Items of physical evidence are not always visible to the naked eye and may be easily overlooked. A methodical approach to collection and preservation of evidence is essential. One exception may be if evidence integrity is at risk. Under those circumstances, it is important that rapid decisions be made to prevent degradation or loss of evidence.

An alternate light source or oblique lighting may be used to identify some types of biological evidence. A sample detected with the ALS should be properly collected and packaged with a label noting that it is a biological sample.

Blood may also be detected with chemical processes such as luminol. Luminol is an investigative aid that can assist in determining the presence of small quantities of blood (human and animal) including where bloodstains may have been cleaned. The luminol reagent reacts with the iron in hemoglobin resulting in a creation of a blue-green, luminescent light.

Precautions to consider when using luminol include the following:

• The chemical reaction can destroy evidence at the crime scene.
• Luminol will react to other substances, including copper and bleach.
• Luminol reactions must be viewed in complete darkness to observe the luminescence.

Based on these considerations, this method can be a valuable tool. It is generally only used after exhausting other options.

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