Advances in Fingerprint Technology.pdf

Intensification
These methods are used to enhance weak Ag-PD prints. There are at least
four methods. The simplest of these is redipping in Ag-PD. This causes
further development on existing silver particles contributing to their growth.
It does not always add new particles to give a greater density of particles and
also it could increase the build-up of silver in the background. Goode and
Morris 13 proposed an intensification method that involves conversion of the
silver to silver sulfide (Ag 2S), followed by silver physical development using
a nitrate/nitrite redox couple. Saunders suggests a rather novel intensification
method that uses the ferrous/ferric (citric acid) redox couple and a dilute
sodium chloride (NaCl) solution. Its mechanism is not fully understood. The
fourth method involves sodium hypochlorite (NaOCl). A 25 to 50% solution
of household bleach, which is 5 to 6% NaOCl, lightens the background and
intensifies the print. The most likely mechanism is that silver oxide (Ag 2O)
is formed. Except for the redipping method, all of these methods change the
composition and surface characteristics of the original silver particle and
both changes contribute to its change and intensification in color.
Radioactive Sulfur Toning and Autoradiography
This method was developed in the U.K. 36 for removing interfering background
from Ag-PD prints. It involves converting the silver to radioactive
silver sulfide (Ag 2 35 S), where 35 S is the β-emitting radioactive isotope of sulfur
used, followed by autoradiography for imaging. As mentioned for the X-ray
methods, if the background contains printing, the ink must not contain materials
that cause silver physical development to occur. If it does, any silver particles
on the printing will also be converted to silver sulfide and be radio-imaged.
Current Research
Non-Silver Physical Development
There are at least two problems of economics with Ag-PDs: the silver is
expensive and about 20 to 50% of it is not used during the useful lifetime of
the developer. 26 The latter fact indicates that the developer is rather inefficient
and that a good portion of silver is thrown away when the exhausted Ag-PD
solution is disposed. At present, there is no way to replenish used Ag-PD. In
1969, Jonker et al. 12 mentioned ways of making non-noble metal physical
developers and later, in 1976, 37 Molenaar et al. worked on optimizing the
copper physical developer. Dr. Kevin Kyle from the Special Technologies
Laboratory, Santa Barbara, CA, assisted the United States Secret Service
(USSS) in adapting the copper physical developer of Molenaar et al. 37 for
visualizing latent prints on paper. Preliminary results show promise, but there