Chapter 22 Laboratory: Forensic Chemistry 3977.ight the burner and use it to gently heat the bottom of L
the beaker. Very soon, the iodine will start to sublimate,
filling the beaker with violet iodine vapor. Continue
heating the beaker gently. If the iodine vapor condenses
on the sides of the beaker, heat the sides gently to force
it back to vapor form.- After 30 seconds to a minute, the fuming phase
should be complete. Allow the iodine vapor to condense
on the sides of the beaker. Examine the sample, which
should now have visible fingerprints. If not, continue
fuming, perhaps after adding a bit more iodine to the
beaker. - Remove the watch glass (being careful not to burn
yourself) and lift the sample out of the beaker. Lay
it on a flat surface and examine it carefully with the
magnifying glass. You should see fingerprints revealed
in considerable detail (Figure 22-4). Shoot an image of
them for the record, if you wish.
Spray the sample with the cornstarch solution to develop
the prints. You needn’t drench the sample; a gentle
misting is sufficient. As the iodine reacts with the starch,
you should see the color of the prints change to a deep
blue-black.
Record your observations and, if you wish, tape or staple
the developed sample to your record.
10.
11.
FIGURE 22-3:
Iodine fumes in a beaker developing latent fingerprints
FIGURE 22-4:
Fingerprints
developed by
iodine fuming
PII:RTA NdNHI y RIN SpRAy
- If you have not already done so, put on your splash
goggles, gloves, and protective clothing. - Place one of your samples on a flat surface and spray it
with the ninhydrin solution. (Use several thicknesses of
newspaper to protect the surface against the ninhydrin.)
Again, a gentle misting is sufficient, just enough to
dampen the sample without drenching it. Record the time
when you sprayed the sample, and check the sample
every few minutes to determine how far development
has proceeded. At room temperature, ninhydrin normally
takes an hour or two to develop most prints, although
some prints may require 24 hours or more. Figure 22-5
shows a fully developed ninhydrin test. - Heat and humidity greatly increase the rate of print
development with ninhydrin. If you want to examine
the effect of heat on the development of the latent
fingerprints, place the original or a second sample
that has been sprayed with ninhydrin into an oven set
to 200°F. Check the sample every minute or two to
determine the reaction rate at the higher temperature.
Alternatively, you can place the sample between sheets
of paper towel or blotter paper and use a clothes iron on
medium heat to develop the prints using steam, which
requires only a few seconds.
FIGURE 22-5:
Latent fingerprints after treating with ninhydrin