Week 7 - 6 Alcohol

Loading...

En provenance du cours de Nanyang Technological University, Singapore

Introduction to Forensic Science

358 notes

Nanyang Technological University, Singapore

Introduction to Forensic Science

358 notes

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.

À partir de la leçon

Toxicology

Week 7 - 1 Introduction to Toxicology16:51

Week 7 - 2 Deliberate & Accidental Poisoning9:36

Week 7 - 3 Toxins & Biological Poisons7:44

Week 7 - 4 LD5012:24

Week 7 - 5 Forensic Toxicology13:54

Week 7 - 6 Alcohol20:06

Week 7 - 7 Inorganic Poisons - Arsenic14:55

Week 7 - 8 Inorganic Poisons - Thallium5:17

Week 7 - 9 Inorganic Poisons - Barium5:00

Week 7 - 10 Nerve Agents13:24

Week 7 - 11 Georgi Markov Case4:16

Week 7 - 12 Alexander Litvinenko Case; Summary6:14

Rencontrer les enseignants

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

[MUSIC]

Alcohol deserves a special section all to itself,

and this is because of all the molecules, all the

poisons, that have to be dealt with by forensic scientists,

by far it's alcohol that generates the most work.

Alcohol is an interesting and controversial molecule.

For some societies, for some cultures, alcohol is

an essential lubricant that keeps the wheels turning.

And yet in other cultures, it's an evil

molecule which is reviled and must never be consumed.

So, this is a famous picture.

This is a picture titled Gin Lane, which

was produced by the 18th century artist Hogarth,

and it's intended to show the evils

of alcohol. Well actually, more specifically, it

shows the evils of the very strong spirit, gin, because at that time in England,

gin consumption was enormous. Well, Hogarth

actually wasn't opposed to alcohol itself.

He just felt the consumption of gin was very harmful.

And this picture is one of a pair, the other picture is called Beer Street.

Beer Street, beer contains much less alcohol

and therefore is much less damaging than gin,

and Hogarth and his friends wanted to encourage

people to switch from one to the other.

[BLANK_AUDIO]

Now, what happens when someone drinks alcohol?

Well of course, the liquid goes down into the stomach.

Some of it is absorbed by the stomach, and

the bulk of it is absorbed in the small intestine.

It's absorbed very fast, it will appear in the blood almost immediately,

and it will be almost completely absorbed into the body within one hour.

There is some variation of the rate of absorption, depending on the kind of

drink and what the stomach contents are,

but it's not a particularly big difference.

Once absorbed into the blood, alcohol will be

distributed throughout the body via the cardiovascular system.

And it will, of course, penetrate into

the brain, the blood-brain barrier doesn't stop it.

And the effect of alcohol is actually to depress the Central Nervous System.

Now, this seems a bit illogical, because normally when

people drink some alcohol, they don't seem to be depressed.

This is because its not depressing the person, its depressing the nervous system.

Part of that depression of the nervous system is the depression of

inhibition, so that makes people feel

happier, but it doesn't just depress inhibitions.

It depresses the ability to make judgements,

it depresses the ability to react to situations.

it depresses reflex times and this is why alcohol, when combined with

machineries such as motor vehicles, can be a very, very dangerous combination.

Because of this danger, pretty much all

countries around the world limit the amount of

alcohol that can be present in a person's

body while they are driving a motor vehicle.

Now, the question is how much is too much?

What concentration of alcohol is there going to

be in the blood after someone takes a drink?

Because

you don't know what's in your blood,

all you know is what is in the glass in front of you.

Well, we can calculate the likely blood alcohol

concentration from the amount of alcohol taken in.

So, suppose someone drinks something that contains a certain amount of alcohol.

So let's say the amount of alcohol in grams is a.

This amount of alcohol is then going to be distributed over the person's body,

so, divide by the person's weight in kilograms, which is p.

However, as we shall see, alcohol is not distributed uniformly.

So we have to allow for the fact that

it's only spread over a certain proportion of the body,

and so we have to put in a fudge factor,

and this is called the Widmark factor.

And there are different Widmark factors for men and women.

So r, the Widmark factor, is 0.68 for men and 0.55 for women.

And this is because Widmark considered that women's bodies

have a higher proportion of fatty tissue than men's bodies,

and that is a scientific and not a personal comment.

So when we use this formula, we can calculate the maximum concentration

of blood alcohol, c, as being equal to a divided by pr.

So, let's see how this works.

Suppose you have a 70 kilogram man who drinks a double whisky.

How high is his blood alcohol concentration going to be after this?

Well, if whisky is about 43% alcohol

and a single is 25 cubic centimetres, taking

into account that the density of alcohol is

0.79 grams per cubic centimetre, then we can

calculate that he has ingested 17 grams of

alcohol.

So, if we put 17 grams of alcohol in

a 70 kilogram man, so the Widmark factor is 0.68,

then we can estimate that the blood alcohol concentration

is going to be 0.36 grams per litre of blood.

So alcohol concentration is usually expressed

as milligrams per 100 ml of blood.

So when we correct for the units, this will be 36 milligrams per 100 ml of blood.

Now, what is the effect of alcohol? In particular,

what is the effect of alcohol on the brain?

Now, different parts of the brain are not effected equally,

so the effects vary according to the amount of alcohol.

So below about 50 milligrams per 100 mil,

for most people there would be no obvious effect.

As the blood alcohol concentration goes higher, as the person has more drinks,

then there are noticeable effects, loss of

coordination, and of course slurring of speech.

Increase the concentration further and these effects get worse.

Marked loss of coordination, poor sensory perception and in a lot

of people, starting to feel ill and wanting to be sick.

Say at 150 to 200 mg per 100 ml.,

now we have drunkenness, nausea, ability, inability to stand up, and vomiting.

So, even after that, some people will keep drinking, and this will lead to

coma, problems with the circulatory and respiratory

system. And at high enough blood alcohol concentrations,

this can lead to death.

Now, alcohol has always been there in nature.

For instance, the fermentation of fruits which can occur naturally

means that alcohol has always been there as humanity has evolved.

And therefore, our body has a biochemical mechanism for eliminating alcohol.

In particular, there's an enzyme called alcohol

dehydrogenase present in the liver and the small intestine.

And this enzyme, alcohol dehydrogenase, will convert ethanol

into a higher oxidation state molecule which is acetaldehyde.

Acetaldehyde is also toxic,

so we have a second enzyme called aldehyde dehydrogenase,

which will further oxidize that acetaldehyde to acetic acid.

And acetic acid, that is naturally present as part of our metabolism,

our body can burn up acetic acid, and it will

end up as carbon dioxide and be exhaled from the lungs.

And almost all alcohol that is taken into the body will end up as carbon dioxide.

A very small amount will be lost through the kidneys

or breathed out through the lungs in

the form of alcohol, the unchanged ethanol molecule.

So when you smell alcohol on someone's breath, it's that

small percentage that doesn't get burned up by the enzymes.

Now, when the alcohol is in the body, it is pretty uniformly distributed.

At least it's pretty uniformly distributed over all parts

of the body where there's a lot of water.

The parts of the body where there isn't much water do not get alcohol,

that is the bones, the fat and the hair.

And this is the reason why we have the Widmark factor.

So there's a little difference between arterial

and veinous blood during the absorption

phase, and that's simply because the

alcohol hasn't been completely distributed yet.

So in order to know how much alcohol is in someone's

body, we can measure the amount of alcohol in any body fluid.

So typically, blood alcohol is measured, and we can assume that whatever level of

alcohol is in the blood, it will be the same level inside the brain.

So to measure blood alcohol, a blood sample has to be taken

and then this can be analyzed by a technique such as Gas Chromatography

which we discussed in an earlier lecture.

Now, to measure someone's blood alcohol level, this has to be done by

a medically trained person, it has to be done in a proper place.

But if a motorist is pulled over

by the traffic police because he's driving erratically,

how can the traffic police determine whether this guy's got too much alcohol?

And the technique used is to measure the level of alcohol in the breath.

Of course, if there's a traffic accident, and

someone's dead, then you can choose, as I said,

any of the body fluid you like.

Now,

if the traffic police can measure the breath alcohol, how can we

relate this to the blood alcohol, which is actually the important factor?

The two are closely linked and they're closely linked because of Henry's Law.

And Henry's Law says that if you have a solution of something, a solution of a

volatile substance in a liquid, then the vapour

pressure, that is the amount of that volatile substance

in the the vapour phase above the liquid,

will be proportional with concentration in the liquid.

So if someone has a lot of alcohol in their blood,

then there's going to be a lot of alcohol in their breath.

If they only have a little alcohol in their blood,

there will only be a little alcohol in their breath.

And these are strictly proportional, and that ratio

is typically taken as 2300 : 1. So you can

measure the breath alcohol, multiply it, and then you have the blood alcohol level.

Now, breath alcohol can be very easily measured, as we've

said, at the roadside using these kind of breathalyzer devices.

And typically, these breathalyzer devices rely

on a little bit of chromium chemistry.

Potassium dichromate is a very, very nice bright orange crystalline substance.

Very nice colour indeed.

But if you take potassium dichromate and you have some acid present, and then you

add alcohol, so the suspect motorist breathes into the device,

the alcohol in his breath will react

with the potassium dichromate. And the chromium is

reduced and ends up as chromium sulfate, whereas

the ethanol is oxidized to acetic acid.

Now, chromium sulphate is green, it's

a quite different colour to potassium dichromate.

And we can simply measure the amount of potassium dichromate that is converted

to chromium sulphate by measuring the loss of absorbance at 420 nanometres.

Well, it's not always as simple as

just measuring the breath or blood alcohol levels.

Suppose there has been an accident,

and one of the cars involved in the accident drives

off, and the driver is not apprehended until some hours later.

Then you're not interested in the amount of blood alcohol the guy has

at the time he's arrested, what you really want

to know is how much alcohol was in his blood

when the accident happened. And as we know, the

body is working hard to eliminate alcohol from the blood.

So what we need to do is to be able to calculate

how much blood alcohol somebody had in the past,

a certain number of hours, after they are actually measured.

So suppose, someone takes a drink at time 0.

Okay, at time 0, all the alcohol is in their stomach, none is in their blood.

Very rapidly, that alcohol starts to move into their

blood, so their blood alcohol concentration starts to go up.

At some point, it will reach a maximum and

the biochemical machinery will be eliminating it, so it will reach a

maximum and then it will drop off, and after some hours, we'll reach 0.

The question is, how steep is the slope of this graph?

How fast does the body get rid of alcohol?

Let's take an example.

Suppose there's an accident at 3: 00 a.m.

The man is arrested some hours later, and eye witnesses

confirm that he had had a drink before the accident.

So the question is, what is his blood alcohol

level at 3:00 a.m at the time of the accident?

Well, suppose it takes 2 hours to arrest him and take him

to a police station and take the blood sample and do the measurement.

So, we can only measure the blood alcohol level at 5 a.m.

Now, at 5 a.m., let's suppose that the measurement is 70 mg per 100 mil.

So in most countries, including Singapore, 70 mg

per 100 mil is below the legal limit.

But what we need to do is to calculate his blood alcohol level

two hours before, at 3 a.m.

We're going to do this two ways.

First way we're going to do this is using algebra.

Now, what we need to know is how fast do people's bodies eliminate ethanol?

Well, it's found that the slowest elimination rate is something

like 12.5 milligrams of ethanol per 100 mil blood per hour.

So let's take the equation Ct = C0 minus t beta,

where Ct is the concentration of blood alcohol at the time of measurement,

C0 is the concentration of blood alcohol at the time of interest i.e.

the time of the accident. t is the time elapsed between the two,

and beta is the elimination rate.

If we rearrange this equation, then we get C0 equals Ct plus t beta.

Now we can plug in the numbers.

Ct is the 70 mg per 100 mil that was measured at the police station. t is two,

that's the two hours between 3 a.m. and 5 a.m.

And the low elimination rate of 12.5 gives

us a concentration of 95 mg per 100 mil.

Now, people eliminate alcohol at different rates.

So, this is the calculation assuming a slow elimination rate of 12 and a half.

A high elimination rate will be 25 mg of ethanol per 100 mil of blood per hour.

So if we use the same algebra and put in this new value for beta of 25 mg,

then we calculate that the concentration at the time

of the accident is 120 mg per 100 mil.

Well, whichever value of beta we use in

this case, the man was above the legal limit.

If you don't like algebra,

you can do it this

way.

If we assume the low elimination rate of 12.5 mg per 100

mil of blood per hour, 12.5 mg for 2 hours is 25 mg.

Add that back on to the 70 that we

measured, and you get the 95 mg per 100 mil.

The same logic applies to the higher elimination rate of

25 mg of ethanol per 100 mil of blood per hour,

so 25 mg times 2 hours is 50 mg.

Add that onto the 70 mg measured at 5 a.m.

and you get 120 mg per 100 mil. So

however you do the math, you get the same conclusion

that this man was above the legal limit.

So as I said in the beginning, alcohol is

the molecule that generates the most work for toxicologists.

So, to your good health.

[BLANK_AUDIO]

Explorer notre catalogue

Rejoignez-nous gratuitement et obtenez des recommendations, des mises à jour et des offres personnalisées.

Coursera propose un accès universel à la meilleure formation au monde, en partenariat avec des universités et des organisations du plus haut niveau, pour proposer des cours en ligne.

© 2018 Coursera Inc. Tous droits réservés.

Télécharger dans l'App Store

Disponible sur Google Play