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Strengthening Forensic Science in the United States: A Path Forward
Sample Data and Collection
At the crime scene, biological evidence is located, documented, collected, and preserved for subsequent analysis in the crime laboratory. Locating and recognizing biological evidence can be more difficult than a layperson would presume. For example, blood is not always red, some red substances are not blood, and most biological evidence, such as saliva or semen, is not readily visible. Crime scene investigators locate biological evidence through tests that screen for the presence of a particular biological fluid (e.g., blood, semen, saliva), and investigators have a choice of techniques.4 For blood they might use an alternate light source (ALS) at 415nm, the wavelength under which bloodstains absorb light and are thus more visible to the naked eye. Most commonly, though, the screening test for blood is a catalytic chemical test that turns color or luminesces in the presence of blood. Scene investigators may also use Luminol, fluorescein, or crystal violet to identify areas at the scene where attempts were made to clean a bloody crime scene.
These tests for blood may also locate other evidence that should be collected and taken to the laboratory for analysis. Recently, immunological tests that can identify human hemoglobin or glycophorin A have become available. These are blood-specific proteins that can be demonstrated to be of human origin. At some point in the future, these immunological tests may replace standard chemical tests, and, although more expensive, they are more specific because they identify blood conclusively instead of just presumptively. Investigators also have several techniques for locating semen at the crime scene. Commonly they rely on an ALS, under which semen, other biological fluids, and some other evidence will luminesce. More recently, immunological tests can be used to identify seminal plasma proteins, for example, prostate specific antigen (p30 or PSA) or semenogelin.5
Finding saliva at the scene is mostly happenstance. Although it luminesces with the ALS at specific wavelengths, the glow is not as strong, and a weaker ALS light source may not highlight it well and possibly not at all. Thus, it can be easily missed. Screening tests for saliva are chemical tests that identify amylase, an enzyme occurring in high concentrations in saliva. But the screening is not definitive, because other types of tissue also
Interpreting the results of any screening test requires expertise and experience. Many crime scene investigators have the requisite experience, but they may lack a scientific background, and it is not always straightforward to correctly interpret the results of screening tests. Crime scene investigations that require science-based screening tools are most reliable if someone is involved who understands the physics and chemistry of those tools.
I. Sato, M. Sagi, A. Ishiwari, H. Nishijima, E. Ito, and T. Mukai. 2002. Use of the “SMITEST” PSA card to identify the presence of prostate-specific antigen in semen and male urine. Forensic Science International 127(1-2):71-74.