INTRODUCTION TheNear –infrared spectroscopy (NIRS) is a spectroscopic method that uses thenear-infrared region of the electromagnetic spectrum. The near –infrared regionis the region of 700 – 2500 nm (wavelength) or 12500 – 4000 cm–1(wavenumber) of the electromagnetic spectrum. This region lies between the visibleand IR regions. In the year1800 the astronomer Friedrich Wilhelm Herschelanalysed the spectrum of sunlight .He separated the electromagnetic spectrumwith a prism and found out that the temperature increased markedly towards andbeyond the red, i.
e in the region that is now called the near infra red.PRINCIPLE OF NIR NIRspectroscopy is based on the vibrational spectroscopy behaves as a wave withthe properties of simple harmonic motion. The NIR spectrum originates fromradiation energy transferred to mechanical energy associated with the motion ofatoms held together by chemical bonds in a molecule.
The absorption of infraredradiation depends on the net change in the dipole movement of the molecule as aconsequence of its vibration motion. When the vibrations are accompanied bychange in dipole movement and when the frequency of vibration matches thefrequency of infrared radiation, a transfer of net energy from the radiation tothe molecule will be observed. This results in a change in the amplitude of themolecular vibrations. That is, the vibration absorbs the infrared radiation andthe molecule is excited to a higher energy level. This energy transmission canbe measured as the plot of energy (reflectance, absorption or transmittance)versus wavelength, which is called as spectrum .Electromagnetic response in theNIR region is primarily from the vibration response of O-H, C-H, C-O and N-Hmolecular bonds.
Hydrogen bond has more spectral stability in NIR region. Thishelps greatly in food quality analysis and possible information of the chemicalcomposition present in the food. In this way NIR spectrophotometer play keyrole in identification and quantization of sample. INSTRUMENTATION There is asource, 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 ortransmission. Common incandescent or quartz halogen light bulbs are most oftenused as broadband sources of near-infrared radiation for analyticalapplications. The available spectralregion of the radiation is determined by the type of source, beam splitter usedin the interferometer and detector.
Light-emitting diodes (LEDs) are also used;they offer greater lifetime and spectral stability and reduced powerrequirements. The data acquired from a sample by an NIR spectrometer containsspectral information related to the sample composition. Different chemicalentities have absorption at specific wavelengths which can be used to definethe chemical composition of different substances. However, absorption peaks ofdifferent chemical molecules overlaps in several parts of the spectral region. Theabsorption signal is often weak compared to other phenomena that intervene inthe interaction between light and particle like all the scattering effects. Forthese reasons, it is necessary to mathematically process spectral data in orderto extract valuable information on the chemical properties of samples.
CALIBRATION PROCEDURE Chemometric methodsare used to extract information from NIR. Chemometric is the chemicaldiscipline that uses mathematical and statistical methods to design or selectoptimal procedures and experiments and to provide maximum chemical informationby analyzing chemical data.The most important part in the development of an NIRmethod is building the predicting model generally called calibration. The calibrationis simply a regression model that will allow the prediction of chemicalcomposition based on spectral data.ADVANTAGE The spectralmeasurements is really rapid one sample can be scanned in less than 1min.Less expensivebecause there isn’t any use of chemical reagents and a single operator cananalyze a large number of samplesSeveral scanscan be made on the same object, which permits to obtain a more representativesample composition and a more accurate result of analysisSamplerequires minimal (drying and grinding) or no preparationSeveralconstituents of the same sample can be measured at the same timeEasilyapplicable in different environments (like industry, laboratory, harvesters,etc.) measurements can also be carried out on/in/at line Theopportunity to use optical probes makes it possible to analyse the sample in-situthe availability of portable instruments permits to obtain spectra directly inthe field, useful to follow process like ripening.DISADVANTAGE Lowsensitivity of the signal which can limit the determination of substances withconcentration below 0.1%.It is asecondary analytical method, so it requires an accurate chemical an physicalanalysis as reference samples.Development ofcalibration models require high trained personnelAccurate androbust calibration require a large data set incorporating large variation,which is often difficult to obtain.It requires acontinuous maintenance of the calibration data set.With somehardware it is difficult to transfer calibration between instruments of the same manufacture or between differentmanufactures .Although NIR technique has low measuring cost, the initial highfinancial investment for the instrumentation represents a important obstaclefor the purchaseAPPLICATIONS Analysis ofproximate composition of feed ingredients starch estimation, total sugaranalysis ,quality control of oils, urea estimation, salt analysis ,fatty acidcomposition, amino acid composition, digestibility measurements etc.,