Near infrared reflectance spectroscopy (NIRS) is a nondestructive and rapid technique applied increasingly for food quality evaluation in recent years. The most wide spread use of NIRS has been for determination of protein, moisture, starch, lipids, ash, oil, and salts. NIR spectroscopy utilizes the spectral range from 780 to 2500 nm and provides much more complex structural information related to the vibration behavior of combination of bonds.

1. Mr. Sanket P. Shinde
Assistant Professor
Pune-Maharashtra.

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Mr. S. P. Shinde
Introduction
❑ Near infrared reflectance spectroscopy (NIRS) is a nondestructive and
rapid technique applied increasingly for food quality evaluation in
recent years.
❑ The most wide spread use of NIRS has been for determination of
protein, moisture, starch, lipids, ash, oil, and salts.
❑ The resulting bands in the near infrared are usually weak in intensity
and the intensity generally decrease.
❑ The bands in the near infrared are often overlapped making them less
useful than the mid-infrared region for qualitative analysis.

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Near Spectra

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❑ NIR spectroscopy utilizes the spectral range from 780 to 2500 nm and
provides much more complex structural information related to the
vibration behavior of combination of bonds.
❑ Near-IR is a spectroscopic method is based on molecular over tones and
combination vibrations of O-H, C-H, N-H.
❑ Such vibrations excites from ground state to second excited state to give
first overtone and to third vibrational state to give second overtone.
❑ The energy absorption of organic molecules in NIR region occurs when
molecules vibrate or is translated into an absorption spectrum within the
NIR spectrometer.
❑ These special bonds also play an important part in the field of chemical
food chemical analysis, and could extract information to analyze the
chemical structures.
Principle of NIR

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Molecules that absorbs wavelength near infrared- energy vibrate in 2 modes:
1. Stretching Defines the continuous change in the intra atomic distance
along the axis of bonds between the two atoms
2. Bending Defines change in bond angle

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❑ NIR instrumentation is not much differs from the conventional IR
spectrometers.
❑ The instrumentation which is used for the visible or infrared regions can
also be used for the NIR region, depending on purpose or on
environment.
❑ Visible or an ultraviolet (UV) spectrometers are generally used in the
short-wavelength region (12 μm) of NIR, whereas, infrared spectrometers
are used in the long-wavelength region (>2 μm) of NIR by modifying
some components.
❑ An NIR instrument consists of the following general components :
• Light source
• Wavelength selector
• Sample holder
• Detector
Instrumentation

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Light Source
❑ The most important source of Near Infra red light is light emitting diodes
(LED).
❑ It consists of Gallium arsenide which is used as semiconductor for near
infrared light emission.
❑ LEDs emit radiation at specific wavelength, and relatively broad energy
emitted.
❑ They do not produce the radiation in the region of 1700 and 2500 nm thus
the use of filters to eliminate radiations of this region can be avoided.
❑ LEDs have long life due to their low power requirement. The tungsten
lamps (incandescent bulbs) are also used as light source.
❑ These lamps produce heat up to 1100K.

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❑ The disadvantages of tungsten lamps are that they emit visible radiation
along with considerable NIR radiation thus filters are needed to eliminate
visible radiation.
❑ These lamps are cheap in cost and are readily available and these lamps
cannot increase their energy output as the voltage of source is increased.
❑ Most of the applications requires only selected wavelength.
❑ In some applications there is need to measure the response at many wave
lengths.
❑ The existing method of wave length selection is a tool to determine the
capacity of NIR instrument because it is possible to select the wavelength
of interest.
Continue…

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Wavelength Selector
❑ Wavelength selectors are of different types depending on the type of
instruments.
❑ It may be a grating, beam splitter or Acoustic Optical Tunable and Filter
tilting filter.
❑ LEDs combined with filters are also used as wavelength selector.
a) Gratings:
• Gratings are made up of metal or glass.
• They are very tiny materials consisting of minute grooves engraved on
their surface.
• When the light beam strikes the surface it falls on the different grooves
and diffracted.
• This diffraction leads to various wavelengths.

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b) Beam splitter
• Beam splitters are used as wavelength selector in interferometer.
• When the light beam is allowed to strike on the beam splitter, it is
splitted into two separate beams.
• These separated light beams passed through fixed and moving mirrors
and allowed to strike on the beam splitter again.
• Now these two separated beams are combined in the beam splitter and
taken to the sample.
• Beam splitter is made up of different materials such as KBr, CaF2,
quartz etc.
• Depending on the material used, the beam splitter acts at specific
wavelength.

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c) Acoustic Optical Tunable Filter
• A piezoelectric material bonded to the crystal which produces a high
radio-frequency acoustic wave.
• When the light beam directed to the crystal, the acoustic waves that
propagate through the crystal, split the light beam to two monochromatic
beams.
• The generated beam polarized in a different direction. Thus it acts as
wavelength selectors.
• Optical fiber is used to couple these monochromatic beams and taken to
the sample holder.
Advantages :
It has no moving parts, adjustable intensity and gives narrow beams.
Disadvantages :
It covers spectrum at limited wavelength range (1000-2000nm) and
difficulties when measuring highly absorbing samples.

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Acoustic Optical Tunable Filter

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d) Tilting filter
It comes under the category of interference filter. It consists of a encoder
wheel. Inside the encoder wheel around 3-7 interference filters are attached in
a circular fashion. The light beam approaches the interference filter at
different angle. The wavelength is depending on the incident angle of light on
the filter.

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Sample Holder
❑ Sample holders are generally made up of quartz or glass.
❑ The size and shape of the sample holders are designed depending upon
the instrument.
❑ The sample holder cells may be round shaped cells for dry solid and
grained samples.
❑ Transport cells are designed for analyzing the bulk samples.
❑ Round cups are often rotated in order to scan maximum amount of
sample and to eliminate the lack of sample homogeneity.
❑ Transport cells move vertically where the scanning beam remains
stationary.

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Detectors
❑ Detectors used in NIR spectrometers are Lead sulphide detectors (PbS),
Lead selenide detectors (PbSe), Silicon detectors, Indium antimonide
Detectors (InSb), Indium gallium arsenide (InGaAs) and Common
Charged Coupled Devices (CCD).
❑ The choice of detectors depends on wavelength range, spectrometer
design characteristics, detector characteristics such as photosensitivity
(responsivity) and noise equivalent power (NEP) detectivity.

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Detectors
Wavelength
Range (nm)
Region
Responsivity/
Detectivity
Remark
PbS
1100-2500
400-2600
1100-4500
NIR
UV-NIR
NIR-MIR
Intermediate/
Intermediate
PbS sandwiched with silicon
photodiodes are often used
for VIS-NIR
PbSe 1100-5000 NIR-MIR Fast/ high
The detector must be cooled
with liquid nitrogen
InGaAs 700-1700
NIR
NIR
Raman
Fast/ very high
Linear arrays high sensitivity
dynamic range, signal-to-
noise performance and
stability FT-NIR Diode
arrays spectrometers
InSb/InAs 1000-5500
NIR
MIR
IR
Fast/ very high
High quality detector
photodiodes
CCD 800-2200 NIR Fast/ high
High performance detector
applied in cameras diode
arrays spectrometers
Detectors

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Types of NIR instruments
1. Scanning spectrophotometers
2. Fourier transform spectrophotometers
3. Acoustic optical tunable filter spectrophotometers
4. Photo diode array spectrophotometers

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Scanning spectrophotometers
A. Pre-dispersive instruments
In pre-dispersive instruments dispersed light from gratings is transmitted
through sample, then passes through slit and reaches the detector.
B. Post- dispersive instruments
In post-dispersive instruments light is passed directly to the sample then
directed to grating and gets dispersed. The dispersed light then passes
through slit and reaches detector.

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Photo diode array spectrophotometers
❑ In Photo diode array spectrophotometers detector is placed away from a
diffraction grating.
❑ Silicon PDAs below 1000nm are used.
❑ Recently available detectors cover the range from 900-2200nm.
❑ In this type optical interference filters are used which allow narrow
wavelength range.
❑ The most popularly used optical element for NIR spectroscopy is an
interference filter

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Applications of NIR Spectroscopy
❑ Near-infrared (NIR) spectroscopy is very useful for the investigation of
hydrogen bonds, hydration and self-association of molecules.
❑ It is useful in the petrochemical industry for improved process stability,
better pollution control and more accurate blending
❑ It is widely used quality control tool for raw materials in the chemical,
petrochemical, polymer and food industries.
❑ NIR spectroscopy is useful to monitor the orientational and crystallisation
changes in the film-forming process of polymeric films.
❑ It is useful in monitoring of polyethylene/ polypropylene (PE/PP)
extrusion-blending procedures.
❑ It is useful as a tool for sorting polymeric waste in recycling operations.
❑ Near-infrared (NIR) spectroscopy is widely used in the food industry as a
fast routine analytical method for the quantitative measurement of water,
proteins, fats and carbohydrates.

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❑ Whiteness of rice can be measured by NIR spectroscopy
❑ NIR is useful for the analysis of vegetables and fruits such as determination
of dry matter in onion, analysis of peaches, apples, potatoes, melons,
strawberry.
❑ It has been used for many years in the dairy industry to examine milk. It is
used as standard method for determination of fat, lactose, total solids in milk.
❑ It is useful for the determination of sodium chloride in hams and sausage,
soybean flour in ground beef, starch content in gravy, calories in raw pork
and beef, pH in ham, the physical & chemical characteristics of beef cuts.
❑ Wine analysis is also feasible with near-infrared spectroscopy, analysing
ethanol in wine. Methanol is easily distinguished from ethanol in the NIR.
❑ NIR finds its use in analysis of marine products. It has been applied to the
determination of moisture, protein, oil and salt content in fish meat.
Continue…

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Mr. S. P. Shinde
Introduction
❑ Infrared (IR) absorption and Raman scattering are the main basic
processes that are employed to detect vibrations in molecules.
❑ Scattering
o When a monochromatic light irradiate a substance, the light may get
absorbed or transmitted or scattered.
o The direction of the pathway of light is diverted by the substances at
different angles of radiations with same or different frequency with or
without change in energy, this process is called as scattering.
o Usually if the energy of the incoming radiation is high then scattering
occurs in forward direction and if the energy of the incoming radiation
is low then scattering occurs in backward direction.

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Raman Scattering
❑ when a monochromatic light passes through a sample, the scattered light
by the molecule contains some different frequency compared to the
frequency of incident light rays, this type of scattering is known as
Raman scattering.
❑ In Raman spectroscopy samples of any physical state solids, liquids and
gas can be examined. Raman scattering was comparatively used less than
infrared absorption, largely due to problems with sample degradation and
fluorescence.
When light strikes the sample, it is absorbed or scattered

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❑ When monochromatic radiation is incident upon a sample then this
light will interact with the sample. It may be reflected, absorbed or
scattered in some manner. It is the scattering of the radiation that
occurs which gives information about molecular structure.
❑ Raman is based on scattering. The sample is irradiated with a coherent
source, typically a laser. Most of the radiation is elastically scattered
(called the Rayleigh scatter).
❑ A small portion is inelastically scattered (Raman scatter, composed of
Stokes and anti-Stokes portions). This latter portion is what we are
particularly interested in because it contains the information in which
we are interested.
Principle

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❑ Sometimes lines of higher frequency are also obtained that of the
incident beam will be scattered. It is called Raman scattering.
❑ The line with lower frequency are called Stoke’s lines.
❑ Also, the line with higher frequency are called Antistoke’s lines.
❑ The line with the same frequency as that of the incident light is called
Rayleigh line.
Raman effect (Inelastic)

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Instrumentation

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Schematic diagram of Raman Spectrometer

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❑ The sources used in modern Raman spectroscopy are nearly always
lasers because their high intensity is necessary to produce Raman
scattering.
❑ Mercury arc is also used and the line corresponding to 4358Aº is
commonly used.
❑ Example of other lines used in Raman spectroscopy include, Argon
ion (488 or 514.5 nm), Krypton ion (530.9 or 647.1 nm), Helium/
Neon (632.8 nm), Diode laser (782 or 830 nm) and Nd; YAG
(1064nm).
Source

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❑ If polychromatic light sources are used then the filters are used to
isolate the monochromatic radiations.
❑ The incident polychromatic radiations will result in overlapping of
lines in the spectrum which will make the interpretation more difficult.
❑ Hence monochromatic radiation is always preferred otherwise use of
filters is required.
❑ Filters made of quartz glass or nickel oxide glass is used for getting
monochromatic radiations.
Monochromators :
o Monochromator can also be used to produce monochromatic light.
o Diffraction grating is used in most of the Raman spectrometers as the
dispersing element.
o Sometime a double monochromator is used to avoid stray light
problem from scattering by dust particles in the sample.
Filters

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Sample Holder
The sample holder depends upon the nature of samples used.
Liquid Samples
For liquid samples, glass or silica sample holders or capillaries are used.
A glass melting-point capillary is used for non-absorbing liquid.
Solid Samples
For solid samples unlike IR, no medium such as null, KBr or solvent is
needed. Solids as polycrystalline material or as a single crystal can be
analysed by Raman technique.
Gas Samples
The Raman spectra of gases are generally weaker than those of liquids or
solids and hence may require cells of larger path length. The gas may be
filled in a glass or silica tube of 1 to 2 cm diameter.

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❑ Detection system involves the use of photographic plate or automatic
recording devices.
❑ In photographic plate detection, the scattered light emerging through
a glass window is passed through a prism or grating and then focused
on a photographic plate.
❑ The plate is then developed and both the line frequencies and
intensities can be measured using external equipments.
Detectors

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Applications
1. Raman spectroscopy is commonly used in organic chemistry to identify
the molecular structure of organic compounds. Raman lines generally lie
in the region of 500-3500 cm-1.
2. It is used to determine the structures of cis and trans isomers.
3. The technique is used is to study changes in chemical bonding, when a
substrate is added to an enzyme.
4. Raman spectroscopy is used to identify active pharmaceutical
ingredients (APIs) and their polymorphic form thus useful in solid state
chemistry and the bio-pharmaceutical industry.
5. Raman gas analyzers are used in medicine for real-time monitoring of
anesthetic and respiratory gas mixtures during surgery.
6. Raman scattering gives information on the crystal orientation.

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