Biology Reference
In-Depth Information
found at approximately 12% of investigated crime scenes; this figure can go up
significantly if the crime scene is exhaustively searched [58].
The methods used for collection will vary depending on the type of sample. Dry
stains and contact marks on large immovable items are normally collected using a
sterile swab that has been moistened with distilled water [59, 60]; in other cases,
scraping or cutting of material may be more appropriate. Lifting from the surface
using high-quality adhesive tape is an alternative method for collecting epithelial
cells [52]. Liquid blood can be collected using a syringe or pipette and transferred to
a clean sterile storage tube that contains anticoagulant (ethylene-diamine tetraacetic
acid (EDTA)), or by using a swab or piece of fabric to soak up the stain, which
should be air dried to prevent the build up of microbial activity [8]. Liquid blood
can also be applied to FTA
paper, which is impregnated with chemicals to prevent
the action of microbial agents and stabilize the DNA. (FTA
paper was developed
by
Flinders
Technology Associates.)
Smaller movable objects, such as weapons, which might contain biological material
are packaged at the scene of crime and examined in the controlled environment of
the forensic laboratory. The same range of swabbing, scraping and lifting techniques
as used in the field can be employed to collect the biological material. Clothing taken
from suspects and victims presents an important source of biological evidence. This
is also analysed in the forensic biology laboratory, where stains and contact areas
can be recorded and then cut out or swabbed.
Sexual and physical assault
Following sexual assaults, the victim should be examined as soon after the event as
possible. Semen is recovered by a trained medical examiner using standard swabs;
fingernail scrapings can be collected using a variety of swabs; combings of pubic
and head hair are normally stored in paper envelopes. The samples collected by
the medical examiner are dependent on the nature of the allegation and information
given by the alleged victim. Contact marks, for example bruising caused by gripping
or bite marks, can be swabbed for DNA. The same types of evidence (except semen)
can be taken after cases of physical assault [8].
Reference samples
In order to identify samples recovered from the scene of crime, reference samples are
needed for comparison. Reference samples are provided by a suspect and, in some
cases, a victim. Traditionally, blood samples have been taken and these provide
an abundant supply of DNA; however, they are invasive and blood samples are a
potential health hazard. Buccal swabs that are rubbed on the inner surface of the
cheek to collect cellular material have replaced blood samples in many scenarios.
In some circumstances plucked hairs may be used, but this source of material is not
commonly used.
