Blood and hair analysis
To understand the limitations of forensic evidence, it is necessary to distinguish between class evidence and individual evidence. Class evidence consists of substances such as blood and hair, which can be used to place an individual in a general class but cannot be used to identify an individual. For example, blood typing can be used to establish whether someone has A, B, AB, or O blood, but cannot point to a person.
A common method to test for blood is to spray an area with a solution of luminol and hydrogen peroxide. If blood is present, the iron atoms within the hemoglobin molecules in red blood cells act as a catalyst, causing the luminol to emit an eerie blue glow. This chemical reaction is a great way to test for blood stains. However, other compounds can also catalyze this reaction. A number of other substances, from bleach to horseradish, can also produce an eerie blue glow, leading a forensic investigator to conclude that blood is present when, in fact, there is none.