Questioned document analysis-Richard Flashcards
Name 5 techniques for QDA?
VSC CIE LAB colour analysis raman spectroscopy TLC ESDA
Ballpoint pen
rotating steel ball tip paste ink composition characteristic striations gooping of ink furrowed
Fibre tip pen
fibrous tip with ink reservoir
very solid deposition
near brush like stroke
less furrowing or unfurrowed
roller ball pen
similar to ballpoint more fluid ink type greater ink saturation pigment based no striations or glooping furrowed
4 non destructive techniques for ink comparison
microscpoic examination of text
video spectral comparison
CIE Lab colour analysis
raman spectroscopy
3 destructive techniques for ink comparison
TLD
HPLS/CE
FTIR of ink
video spectral comparison
a white item such as piece of paper is white because reflects equally
a black item is black because absorbs equally
that doesn’t mean it responds in the same way to wavelengths outside of this region
-infrared
-UV
Raman spectroscopy
drugs, explosies, firearms chemistry
can be used to differentiate ink types
non destructive
What is raman SERRS?
surface enhances - possible localised damage
- requires the addition of colloid reagents
- a means of increasing sensitivity
how does raman spectroscopy work?
uses a mono-chrome laser or high intensity light source to illuminate sample
light interacts with sample in variety of ways
- reflection
- absorption
- scattering
- fluorescence
scattered light which has interacted with the chemical structures within out sample can provide insight into the composition of the sample
the spectroscopist is interested only in a portion of the scattered radiation
- raman/stokes radiation -photons
- a different wavelength to the original light source
- gives info of chemical and molecular structure
Lab* colour
method of determination of colour coordinates
- dye
- paint
- food
human eye too insensitive for eyeball analysis
- variations in colour appreciation
- male colour gamut generally lower than female
colour often defined by:
- red/green/blue
- hue/saturation/luminance
- cyan/magenta/yellow/key
device dependent
- each pixel has to be translated into the colourspace of whatever we are viewing out image on
- conversion losses-gamut
- gamma correction
CIELab*
device dependent
used as basis for colour conversion
L=lightness
A=red/green
B=blue/yellow
lightness is independent of colour
Thin layer Chromatography
separates ink into its consistent components
small sample of ink is removed from the document using a micro-punch
then dissolved in appropriate solvent e.g methanol
can be further concentrated
TLC -Preparation
samples loaded onto TLC plate
- normally silica
- various mesh sizes
- loaded using glass capillary
placed in a saturated solvent tank
TLC - Separation
solvent travels up 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
separation produces a characteristic pattern of bands
- can be used comparatively
- R.F value allow lab/lab comparison or qualitative analysis
advantages of TLC
simple
rapid
effective
cheap
disadvantages of TLC
chemical development
visualisation
may not differentiate between similar inks
what does HPLC stand for?
High Performance Liquid Chromatography
HPLC
similar to TLC uses packed column excellent resolution variety of detectors may require greater sample conc
what does EDX stand for?
Energy Dispersive X-ray spec
ESDA
develop indented marks on paper
non-destructive
can provide results from pages up to 5 sheets below
document is first humidified for several mins
ESDA process
vacuum switched on
document placed on sample stage and a think mylar film is pulled over the top
the two are intimately held together by vacuum
electrostatic wand moved over stage
- uses a corona discharge wire
- creates static charge across document
glass beads carrying toner powder are then placed on stage
- black toner collects in areas of higher static charge
- indented areas
Advantages of ESDA
non-destrictuve
sensitive
inexpensive
disadvantages of EDSA
may be affected by fingerprint techniques
may be affected by very high humidity
affected by type of pen used and the pressure applied
