What is the difference between ir and nir

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You can read FutureLearn's Cookie policy here. Content was interesting but not good enough for the course, i would love to learn different methods in order to find food frauds in products. Hope to learn more from here.

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Learn more about this course. This article compares the two approaches. Share this post. In one method, called the dispersive method, individual wavelengths which are produced by shining a broadband source on a diffraction grating or a prism are impingent on a material and absorption at each wavelength is measured. Near Infra-red diffuse reflectance method utilizes this technique.

In the second method, the full beam of the broad-band source shines on the input of an interferometer where the sample whose absorption is to be measured is placed in one arm. The interferometer consists of a stationary mirror and a moving mirror and the scanning is done through the movement of the moving mirror. An interference pattern is hence recorded whose Fourier transform regenerates the spectral signal.

The same measurement is done without the presence of the absorption material in one arm and by dividing the former by the latter, an absorption spectrum for the material is generated. Figure 2 shows the interferometer for a FT-IR method:. Figure 3: ATR method. In this method a crystal of high refractive index is in contact with the sample surface.

An infrared beam undergoes total internal reflection and generates an evanescent wave which penetrates into the material through depth of few microns.

Each bounce produces some absorption of the beam and after several bounces, the IR beam reaches a detector. Since the absorption is strong, this technique has the advantage of probing several locations of the sample surface. One can see that this method is effective to probe only a thin layer on the surface of a material and if the bulk of the material needs to be probed, this method will fail.

NIR is the choice for this seedy, watery material because the nir beam can penetrate through the material and bounce several times before reaching the detector. The technique of trans-reflectance is shown in Figure 4. After undergoing multiple reflection through the volume of the sample, the beam finally finds it way to the detector and hence absorption spectra represent the bulk of the sample rather than just the touch of the surface as was done with ATR sampling technique used for FTIR.

The trans-reflectance method has the disadvantage that the source should be separate from the detector and can not be embedded within the spectrometer. It is a striking difference in the dependence of the quality vs. This result may be explained based on direct comparison of the molecular motions corresponding to the vibrations being influential for the IR and NIR regions. Vibrations involving out-of-plane atomic motions strongly affect the IR region of melamine.

Therefore, it is essential to incorporate a proper description of the inter-plane interactions of melamine molecules as they appear in the crystalline lattice. The neglecting of the neighboring molecules in proximity planes leads to a completely incorrect calculated IR spectrum. In contrast, the in-plane vibrations of melamine are less affected by inter-molecular interactions. The vibrations involving in-plane atomic displacements are the most essential for the NIR spectrum of melamine. On the other hand, the out-of-plane motions are either suppressed in intensity or they are located outside of the NIR region.

Hence, the calculated NIR spectrum of melamine is not significantly affected by a radical simplification of the molecular model. Even a single molecule model provides accurate reproduction of NIR spectrum of crystalline melamine. It may be concluded that the long-range ordering and, in particular, inter-plane interactions in the crystal lattice of melamine are significantly less important factors for NIR modes than for IR modes. From this observation, another important conclusion may be drawn.

Due to the very significant computational cost of anharmonic calculations, the possibility to reduce the complexity of the molecular model i. Our findings shed light on the spectral features of melamine that have been reported in analytical spectroscopic studies of melamine as contaminant. In particular, the concluded in literature superiority of NIR spectroscopy in the analysis of the samples with relatively higher content of melamine was explained.

Table S1: Projection of normal coordinates of melamine onto natural internal coordinates. Conceptualization, K.

The authors declare no conflict of interest. The funders had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript, or in the decision to publish the results. Sample Availability: Sample of the compound melamine is available from the authors.

National Center for Biotechnology Information , U. Journal List Molecules v. Published online Apr Justyna Grabska , 1 Krzysztof B. Kirchler , 1 Yukihiro Ozaki , 3 and Christian W. Huck 1. Find articles by Justyna Grabska. Krzysztof B. Christian G.

Find articles by Christian G. Christian W. Find articles by Christian W. Author information Article notes Copyright and License information Disclaimer. Received Mar 14; Accepted Apr 7. This article has been cited by other articles in PMC. Associated Data Supplementary Materials moleculess Abstract Melamine IUPAC: 1,3,5-Triazine-2,4,6-triamine attracts high attention in analytical vibrational spectroscopy due to its misuse as a food adulterant.

Results and Discussion 2. Experimental and Simulated IR Spectra of Crystalline Melamine The simulation of the IR spectrum of melamine in polycrystalline state requires a proper representation of the long-range ordered structure Figure 1. Open in a separate window. Figure 1. Figure 2. Figure 3. Non-Scaled Calc. Figure 4. Figure 5. New Insights on the Quantitative Analytical Spectroscopy of Melamine Melamine is known food adulterant, and for this reason, it has been frequently focused on analytical IR and NIR spectroscopy [ 9 , 42 , 43 , 44 , 45 , 46 ].

Materials and Methods 3. Quantum Mechanical Calculations 3. IR spectrum Calculation in 3D Periodic Approximation The simulation of IR spectra of crystalline melamine was based on harmonic analysis that was executed in three-dimensional periodic representation of crystal structure in Crystal 09 software Aethia Srl, Italy [ 59 ].

Conclusions Quantum mechanical calculations of IR spectra of crystalline melamine were successfully carried out for an infinite 3D periodic lattice model.

Click here for additional data file. Author Contributions Conceptualization, K. Conflicts of Interest The authors declare no conflict of interest. Footnotes Sample Availability: Sample of the compound melamine is available from the authors. References 1. Deim H. Amino Resins.

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The China melamine milk scandal and its implications for food safety regulation. Food Policy. Cantor S. Analytical methods for the evaluation of melamine contamination. Henn R. Comparison of sensitivity to artificial spectral errors and multivariate LOD in NIR spectroscopy—Determining the performance of miniaturizations on melamine in milk powder.

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