Fingerprint Region
Definition
The 1500 to 400 cm⁻¹ stretch of an infrared spectrum where bending and combination bands produce a pattern unique to a given compound. Library matching is dominated by this region rather than by the group-frequency region above 1500 cm⁻¹.
- Wavenumber range
- 1500 to 400 cm⁻¹
- Type of vibrations
- Coupled and skeletal vibrations, bending and combination bands
- Primary application
- Identity confirmation against spectral libraries
Common questions
What wavenumber range defines the fingerprint region in infrared spectroscopy?+
The fingerprint region spans 1500 to 400 cm⁻¹. This range is where coupled vibrations and skeletal movements of molecules produce complex patterns. Each compound has its own unique pattern in this range, making it reliable for identification against spectral libraries.
Why is the fingerprint region better than the group-frequency region for compound identification?+
Library matching works more effectively in the fingerprint region because the complex patterns created by bending and combination bands are highly specific to individual compounds. The group-frequency region above 1500 cm⁻¹ is broader and less distinctive, so forensic and analytical labs rely more on the fingerprint region for definitive identification.
How is the fingerprint region used in forensic analysis?+
In forensic spectroscopy, the fingerprint region allows chemists to compare an unknown sample against reference libraries. The unique molecular vibration pattern in this region acts like a fingerprint, confirming the identity of unknown substances or materials in evidence.
Related terms
- ATR (Attenuated Total Reflectance)
- An FTIR sampling mode in which the sample is pressed against a diamond or ZnSe crystal and the evanescent wave probes the...
- ATR-FTIR
- Attenuated Total Reflectance Fourier-Transform Infrared Spectroscopy: a non-destructive mid-infrared method that analyses a single drop of milk pressed against an ATR crystal....
- 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...
- Electron Impact (EI) Ionisation
- A hard ionisation mode at 70 eV used in GC-MS that produces extensive, reproducible fragmentation. The fragmentation pattern is matched against libraries...
- FTIR
- Fourier-transform infrared spectroscopy. A Michelson interferometer modulates all wavelengths simultaneously, the interferogram is Fourier-transformed into the conventional absorbance versus wavenumber spectrum.
- Group Frequency
- A characteristic absorption band associated with a specific functional group (carbonyl about 1700 cm-1, hydroxyl 3200 to 3600 cm-1) used to read...
- Internal Standard (IS)
- A compound chemically near-identical to the analyte, often a deuterated isotopologue (d3-morphine, d5-EDDP, d5-alprazolam, pyrene-d10), spiked into every sample at a known...
- KBr Pellet
- A classical solid sample mode: 1 to 2 mg analyte ground with about 200 mg dry KBr and pressed in a 13...
- Limit of Detection (LOD) and Quantitation (LOQ)
- LOD is the lowest concentration at which the signal-to-noise ratio is 3:1; LOQ is where it is 10:1. LC-MS/MS LOQs for drugs...
- MRM (Multiple Reaction Monitoring)
- A targeted MS/MS scan mode on a triple quadrupole that monitors specific precursor-to-product ion transitions, with two transitions per analyte (quantifier and...
- Wavenumber
- The reciprocal of wavelength expressed in cm-1, the universal IR axis. Higher wavenumber means higher photon energy. The mid-IR window is 4000...
Explained in these topics
- Infrared Spectroscopy: FTIR and ATR-FTIRThe 1500 to 400 cm-1 range where coupled and skeletal vibrations produce a complex pattern unique to each molecule, used for identity confirmation against a li...
- Spectrometric Detection: UV-Vis, FTIR and Mass SpectrometryThe 1500 to 400 cm⁻¹ stretch of an infrared spectrum where bending and combination bands produce a pattern unique to a given compound. Library matching is domi...