Infrared Spectroscopy

Infrared spectroscopy is a branch of spectroscopy that examines the interaction of matter with infrared electromagnetic radiation. In chemical analysis, it principally measures absorption associated with vibrations of molecules, producing spectra that reveal chemical groups and help identify substances. Measurements can also involve emission or reflection. Without further qualification, the term commonly denotes mid-infrared vibrational spectroscopy, covering approximately 4,000–400 cm⁻¹, or wavelengths of 2.5–25 micrometers. (goldbook.iupac.org)

Physical principles

Molecular vibrations include stretching, which changes bond lengths, and bending, which changes bond angles. Absorption occurs when incident radiation supplies the energy required for a transition between vibrational states. The interpretation rests on quantum mechanics: molecular absorption involves transitions between discrete energy levels rather than arbitrary increases in vibrational energy. Different chemical bonds and molecular arrangements therefore produce characteristic absorption patterns. (sites.science.oregonstate.edu)

For an ordinary electric-dipole vibrational transition to be infrared-active, the vibration must change the molecular electric dipole moment. A molecule need not possess a permanent dipole to absorb infrared radiation. For example, certain vibrations of carbon dioxide are active, although its equilibrium structure has no permanent dipole. Conversely, the fundamental stretching vibrations of isolated nitrogen and oxygen molecules are inactive under this selection rule. (sites.science.oregonstate.edu)

Spectra usually express position as wavenumber, the reciprocal of wavelength, in inverse centimeters. Higher wavenumbers correspond to higher frequencies and photon energies. Spectroscopists distinguish near-, mid-, and far-infrared regions; exact boundaries depend on convention. Near-infrared measurements commonly cover about 12,500–4,000 cm⁻¹ and emphasize overtones and combination bands, whereas mid-infrared spectra principally reveal fundamental molecular vibrations. (bruker.com)

Spectral interpretation

An infrared spectrum plots transmittance or absorbance against wavenumber. Transmittance describes the fraction of incident radiation transmitted through the sample; decadic absorbance is its negative base-ten logarithm. Thus, an absorption band appears as a downward feature in a transmittance spectrum and an upward feature in an absorbance spectrum. (webbook.nist.gov)

In organic chemistry, the region above approximately 1,500 cm⁻¹ is often called the functional-group region. A functional group can contribute recognizable bands: carbonyl stretching commonly occurs within approximately 1,600–1,800 cm⁻¹, while alcohol O–H stretching commonly appears around 3,300–3,600 cm⁻¹. These are indicative ranges, not universal fixed positions. Hydrogen bonding can broaden O–H bands, illustrating how spectra reflect molecular surroundings as well as bond identity. (sites.science.oregonstate.edu)

The lower-wavenumber portion of a conventional mid-infrared spectrum is known as the fingerprint region. Its complex pattern is useful for comparison with reference spectra. Identification normally combines characteristic bands, the overall spectral pattern, and independent information; one peak alone seldom establishes a complete molecular structure. Reference measurements must also be interpreted in context, because solvents and other chemical environments can change spectral details. (sites.science.oregonstate.edu)

Instrumentation and measurement

A spectrometer requires a radiation source, a means of resolving spectral information, suitable sample optics, and a detector. Dispersive instruments separate wavelengths sequentially. Fourier-transform infrared spectroscopy (FTIR) instead records information from many frequencies through an interferometer, then applies a Fourier transform to recover the spectrum. Many FTIR instruments use a Michelson interferometer, whose changing optical path difference produces an interferogram. (bruker.com)

Sampling geometry is distinct from the choice between dispersive and Fourier-transform instrumentation:

• Transmission: Radiation passes through the sample. Excessive thickness or strong absorption can saturate bands, so sample thickness and dilution affect spectral quality.
• Attenuated total reflectance (ATR): Radiation undergoes total internal reflection inside a high-refractive-index crystal. An evanescent field extends into the contacting sample, probing a shallow region. Diamond, germanium, and zinc selenide are common crystal materials.
• Reflection: The instrument measures radiation reflected from a sample. Specular and diffuse-reflection arrangements accommodate different surfaces and materials. (bruker.com)

ATR often requires little sample preparation, but reliable solid-sample measurements depend on contact with the crystal. Its effective penetration varies with wavelength and optical properties, so ATR and transmission spectra can differ in relative intensities and band positions. Those differences matter when comparing measurements with spectral libraries. (bruker.com)

Quantitative analysis and limitations

Under suitable conditions, transmission absorbance follows the Beer–Lambert law:

where $c$ is concentration, $l$ is optical path length, and $\varepsilon$ is the decadic molar absorption coefficient. Quantitative measurements therefore require appropriate calibration and knowledge of measurement conditions. NIST’s quantitative gas-phase infrared database provides absorption-coefficient spectra derived from primary gas standards, together with uncertainty estimates. (webbook.nist.gov)

Reference libraries are not interchangeable with quantitative calibrations. Some collections contain only relative spectral intensities suitable for identification, not absolute concentration measurement. Sample environment and measurement geometry can further limit direct comparison. (nist.gov)

Atmospheric water vapor and carbon dioxide can introduce unwanted bands when conditions change between background and sample measurements. Instrument purging and atmospheric correction are methods used to reduce these contributions. Temperature also affects some sample spectra: near-infrared water bands can shift as hydrogen-bonding conditions change. (shimadzu.com)

Applications and complementary methods

Infrared spectroscopy is widely used in analytical chemistry for material identification, contaminant analysis, quality control, and compositional measurement. FTIR is particularly useful for polymers, whose vibrational bands reveal chemical characteristics. Coupling infrared measurements with microscopy enables spatially resolved analysis, including examination of small particles and heterogeneous samples. (nist.gov)

Infrared analysis can also accompany separation techniques. NIST’s gas-phase reference collection includes spectra obtained during gas-chromatographic measurements, with corresponding mass-spectral identifiers where both measurements were acceptable. Such combinations connect spectral identification with separated components. Raman spectroscopy provides another route to molecular vibrational information and is frequently considered alongside infrared methods. (nist.gov)