Advances in Fingerprint Technology.pdf

Test of Effectiveness
Currently in our laboratory the effectiveness of an Ag-PD is crudely tested
using a test paper strip with a spot of gold chloride on it. The density of the
gold chloride spot (gold chloride/area) is high because it is made with a
relatively concentrated (20 mg/l) gold chloride solution. The Ag-PD reduces
the gold chloride to elemental gold and this catalyzes the deposition of silver.
It only tests if the Ag-PD deposits silver on nucleation sites — not how well
or how fast. Barford et al. 26 observed that the ferrous ion and silver ion
concentrations were the most critical factors affecting the effectiveness of an
Ag-PD. They found that the UK-PD performed best (with development time
≤20 min) when the silver ion concentration does not fall below 50% of its
original concentration and the ferrous ion concentration does not fall below
60% of its original concentration. Seeing the importance of obtaining these
concentrations, they determined the ferrous ion concentration using potentiometric
titration and the silver ion concentration using a silver/sulfide solidstate
electrode. Currently, Dr. Kyle is working on an alternate way of obtaining
the ferrous ion concentration. 41 Also, Cantu is currently working with the
Institut de Police Scientifique et de Criminologie (University of Lausanne, Lausanne,
Switzerland) in developing a test strip containing gold chloride spots of
equal size but of decreasing density. The test strip is placed into the Ag-PD at
two different times to estimate the rate and sensitivity of development.
Conclusion
Silver physical developer is a powerful reagent for recovering latent prints on
porous surfaces that the amino acid visualizing reagents failed to develop. It
works on the water-insoluble components of latent print residue and, consequently,
works on water-soaked evidence. When it is used after a colloidal gold
pretreatment, its working ability is extended to nonporous surfaces. A mechanism
by which it works has been proposed. It is built on several observations
and offers avenues one can take to improve the current or create new physical
developers. At present, the U.S. Secret Service uses about 1600 liters (423 gallons)
of Ag-PD annually. It has increased the number of prints found on evidence
and brought about more convictions than when ninhydrin was used alone.
References
1. Cantu, A.A., On the composition of silver physical developers used to visualize
latent prints on paper, Forensic Sci. Rev., 13, 29, 2001.
2. Phillips, C.E., Cole, D.O., and Jones, G.W., Physical developer: a practical and
productive latent print developer, J. Forensic Ident., 40, 135, 1990.