Transcript Food Quality Evaluation Methods - Lecture 2

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.
Contents
• This lecture will cover:
– An introduction to NIR spectroscopy
– Principles of NIR spectroscopy
– Devices and apparatus
– Chemometrics
Food Quality Evaluation Methods– Department of Biosystems Engineering – University of Kurdistan
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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.
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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
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Introduction: Spectroscopy definition
 The electromagnetic spectrum
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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.
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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.
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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.
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Introduction: NIR spectroscopy
 The spectral region of the NIR radiation: 750 - 2,500 nm (13,300 - 4,000 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).
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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.
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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.
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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.
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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, 2.997 × 108 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.
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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.
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Principles of technique: Physical and spectroscopic principles
E2
Emission
Absorption
E1
Energy (J)
Planck’s constant,
6.626 × 10–34 (J.s)
Frequency (Hz)
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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.
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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.
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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.
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Principles of technique: Physical and spectroscopic principles
 The absorption spectra for monatomic particles can be plotted for a few welldefined frequencies.
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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
vibrations,
∆Evibrational;
energy
from
the
molecule’s
various
atomic
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.
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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.
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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
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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.
Food Quality Evaluation Methods– Department of Biosystems Engineering – University of Kurdistan
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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.
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Principles of technique: Physical and spectroscopic principles
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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.
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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
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Principles of technique: Physical and spectroscopic principles
 Distribution of incident light in biological materials
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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.
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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.
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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:
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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.
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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
A triangular prism, dispersing light; waves shown to
illustrate the differing wavelengths of light.
shorter wavelengths (violet).
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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
Diffraction grating
(red).
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Device and apparatus: Slit location
Before dispersing element
After dispersing element
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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.
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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 - 2500 nm using different controls and calibrations.
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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.
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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.
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Chemometrics methods
 Methods for collection appropriate data:
- Experiments design
- Calibration
- Signal processing
 Methods for knowledge extraction from raw data:
- Statistics
- Pattern recognition
- Modeling
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KurdistanFood
Nature
Quality
Zrebar Lake, Marivan
Evaluation Methods– Department of Biosystems Engineering – University of Kurdistan
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