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c.
Types of impurities and their consequences
We need to differentiate among chemical, biological, and human impurities.
All of these may have different consequences for the interpretation and sig-
nificance of the evidence, and the consequences will differ depending on the
nature of the evidence. In the following section, we summarize some general
consequences of various types of impurities for biological and nonbiological
evidence and provide a few examples. Obviously this discussion is nowhere
near exhaustive and each case must be considered on its own merits.
i.
Nonbiological impurities
For nonbiological samples
Chemical substances unrelated to the crime certainly have the potential to
influence the results obtained from various types of nonbiological evidence.
Generally, a contaminant that causes a problem in nonbiological evidence
will be of like nature to the evidence and has the potential to produce a false-
positive result.
A mundane example might be a crime scene investigator tracking soil
from outside a crime scene into one, or worse yet, from one crime scene to
another. In a more inflammatory example (as it were), it has been suggested
that improperly stored explosives evidence was responsible for false positives
in the Oklahoma City bombing case. The allegation involved storing improp-
erly sealed volatiles from numerous cases in the same physical space, thus
introducing the potential for cross-contamination (
United States v. Terry
Nichols,
December 1).
Historically, one of the most infamous examples of a test that routinely
gave false-positive results was the paraffin (diphenylamine, dermal nitrate)
test used to detect gunpowder on a suspect’s hands. Detection of nitrate
residue was meant to suggest recent discharge of a firearm. Unfortunately,
many other substances, such as tobacco, fertilizer, and some plastics, also
produce positive results with this test. The test long ago fell out of favor with
forensic scientists everywhere. It was certainly responsible for a number of
miscarriages of justice (Mullin, 1986); (see Sidebar 1 in Chapter 2).
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For biological samples
Although nonbiologicals won’t produce false-positive results in biological evi-
dence, they may interfere with an analysis by preventing the various biochemical
components from working properly. Chemicals such as dyes, oils, soaps, and
others are commonly encountered in forensic samples. Part of the sample
preparation is to remove or mitigate these potentially interfering agents.
Chemical impurities might also be introduced by an analyst or technician
in an attempt to locate evidence, including biological evidence. For example,
a crime scene will be routinely dusted for fingerprints or blood-detecting
reagents such as luminol will be used to detect latent bloodstains. Because
the analyst knows of the existence of these impurities, accommodations to
the analysis or interpretation can be made as appropriate (Gross et al., 1999).