Question

Why would the quantative analysis on Ethanol in spirits be difficult to do with infrared spectroscopy (IR) vs doing with Raman?

Answer 1

The quantitative analysis of ethanol by spectroscopic method is governed by the Beer's Law. By plotting Absorbance vs concentration graph we can easily determin the amount of ethanol present in spirit by compairing with a standard.

In the spectra the concentration of ethanol depends on O-H signal . In IR spectra the O-H absortion peak (3600-3045 cm-1) and area under the combined alken C-H absorption (3045-2760 cm-1)are very close to each .So it is very difficult to separate the absorption peaks .On the other hand ethanol can undergo intermolecular H bonding which makes it more difficult to identify the absorption peak .Thus it is difficult analys ethanol quantitatively.

Raman spectroscopy has proved to be an excellent tool, not only for quantitative purpose but also for quantitative analysis of water rich samples like ethanol.There are several advantages using Raman spectroscopy.Since band intensities caused by -OH stretching vibration are week in Raman spectra , water rich sample such as spirits can be directly analysed.here we get only one intense peak at 880cm-1 which could be attributed to the c-c-o stretch of ethanol.from which conc, o