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The most common type of spectroscopy used for structure determination is called absorption spectroscopy. The basis of absorption spectroscopy is that substance absorb energy from certain wavelength of electromagnetic radiation. 
In an absorption spectroscopy experiment, this absorption is determined as a function of wavelength, frequency or energy in an instrument called spectrophotometer or spectrometer (fig.1.1).


Fig.1.1 an absorption  spectroscopy experiment



The experiment requires, first a source of electromagnetic radiation. We place the material to be examined (know as sample), the radiation beams.

 A detector measures the intensity of radiation that passed through the sample unabsorbed; when the intensity of the source through the amount of radiation absorbed by the sample is known.

We then vary the wavelength of the radiation falling on the ration and the observed at each wavelength is recorded as a graph of either radiation transmitted or radiation absorbed versus wavelength or frequency .This graph is commonly known as a spectrum of the sample (figs. 1.2 and 1.3)


Fig.1.2 Infrared Spectrum of nonane 
Fig.1.3 The UV spectrum of isoprene dissolved in methanol shows Λmax=222nm,ε=20,000

There are many types of spectroscopy. The three types of greatest use to organic chemist for structure determination are NMR, IR and UV and spectroscopy.


These type of spectroscopy differ            conceptual only in the type of radiation used, although the kind of spectrometer required for each are quite different.Mass spectroscopy is fourth physical technique but it is not a type of absorption spectroscopy and thus fundamentally different from NMR ,IR and UV spectroscopy.