Studies On Fundmental Issues Of Near-Infrared Spectroscopy: In-Line Analysis, Multi-Component Anlysis And Spatial Effect

The presented thesis focuses on some fundamental issues of near-infrared spectroscooy (NIR), including studies on themodynamics, kinetics, and adsorption state of a gas-solid adsorption process through non-invasive monitoring the process, quantitatively analyzing a multi-component system with a small-scale calibration set, as well as the spatial effect of NIR that was experimentally observed by the author. They can be concisely described as follows:1) Through non-invasive monitoring the uptake of aniline vapor by active alumina in a differential adsorption bed (DAB) with near-infrared diffuse reflectance spectroscopy (NIR-DRS), we have studied several features of the adsorption, including isotherm, kinetics and the chemical state of aniline molecules in aniline-alumina system. What is perhaps more important, since the information above is obtained synchronously, the proposed methodology could provide information about the type of adsorption (chemical or physical adsorption), the change of chemical state of aniline in the system during the adsorption process, whether the chemical adsorption and physical adsorption took place simultaneously, the rate of the chemical and physical adsorption, and so on.2) We have studied the uptake of orthoxylene / isoamyl alcohol on silica gel as an example to demonstrate a novel methodology for studying multicomponent gas adsorption on solid adsorbent. In the method, the solid adsorbent was filled into a differential adsorption bed, and the bed was in-line monitored with near-infrared diffuse reflectance spectroscopy continuously when the adsorption process was taking place. The spectral data recorded during the process were treated with algorithm of locally weighted regression, which constructed a series of partial least squares models to more accurately predict concentrations of each adsorbates on the adsorbent. These efforts made the method feasible to obtain more thermodynamic and kinetic information about the adsorption process in a more convenient, rapid, economical as well as straightforward way.3) Through randomly arranging samples of a calibration set, treating their NIR spectra with orthogonal discrete wavelet transform, and selecting suitable variables in terms of correlation coefficient test (r-test), it is possible to extract features of each component in a multi-component system respectively, and partial least squares (PLS) models based on these features are capable of predicting the concentration of each component. What is perhaps more important, with the proposed strategy, the predictive ability of the model is at least not impaired while the size of the calibration set can be substaintially reduced. Therefore, it provides a more economical, rapid, as well as convenient approach of NIR quantitative analysis for multi-component system. In addition, all important factors and parameters related to the proposed strategy are discussed in detail.4) The paper focuses on solving a common problem of NIR quantitative analysis for a multi-component system: how to significantly reduce the size of the calibration set. To cope with the problem orthogonal discrete wavelet packet transform, least correlation design and correlation coefficient test (r-test) have been combined. A two-component carbon tetrachloride system and a two-component aqueous system have been treated with the proposed strategy where only 21 calibration samples are needed, and the result showed that the predictive ability was not obviously impaired for the former system while being clearly strengthened for the latter one in comparison with a conventional way that required a calibration set of 100 samples.5) An unlooked-for experimental observation that in near-infrared spectroscopy (NIR) the second overtone of aniline adsorbed by 13X molecular sieve nearly disappeared led us investigate a fundamental problem: the behavior of NIR when the outside space surrounding a molecule is too small to allow the molecule to vibrate freely. Through NIR of various organic compounds adsorbed by different porous inorganic materials like 13X molecular sieve, silica gel and active aluminium oxide, and NIR of supramolecular cyanuric acid-melamine, we can reasonably conclude that in the micro-environment above, all intensities of NIR absorbance decrease, and the second overtone decreases more than the first overtone does. Furthermore, one distinct feature of NIR, higher sensitivity to the size of micro-environment space as compared with mid-infrared (MIR), and its application to the study of supramolecular structures are outlined by our experiments.