INTRODUCTION

              The
Near –infrared spectroscopy (NIRS) is a spectroscopic method that uses the
near-infrared region of the electromagnetic spectrum. The near –infrared region
is the region of 700 – 2500 nm (wavelength) or 12500 – 4000 cm–1(wave
number) of the electromagnetic spectrum. This region lies between the visible
and IR regions. In the year1800 the astronomer Friedrich Wilhelm Herschel
analysed the spectrum of sunlight .He separated the electromagnetic spectrum
with a prism and found out that the temperature increased markedly towards and
beyond the red, i.e in the region that is now called the near infra red.

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PRINCIPLE OF NIR

               NIR
spectroscopy is based on the vibrational spectroscopy behaves as a wave with
the properties of simple harmonic motion. The NIR spectrum originates from
radiation energy transferred to mechanical energy associated with the motion of
atoms held together by chemical bonds in a molecule. The absorption of infrared
radiation depends on the net change in the dipole movement of the molecule as a
consequence of its vibration motion. When the vibrations are accompanied by
change in dipole movement and when the frequency of vibration matches the
frequency of infrared radiation, a transfer of net energy from the radiation to
the molecule will be observed. This results in a change in the amplitude of the
molecular vibrations. That is, the vibration absorbs the infrared radiation and
the molecule is excited to a higher energy level. This energy transmission can
be measured as the plot of energy (reflectance, absorption or transmittance)
versus wavelength, which is called as spectrum .Electromagnetic response in the
NIR region is primarily from the vibration response of O-H, C-H, C-O and N-H
molecular bonds. Hydrogen bond has more spectral stability in NIR region. This
helps greatly in food quality analysis and possible information of the chemical
composition present in the food. In this way NIR spectrophotometer play key
role in identification and quantization of sample.

INSTRUMENTATION

There is a
source, a detector, and a dispersive element 
to allow the intensity at different wavelengths to be recorded.
Depending on the sample, the spectrum can be measured in either reflection or
transmission. Common incandescent or quartz halogen light bulbs are most often
used as broadband sources of near-infrared radiation for analytical
applications.  The available spectral
region of the radiation is determined by the type of source, beam splitter used
in the interferometer and detector. Light-emitting diodes (LEDs) are also used;
they offer greater lifetime and spectral stability and reduced power
requirements. The data acquired from a sample by an NIR spectrometer contains
spectral information related to the sample composition. Different chemical
entities have absorption at specific wavelengths which can be used to define
the chemical composition of different substances. However, absorption peaks of
different chemical molecules overlaps in several parts of the spectral region. The
absorption signal is often weak compared to other phenomena that intervene in
the interaction between light and particle like all the scattering effects. For
these reasons, it is necessary to mathematically process spectral data in order
to extract valuable information on the chemical properties of samples.

CALIBRATION PROCEDURE

                           Chemometric methods
are used to extract information from NIR. Chemometric is the chemical
discipline that uses mathematical and statistical methods to design or select
optimal procedures and experiments and to provide maximum chemical information
by analyzing chemical data.The most important part in the development of an NIR
method is building the predicting model generally called calibration. The calibration
is simply a regression model that will allow the prediction of chemical
composition based on spectral data.

ADVANTAGE

The spectral
measurements is really rapid one sample can be scanned in less than 1min.

Less expensive
because there isn’t any use of chemical reagents and a single operator can
analyze a large number of samples

Several scans
can be made on the same object, which permits to obtain a more representative
sample composition and a more accurate result of analysis

Sample
requires minimal (drying and grinding) or no preparation

Several
constituents of the same sample can be measured at the same time

Easily
applicable in different environments (like industry, laboratory, harvesters,
etc.) measurements can also be carried out on/in/at line

The
opportunity to use optical probes makes it possible to analyse the sample in-situ
the availability of portable instruments permits to obtain spectra directly in
the field, useful to follow process like ripening.

DISADVANTAGE

Low
sensitivity of the signal which can limit the determination of substances with
concentration below 0.1%.

It is a
secondary analytical method, so it requires an accurate chemical an physical
analysis as reference samples.

Development of
calibration models require high trained personnel

Accurate and
robust calibration require a large data set incorporating large variation,
which is often difficult to obtain.

It requires a
continuous maintenance of the calibration data set.

With some
hardware it is difficult to transfer calibration between instruments of  the same manufacture or between different
manufactures .

Although NIR technique has low measuring cost, the initial high
financial investment for the instrumentation represents a important obstacle
for the purchase

APPLICATIONS

Analysis of
proximate composition of feed ingredients starch estimation, total sugar
analysis ,quality control of oils, urea estimation, salt analysis ,fatty acid
composition, amino acid composition, digestibility measurements etc.,

 

 

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