6.2: : Extraction of inks and separation by TLC of the Suicide Note
- Page ID
- 364579
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D2: Extraction of inks and separation by TLC of the Suicide Note
The chemicals used in this experiment pose little hazard provided that routine laboratory precautions are taken to avoid ingestion and skin contact. Care should be taken when handling and disposing of flammable organic solvents. When using an UV lamp, do not look directly at the lamp under any circumstances.
Sample
Various pens and ‘suicide’ note
Reagents
Ethanol
Mobile Phases
5:3:2 ethyl acetate: ethanol: water
Apparatus
Test tubes Test tube rack Silica Gel GF254 TLC plates Two TLC tanks (large beakers) Filter paper
Visual examination of the suspect note and pens
- Using each of the supplied pens, write specimen words on a sheet similar to that used for the note.
- Using a low power microscope, compare these specimens with the writing on the note.
- Look at color of ink and thickness of lines and any other visible characteristics, such as blotchiness etc.
- Identify which pens give marks of similar appearance to the writing on the note.
- Try to identify any areas of the note that show signs of possible alteration.
Extraction of ink
- Having identified areas of the note worthy of further scrutiny, carefully cut out several letters and place in small glass sample tubes.
- Also transfer to a tube a similar-sized piece from a control section of paper free of ink to act as a blank.
- Add 1–2 drops of ethanol to each tube.
- Allow 15 minutes for the ink to dissolve.
- Perform the same extraction as above with each of the specimen inks of similar color to those on the note.
Procedure
- Make up the eluent ethyl acetate : ethanol : water (5:3:2).
- Pour into the chromatographic tank to a depth of less than 1 cm.
- Cover the tank and leave to equilibrate with the solvent for about 20 minutes.
- Cut the aluminium-backed silica gel with fluorescent indicator to appropriate size.
- Mark the base line approx. 1-cm from the bottom of the plate with a pencil.
- Using a narrow capillary tube, apply a small spot of each ink solution plus the blank, spacing the spots at equal distances along the base line of the TLC plate.
- Allow to dry in the fume cupboard.
- Stand the plate in the tank and develop it, allowing solvent to migrate to within 1 cm of the top edge of the plate.
- Mark the solvent front with a pencil.
- Air-dry the plate in the fume cupboard.
- Note the appearance and positions of all spots and check under the UV lamp (take care not to look at the lamp directly) as well as any matching and non-matching inks.
- If more than one pen was used to write the note, draw conclusions as to whether the note has been altered. If so, are you able to decipher the text of the original note?
Thinking about Making a Valid Measurement
How far apart do the spots need to be for the analyst to be confident that adequate separation has been achieved? What experiments could be performed to check your prediction?
Improving Your Practical Technique
- Work in a clean environment to avoid contamination.
- Only handle the TLC plate by its top corner.
- Only lay the TLC plate down on a clean surface.
- Put identifying marks on the top of the TLC plate using a pencil.
- Consider doing duplicate samples.
- Prepare a relatively concentrated solution from the original samples otherwise you will not be able to see minor components on the TLC plate.
- Cover the tank and line it with solvent soaked paper to provide an atmosphere saturated with solvent vapor.
- Select a solvent in which the solvent dissolves.
- Use reference materials (e.g. a pure substance) to help to identify the components in the sample.
- Allow the spots to dry before putting the spotted TLC plate in the tank.
- When examining a plate under UV light, the components show up as dark spots against a white background. Never look at the UV light directly.
- The intensity of the spots can be used as a crude guide as to the relative amount of components in the sample.