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Author Topic: Luminol information  (Read 7490 times)

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MysteryAdmin

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Luminol information
« on: October 03, 2006, 04:34:37 PM »

Police Procedures
From: John Rickards yemighty@hotmail.com
Date: Friday, December 06, 2002 03:35 AM

Dug out some more of my old notes on the subject, which are below. Long, technical and a little dull, I'm afraid. :-(

I went through a similar 'oh crap, how can you test for blood? how does it all work?' thing for a book of mine, which is the only reason I've got this stuff. Feel free to ignore it - I'm a writer, not a forensic tech! :-)

If you want to confirm any information, my advice would be to sign up to the Forensic Science or ICSIA Public Forum Yahoo groups (http://groups.yahoo.com) and ask them. They'll be happy for a query that's not just "how do I study forensics at college?" :)

Below, my (next) 2c,

John Rickards

-- WINTER'S END coming March 2003 (UK)/Fall 2003 (US) --

============

Luminol uses metal ions such as the iron in blood (it will also react with any copper and quite possibly other metallic stuff in the blood as well, but iron is the most common) as catalysts for the chemical's oxidation with the solvent it's kept in (usually hydrogen peroxide). Effectively, luminol breaks down in the presence of metal ions and this breakdown produces light. The amount of light and the duration of the glow produced varies according to what chemical is present.

Luminol doesn't react directly with the metal - handy for blood because the test isn't destructive. It's merely a reaction that relies on the presence of accessible metal ions to work. Iron in haemoglobin, unless it's carrying oxygen, is in the form of an ion - its reactivity is what allows it to carry oxygen in the first place - which means it's ideal for luminol testing.

The reason cleaning chemicals like hypochlorites (bleach) react with it is that they are alternative oxidisers - they're happy to react with luminol in the same way the hydrogen peroxide is. As far as I understand, they can affect the result but not in the 'false positive' way metallic ions do.

In general, the amount of copper or other 'non-blood' metallic ions in a tested area is likely to be low. Road asphalt might contain small amounts of metal contaminants, earth with a high concentration of iron ore or copper minerals, etc. could give false positives, but if you're spraying a sample of clothing, say, it's not likely.

Because luminol will give a positive result with copper or other metal compounds, and because it cannot distinguish between human and animal blood, it is very much a presumptive test. Other chemical tests such as phenolpthalein and the like should be used for confirmation. Certainly in Arkansas - I don't know about elsewhere, this is the only state I've found definite info on - luminol evidence is inadmissable on its own, precisely for this reason. However, luminol's sensitivity and clear positive/negative delinieation make it a very handy tool in the field, even if you'd normally have to perform a second or third test just to confirm what you've found.

==============

There's a short, basic bit on luminol in forensics at:
http://www.deakin.edu.au/forensic/Chemical%20Detective/Luminol_test.htm

And a much longer, more technical piece on luminol as a chemical and an analytical tool for use on things other than blood at:
http://www.shsu.edu/~chm_tgc/JPPdir/JPP1999/
Logged
--
Bob Mueller
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