Question

How is infrared spectroscopy used to identify the presence of organic compounds in water samples?

Answer 1

For IR spectroscopy, it is important to choose an appropriate solvent so that we will have the peaks of the compound we're analyzing instead of the solvent. Also, water has an IR spectrum with strong peaks.

Complete answer:
Let us first discuss about IR spectroscopy:-
Infrared spectroscopy (also known as vibrational spectroscopy) is the measurement of the interaction of infrared radiation with matter by absorption, emission, or reflection. It is used to analyze, study and identify chemical substances or functional groups of the compound given in solid, liquid, or gaseous forms. The method or technique of infrared spectroscopy is conducted with the help of an instrument called infrared spectrometer (or spectrophotometer). It produces an infrared spectrum.
The mid-infrared region has a range from approximately 4000-400 $c{{m}^{-1}}$ ($2.5-25\text{ }\mu m$ ) is used to study and analyze the fundamental vibrations and associated rotational vibrational structure.
As we know, water shows a strong and broad absorbance band in the IR spectrum, around 3600-3200$c{{m}^{-1}}$, along with other combinations and overtone bands. Its bands are strong and broad due to polarity and H-bonding, as a result, the other compound bands are not visible in the IR spectrum. Also, the presence of water in the sample distorts the IR spectrum of the compound. Moreover, KBr and ZnSe optical accessories (prism, beam splitter) used in IR spectrometers are also water sensitive and therefore, it can permanently damage them.
All alcohols, amines, carboxylic acids and other similar functional groups, have peaks lying in the range: 3500-3000$c{{m}^{-1}}$. Using water as a solvent would defeat the purpose of identifying the peaks of these organic compounds.
Therefore, infrared spectroscopy would not be able to identify the presence of organic molecules when the solvent used is water.

Additional Information:
There are many better solvent such as carbon tetrachloride ($CC{{l}_{4}}$), which has a doublet IR peak at 795$c{{m}^{-1}}$ and 762$c{{m}^{-1}}$ , and not much else otherwise. That’s why it's a good solvent and it doesn't significantly overlap with peaks found while analyzing organic compounds.
Examples of some other good solvents are acetone, diethyl ether, etc.

Note:
Before using the IR spectrometer, always check the solvent being used because if you use water as a solvent, you would never receive the desired result.