identification and concentration information about the sample matter. 15

18. Principles of technique: Physical and spectroscopic principles
An emission spectrum is a plot form of the relative power of the emitted radiation as a function of wavelength or frequency. 16

19. Principles of technique: Physical and spectroscopic principles
When the electromagnetic radiation passes through a layer of matter, energy at selected frequencies may be removed via absorption—that is, the energy is transferred to the atoms, ions, or molecules composing the matter.
The amount of light absorption that occurs can be described by a function of wavelength and provides both qualitative and quantitative information about the matter. 17

20. Principles of technique: Physical and spectroscopic principles
The absorption spectra for monatomic particles can be plotted for a few well-defined frequencies. 18

21. Principles of technique: Physical and spectroscopic principles
Absorption spectra for polyatomic molecules can be described by the electronic energy of the molecule that arises from the energy states of its several bonding electrons, ∆Eelectronic; vibrational energy from the molecule’s various atomic vibrations, ∆Evibrational; and rotational energy by the rotational motions within a molecule, ∆Erotational:
The NIR radiation does not provide enough energy for the electronic transitions of polyatomic molecules, but can explain small energy differences between various vibrational and rotational states. 19

22. Principles of technique: Physical and spectroscopic principles
If a molecule has a net change in dipole moment as it vibrates or rotates, it can absorb the NIR radiation. Homonuclear species such as O2, N2, Cl2, or O3 do not exhibit any net change in dipole moment during vibration or rotation, and thus cannot absorb NIR radiation. 20

23. Principles of technique: Physical and spectroscopic principles
Stretching and bending are the basic categories of vibrations.
A stretching vibration is a continuous variation in the interatomic distance along the axis of the bond between two atoms. A bending vibration is a change in the angle between two bonds and includes rocking, scissoring, wagging, and twisting.
Symmetrical stretching
Asymmetrical stretching
Scissoring
Rocking
Wagging
Twisting 21

24. Principles of technique: Physical and spectroscopic principles
In vibrational spectroscopy, nth overtone band is the spectral band that occurs in a vibrational spectrum of a molecule when the molecule makes a transition from the ground state (v=0) to the n+1th excited state (v=n+1), where v is the vibrational quantum number. The transition 0→1 is fundamental. 22

25. Principles of technique: Physical and spectroscopic principles
The HCl molecule as an anharmonic oscillator vibrating at energy level E3. D0 is dissociation energy here, r0 bond length, U potential energy. Energy is expressed in wavenumbers. The hydrogen chloride molecule is attached to the coordinate system to show bond length changes on the curve. 23

26. Principles of technique: Physical and spectroscopic principles24

27. Principles of technique: Physical and spectroscopic principles
Radiant radiation observed after an excited species returns to the ground state may be fluorescence or phosphorescence relaxation.
When the radiation is scattered, if the wavelength of the scattered radiation is the same as that of the source radiation, it is called elastic scattering and can be used for measurements in particle sizing and concentration, such as for nephelometry and turbidimetry. Inelastic scattering produces a vibrational spectrum of sample molecules and is called Raman scattering. 25

28. Principles of technique: Physical and spectroscopic principles
Refraction happens when a light beam passes at an angle through the interface between the two transparent media that have different densities. The difference in density changes the velocity of the light as it travels through the two media, and the direction of the beam is bent. Bending toward the normal to the interface occurs when the beam passes from a less dense to a more dense medium. If the beam passes from a more dense to a less dense medium, the bending is away from the normal.
Snell’s law:
n : Refraction index
V : light velocity 26

29. Principles of technique: Physical and spectroscopic principles
Distribution of incident light in biological materials 27

30. Principles of technique: Measurements of spectrum
Radiant energy converted by a radiation detector into an electrical signal or intensity, I, has been used to determine the radiant power.
In emission, fluorescence, and scattering, the power of the radiation emitted by an analyte after excitation is proportional to the analyte concentration, c :
where k is a constant determined by measuring I for the excitation of the analyte material in one or more reference standards of known concentration. 28

31. Principles of technique: Measurements of spectrum
Absorption and transmittance methods require two power measurements:
- measurement of the light source energy before it falls on the surface of the medium
containing the analyte (I0 ).
- measurement after the energy has passed through the analyte (I ).
From the Beer-Lambert law, the absorption is proportional to the concentration, c, of the absorbing species:
where ε is an absorption coefficient, and b is the thickness of the sample. 29

32. Principles of technique: Measurements of spectrum
The transmittance T of the medium is the fraction after the radiation passes through a medium:
The absorption A of a medium is defined by the equation: 30

33. Device and apparatus
Minimum requirements to breakup the light as spectrum:
- A dispersing element like prism or diffraction grating.
- A slit for light entrance.
Using such system it is possible to discriminate light source spectrum:
- The lack of dispersing element leads no spectrum to be created.
- The lack of slit leads the spectrum to be blurred. 31

34. Device and apparatus: Dispersing element
The dispersing element plays the main role for creation of spectrum.
Dispersive prisms are used to break up light into its constituent spectral colors because the refractive index depends on frequency; the white light entering the prism is a mixture of different frequencies, each of which gets bent slightly differently. Blue light is slowed down more than red light and will therefore be bent more than red light. Longer wavelengths (red) are diffracted more, but refracted less than shorter wavelengths (violet).
A triangular prism, dispersing light; waves shown to illustrate the differing wavelengths of light. 32

35. Device and apparatus: Dispersing element
The dispersing element plays the main role for creation of spectrum.
In optics, a diffraction grating is an optical component with a periodic structure, which splits and diffracts light into several beams travelling in different directions. The directions of these beams depend on the spacing of the grating and the wavelength of the light so that the grating acts as the dispersive element. Because of this, gratings are commonly used in spectrometers. Shorter wavelengths (violet) are refracted less than longer wavelengths (red).
Diffraction grating 33

36. Before dispersing element After dispersing element
Device and apparatus: Slit location
Before dispersing element
After dispersing element 34

37. Spectrometer
A spectrometer is any instrument used to probe a property of light as a function of its portion of the electromagnetic spectrum, typically its wavelength, frequency, or energy. The property being measured is usually intensity of light, but other variables like polarization can also be measured. Technically, a spectrometer can function over any range of light, but most operate in a particular region of the electromagnetic spectrum. 35

38. Device and apparatus
A spectrophotometer is commonly used for the measurement of transmittance or reflectance of solutions, transparent or opaque solids, such as polished glass, or gases. However they can also be designed to measure the diffusivity on any of the listed light ranges that usually cover around 200 nm  nm using different controls and calibrations. 36

39. Spectrophotometer
A spectrograph is an instrument that separates incoming light by its wavelength or frequency and records the resulting spectrum in some kind of multichannel detector, like a photographic plate. Many astronomical observations use telescopes as, essentially, spectrographs. 37

40. Chemometrics
Chemometrics is the science of extracting information from chemical systems by data-driven means. It is a highly interfacial discipline, using methods frequently employed in core data-analytic disciplines such as multivariate statistics, applied mathematics, and computer science, in order to address problems in chemistry, biochemistry, medicine, biology and chemical engineering. 38

41. Chemometrics methods - Experiments design - Statistics 39
Methods for collection appropriate data:
- Experiments design
- Calibration
- Signal processing
Methods for knowledge extraction from raw data:
- Statistics
- Pattern recognition
- Modeling 39

42. Kurdistan Nature
Zrebar Lake, Marivan