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39 Infrared Analysis Methods​

39-00.01 Near-Infrared Methods—Guidelines for Model Development and Maintenance

This guideline provides general information on near-infrared (NIR) model development, performance evaluation, and calibration transfer to instruments of like brand. NIR spectroscopy, applied to the analysis of constituents in cereals, is popular for reasons of simplicity of operation, throughput, objectivity, and accuracy. The absorption response by overtone and combination frequencies of O-H, C-H, and N-H molecular vibrations, abundant in biological matter, allow samples to be analyzed. These signals are discernible by photometric detectors, which inherently have a very high signal-to-noise response. With the aid of a computer for postprocessing, the composition of cereals can be determined from NIR spectra, in terms of the contents of protein, carbohydrate, moisture, lipid, and other constituents. NIR quantitative analysis is a secondary procedure. Linear and nonlinear (less often) regression modeling is used to relate NIR readings to chemical concentrations determined by conventional reference methods. As such, the robustness and accuracy of an NIR model are strongly dependent on the samples chosen for calibration development, on the accuracy of the reference methods, and on the calibration procedure itself.

39-01.01 Evaluation of NIR Instrument Calibration

This procedure covers the general protocol to be adopted when an instrument’s calibration is statistically assessed. It assumes that reference and instrument values have been collected from a common set of samples by an appropriate method. By using regression analysis, the coefficients of the linear relationship between the instrument values and reference values of a set of standard samples are determined. When the reference values are regressed onto the instrument values, the slope is tested to determine whether it is significantly different from a 45° line (for skew), and if it is not significantly different, the significance of the bias (instrument mean minus reference mean) is tested. Where appropriate, precision and accuracy are also assessed according to the procedures specified in Method 39-00.01.

A spreadsheet calculator to evaluate NIR calibration accompanies this method.

39-10.01 Near-Infrared Reflectance Method for Protein Determination in Small Grains

Protein content in small grains may be ascertained by near-infrared reflectance readings of the ground material at either discrete nonadjoining wavelengths or in a continuous wavelength region. Measurement is based upon a calibration to a suitable standard method (Method 46-11.02, 46-12.01, 46-13.01, 46-16.01, 46-19.01, or 46-30.01), whereupon an empirical model is made that relates spectral response to reference value. Near-infrared absorption, attributable to combination or overtone vibrational frequencies of NH from peptide linkages between amino acids, is the primary basis of a protein content model.

39-11.01 Near-Infrared Reflectance Method for Protein Determination in Wheat Flour

Protein content in wheat flour may be ascertained by near-infrared reflectance readings either at discrete nonadjoining wavelengths or in a continuous wavelength region. Measurement is based upon a calibration to a suitable standard method (Method 46-11.02, 46-16.01, or 46-30.01), whereupon an empirical model is made that relates spectral response to reference value. Near-infrared absorption, attributable to combination or overtone vibrational frequencies of NH from peptide linkages between amino acids, is the primary basis of a protein content model.

39-20.01 Near-Infrared Reflectance Method for Protein and Oil Determination in Soybeans

Protein or oil content determination in soybeans may be ascertained by near-infrared reflectance readings of the ground meal either at discrete nonadjoining wavelengths or in a continuous wavelength region. Measurement is based upon a calibration to a suitable standard method (protein from Method 46-11.02, oil from Method 30-25.01, and moisture from Method 44-15.02). Near-infrared absorptions, attributable to combination or overtone vibrational frequencies of NH, CH, OH, and CO, are the primary bases of protein and oil content models.

39-21.01 Near-Infrared Reflectance Method for Whole-Grain Analysis in Soybeans

Protein, oil, or moisture content determination in soybeans may be ascertained by near-infrared reflectance or transmittance through intact seed either at discrete nonadjoining wavelengths or in a continuous wavelength region. Measurement is based upon a calibration to a suitable standard method (protein by Method 46-11.02, oil by Method 30-25.01, and moisture by Method 44-15.02). Near-infrared absorptions, attributable to combination or overtone vibrational frequencies of NH, CH, OH, and CO, are the primary bases of protein and oil content models.

39-25.01 Near-Infrared Reflectance Method for Protein Content in Whole-Grain Wheat

This method determines the protein content in whole-grain wheat (constant moisture basis) based on transmittance or reflectance of near-infrared (850-2,500 nm) energy. Various combinations of energy dispersion, energy capture, and chemometric algorithms are addressed. It is applicable to wheat.

39-70.02 Near-Infrared Reflectance Method for Hardness Determination in Wheat

Near-infrared reflectance (NIR) spectroscopy provides a rapid measurement of certain compositional factors of a ground sample of grain. Reflectance signal is affected by particle size (near-infrared absorption increases with increase in particle size), and particle size of ground wheat increases with hardness. Therefore, NIR can be used to indicate hardness of wheat as well as other factors relating to composition. This method is applicable to all classes of wheat.