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.
Week 2B - 3 Infrared Spectroscopy
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來自 Nanyang Technological University, Singapore 的課程
Introduction to Forensic Science
428 個評分
從本節課中
Chemical Analysis in Forensic Science
與講師見面
Roderick BatesAssociate Professor
Division of Chemistry & Biological Chemistry | School of Physical & Mathematical Sciences
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The different chromatography techniques all achieve the same thing,
that is that they separate the mixture out into its individual components,
and we can get some idea of what some of the components might be,
by comparison to authentic samples.
But to really confirm, sufficient for a court of law, what those components are,
we have to have some more techniques.
We have to positively identify those
components and say absolutely what they are.
And to do this, we can use spectroscopy and spectrometry.
The kind of spectroscopy that is most useful for
the identification of compounds for
forensic purposes is Infrared Spectroscopy.
Now, Infrared Spectroscopy is based on molecular vibrations.
Now, when we look at diagrams of molecules in textbooks, we
always think of them as rigid things, rigid objects, but they're not.
Molecules are flexible.
The bonds connecting the atoms, they stretch and they bend and they wobble,
and the energy changes that corresponds to all
these movements match the energy of infrared light.
So if you take a molecule and you irradiate it with infrared light of the
correct frequency, then it will start to vibrate,
and it's monitoring the absorbance of infrared light
that is the basis of infrared spectroscopy.
Now, if we take a simple molecule where
you have two atoms connected together by a bond,
this behaves very much like a classical simple harmonic oscillator,
and the frequency with which that molecule will
vibrate, the bond will stretch backwards and forwards,
depends on two things.
It depends on the masses of the atoms
involved, it depends on the stiffness of the bond.
Now, chemical bonds in organic molecules come in
three types - single bonds, double bonds and triple bonds.
Single bonds are less stiff than double bonds,
and triple bonds are the most stiff of all.
So we can use these very simple concepts to look at where in the infrared
part of the spectrum different molecules, or
different parts of molecules, will absorb infrared light.
So this is the infrared part of the spectrum.
For historical reasons, chemists use a very odd
unit to measure the frequency of infrared light,
they use the inverse wavenumber, centimetres to the minus one.
And the infrared spectra of organic compounds are typically recorded in
the range of 600 to 4000 wavenumbers inverse centimetres.
Now when we look at the left-hand part of the
chart, we can start to group together particular types of bonds.
For instance, oxygen to hydrogen bonds, nitrogen
to hydrogen bonds, and carbon to hydrogen bonds
are all on the far left-hand side, and you can see
a small difference between the O-H, N-H, and the C-H bonds.
These are all in that particular position, because they
all involve the lightest atom, which is the hydrogen atom.
The stiffest bonds, that is the triple bonds, and
whether they're triple bonds between carbons or nitrogens or oxygens,
they're all in roughly the same place,
and that's at approximately 2000 wavenumbers.
Not quite so stiff are the double bonds -
carbon oxygen double bonds, carbon carbon
double bonds, carbon nitrogen double bonds -
these are all grouped around 1600 to 1800 wavenumbers.
You'll notice that all of these are in the left-hand part of the spectrum,
they are all above 1500 wavenumbers.
This region of the spectrum is characteristic of the class of compounds.
So if you look at the information from
the part of the spectrum above 1500 wavenumbers,
it tells you about the class of compounds to which your sample belongs.
It's not very helpful at telling you which
specific compound in that class your sample is.
So above 1500 wavenumbers,
it's a very useful region for research chemists,
but it's not a very useful region for forensic scientists,
because forensic scientists want to individualize
their compound down to one particular one.
That is best done on the right-hand half of the spectrum, below 1500 wavenumbers.
In this part of the spectrum, you have all the single bond vibrations,
you have all sorts of bending modes,
you have all sorts of complex deformations.
It's a very complex part of the spectrum.
But this means that the spectrum you record below
1500 wavenumbers is characteristic of a specific compound.
In fact, this part of the spectrum acts very much like a molecular fingerprint.
So if you take the infrared spectrum of your unknown compound
and you look in this region and you compare it to the
standard sample, the infrared spectrum of the standard sample, and those two
match, then you've got a very strong identification of that compound.
Here's an example.
It's an infrared spectrum of diamorphine, which is the active ingredient of heroin.
And if we look on the left-hand half of the spectrum
which is outlined in red, you can see a series of bands.
And if you look closely at that part on the left-hand
side, it tells you about some of the parts of the molecule.
For instance, we can see that this molecule has ester groups.
We can see that it has carbon hydrogen bonds, but there's nothing
in the left-hand half of the spectrum that tells us this is diamorphine.
It simply says, this has some of
the chemical functional groups that diamorphine also has.
It's only when we look at the right-hand half of the spectrum
outlined in blue, the so-called "fingerprint region",
that we can say this is diamorphine.
It's as you can see, it's very complex, it's very rich in detail, and comparison
to an authentic sample
proves that this is indeed diamorphine.
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