Fingerprint Region (IR)
Definition
1500 to 400 cm⁻¹. The complex lower-wavenumber zone where the whole-molecule pattern is unique enough to identify a substance by overlay.
- Wavenumber range
- 1500 to 400 cm-1
- Characteristic
- Complex, lower-wavenumber zone
- Use
- Whole-molecule identification by spectral overlay
- Technique
- Infrared spectroscopy
Common questions
Why is this region called the fingerprint region?+
The absorption pattern here results from complex combinations of bond vibrations across the whole molecule rather than isolated functional groups, so it forms a pattern nearly as unique to a compound as a human fingerprint, useful for confirming identity by overlay against a reference spectrum.
How does the fingerprint region differ in use from the higher-wavenumber functional group region?+
The functional group region above roughly 1500 cm-1 is used to spot specific bonds like O-H or C=O, while the fingerprint region is read as a whole pattern match rather than interpreted bond by bond, since individual peaks there overlap too densely to assign reliably.
Related terms
- Atomisation
- Conversion of a sample into free gaseous atoms in the ground state (AAS) or excited state (AES). The mandatory first step of...
- Beer-Lambert Law
- A = εbc. Absorbance equals molar absorptivity times path length times concentration. The linear relation between absorbance and concentration that makes UV-Vis...
- 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...
- Hollow Cathode Lamp (HCL)
- A sealed lamp whose cathode is made of the element being analysed, filled with argon or neon at low pressure. Strikes a...
- ICP Source
- Inductively coupled argon plasma at 6000 to 10000 K. The hottest practical excitation source for AES, giving simultaneous multi-element analysis (ICP-OES, ICP-MS).
- Molar Absorptivity (ε)
- The proportionality constant in the Beer-Lambert law, in L mol⁻¹ cm⁻¹. A property of the analyte at a given wavelength, solvent and...
- Stokes Shift (Raman)
- The Raman-scattered photon at lower energy than the incident photon. The Stokes line is the one routinely measured because it is stronger...