396 DIY Science: Illustrated Guide to Home Chemistry Experiments
AETIN qU fINGERpRINTS
I always smile when I read a mystery novel in which
a detective finds “fresh fingerprints.” As any forensic
scientist will admit, there’s no way to judge the age of a
fingerprint.
In theory, the more volatile components of a latent
fingerprint disappear at a rate that could be calculated
by taking into account the temperature and other
environmental factors, leaving only the nonvolatile sodium
chloride. On that basis, you might expect that fingerprints
that can be developed with iodine fuming or ninhydrin are
“new” and those that can be developed only with silver
nitrate are “old.”
In practice, there’s no feasible way to apply this reckoning
because the type and amounts of residues laid down for
a particular latent fingerprint vary so much from one to
another. Fingerprints that are only hours old may respond
only to silver nitrate, and fingerprints that are months
or years old may respond to iodine fuming or ninhydrin.
Fingerprints have been found in Egyptian pyramids.
Although those latent fingerprints were probably
thousands of years old (based on other evidence), no
forensic scientist could swear under oath that they hadn’t
been left earlier that morning.CUTIOA nS
This experiment uses flame. Be careful with the flame, and
have a fire extinguisher readily available. Iodine crystals
stain skin and clothing. (Stains can be removed with a
dilute solution of sodium thiosulfate.) Iodine vapors are
toxic and irritating. Perform this experiment outdoors or
under a fume hood or exhaust fan. Silver nitrate stains
skin and clothing. Ninhydrin is toxic and irritating. Wear
splash goggles, gloves, and protective clothing.z
blue or blue-purple dye called Ruhemann’s Purple. A 0.5% to
1.0% solution of ninhydrin in ethanol or acetone is sprayed
on the sample. The latent prints develop slowly at room
temperature, requiring an hour or two to be fully revealed. This
process can be hastened by heating the sample in an oven at
80°C to 100°C.
In practice, the order in which these methods are applied
is critical. Iodine fuming is always used first, because it is
completely reversible. If iodine fuming doesn’t provide usable
results, ninhydrin is used next. If ninhydrin fails, the traces of
salt are still present in the latent fingerprints, so the forensic
chemist can attempt to develop the latent prints with silver
nitrate. The silver nitrate solution removes the fatty oils and
amino acids from the sample, so if silver nitrate fails, no other
method can be used subsequently to reveal the prints.
In this lab, we’ll use all three methods to reveal latent
fingerprints. We’re not really forensic chemists, so we’ll make it
easier on ourselves by using three separate samples—one for
each of the methods.
POCEDURER
This laboratory is divided into three parts—one for each of the
methods we cover. Before you dive into the procedures, you need
to create some samples to be tested. We used ordinary copy
paper for the samples. We found that the latent fingerprints were
difficult or impossible to raise using any of the three methods if
we washed our hands soon before touching the sample paper.
Apparently, soap removes all or most of the skin oils, salt, and
other residues that produce latent prints. Our best results were
with samples that we’d touched when our hands were sweaty and
hadn’t been washed recently. There’s a moral here. If you’re going
to forge a document without wearing gloves, wash your hands
first. And afterwards.PI:RTA IodfNEI UmING
1. If you have not already done so, put on your splash
goggles, gloves, and protective clothing.- Set up your tripod stand and gauze square, with the
burner beneath it and the beaker resting on the stand. - Sprinkle a few crystals of iodine into the bottom of the
beaker. You don’t need much; a fraction of a gram
is sufficient. - Use the stiff wire and paperclip to suspend a sample
in the beaker, not touching the sides or bottom of the
beaker. Alternatively, for a larger sample, simply place
the paper against the side of the beaker, as shown in
Figure 22-3. Place the watch glass or other cover on top
of the beaker to contain the iodine fumes. - Observe the sample for 30 seconds or so. At room
temperature, iodine vaporizes quickly enough to
begin fuming the prints within a few seconds. (I didn’t
apply heat to the beaker for Figure 22-3, which shows
the paper after about 30 seconds of fuming at room
temperature.) - If no prints have begun to develop after 30 seconds or a
minute, heat the bottom of the beaker slightly to hasten
vaporization of the iodine. Before you apply heat, if
you’re not working outdoors, turn on the fume hood or
exhaust fan.