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General Chemistry

EPR Signals From Everyday Samples

This laboratory exercise introduces students to the microESR and teaches them the basics of operating the instrument. 

This lab starts by looking at some common everyday radicals. Many radicals are not indiscriminately reactive, and actually have very long life times. For example, the melanin molecule which pigments hair and skin is quite stable; yet it, as we shall see, is a radical. Radicals, like melanin, can also trap radicals like hydroxyl and superoxide which can damage skin. There are many other examples of such stable free radicals. The process of toasting bread or burning wood forms free radicals as does drying tea and roasting coffee. Thermo oxidative decomposition in fuels and lubricants produces a stable radical signal. Asphaltenes and humic and fulvic acids all are stable radicals. Color or F centers have unpaired electrons and are responsible for the color in amethyst, smoky quartz, and fluorite.

Kinetics

The Reduction of TEMPOL by Ascorbic Acid

EPR is one of the best tools for studying reaction mechanisms in which radicals are involved. Antioxidants are an important class of compounds in biology and chemistry. Understanding antioxidant behavior, both beneficial and not, helps us use antioxidants to our benefit, and minimize the adverse effects of some antioxidants. Some antioxidants are stable free radicals and can be observed directly by EPR. There are other free radicals, such as the superoxide anion, that have a lifetime that is too short to observe directly by EPR. There are several EPR methods that can be used to elucidate radical reaction mechanisms.
One method, and the method that will be used in this lab, involves using a stable radical such as a nitroxide. The stable radical has an EPR signal that can be monitored in real time by EPR. The nitroxide can react with other radicals present to produce a non-radical product, or the nitroxide can react with a non-radical, such as an antioxidant, to form a non-radical product. In both cases the EPR signal of the nitroxide will decrease at the same rate it is used up allowing us to monitor the rate of the reaction.