TLC (Thin-Layer Chromatography)
Definition
A planar chromatography technique on silica gel plates that separates ink dye mixtures by polarity. ASTM E2285 governs its forensic application. The Rf values and band colours under visible and UV light constitute the ink's chromatographic fingerprint.
- Governing standard
- ASTM E2285
- Stationary phase
- Silica gel plate
- Separation basis
- Polarity of ink dye components
- Output
- Rf values and band colours under visible and UV light
Common questions
How does TLC fit alongside HPLC and Raman in ink analysis?+
TLC separates and visualises dye components cheaply and quickly, making it a good screening step. HPLC gives more quantitative separation of the same dyes, and Raman probes molecular structure non-destructively, so labs often use TLC first and confirm with the other techniques when a case needs stronger support.
Is TLC a destructive test?+
Yes, TLC requires physically removing a small ink sample, typically a punch or scraping from the writing, so it consumes a portion of the document. This is why non-destructive methods like the video spectral comparator are usually tried first.
What does a mismatch in band colour under UV mean?+
Some dyes look similar under visible light but fluoresce or absorb differently under UV, revealing chemical differences invisible otherwise. A UV band mismatch between two ink entries is strong evidence they were written with different pens, even if the visible colour matches.
Related terms
- ASTM E2285 / E2286
- The two ASTM International standards governing forensic ink examination: E2285 covers TLC-based ink alteration examination; E2286 covers HPLC-based ink comparison. Both are...
- Ballpoint Ink
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- HPLC-DAD
- High-performance liquid chromatography with diode-array detection. Separates ink dyes on a C18 column and records UV-visible spectra at each elution point, providing...
- Iron-Gall Ink
- A historical ink formulation of ferrous sulphate, tannic acid (from oak galls), and gum arabic. The ferrous-tannate complex oxidises to brown ferric-tannate...
- Phenoxyethanol
- The primary high-boiling solvent in most modern ballpoint ink formulations. Its progressive evaporation from the dried ink film after writing is the...
- Raman Microspectroscopy
- A vibrational spectroscopy technique using a focused laser beam to excite molecular vibrations in an ink sample. Non-destructive. Primary method for toner...
- Toner
- A dry-powder printing material consisting of fine styrene-acrylate polymer particles carrying carbon black and organic pigment, electrostatically applied and thermally fused at...
- USSS International Ink Library
- A collection of ~10,000 dated writing ink formulations maintained by the United States Secret Service since 1968, providing a terminus post quem...
- VSC (Video Spectral Comparator)
- A multispectral imaging instrument used in document examination; illuminates the document at multiple wavelengths (UV, visible, near-IR, IR) and captures fluorescence and...