Infrared (IR) radiation is part of the electromagnetic spectrum, interacting with matter by causing molecular bonds to vibrate, stretch, and bend. Infrared spectroscopy measures how a sample absorbs or transmits this radiation, providing a unique molecular fingerprint for identification and analysis. Understanding intensity in this measurement is fundamental to interpreting the resulting data, as it reveals information about the composition and concentration of the substance being studied.
Defining Intensity in Infrared Measurement
Intensity in an infrared spectrum is the measured magnitude of the interaction between the IR radiation and the sample at a specific frequency or wavenumber. The spectrum’s vertical axis represents this intensity, typically shown as either Transmittance or Absorbance. Transmittance is the fraction of incident IR light that passes through the sample without being absorbed. A high percentage of transmittance (near 100%) indicates little absorption, appearing as a high point or flat line on the graph.
Absorbance, conversely, measures how much of the original radiation energy is blocked or absorbed by the molecule. These two values are inversely and logarithmically related. When a spectrum is plotted in Absorbance mode, the points of energy uptake appear as peaks, with taller peaks signifying greater absorption intensity.
The intensity of an absorption peak is a direct measure of the amount of radiation energy removed from the beam by the sample. This magnitude of energy removal determines the peak’s height or depth on the spectrum.
The Molecular Origin of Intensity Variation
The reason some absorption peaks are strong and others are weak or absent is rooted in the molecule’s change in dipole moment during vibration. A dipole moment measures the separation of positive and negative electrical charges within a molecule or bond. For a molecule to absorb IR radiation, its vibration must result in a net fluctuation in this dipole moment.
When a molecular bond vibrates, it creates an oscillating electric field that must interact with the electric field of the incoming infrared radiation for energy absorption to occur. If the vibration causes a large change in the dipole moment, the interaction is strong, resulting in a high-intensity spectral peak. For example, the stretch of a highly polar bond like the carbonyl group (C=O) causes a substantial change in charge distribution, leading to a strong, characteristic absorption band.
Conversely, if a vibration causes only a small change in the dipole moment, the absorption peak will be weak. Symmetric molecules, such as carbon dioxide undergoing a symmetric stretch, may have no change in dipole moment, resulting in no absorption peak or one too weak to detect.
Practical Interpretation of Intensity Data
The intensity of an IR absorption peak provides a powerful tool for quantitative analysis, enabling the determination of how much of a specific substance is present. The amount of radiation absorbed is directly proportional to the number of absorbing molecules in the beam’s path. A taller, more intense peak means a higher concentration of the material responsible for that vibration is present.
This relationship allows researchers to create a calibration curve by measuring the intensity of a characteristic peak for several samples of known concentration. Once established, the concentration of an unknown sample can be quickly calculated by measuring the intensity of the same peak in its spectrum. This quantitative capability is frequently used in industrial settings, such as quality control, to verify the purity of raw materials or to monitor the progress of a chemical reaction.
For instance, in a manufacturing process, a weak peak intensity for a known impurity indicates a high-purity product. Conversely, an unexpectedly strong peak for a reactant can signal that a reaction is incomplete. Interpretation of intensity data moves beyond simple compound identification, providing actionable information regarding the composition and consistency of a sample.