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Contamination
:
Any substance inadvertently introduced into or onto an
item of evidence after its recognition by a responsible party.
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212
Principles and Practice of Criminalistics
Using this definition, our prior illustration of multiple genetic types
found on a bedsheet would actually be considered an example of a 
mixed
sample
. The multiple types might or might not confound the analysis and
interpretation, but they are an integral part of the evidence. It is then the
task of the analyst to sort out the particulars as they relate to the commission
of the crime. Still using the bedsheet example, suppose an evidence technician
collected the sheet and placed it into a plastic bag, where it was stored for
several months at room temperature. During this time, microorganisms grew
and destroyed much of the biological evidence. We would consider the evi-
dence contaminated by the growth of bacteria because it was introduced as
a consequence of improper preservation. In this instance, the contamination
would likely produce no result or a partial result, but would be highly unlikely
to produce an incorrect result, especially a false match to the suspect.
The concept of contamination was not invented to describe the condition
of certain forensic samples. Any physical or biological sample may be asso-
ciated with extraneous substances. However, the consequences hold a greater
significance for forensic evidence samples for the following reasons:
b.
Sources of Contamination
The source of adventitious extraneous material may contribute to its classi-
fication as integral (mixed) or contaminating. Extraneous material may be
introduced into a sample by normal activities of humans prior to, during,
or after the commission of a crime. Because the world is a dirty place, we
have termed this the 
Dirty World Syndrome
or DWS. Substances acquired
through DWS are classified as impurities integral to the evidence. After
recognition as evidence by some responsible party, extraneous material may
Consequences of Contamination for Forensic Samples
• The true nature of the sample is unknown and unknowable. The
contaminant might be mistaken for the true sample.
• Even with the adequate and proper use of controls, no independent
way exists to prove conclusively that the result obtained is the one
relevant to the incident.
• It is often impossible to obtain additional material if it is suspected
that a sample may have been contaminated.
• The sample is often present in limited quantity. Thus a contaminant
could potentially overwhelm the evidence, producing a false result.
For this to be so, the contaminant would have to actually be detect-
able by the test and mimic or fall into the expected range for the
results.
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Good Field Practice — Processing a Crime Scene
213
be introduced either at the scene or in the laboratory (including during
transportation). We define substances acquired in this way as contaminants.
The most egregious type of contamination is introduction of material from
a reference sample either at the scene or in the laboratory; this would be the
scenario most likely to confound the interpretation. Contamination with a
reference sample from the suspect could lead to the worst possible conse-
quence of contamination, a false association between crime scene evidence
and an individual. Fortunately, when good collection, documentation, and
laboratory practices are employed, this possibility is significantly curtailed.