posted by Chelsea .
how could you use infrared spectroscopy to detect the presence of ethanol in an oxygenated fuel?
I think it is either a C-O stretch or
-OH stretch, not sure which one? And I know there is more, but I don't know what to say. Please Help.
Infrared spectroscopy is termed a “chemical fingerprint.” It allows one to determine the presence of certain compounds in a sample by determining possible peak assignments through comparison of experimental and literature frequencies. Ethanol would result in an H-bonded
–OH stretch (3600-3100 cm-1) and a C-O stretch (1300-1000 cm-1).
Oxygenated fuel is fuel that has an added material to increase the oxygen as the fuel (gasoline) burns. In the U. S., that was MTBE until it was outlawed and ethanol has taken the place of MTBE. So if you define oxygenated fuel as a hydrocarbon to which ethanol has been added, then the OH stretching around 3400 cm^-1 would be the one to use. If the fuel is dry (no water), then ethanol is the only absorption in that range (the C-H is close but usually doesn't interfere). You might want to look at the OH bending also.
Here is a table that may be useful. I talk about oxygenated fuel in terms of additives to gasoline but your question states oxygenated FUELS which could be anything for a fuel. If the fuel contains something besides hydrocarbons, the problem is more complicated.
By the way, aromatic hydrocarbons and alkenyl C-H frequencies are in the same range as the OH (which is broad).