Vacuum metal deposition-Katherine Flashcards
method
paper of flat item held to rack with magnets
other forms of evidence suspended from the rack
heat sources loaded with metals to be evaporated
metals can be evaporated from two types of sources, foil boats or crucible heaters (made from thin refractory metal stampings)
dimples in boat hold the evaporation material
crucible heaters
open, circular wound filaments with crucibles inside the windings
made from alumina, carbon, quartz and boron nitride
have insulating properties which allow a uniform melt temperature
pumps sweep the air from the chamber to provide a vacuum (takes 45 mins)
metals evaporated
is it better for the metals to be under or overdeveloped
better to be under as can reprocess, overdeveloped cannot be reversed
choice of metal for evaporation
best is gold or silver followed by cadmium or zinc
gold preferred over silver as silver can be degraded by fingerprint secretions and atmospheric pollutants
zinc will not condense on grease
zinc will deposit on small nuclei of metal
gold diffuses into the fat and hence no gold nuclei close to surface
when zinc deposited, it condenses on regions of gold nuclei but not on the fatty deposit
comparison of VMD with other techniques
others give better results on paper
VMD gives good results on nonporous substrates
better than superglue for marks exposed to water and high humidity
VMD detrimentally affected by presence of body fluids
difficult to develop prints on heavily plasticised polymers (single silver process better)
silver nitrate
silver ions react with chloride ions in salt contained in the latent print residue to form silver chloride, soluble salt
AgCl=white precipitate, turns purple/grey on exposure to sunlight
Ag=on paper looks brown/black as silver is nanoparticles
drawback of silver nitrate method
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
used last as visualisation method as can not use another method after it
used on metal surfaces depositing silver everywhere
metal must be above silver in the electrochemical series
silver physical developer and multimetal deposition methods more commonly used for water-insoluble components but do not target chloride ions
physical developer
citric acid added to form ferric citrate and shifts equilibrium to the right
citric acid ionises to give citrate ions and protons in water reducing the pH
Ag particles are colloidal and adhere to fatty acid and lipid constituents of latent fingerprint
if conc of Ag gets too large the particles coalesce and precipitate out
overcome by use of cationic surfactant (n-dodecylamine acetate)
the cations engulf the Ag nanoparticles which then repel each other preventing coalescence into a precipitate
silver physical developer is acidic
amine groups in proteins of fingerprint residue have positive charge, these displace cationic surfactant
Ag nanoparticle attracted to protonated fingerprint residue
each silver nanoparticle is a nucleating site - autocatalytic
critical issues
paper must be prewashed to remove/neutralise alkali fillers
glassware must be clean
distilled is better than de-ionised water
type of surfactant is critical and must be very pure
non-ionic surfactant also added to prevent precipitation of the cationic surfactant
process must be below 20 degrees to prevent cloudy solution
advantages
sensitive to sebaceous residues
can be used after DFO and ninhydrin
disadvantages
delicate and time consuming to prepare reagents
expensive
solution has short shelf life -less than 2 weeks
can get irreversible overdevelopment
destructive - results in permanent stain so no further fingerprint development possible
multimetal deposition methods
gold binds to biomolecules in the fingerprint residue
step 1: gold acquires -ve charge from citrates and binds to the +ve charge of amino acid
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
electrostatic attraction between colloidal gold articles with a -ve charge and the amino acid with a +ve charge
colour of ridges is pale but can be darkened by Ag particles
MMD process
develops the water-insoluble components of latent print residue
appears light on dark background and dark on light background
sudan black
a dye used for chemical screening for fatty components
detection of sebaceous components on nonporous and some semi-porous substrates
