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22 Spectroscopy 26(6) June 2011 www.spectroscopyonline.com Chemometrics in Spectroscopy Classical Least Squares, Part VI: Spectral Results We continue to examine in detail the spectral behavior of three-component mixtures. Howard Mark and Jerome Workman, Jr. This column is the continuation of our discussion of the classical least squares approach to calibration (1–5). In this installment, as in the previous one (5), we will be dealing largely with figures, and the spectra therein. So far, we have looked at the spectra of the pure components comprising the ternary mixtures we are working with. We also have looked at the spectra of all the mixtures overlaid on each other. One important reason to perform this extensive exercise of examining spectra is to address, and hopefully clear up, a misconception some people have about nearinfrared (NIR) spectra. Because of its history, NIR spectroscopy sometimes is believed to rely on, and require, the “magic” of chemometric analysis to obtain meaningful scientific results. But that’s not always true. That perception has arisen because NIR measurements most often are made on samples that are powdered solids, part of a dynamic mechanism, or involve other difficult conditions that often make other types of spectral measurements impossible. This has created an impression that NIR exists in a Celebrating 25 Years The editors congratulate Howard Mark and Jerry Workman for 25 years of statistics and chemometrics columns in Spectroscopy. universe of its own, disconnected from the rest of science. Therefore, we take this opportunity to emphasize (and maybe overemphasize) the fact that NIR is not some “magical” technique that is different from the rest of the universe, but in fact is the same spectroscopy we are all used to seeing in other spectral regions. When samples are presented to a spectrometer, they behave the same way in the NIR as they do in the UV, visible, mid-IR, or any other region. In the current experiment, we have a situation where clear liquid samples are used, and measured in a cuvette having plane parallel windows — the near-ideal conditions that are normally used for any spectral region. In this way, we can demonstrate that NIR spectra, under those conditions, do indeed behave the same way as spectra in other spectral regions do. Let us look at the three sets of two-component mixtures that the experimental design includes. These are presented in Figures 1–3. In these figures, we first present the full spectrum, and then for each mixture we present the spectra of the mixtures in the several subranges that contain the useful absorbance bands. Examining these spectra, we see some features that were partially obscured when the spectra from all the mixtures were plotted together. One effect that was hidden by the overlapping spectra was the presence of isosbestic points. Although Figure 5e from part V of this column series (5) shows a ternary isosbestic point, that is rare; for the most part isosbestic points are not seen
www.spectroscopyonline.com June 2011 Spectroscopy 26(6) 23 (b) (a) 1 0.9 -0.2 -0.1 4000 5000 6000 7000 8000 9000 10,000 4000 4100 4200 4300 4400 4500 4600 4700 4800 4900 5000 0.8 0.8 0.7 0.6 0.6 0.5 0.4 0.4 0.3 0.2 0.2 0.1 0 0 Wavelength Transmittance (c) 1 (d) 1 Transmittance 0.9 0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0 5000 5500 6000 6500 Wavelength Transmittance 0.95 0.9 0.85 0.8 0.75 0.7 0.65 6500 6600 6700 6800 6900 7000 7100 7200 7300 7400 7500 Wavelength (e) 1 0.95 0.9 Transmittance 0.85 0.8 0.75 0.7 0.65 7500 8000 8500 9000 Wavelength Figure 1: Transmission spectra of mixtures of toluene and dichloromethane: (a) full spectrum from 4000 to 10,000 cm -1 ; (b) 4000–5000 cm -1 ; (c) 5000–6500 cm -1 ; (d) 6500–7500 cm -1 ; (e) 7500–9000 cm -1 .
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Experimental - Spectroscopy

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