Monograph
46Pages

NIR Spectroscopy A guide to near-infrared spectroscopic analysis of industrial manufacturing processes

A guide to near-infrared spectroscopic analysis of industrial manufacturing processes All rights reserved, including translation rights. Printed in Switzerland by Metrohm AG, CH-9101 Herisau, Switzerland. 8.108.5026EN – 2014-02

A guide to near-infrared spectroscopic analysis of industrial manufacturing processes

Near-infrared spectroscopy (NIRS) is a simple, quick (< 30 s analysis time), nondestructive technique that provides multiconstituent analysis on virtually any matrix with levels of accuracy and precision that are comparable to primary reference methods. As another great benefit, near-infrared (NIR) analyses require no sample preparation or manipulation with hazardous chemicals, solvents, or reagents: NIRS thus belongs to the reagent-free methodologies, for which reason it is an exemplary analytical contribution to the Environmental Protection Agency’s (EPA) green chemistry attempt. The...

Theory of vibrational spectroscopy Theory of vibrational spectroscopy Names as «overtone vibrational spectroscopy» and «anharmonic vibrational spectroscopy» illustrate that the understanding of the NIRS theory is by far not easy. Although it is beyond the scope of this monograph to provide an overview of the theoretical background of vibrational spectroscopy, it cannot do without delving into some basic principles that facilitate spectra interpretation. Interaction of light and physical matter Spectroscopy uses light – electromagnetic radiation – to analyze materials by describing the...

Theory of vibrational spectroscopy According to Hook’s law, the potential energy of the harmonic oscillator is a quadratic function of the displacement of the vibrating atoms. The parabolic function is symmetrical about the equilibrium bond length . (Eq. 3) – potential energy – internuclear distance – internuclear distance at equilibrium – displacement of vibrating atoms Quantum mechanical considerations using the Schrödinger equation show that the vibrational energy for the harmonic oscillator has no continuum for vibrational energy levels, but only certain discrete energy levels. ( (Eq....

Theory of vibrational spectroscopy A MetrohmNIRSystems Anharmonic oscillator The potential energy curve of the harmonic oscillator (Eq. 3) is parabolic and only transitions between consecutive energy levels that additionally cause a change in dipole moment are possible. According to this model, infinite energies can be stored in the molecule without bond breaking. Experience teaches, however, that all molecules dissociate, when only enough energy is applied and the vibrating bond is extended. Moreover, strong repelling forces are observed, when the atoms are pressed together. Due to this...

A MetrohmNIRSystems 3 NIRS, MIR, and Raman Spectroscopic techniques excel by their possibility to gain rapid and accurate information from the high-resolution spectra of solid and liquid samples without prior (NIRS) or reduced (Raman) sample preparation. The advantages of NIRS, mid-infrared (MIR), and Raman are numerous (Table 1). All three spectroscopic techniques are economic and facilitate qualitative and quantitative as well as noninvasive and nondestructive analysis. Further advantages include that they are reagent- and waste-free and require no additional auxiliary chemicals. For all...

NIRS, MIR, and Raman NIRS can analyze through some translucent packaging materials and is thus ideally suited for quality control of raw materials. The penetration depth also allows to analyze bulk material. Intermolecular hydrogen bondings and dipol interactions alter vibrational energy and thus lead to shifts in existing absorption bands or even the formation of new absorption bands. The fact that the spectra are strongly impacted by physical parameters (e.g., particle size, density, and moisture content) is on the other hand the reason, why NIRS is ideally suited for the capture of these...

This monograph gives an impression of some of the chemometric procedures on a basic level. If necessary, a deeper insight into chemometrics can be gained by studying the literature listed in chapter 12. Whether or not a specific technique can or should be applied depends on the particular problem and the software that is used for data analysis. Data analysis and chemometrics A good definition of chemometrics stems from Massart (2003): «Chemometrics is a chemical discipline that uses mathematics, statistics and formal logic (a) to design or select optimal experimental procedures; (b) to...

Data pretreatment Nonhomogeneous distribution of particles in a sample, particle size differences, sample density variations, sample morphology differences (shape and roughness of sample surface), … lead to sample-to-sample variations in the overall path-length the photons have to travel before they reach the detector. These small physical differences from sample to sample lead to light scattering effects that influence the measured NIR spectra and result in baseline shifts and scaling variations (intensity variations). If the physical appearance of a sample is not of interest (e.g., as the...

Chemometrics component and is calculated to have the highest remaining variance possible; this is done until only noise remains. «Overfitting» (choosing too many principal components so that one or more of them only represent noise or unwanted interferences) has to be carefully avoided as it is detrimental for the robustness and prediction performance of the involved model. It is well possible that an overfitted model may show excellent results for samples that are part of the calibration set but at the same time fails to predict properties of samples that are part of an external validation...

Chemometrics Literature describes the use of the ANN technique for NIR calibration when the relationship between the measured spectral data and the quantitative sample property to be analyzed is nonlinear. The linear calibration methods PCR and ordinary PLS would not work well under this condition. It goes beyond the scope of this monograph to cover every mathematical detail of the different calibration models. Moreover, it is not necessarily a must to be a math wizard who has to know every detail about a used algorithm. It is the more important to know under which circumstances a model can...