vacuum metal decomposition

Question 1

history

Answer 1

1963 – Tolansky discusses the possibility of depositing thin metal films on latent prints with the UK Home Office
1968 Theys et al. used a mixture of metal powders (zinc/antimony/copper) to develop LP on paper
1972 – Hambley publishes work from his dissertation which described using gold and cadmium metal for LP on fabric/cloth
1991 – First print identified using VMD in North America (Canada)

Question 2

method

Answer 2

• paper or flat item is held to rack with magnets.
• other forms of evidence are suspended from the rack.
• The heat sources are loaded with the metals to be evaporated.
• The metals can be evaporated from two types of sources, foil boats or crucible heaters.

Question 3

crucible heaters

Answer 3

• open, circular wound filament with crucibles inside the windings.
• Crucibles have insulating properties

pumps sweeps the air from the chamber to provide a vacuum - takes ~45mins

Then metals evapourated – skilled job
Underdeveloped – can reprocess
Overdeveloped – serious problem- can not be reversed

Question 4

choice of metal for evaporation

Answer 4

combination of gold or silver followed by cadmium (toxic) or zinc

• gold preferred over silver (silver can be degraded)
• zinc (and cadmium) will not condense on grease
• zinc deposits on small nuclei of metal
• gold diffuses into the fat
• when zinc is deposited it condenses on the gold nuclei bu not the fact deposit

Question 5

comparing VMD with other techniques

Answer 5

• other process give better results on paper
• vmd gives excellent results on non-porous substrates
• vmd best for developing marks on plastic bags
• better than superglue for marks exposed to water and high humidity
• vmd detrimentally affect by presence of body fluids
• difficult to develop Prints on heavily plasticised polymers

Question 6

silver nitrate

Answer 6

1873 – Camillo Golgi used a mixture of potassium dichromate and silver nitrate to stain tissue samples
1877 – Pierre Aubert uses silver nitrate to detect latent prints
1981 – Kerr et al. evaluate perchlorate and chromate silver salts

Question 7

silver nitrate and chloride ions

Answer 7

reacts to form a highly insoluble salt of silver chloride, white ppt turns purple/grey on exposure to sunlight

• trapped in pores of porous surfaces
• used on water insoluble components and metal surfaces

Question 8

drawback of silver nitrate

Answer 8

• chloride ions in the latent print residue diffuse over time, accelerated by humidity – lose detail.
• prints no older than 1 week develop well
• prints on porous surfaces last longer in the winter than in the summer
• used on porous and glossy surfaces
• can not use another method after it
• used successfully to develop latent prints that are even centuries old- advantage

Question 9

physical developer

Answer 9

1969 – Jonker and Molenaar develop stabilized physical developers
1972 – Collins/Thomas use unstable physical developers to amplify VMD prints
1975 – Morris is the first to use stabilized PD to visualize LP
1981 – First operational user’s guide prepared by Hardwick (PSDB)
2000s – Improvements include addition of Tween 20 and RO/DI water

Question 10

physical developer

Answer 10

• ag particals are colloidal and adhere to the fatty acid and lipid constituents of latent fingerprints
• Ag nanoparticle attracted to protonated fingerprint residue

Question 11

critical issue physical developer

Answer 11

Paper must be prewashed

Glassware must be scrupulously clean

Distilled is better than de-ionised water

Type of surfactant is critical and must be very pure. Non-ionic surfactant also added

Process must be below 20°C

Question 12

pros and cons of physical devopler

Answer 12

pros

• Sensitive to sebaceous residues
• can be used after DFO and ninhydrin

cons

• Delicate and time consuming to prepare reagents
• Expensive
• Solution has short shelf life
• Can get irreversible overdevelopment
• Destructive- results in permanent stain

Question 13

multi metal deposition methods

Answer 13

Step 1- gold acquires –ve charge from citrates and binds to the +ve charge of amino acid , protein etc.

Step 2- acts as nucleation site for silver physical developer to give enhanced prints

The colloidal gold particles are produced by the reduction of tetrachloroauric acid, by citric acid.

Question 14

MMD

Answer 14

• like silver physical developer mmd developer the water insoluble components of latent print residue
• mmd developed prints on variety of surfaces
• appears light on dark background and dark on light backgrounds
• can be used on bloody surfaces

Question 15

sudan black

Answer 15

• dye used for chemical screening for fatty components

- detection of sebaceous components on nonporous and some semipros substrates