Mid-Infrared (Mid-IR)
Definition
The 2500-25000 nm (400-4000 cm^-1) spectral region where functional-group vibrational absorptions serve as chemical fingerprints; the operating region of FTIR spectroscopy.
- Wavelength range
- 2500 to 25000 nm
- Wavenumber range
- 400 to 4000 cm^-1
- Physical basis
- Functional-group vibrational absorption
- Primary technique
- FTIR spectroscopy
Common questions
Why is mid-IR described as a chemical fingerprint region?+
Nearly every organic functional group, such as carbonyl, hydroxyl, or amine bonds, absorbs at characteristic mid-IR frequencies, so the resulting absorption pattern is specific enough to identify or compare unknown compounds against reference spectra.
How does mid-IR relate to near-IR used in other forensic instruments?+
Near-IR sits at shorter wavelengths and probes weaker overtone and combination vibrations, giving broader and less distinct bands, while mid-IR captures the fundamental vibrations directly, which is why FTIR rather than near-IR is the standard for compound identification.
Related terms
- Chromophore
- The molecular subunit responsible for a dye's colour, typically a conjugated double-bond system; chemical erasure destroys the chromophore while leaving the decolourised...
- Ionising Radiation
- Electromagnetic radiation whose photon energy exceeds the ionisation potential of atoms (typically above ~10 eV); includes gamma rays and X-rays. UV below...
- Photon
- A quantum of electromagnetic radiation; a discrete energy packet with energy E = hν where h is Planck's constant and ν is...
- Photon Energy (eV)
- Photon energy expressed in electron volts; UV photons carry 3-12 eV (chemically reactive), visible 1.8-3.1 eV, mid-IR 0.05-0.5 eV (vibrational excitation only).
- Planck-Einstein Equation
- E = hν = hc/λ; relates photon energy to frequency and wavelength. Foundation of all spectral energy calculations.
- Refractive Index
- A physical property of glass (and other transparent materials) that describes how much light bends when passing through it. Forensic glass comparison...
- Soret Band
- The intense porphyrin absorption band of haemoglobin at 415 nm; the spectral basis for bloodstain contrast enhancement.
- UV-a
- The ultraviolet band from 315 to 400 nm; the primary band used for UV-reflectance forensic imaging of bruises, GSR, and bite marks...
- UV-C
- Ultraviolet radiation in the 100-280 nm band; the 254 nm germicidal line excites biological-fluid fluorescence in forensic screening.
- Wave-Particle Duality
- The quantum-mechanical property of electromagnetic radiation: it behaves as a wave (diffraction, interference, polarisation) and as a stream of photons (photoelectric effect,...
- Wavenumber (Cm^-1)
- Reciprocal of wavelength in centimetres, the standard unit for mid-infrared spectra; 1 cm^-1 = 10^7 / λ(nm).