Week 3B - 2 Tests for Blood

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Del curso dictado por Nanyang Technological University, Singapore

Introduction to Forensic Science

361 calificaciones

Nanyang Technological University, Singapore

Introduction to Forensic Science

361 calificaciones

We have all seen forensic scientists in TV shows, but how do they really work? What is the science behind their work? The course aims to explain the scientific principles and techniques behind the work of forensic scientists and will be illustrated with numerous case studies from Singapore and around the world. Some questions which we will attempt to address include: How did forensics come about? What is the role of forensics in police work? Can these methods be used in non-criminal areas? Blood. What is it? How can traces of blood be found and used in evidence? Is DNA chemistry really so powerful? What happens (biologically and chemically) if someone tries to poison me? What happens if I try to poison myself? How can we tell how long someone has been dead? What if they have been dead for a really long time? Can a little piece of a carpet fluff, or a single hair, convict someone? Was Emperor Napoleon murdered by the perfidious British, or killed by his wallpaper? *For Nanyang Technological University (NTU) students, please be noted that this course will no longer be eligible for credit transfer.

De la lección

Time of Death; Blood

Week 3A - 1 Recent Deaths10:39

Week 3A - 2 Decomposing Bodies I (Putrefaction)6:13

Week 3A - 3 Decomposing Bodies II (Forensic Entomology)11:50

Week 3A - 4 Analysis of Skeletal Remains I8:29

Week 3A - 5 Analysis of Skeletal Remains II11:37

Week 3A - 6 Ötzi Case; Summary3:36

Week 3B - 1 Blood13:02

Week 3B - 2 Tests for Blood6:54

Week 3B - 3 Precipitin Technology3:53

Week 3B - 4 Blood Spatter Analysis8:17

Week 3B - 5 Lord Lucan Case; Summary7:04

Conoce a los instructores

Roderick Bates
Associate Professor
Division of Chemistry & Biological Chemistry | School of Physical & Mathematical Sciences

[MUSIC]

Well, if we have a stain on a suspect or

at a crime scene that we suspect might be blood,

how do we demonstrate that it is blood?

Well, there's a number of quite simple chemical tests

which we can use as presumptive tests for blood.

One of the old ones is called the Benzidine test.

This tends to be not used anymore because of the toxicity of the chemicals involved.

An excellent test that gives a nice

colour reaction is the Kastle-Meyer test using phenolphthalein,

and another very good reagent for testing for blood is Luminol.

All

of these tests are based on the chemical property of blood, that it can catalyze

the break down of hydrogen peroxide to water and oxidizing species.

[BLANK_AUDIO]

Let's take a look at the Kastle-Meyer test.

What you need for doing the Kastle-Meyer test is reduced phenolphthalein.

Now on the left, this is phenolphthalein in

its acidic form, which is a colourless material.

But in alkali, phenolphthalein loses a proton and turns into

its alkali form, which is a very bright pink colour.

Now if this alkaline form is reacted with zinc powder, it's reduced,

and this reduced form is, once again, colourless.

Now, in the presence of an oxidizing agent, that

reduced form is converted back into the bright pink form.

So if we give it hydrogen peroxide, this is an oxidizing agent, but hydrogen

peroxide alone cannot turn the reduced form back into the pink alkali form.

What it needs is a catalyst and

one excellent catalyst for this reaction is blood.

So if we have a suspected blood stain, we treat it with reduced phenolphthalein and

hydrogen peroxide, and we see the colour change to pink, then it probably is blood.

But it is still only a presumptive test for blood,

it doesn't prove it's blood.

It just says, it's most likely to be blood.

This is because there are other materials,

not just blood, that can catalyze this reaction.

For instance, a potato, it turns out that there are

very powerful enzymes in potatoes which are excellent catalysts for this reaction.

So, potatoes are one example of something that can give

what we call a false positive in the Kastle-Meyer test.

There's a mysterious stain on this piece of tissue paper here.

Could be a blood stain,

but then, old blood stains just look like brown stains, so

it could also be paint or chocolate or something like that.

But what we are going to do is to use the

Kastle-Meyer test to find out if that is a blood stain.

This is just a little bit of ethanol.

This is the Kastle-Meyer reagent from reducing

alkaline phenolphthalein with zinc,

and you can see a little bit of the zinc in the bottom of the vial.

[BLANK_AUDIO]

And you notice, when we

add the Kastle-Meyer

reagent, nothing happens.

That's because we haven't added the

oxidizing agent, which is hydrogen peroxide.

And this isn't the dilute kind of hydrogen peroxide

that some people might use to bleach their hair.

This is 30% hydrogen peroxide, and this will burn holes in your epidermis.

And you immediately see the pink colour, as

the hydrogen peroxide re-oxidizes the reduced form of phenolphthalein,

a reaction that is catalyzed by blood.

So probably, that brown stain on that tissue is blood.

Well, the Kastle-Meyer test is not perfect because

a number of substances will also give positive results.

That is so-called "false positive" and one of

these substances that will do this is the humble potato.

So, here is a potato.

I am going to treat it in exactly the same way.

[BLANK_AUDIO]

That was the ethanol.

This is the Kastle-Meyer reagent here.

[BLANK_AUDIO]

And finally, some hydrogen peroxide.

[BLANK_AUDIO]

So, potato's a little bit slow until it goes pink.

[BLANK_AUDIO]

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