Testing for Blood

You've probably all seen it in TV dramas; the crime scene looks clean as a whistle, not a trace left. That is until the CSI unit rolls in, sprays a mysterious liquid all over the place, off go the lights, out come the UV lamps and suddenly the room is covered in ethereal blue stains. Well you may be happy to learn that that's one of the few things that forensics shows largely get right.

Presumptive tests for blood reveal blood where it may not have been found with the naked eye. This could be due to it blending well with the background, seeping into cracks, or possibly just occuring in trace amounts. Luckily presumptive tests can help a forensic scientist find and identify blood at a crime scene.

Presumptive tests for blood are usually based on haemoglobin's ability to catalyse the oxidation of certain reagents. The most common reagent is Hydrogen Peroxide Solution (H202(aq)). These tests also commonly use reagents that change colour on oxidation. Phenolphthalein which goes from colourless to pink when oxidised.

To test a stain, a small circular piece of absorbant card (diameter=25mm) is folded twice to form a point. A small amount of the stain being tested is then scraped onto this point. a drop of phenophlthalein is added by a drop of hydrogen peroxide, with the bright pink colour showing a positive result. It's important to note that this test does not differentiate between human blood and other kinds of blood. 

However in some situations the colour change tests are not appropriate. Maybe the crime scene has been scrubbed clean, or you need the blood 'intact' for DNA profiling. This is were luminol comes into play. In the luminol test an alkaline solution containing both luminol and an oxidising agent (for example Hydrogen Peroxide), is sprayed onto the area being tested. Where blood is present luminol is catalytically oxidised and produces a distinct glow. 

Luminol before and after

I hope this has helped to illuminate the basics of blood identification. Until next time! Over and out!