Advances in Fingerprint Technology.pdf

weeks. 42,46,49 Exposure times and temperatures vary considerably among the
various groups. Oden and von Hofsten suggest heating to 80°C for a few
minutes. 36 O’Hara recommends a much higher temperature (140°C). 60
Crown suggests processing at 100°C 43 for his petrol ether formulation,
whereas Kent recommends 80°C and warns against heating certain items
above 50°C. 49 It is clear, however, that if time permits, room-temperature
development will always give the best “signal-to-noise” ratio, and this is the
recommended technique. 46
A number of groups have studied the influence of humidity on the quality
of developed prints. In the mid-1960s, both Crown 61 and Mooney 62 reported
that the presence of water vapors in the processing oven improves results.
Moenssens found that best results are obtained during periods of high relative
humidity and that “slow cure” at room temperature is preferable. 63 A study
by Lesk established that optimum development is obtained by processing the
ninhydrin-treated prints at 65 to 80% relative humidity. 64 In tests conducted
by the U.S. Army Criminal Investigation Laboratory, the best results were
obtained when ninhydrin-treated documents were maintained at 80%
humidity and a temperature only slightly above room temperature. 1 The
Royal Canadian Mounted Police (RCMP) laboratories in Ottawa recommend
the use of a saturated sodium chloride solution in the development chamber
to maintain the 75% relative humidity that is most desirable for the ninhydrin
process. 65 A steam iron can be effectively used to accelerate the ninhydrin
development of prints. The iron must not touch the paper surface, but be
moved over the surface at a distance of 1 to 2 cm. Olsen, however, reports
better contrast in humidity cabinet development. He also suggests not to use
the iron method on cardboard or coated paper because steam condenses on
such surfaces. 1 Development by placing the object over boiling water has
been suggested by Rispling, 66 who also advises to watch the exhibit closely
until coloration ceases. In 1976, Connor reported the results of a collaborative
study that examined the effect of a steam iron on ninhydrin-treated prints
on bond and newsprint papers. His study shows that latent prints are developed
by this method within minutes after the ninhydrin treatment and that
the mode of the ninhydrin application has very little effect on the results. 67
Consequently, the Association of Official Analytical Chemists (AOAC)
adopted this method that same year. 68
In their late 1970s studies, Morris 69 and then Jones and co-workers 70,71
reached the firm conclusion that steaming and heating after ninhydrin treatment
of papers can lead to a significant increase in the quality and contrast
of the marks revealed. Even when this improvement does not occur, there is
no disadvantage in using this method; thus, they confidently recommend it
for general use. The use of a microwave oven for steaming after ninhydrin
treatment is recommended by Margot, Lennard, and co-workers. 72