Chromophore
Definition
The molecular subunit responsible for a dye's colour, typically a conjugated double-bond system; chemical erasure destroys the chromophore while leaving the decolourised molecule in the paper fibre, where it may retain IR absorption.
- Common types
- Conjugated double bonds, aromatic rings, carbonyls, azo groups, and porphyrin rings
- Light range
- Absorbs ultraviolet and visible light (UV-Vis region)
- Forensic use
- Spectrophotometry, bloodstain analysis, document examination, chemical identification
Common questions
What exactly is a chromophore?+
A chromophore is the specific part of a molecule that absorbs light in the UV or visible range. Common examples are conjugated double bonds (C=C), carbonyl groups (C=O), azo groups (N=N), and aromatic rings. When light hits these groups, electrons jump to higher energy levels, giving the molecule its color.
Why do forensic scientists care about chromophores?+
Chromophores are the key to identifying substances using spectrophotometry. In bloodstain analysis, haemoglobin's porphyrin ring is the dominant chromophore. When you shine UV or visible light on evidence, the chromophore tells you what is absorbing that light and helps confirm what chemical compound is present.
How do chromophores help in document examination?+
When someone chemically erases writing, the erasure agent destroys the chromophore of the dye, removing the color. However, the decolorized molecule stays behind in the paper fiber and may retain IR absorption. This is why IR imaging can reveal chemically erased text that is invisible to the naked eye.
Related terms
- 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...
- 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...
- Absorbance (A)
- The base-10 logarithm of the ratio of incident to transmitted light intensity, A = log10(I0/I). Dimensionless. Linear with concentration in the working...
- Absorption
- The purification technique in which a crude antiserum is mixed with antigen preparations from cross-reacting species. Cross-reactive antibodies bind those antigens and...
- Atomisation
- Conversion of a sample into free gaseous atoms in the ground state (AAS) or excited state (AES). The mandatory first step of...
- Bandpass and Slit Width
- The wavelength interval the monochromator delivers to the sample. Routine forensic UV-Vis runs at 1 nm or 2 nm bandpass; pharmacopoeial methods...
- Chemical Erasure
- Destruction of an ink's chromophore by oxidising or reducing agents (bleach, permanganate, borohydride) without physically removing paper fibres, leaving altered UV fluorescence...
- Diode Array Detector (DAD)
- A UV-Vis detector that records the full 190 to 800 nm absorbance spectrum of every peak as it elutes, instead of a...
- Fingerprint Region (IR)
- 1500 to 400 cm⁻¹. The complex lower-wavenumber zone where the whole-molecule pattern is unique enough to identify a substance by overlay.
- Fluorescence Lifetime
- The average time a fluorophore spends in the S1 excited state before emitting a fluorescence photon; typically 1-10 ns for organic fluorophores,...
- Fluorescence Quantum Yield (Phi)
- The fraction of absorbed photons that result in fluorescence emission (0 to 1); determines the brightness of a fluorescent signal and is...
- 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...
Explained in these topics
- Light-Matter Interaction: Absorption, Reflection, FluorescenceA molecular group or conjugated pi-electron system responsible for absorbing visible or near-UV light; the porphyrin ring of haemoglobin is the dominant forens...
- Light, Photons and the Forensic EM SpectrumA molecular group responsible for light absorption in the UV-visible range; haemoglobin's porphyrin ring is the dominant chromophore in bloodstain analysis.
- Mechanical and Chemical Erasures, Obliterations and InterlineationsThe molecular subunit responsible for a dye's colour, typically a conjugated double-bond system; chemical erasure destroys the chromophore while leaving the de...
- Spectrophotometry for Forensic Science: UV, Visible, IR, Raman, AAS and AESThe functional group inside a molecule responsible for absorption in the UV-Visible region (C=C, C=O, N=N, aromatic ring).
- UV-Vis Spectrophotometry and the Beer-Lambert LawThe structural unit in a molecule responsible for its absorption in the UV or visible region. Conjugated double bonds, aromatic rings, carbonyls and azo groups...