Chapter 22 Laboratory: Forensic Chemistry 395LABORATORY 22.3:
REvEAL LATENT fINGERpRINTS
Developing methods to reveal latent
fingerprints was one of the early focuses of
forensic chemistry. Latent fingerprints on glass,
polished metal, and other glossy surfaces can
be revealed by dusting them with a fine powder
whose color contrasts with the background
surface. But latent fingerprints on paper and
similar materials are difficult or impossible to
reveal by dusting.
RIREEqU d EqUIpmENT ANd SUppLIES£ goggles, gloves, and protective clothing£ gas burner or alcohol lamp£ ring stand£ support ring£ wire gauze square£ large beaker with watch glass or other cover£ stiff wire and paperclip (to suspend sample in beaker)£ magnifying glass or loupe to examine samples
(optional)£ small sprayer bottles (3)£ samples of paper with latent fingerprints (3)£ iodine crystals (~1 g)£ starch solution (~1 g of cornstarch in 25 mL
water; filter or decant off clear liquid)£ silver nitrate solution (0.3 g silver nitrate in
10 mL distilled water)£ ninhydrin solution (0.1 g ninhydrin in 10 mL of
95% ethanol)Iodine fuming was the first method developed for revealing
latent fingerprints on such surfaces. The paper or other
material to be treated is placed in a chamber that also contains
a few crystals of iodine. As the iodine is heated, it sublimates
as a violet vapor, which is deposited on the material being
treated. The iodine condenses to a solid and selectively
adheres to the oils present in the fingerprints, revealing the
fingerprints as faint orange marks. Because it depends on the
physical binding of a marking agent to the latent fingerprints,
iodine fuming is conceptually similar to dusting, with the iodine
vapor acting as an extremely fine “dust.”
Iodine fuming is still used today because it is nondestructive.
Latent fingerprints revealed by iodine vapor gradually
disappear as the iodine sublimates, leaving the sample in its
original state. The revealed prints can be photographed before
they disappear or they can be developed by treating them with
a 1% aqueous solution of starch. The iodine-starch reaction
turns the prints a darker blue color, which is persistent for
weeks or months, depending on how the sample is stored. If
preserving the original state of the sample is unimportant,
the revealed prints can be developed with a dilute solution of
benzoflavone, which reacts with the iodine to make the prints
permanent.
Development with silver nitrate was another early method used
to reveal latent fingerprints on paper and similar materials. A
3% solution of silver nitrate is sprayed or gently brushed onto
the sample. The silver nitrate reacts with the salt present in the
latent fingerprints to produce silver chloride.
NaCl(s) + AgNo 3 (aq) → AgCl(s) + NaNo 3 (s)
The sample is sometimes then washed gently with distilled
water to remove excess silver nitrate, after which it is exposed
to sunlight or an ultraviolet lamp. The UV reduces the silver
ions present in the silver chloride to metallic silver, which is
visible as orange or black marks. The silver nitrate method
is used less often than the iodine fuming method, because
treating the sample with silver nitrate causes irreversible
changes.The final method of revealing latent fingerprints that we
examine in this lab depends on the fact that fingerprints
contain trace amounts of amino acids. Ninhydrin
(triketohydrindene hydrate, C 9 H 6 O 4 ) reacts with the
breakdown products of these amino acids to produce a deepSBSTITUTIU oNS ANd modIfICATIoNS- You may substitute discarded decongestant nasal
sprayers or similar for laboratory spray bottles.