Infrared (IR) spectroscopy is an analytical technique used to identify and characterize the chemical composition of organic and inorganic compounds. This method measures the interaction between a molecule and infrared radiation, a form of light with a lower energy level than visible light. The resulting measurement, called a spectrum, provides a unique molecular fingerprint based on how the sample absorbs energy at specific frequencies. Analyzing this pattern of light absorption allows scientists to determine the types of chemical bonds and functional groups present within the sample structure.
The Underlying Principle of IR Absorption
Molecular bonds are constantly in motion, stretching and bending in movements known as molecular vibrations. These vibrations occur at specific, natural frequencies unique to the bond type, such as a carbon-oxygen double bond (C=O) or a carbon-hydrogen single bond (C-H). When the frequency of the incoming IR radiation exactly matches a bond’s natural vibrational frequency, the molecule absorbs that energy. This absorption increases the amplitude of the bond’s motion, which is the event recorded by the spectrometer.
For a bond to absorb infrared energy, its vibration must result in a temporary change in the molecule’s dipole moment. The dipole moment measures the separation of positive and negative charges within the molecule. If a bond is polar, its stretching creates an oscillating electrical field that interacts with the electrical field of the IR light.
Molecules like oxygen (O2) or nitrogen (N2) do not change their dipole moment when they vibrate, making them “IR inactive” and unmeasurable by this technique. Conversely, bonds with significant charge separation, such as the O-H bond in an alcohol, are highly IR active and produce strong absorption signals. The absorbed energy causes a transition between vibrational energy states, not an electronic transition.
Sample Preparation Methods
Sample preparation is a step because the material used to hold the sample must be transparent to the infrared light beam. The technique chosen depends on the physical state of the compound (solid, liquid, or viscous material). Materials like sodium chloride (NaCl) or potassium bromide (KBr) are commonly used as plates or matrices because they do not absorb light in the mid-infrared region. It is important to eliminate water and carbon dioxide from the sample and the instrument environment, as both atmospheric components produce strong, interfering absorption peaks.
Solid Samples (KBr Pellet)
For solid samples, the most common method is creating a KBr pellet. This requires grinding a small amount of the sample with spectroscopic grade KBr powder. The mixture is then compressed under high pressure (typically 8 to 15 tons) to form a clear, thin disk. The resulting transparent pellet is mounted directly into the spectrometer’s beam path for analysis. Since KBr readily absorbs moisture, preparation must be performed quickly, and components must be kept in a desiccator to prevent water contamination.
Liquid and Viscous Samples
Liquid samples, especially non-volatile ones, are typically prepared by creating a thin film between two salt plates. A single drop of the liquid is placed onto one polished salt plate, and a second plate is pressed against it to create a thin layer of sample. For solids that are oily, viscous, or react with KBr, an alternative method called a Nujol mull is used. The solid sample is finely ground with a small amount of Nujol (liquid paraffin or mineral oil) until a thick, uniform paste is formed.
This viscous paste is then pressed between two salt plates to create a thin film suitable for measurement. The Nujol itself has characteristic C-H absorption bands that will appear in the final spectrum and must be accounted for during interpretation. If the sample is soluble in a volatile solvent, a thin film can be prepared by dissolving the sample, applying the solution to a salt plate, and allowing the solvent to evaporate completely.
Operating the Spectrometer and Data Collection
Modern IR instruments are typically Fourier-Transform Infrared (FTIR) spectrometers, which use an interferometer to rapidly measure all infrared frequencies simultaneously. The procedure begins by powering on the instrument and allowing it time to stabilize internal components, such as the source lamp and the laser. After initializing the software, the first measurement collected is the background scan, performed with no sample present in the beam path.
The background scan records the absorption signature of the air in the sample compartment, primarily water vapor and carbon dioxide, and inherent instrument noise. This signal is automatically subtracted from the subsequent sample scan to isolate the absorption peaks belonging only to the compound of interest. Scan parameters are then set, including the spectral range, resolution, and the number of scans to be averaged (often 4 to 32 for a good signal-to-noise ratio).
Once the background is collected, the prepared sample is carefully placed into the sample compartment, ensuring correct positioning in the infrared beam path. If using an Attenuated Total Reflectance (ATR) accessory, the sample is placed directly onto the crystal and compressed to ensure maximum contact. The instrument then collects the data, which is processed using a mathematical function called a Fourier transform to convert the raw signal into the final spectrum.
Basic Data Analysis and Interpretation
The final output is a graph plotting the intensity of infrared absorption (percent transmittance or absorbance) against the wavenumber, measured in inverse centimeters (cm⁻¹). The spectrum is conventionally divided into two main regions, with the division occurring around 1500 cm⁻¹.
The Functional Group Region
The region from 4000 cm⁻¹ down to 1500 cm⁻¹ is known as the functional group region, which is the primary area for identifying bond types. Characteristic stretching vibrations of common bonds appear here at predictable locations. For example, a strong, sharp peak around 1715 cm⁻¹ indicates a carbonyl group (C=O), such as those found in ketones or aldehydes. A broad absorption band appearing between 3200 and 3600 cm⁻¹ signifies the presence of an O-H stretching vibration, characteristic of alcohols or carboxylic acids.
The Fingerprint Region
The lower half of the spectrum, from 1500 cm⁻¹ down to about 400 cm⁻¹, is called the fingerprint region. This area contains a complex pattern of peaks arising from various bending and stretching vibrations. While too complicated for simple functional group assignment, the overall pattern is unique to every single molecule. This region is primarily used for confirming the identity of an unknown compound by comparing its exact pattern to the spectrum of a known reference material.