question document examination Flashcards
techniques
- VSC (video spectral comparison)
- CIE LAB colour analysis
- raman spectroscopy
- TLC- thin Laye chromatography
- ESDA- electrostatic detection apparatus
pen type characteristics
- There are a wide array of different types of ink delivery system
- Fairly simple task to determine what type of pen was used in questioned and non-questioned areas microscopically
ink delivery
Ball-Point
- Rotating steel ball tip
- Paste ink composition – -‘Solvent/Oil’ based
- Characteristic striations
- ‘Gooping’ of ink – Dot deposits or striated defects
- Furrowed
Fibre Tip
- Fibrous tip with ink reservoir
- Very solid deposition
- near brush like stroke
- Less furrowing or un-furrowed
Roller Ball
- Similar to ball-point
- More fluid ink type
- Greater ink saturation
- Generally pigment based
- No striations or ‘gooping’
- Furrowed
techniques for ink comparison
Non-Destructive
- Microscopic examination of text – To determine similarity and difference
- Video Spectral Comparison
- CIE Lab* Colour analysis
Raman Spectroscopy – Potentially semi-destructive
Destructive
- TLC – Inks must be dissolved
- HPLC/CE – Inks must be dissolved
- FTIR of ink – Not all that successful!
VSC
Although black ink absorbs all (or nearly all) wavelengths of light in the visible range…
- That doesn’t mean to say that it responds in the same way to wavelengths outside of this region
- Infra-red
- Ultra-Violet
It may be that different inks respond differently to light in these regions
raman spectroscopy
-differentiate ink types
- identify ink type
Comparison to a database of inks
- Raman – Non destructive (or perhaps semi-destructive)
- Raman SERRS (Surface enhanced) – Possible localised damage
raman spectroscopy- how does it work
Uses a mono-chromatic laser or high intensity light source to ‘illuminate’ the sample
The light interacts with the samples in a variety of ways Reflection Absorption Scattering Fluorescence – Not good!
- provides insight of the composition of the sample, a different wavelength to the original light source gives info on chemical and molecular structure
Lab* colour
A method for determination of colour coordinates
Are device dependent
- May not look the same on different platforms
CIELab* is different
Device independent
Often used as the basis for colour conversion
Lab* colour analysis
Using Photoshop to convert image to Lab* Decompose to Lab* This creates 3 monochrome layers L – Lightness a – Green/Magenta b – Blue/Yellow
Differences in colour show up as differences in brightness in the a and b layers
thin layer chromatography
Separates ink into its constituent components
A small sample of ink is removed from the document using a micro-punch
- This is then dissolved in a appropriate solvent such as methanol
Preparation
-Samples are loaded onto a TLC plate
Various types – Normally Silica
Placed in a saturated solvent tank
- Solvents systems designed to give best separation/resolution
Separation
- Solvent travels up the plate by capillary action
Separation is driven by the differential solubility of components in
The mobile phase – Solvent
The stationary phase – Silica
Polar compounds tend to have a stronger interaction with the silica stationary phase
R.F values allow lab/lab comparison or qualitative analysis
TLC
Simple – Anyone can do it…
Rapid – 20 minutes
Effective – High DP
Cheap – A few pounds
- May require chemical development
- Visualisation
- May not differentiate between very similar inks
other techniques
HPLC – High performance liquid chromatography
- Similar in principle to TLC
- Uses packed column
- Excellent resolution
- Variety of detectors
- May require greater sample conc.
EDX – Energy dispersive X-ray spec.
- Elemental composition of inks
ESDA
A technique used to develop indented marks on paper
Extremely successful non-destructive technique
- Can be used to examine writing or other indents where original has been removed
- Can be used to link documents
- Can provide results from pages up to 5 sheets below original text
- sensitive
- inexpensive
ESDA process
-Vacuum switched on
- Document placed on a sample stage and a thin film is pulled over the top
- The two are intimately held together by vacuum
-Film follows the contours of the paper accurately
- An electrostatic wand is moved over the stage
- This creates a ‘static’ charge across the document
-This is left to develop for several minutes
- Glass beads carrying toner powder are then
cascaded over the stage
ESDA
- may be affected by fingerprint techniques
- may be affected by high humidity
- may be affected by type of pen used and pressure
