| Because chemometric methods have been applied to transmission measurements in the near-infrared and visible wavelength region, it is a natural extension to apply these same techniques to optical-fiber transmission measurements. Near-infrared spectra are made up of combinations and overtones of the fundamental bands, which tends to make the peaks highly overlapped. This makes peak assignments extremely difficult. Spectra obtained over optical fibers are more difficult to interpret because the optical fibers contain large background absorbances. These absorbances limit the wavelength range which can be measured in a spectrum. Many times, only a shoulder of a peak can be obtained. It is shown that the chemometric methods are still able to find a linear relationship to the analyte in spite of the apparent lack of information.; Spectroscopy over optical fibers provides a tool for performing process analysis. This method is non-destructive, can be done remotely, and is reproducible. Linear results are obtained with near-infrared and visible optical-fiber transmission analysis. A diffuse reflectance study of valine, performed in the optical-fiber region, shows stepwise multiple linear regression (SMLR) can be used to determine the enantiomeric purity. This study is used to illustrate the ability of chemometrics to find linear relationships. SMLR is also used to measure the weight percent of methanol in gasoline via optical fibers. Partial least squares (PLS), a global calibration method, was used in the quantitation of ethanol in wine and the classification of fuels according to type. These final two studies were also performed over optical fibers.; A way to improve the PLS method is presented. An F test and a multiple correlation coefficient are used to detect the introduction of chemical species unknown to the calibration. Once the unknown has been detected, other means can be used to identify it. These statistics have been tested with calibrations which used visible optical-fiber transmission spectra of plutonium. |
