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Chapter 5:
The Shapes of Spectral Signals
Abstract
One of the powers of spectroscopy is the ability to deduce the quantity of an absorbing material in a sample. As mentioned in the last chapter, this concept is called Beer's law. Here, we will take a closer look at Beer's law, particularly from a historical perspective. Beer's law relates the absorbance of a signal to the concentration of the absorbing species. This implies that we are interested in the height of the line. So aside from the straightforward relationship in Beer's law, what are the factors that contribute to the height of a spectral signal? We will explore some of those factors here. Also, we should recognize that spectral lines do not appear at certain, exact frequencies. We sometimes speak of them that way, such as "€œwe have an absorption at 687.22 nm,"€ as if the signal were perfectly monochromatic. The reality is, signals are not perfectly monochromatic, and a signal will, upon close inspection, exist over some range. That is, all spectral lines have some width to them. We will also consider factors that impose a width to spectral signals.
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CHAPTER 5
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