3 - 8 - Week 3B - 2 Tests for Blood (06 54), kryminalistyka, introduction to forensic science
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[MUSIC]Well, if we have a stain on a suspect orat a crime scene that we suspect might beblood,how do we demonstrate that it is blood?Well, there's a number of quite simplechemical testswhich we can use as presumptive tests forblood.One of the old ones is called theBenzidine test.This tends to be not used anymore becauseof the toxicity of the chemicals involved.An excellent test that gives a nicecolour reaction is the Kastle-Meyer testusing phenolphthalein,and another very good reagent for testingfor blood is Luminol.Allof these tests are based on the chemicalproperty of blood, that it can catalyzethe break down of hydrogen peroxide towater and oxidizing species.[BLANK_AUDIO]Let's take a look at the Kastle-Meyertest.What you need for doing the Kastle-Meyertest is reduced phenolphthalein.Now on the left, this is phenolphthaleininits acidic form, which is a colourlessmaterial.But in alkali, phenolphthalein loses aproton and turns intoits alkali form, which is a very brightpink colour.Now if this alkaline form is reacted withzinc powder, it's reduced,and this reduced form is, once again,colourless.Now, in the presence of an oxidizingagent, thatreduced form is converted back into thebright pink form.So if we give it hydrogen peroxide, thisis an oxidizing agent, but hydrogenperoxide alone cannot turn the reducedform back into the pink alkali form.What it needs is a catalyst andone excellent catalyst for this reactionis blood.So if we have a suspected blood stain, wetreat it with reduced phenolphthalein andhydrogen peroxide, and we see the colourchange to pink, then it probably is blood.But it is still only a presumptive testfor blood,it doesn't prove it's blood.It just says, it's most likely to beblood.This is because there are other materials,not just blood, that can catalyze thisreaction.For instance, a potato, it turns out thatthere arevery powerful enzymes in potatoes whichare excellent catalysts for this reaction.So, potatoes are one example of somethingthat can givewhat we call a false positive in theKastle-Meyer test.There's a mysterious stain on this pieceof tissue paper here.Could be a blood stain,but then, old blood stains just look likebrown stains, soit could also be paint or chocolate orsomething like that.But what we are going to do is to use theKastle-Meyer test to find out if that is ablood stain.This is just a little bit of ethanol.This is the Kastle-Meyer reagent fromreducingalkaline phenolphthalein with zinc,and you can see a little bit of the zincin the bottom of the vial.[BLANK_AUDIO]And you notice, when weadd the Kastle-Meyerreagent, nothing happens.That's because we haven't added theoxidizing agent, which is hydrogenperoxide.And this isn't the dilute kind of hydrogenperoxidethat some people might use to bleach theirhair.This is 30% hydrogen peroxide, and thiswill burn holes in your epidermis.And you immediately see the pink colour, asthe hydrogen peroxide re-oxidizes thereduced form of phenolphthalein,a reaction that is catalyzed by blood.So probably, that brown stain on thattissue is blood.Well, the Kastle-Meyer test is not perfectbecausea number of substances will also givepositive results.That is so-called "false positive" and oneofthese substances that will do this isthe humble potato.So, here is a potato.I am going to treat it in exactly the sameway.[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 itgoes pink.[BLANK_AUDIO] [ Pobierz całość w formacie PDF ]
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[MUSIC]Well, if we have a stain on a suspect orat a crime scene that we suspect might beblood,how do we demonstrate that it is blood?Well, there's a number of quite simplechemical testswhich we can use as presumptive tests forblood.One of the old ones is called theBenzidine test.This tends to be not used anymore becauseof the toxicity of the chemicals involved.An excellent test that gives a nicecolour reaction is the Kastle-Meyer testusing phenolphthalein,and another very good reagent for testingfor blood is Luminol.Allof these tests are based on the chemicalproperty of blood, that it can catalyzethe break down of hydrogen peroxide towater and oxidizing species.[BLANK_AUDIO]Let's take a look at the Kastle-Meyertest.What you need for doing the Kastle-Meyertest is reduced phenolphthalein.Now on the left, this is phenolphthaleininits acidic form, which is a colourlessmaterial.But in alkali, phenolphthalein loses aproton and turns intoits alkali form, which is a very brightpink colour.Now if this alkaline form is reacted withzinc powder, it's reduced,and this reduced form is, once again,colourless.Now, in the presence of an oxidizingagent, thatreduced form is converted back into thebright pink form.So if we give it hydrogen peroxide, thisis an oxidizing agent, but hydrogenperoxide alone cannot turn the reducedform back into the pink alkali form.What it needs is a catalyst andone excellent catalyst for this reactionis blood.So if we have a suspected blood stain, wetreat it with reduced phenolphthalein andhydrogen peroxide, and we see the colourchange to pink, then it probably is blood.But it is still only a presumptive testfor blood,it doesn't prove it's blood.It just says, it's most likely to beblood.This is because there are other materials,not just blood, that can catalyze thisreaction.For instance, a potato, it turns out thatthere arevery powerful enzymes in potatoes whichare excellent catalysts for this reaction.So, potatoes are one example of somethingthat can givewhat we call a false positive in theKastle-Meyer test.There's a mysterious stain on this pieceof tissue paper here.Could be a blood stain,but then, old blood stains just look likebrown stains, soit could also be paint or chocolate orsomething like that.But what we are going to do is to use theKastle-Meyer test to find out if that is ablood stain.This is just a little bit of ethanol.This is the Kastle-Meyer reagent fromreducingalkaline phenolphthalein with zinc,and you can see a little bit of the zincin the bottom of the vial.[BLANK_AUDIO]And you notice, when weadd the Kastle-Meyerreagent, nothing happens.That's because we haven't added theoxidizing agent, which is hydrogenperoxide.And this isn't the dilute kind of hydrogenperoxidethat some people might use to bleach theirhair.This is 30% hydrogen peroxide, and thiswill burn holes in your epidermis.And you immediately see the pink colour, asthe hydrogen peroxide re-oxidizes thereduced form of phenolphthalein,a reaction that is catalyzed by blood.So probably, that brown stain on thattissue is blood.Well, the Kastle-Meyer test is not perfectbecausea number of substances will also givepositive results.That is so-called "false positive" and oneofthese substances that will do this isthe humble potato.So, here is a potato.I am going to treat it in exactly the sameway.[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 itgoes pink.[BLANK_AUDIO] [ Pobierz całość w formacie PDF ]