Spectroscopy in Chemistry
Since the early 1960's, chemists have come to depend very heavily upon spectroscopy as a diagnostic tool. Many people who today call themselves chemists spend almost all of their time analyzing spectra rather than making or isolating new molecules. This is a large shift in procedure from older times, but reflects the inestimable value of spectroscopy to the molecular sciences. The key to the widespread use of spectroscopy in chemistry is that it permits one to probe the microscopic structure of molecules, the ultimate building blocks of the chemical sciences.
Spectroscopy is used for a wide variety of procedures in chemistry. At the simplest level, spectroscopy is now routinely used for identification of new molecules. In the early 1950's, certain identification of a new molecule by melting point/chemical analysis/derivative formation typically took 2-7 days, and could take much longer. Now, the same identification typically takes 15 minutes to one day, depending on complexity and availability of spectroscopic tools. In addition, the identification is much more certain.
Many chemists use spectroscopy for quantitative analysis, i.e., to tell how much of a specific substance is in a mixture. When you look at a glass of colored soda into which ice has melted, you know at once that it is diluted relative to the original soda. You can tell this by the color of the sample. Spectroscopy of various sorts can be used to do exactly the same thing, but with quantitative determination of how strong the absorption is at a certain wavelength. So long as one knows the amount of substance that gives a certain amount of absorption, one can use spectroscopy for analysis of concentration of that substance. One also needs to know the pathlength of the sample (longer pathlengths give apparently deeper colors). In the accompanying ultraviolet-visible spectrum (see below), note that the sample has a maximum absorption wavelength at 512 nanometers (10-9 meters), which turns out to be in the green color region of the human eye. Note also that sample A, with twice the concentration of sample B, also has twice the absorbance (amount of light absorbed): this measurement require both A and B to be placed in tubes with the same pathlength.
