Infrared spectroscopy provides information about the structure of molecules from the absorbance of infrared light leading to bond vibrations. The acquisition of IR spectra is covered on the instrumentation pages with this section covering the interpretation of results. 

There are a number of types of IR data collection listed below. There are some differences in the interpretation of data from the different routes and the information that can be obtained from each method. The more common routes have more detailed pages covering the interpretation of data collected with the method.

ATR of solids or liquids

Attenuated Total Reflectance IR spectroscopy sees widespread use in analysing compounds, providing information on the presence or absence of functional groups, and can provide compound identification from fingerprint matching against databases of known compounds.

The page linked below provides guidance for the interpretation of ATR-IR spectra.

Transmission measurement of liquid or thin film between salt discs

This is an older method of data acquisition, where a compound is placed between two salt discs, and a transmission IR is recorded by placing the discs with compound within the IR beam. The methods have largely been replaced by ATR-IR, although in most cases the resulting spectra are comparable in data, and many of the same approaches for interpretation can be followed.

Transmission measurement samples are typically prepared in two ways:

• Liquids: A drop of liquid is placed onto one of the salt discs, and the second disc is then placed onto the first and twisted to form a thin film of the compound.

• Solids: A drop of a solution of the compound is placed onto one of the salt discs and an analogous process to liquids is followed. Once the solution has been formed into a thin film, the discs are separated and the solvent allowed to evaporate prior to bringing both discs back together and recording the spectrum.

Nujol mull transmission IR

Nujol is a brand of light paraffin oil produced by Schering-Plough, consisting of a mixture of saturated hydrocarbons and the use of Nujol mulls was widespread until being displaced by ATR-IR during the 2000s.

Consequently, comparing data to reported literature recorded as Nujol mulls remains commonplace, and understanding how the technique works is important to allow effective data comparisons to be made.

Samples were prepared by grinding solids using a pestle and mortar to a fine powder before adding drops of Nujol oil to form a paste. This paste would be smeared onto a salt disc and the second disc placed against the first and twisted to form a thin film. Consequently, the data recorded arises from the compound of interest along with peaks arising from Nujol. Analysis of Nujol derived spectra is similar to ATR-IR, with the additional step of needing to discount peaks from the oil. Note that Nujol bands may obscure bands from the compound of interest.

Solution phase transmission IR

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Gas phase transmission IR

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