Blood+Dection+Outline-HEB

Blood detection by chemical methods outline

A. Blood detection with chemicals 1. Crystal tests - most common when Haem forms crystals when reacted with certain reagents - now rarely used -A negative result does not necessarily show that blood is absent - it may, for example, indicate faulty technique - and a positive control should always be run for comparison. Bloodstains up to 20 years old have given positive results in crystal tests.

2. Catalytic tests - These tests rely on the fact that haem can catalyze the breakdown of hydrogen peroxide. As the H2O2 breaks down, another substance in the reaction mixture is oxidised, producing a colour change. -extreamly sensitive - here has to be an awareness of this when results are interpreted, particularly when testing outdoors, where many types of plant material can be present, or testing in vehicles, where metal surfaces can interfere. The general principle is that if the test is negative, blood is absent, but that if the test is positive, blood is probably, not definitely present. For this reason the tests are often described as "presumptive" tests.

3. Instrumental methods - used to identify the presence of haemoglobin -HPLC - used to confirm the identity of blood using the absorbance of haemoglobin for detection. This method can also be used to identify the species of origin from variations in the globin chains4, to distinguish foetal haemoglobin from adult haemoglobin5, and to give an estimate of the age of a bloodstain6.

B. APPLICATION -This is largely carried out using either the "Sangur" sticks mentioned earlier, or using the Kastle-Meyer test. The Sangur sticks, in which the detecting reagent is in an immobilised form, require only to be rubbed gently on the stain and moistened.

C. Detection of non-visible blood - Luminol's major application is in areas where blood may be present but is difficult to see, such as outdoors among vegetation, or where attempts have been made to clean up blood and traces are still present. A positive reaction can also sometimes be given by bloodstained clothing which has been washed.

D. Enhancement of blood stains -A further area in which blood detection reagents have proved useful has been in the enhancement of existing bloodstains. In cases of partial shoeprints or fingerprints in blood there is often more of the print present than can be seen, and treatment of the print with a chemical which reacts with blood can often produce a much more detailed print. This can then be photographed and subsequently compared with a suspect's shoe or fingerprint. Luminol has been used for this purpose but, as noted earlier, detail can be lost by excessive spraying of the stain, and photography is often difficult.

E. Subsequent reactions of stains treated with blood detecting reagents Once it is determined that a stain is probably blood, the next question one asks is "whose?" It is therefore necessary to ensure that the presumptive tests for blood do not interfere with subsequent tests used to "type" or "group" the blood.