A beam of infrared light is produced and split into two separate beams. One is passed through the sample, the other passed through a reference which is often the substance the sample is dissolved in. The beams are both reflected back towards a detector, however first they pass through a splitter which quickly alternates which of the two beams enters the detector. The two signals are then compared and a printout is obtained.
A reference is used for two reasons:
* This prevents fluctuations in the output of the source affecting the data
* This allows the effects of the solvent to be cancelled out (the reference is usually a pure form of the solvent the sample is in)
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