Luminol is used as a last resort in criminal investigation.Liminol in itself is not used as conclusive proof that a crime has been committed. The alleged blood evidence will be collected and sent to a laboratory to be tested, to see if it is indeed blood or some other bodily fluid, and be typed and tested for DNA. But a simple positive hit with luminol is not used to conclusively prove guilt because it is known by law enforcment that there are other substances that can cause a positive hit. found these references on the internet.
"If luminol reveals apparent blood traces, investigators will photograph or videotape the crime scene to record the pattern. Typically, luminol only shows investigators that there might be blood in an area, since other substances, including household bleach, can also cause the luminol to glow. Experienced investigators can make a reliable identification based on how quickly the reaction occurs, but they still need to run other tests to verify that it is really human blood.
Luminol in itself won't usually solve a murder case. It's only one step in the investigative process. But it can reveal essential information that gets a stalled investigation going again. For example, hidden blood spatter patterns can help investigators locate the point of attack and even what sort of weapon was used (a bullet makes blood splatter very differently than a knife does). Luminol may also reveal faint bloody shoe prints, which gives investigators valuable information about the assailant and what he or she did after the attack. "
-------------------------------------------------------------------------------------
http://www.sas.upenn.edu/~willard2/Visi ... tains.html
"The luminol test is used in the field of forensic science to detect trace amounts of blood at crime scenes. A reaction between luminol and the hemoglobin in blood causes chemiluminesence to occur. A spray containing the luminol and hydrogen peroxide is used by forensic scientists to detect blood at crime scenes. Hemoglobin is able to catalyse the breakdown of hydrogen peroxide into oxygen, which can oxidize the luminol. When the luminol breaks down light is emitted showing the presence of blood. Since the human eye is very good at detecting even a small amount of light in a dark room it is possible to detect tiny drops of blood at a crime scene. A reaction very similar to this is used by fireflies to emit light.
It works well with both fresh and dry blood, and can be applied several years after the incident. It is usually used at a crime scene after it escaped detection during the first crime scene processing assignment. The room is photographed and a crime scene sketch is made. The room is then darkened as much a possible. The luminol solution is fine sprayed over the suspected area of a room or object in the room.
Luminol is hazardous so protective clothing should be worn, including goggles, rubber gloves, boots and an organic vapor mask.
The use of luminol in this field was first suggested by W. Specht in 1937. Unfortunately this test is only a presumptive test for the detection of blood as chemiluminesence can occur when the luminol solution reacts with other common substances, such as household bleach, plant and vegetable peroxidases and copper metal,as well as its salts and its alloys.
Once the blood stain has been identified it is then collected. To collect the bloodstain the FBI recommends you absorb the dried bloodstains on immovable objects onto a clean cotton cloth or cotton swab moistened with distilled water. Leave a portion of the cloth or swab unstained as a control. Air dry the cloth or swab and pack in clean paper or an envelope with sealed corners. Do not use plastic containers.
The blood is then sent to the crime lab for both type and DNA examination. "
-------------------------------------------------------------------------------------
"How long does DNA remain after death? Depending on conditions such as moisture and temperature, it may remain for millions of years. Scientists have claimed extraction of DNA from a variety of ancient sources, including the remains of a quagga, a zebra-like animal that became extinct 145 years ago; ancient Egyptian mummies; an American mastodon that lived 46,000 years ago; Ice Age hunters frozen about 10,000 years ago, and 40-million-year old bacteria isolated from the gut of bee.
No matter how well preserved an ancient organism may be, scientists think that most DNA deteriorates after 50,000 to 100,000 years.