Transcript Lecture 18_Anal_Tech_Part1

Nanochemistry
NAN 601
Instructor:
Dr. Marinella Sandros
Lecture 18: Analytical Techniques Part 1
1

Nuclear Magnetic Resonance (NMR) Spectroscopy

Fourier Transform Infrared (FTIR) Spectroscopy

Dyanmic Light Scattering
Spectroscopy
“seeing the unseeable”
Using electromagnetic radiation as a probe to obtain
information about atoms and molecules that are too
small to see.
Electromagnetic radiation is propagated at the speed of
light through a vacuum as an oscillating wave.
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NMR is the most powerful tool available for
organic structure determination.
It is used to study a wide variety of nuclei:
◦ 1H
◦ 13C
◦ 15N
◦ 19F
◦ 31P
4
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A nucleus with an odd atomic
number or an odd mass
number has a nuclear spin.
The spinning charged nucleus
generates a magnetic field.
If we place these nuclei in a
magnetic field, they can line
up with or against the field by
spinning clockwise or counter
clockwise.
5
N
N
N
S
- spin state,
favorable,
lower energy
S
A spinning nucleus with it's magnetic field
aligned with the magnetic field of a magnet
S
N
- spin state,
unfavorable,
higher energy
S
A spinning nucleus with it's magnetic field
aligned against the magnetic field of a magnet
• Alignment with the magnetic field (called ) is lower energy than
against the magnetic field (called ).
• Note that for nuclei that don’t have spin, such as 12C, there is no
difference in energy between alignments in a magnetic field since
they are not magnets. As such, we can’t do NMR spectroscopy on
12C.

Imagine placing a
molecule, for example,
CH4, in a magnetic field.
We can probe the energy
difference of the - and
- state of the protons
by irradiating them with
EM radiation of just the
right energy.
In a magnet of 7.05 Tesla, it takes EM
radiation of about 300 MHz (radio waves).
So, if we bombard the CH4 molecule with
300 MHz radio waves, the protons will
absorb that energy and we can measure
that absorbance.
 In a magnet of 11.75 Tesla, it takes EM
radiation of about 500 MHz (stronger
magnet means greater energy difference
between the - and - state of the protons)

at no magnetic field,
there is no difference beteen
- and - states.
 proton spin state
(higher energy)
Graphical relationship between
magnetic field (B o) and frequency ( 
E = h x 300 MHz
E
E = h x 500 MHz
for 1 H NMR absorptions
proton spin state
(lower energy)
0T
7.05 T
11.75 T
Bo
But there’s a problem. If two researchers want to compare their data
using magnets of different strengths, they have to adjust for that
difference. That’s a pain, so, data is instead reported using the
“chemical shift” scale as described on the next slide.
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Here’s how it works. We decide on a sample
we’ll use to standardize our instruments. We
take an NMR of that standard and measure its
absorbance frequency.
We then measure the frequency of our sample
and subtract its frequency from that of the
standard.
We then then divide by the frequency of the
standard. This gives a number called the
“chemical shift,” also called d, which does not
depend on the magnetic field strength.
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Imagine that we have a magnet where our standard absorbs at
300,000,000 Hz (300 megahertz), and our sample absorbs at
300,000,300 Hz. The difference is 300 Hz, so we take
300/300,000,000 = 1/1,000,000 and call that 1 part per million (or
1 PPM).
Now lets examine the same sample in a stronger magnetic field
where the reference comes at 500,000,000 Hz, or 500 megahertz.
The frequency of our sample will increase proportionally, and will
come at 500,000,500 Hz. The difference is now 500 Hz, but we
divide by 500,000,000 (500/500,000,000 = 1/1,000,000, = 1 PPM).
Of course, we don’t do any of this, it’s all done automatically by the
NMR machine.
CH3
H3C
Si CH3
CH3
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TMS is a common standard.
Since silicon is less electronegative than
carbon, TMS protons are highly shielded.
Signal defined as zero
Organic protons absorb downfield (to the
left) of the TMS signal.
12
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NMR would not be very valuable if all protons absorbed at the
same frequency.

What makes it useful is that different protons usually appear
at different chemical shifts (d.
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So, we can distinguish one kind of proton from another.

Why do different protons appear at different d?

There are several reasons, one of which is shielding.

The electrons in a bond shield the nuclei from the magnetic
field. So, if there is more electron density around a proton, it
sees a slightly lower magnetic field, less electron density
means it sees a higher magnetic field:
Z
C
H
This
represents
the electronthe
density
of C-H bond.
How
This
represents
electron
density
much
electron is
density
on the
proton depends
on what
density
on isthe
proton
depends
on
else is attached to the carbon. If Z is an electronegative
is ,an
elelctronegative
atom, and
thepulls
carb
atom
the carbon
becomes electron deficient
some
the density
electron
density
some
of theof
electron
away from
the H. Ifaway
Z is an fr
electron
donating
group,
more electron
density ends
up on
group,
more
electron
density
ends
up
the H.
 How do the electrons shield the magnetic field?
 By moving.
 A moving charge creates a magnetic field, and the field created by the
moving electrons opposes the magnetic field of our NMR machine.
 It’s not a huge effect, but it’s enough to enable us to distinguish between
different protons in our sample.
The Hard Part - Interpreting Spectra
Learning how an NMR machine works is straightforward. What is less
straightforward is learning how to use the data we get from an NMR
machine. That’s because each NMR spectrum is a puzzle, and there’s
no single fact that you simply have to memorize to solve these
spectra. You have to consider lots of pieces of data and come up
with a structure that fits all the data. What kinds of data do we get
from NMR spectra?
For 1H NMR, there are three kinds each of which we will consider each
of these separately:

Chemical shift data - tells us what kinds of protons we have.

Integrals - tells us the ratio of each kind of proton in our sample.

1H
- 1H coupling - tells us about protons that are near other
protons.
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More electronegative
atoms deshield more
and give larger shift
values.
Effect decreases with
distance.
Additional
electronegative atoms
cause increase in
chemical shift.
17
Depending on their chemical environment,
protons in a molecule are shielded by
different amounts.
Chapter 13
=>
19
Nonequivalent protons on adjacent carbons
=>
Chapter 13
20
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Infrared radiation lies between the visible and microwave portions of
the electromagnetic spectrum.
Infrared waves have wavelengths longer than visible and shorter than
microwaves, and have frequencies which are lower than visible and
higher than microwaves.
The Infrared region is divided into: near, mid and far-infrared.
◦ Near-infrared refers to the part of the infrared spectrum that is
closest to visible light and far-infrared refers to the part that is
closer to the microwave region.
◦ Mid-infrared is the region between these two.
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The primary source of infrared radiation is thermal radiation. (heat)
It is the radiation produced by the motion of atoms and molecules in
an object. The higher the temperature, the more the atoms and
molecules move and the more infrared radiation they produce.
The bonds between atoms in the molecule stretch
and bend, absorbing infrared energy and
creating the infrared spectrum.
Symmetric Stretch
Antisymmetric Stretch
Bend
A molecule such as H2O will absorb infrared light when
the vibration (stretch or bend) results in a molecular
dipole moment change
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Plot IR energy vs. %transmittance (%T)
– Energy scale in wave numbers, wn (cm-1)
– %T scale
Compares intensity of IR striking sample (Iin)
with intensity of IR leaving sample (Iout)
100%T no light absorbed by sample
0% all light absorbed by sample
A molecule can be characterized (identified) by its
molecular vibrations, based on the absorption and
intensity of specific infrared wavelengths.
For isopropyl alcohol, CH(CH3)2OH, the infrared absorption bands
identify the various functional groups of the molecule.
 Identification
and quantitation of organic solid, liquid or
gas samples.
 Analysis
of powders, solids, gels, emulsions, pastes,
pure liquids and solutions, polymers, pure and mixed
gases.
 Infrared
used for research, methods development,
quality control and quality assurance applications.
 Samples
range in size from single fibers only 20
microns in length to atmospheric pollution studies
involving large areas.
 Pharmaceutical
research
 Forensic investigations
 Polymer analysis
 Lubricant formulation and fuel additives
 Foods research
 Quality assurance and control
 Environmental and water quality analysis
methods
 Biochemical and biomedical research
 Coatings and surfactants
 Etc.
Dispersion Spectrometer
In order to measure an IR spectrum, the dispersion
Spectrometer takes several minutes. Also the detector
receives only a few % of the energy of original light
source.
To separate IR light, a grating is used.
Grating
Detector
Slit
Sample
Light source
To select the specified IR light,
A slit is used.
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FTIR was developed in order to overcome the limitations encountered with
dispersive instruments.
The main difficulty was the slow scanning process.
A method for measuring all of the infrared frequencies simultaneously, rather
than individually, was needed.
A solution was developed which employed a very simple optical device called
an interferometer.
The interferometer produces a unique type of signal which has all of the
infrared frequencies “encoded” into it. The signal can be measured very
quickly, usually on the order of one second or so.
Thus, the time element per sample is reduced to a matter of a few seconds
rather than several minutes.
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Most interferometers employ a beamsplitter which
takes the incoming infrared beam and divides it
into two optical beams. One beam reflects off of a
flat mirror which is fixed in place. The other beam
reflects off of a flat mirror which is on a
mechanism which allows this mirror to move a very
short distance (typically a few millimeters) away
from the beamsplitter.
The two beams reflect off of their respective
mirrors and are recombined when they meet back
at the beamsplitter. Because the path that one
beam travels is a fixed length and the other is
constantly changing as its mirror moves, the signal
which exits the interferometer is the result of these
two beams “interfering” with each other.
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The resulting signal is called an
interferogram which has the unique
property that every data point (a function
of the moving mirror position) which
makes up the signal has information
about every infrared frequency which
comes from the source.
This means that as the interferogram is
measured, all frequencies are being
measured simultaneously. Thus, the use
of the interferometer results in extremely
fast measurements.
IR and laser interferograms
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IR interferogram is recorded
after the IR beam passes
through the interferometer and
sample cell
Laser interferogram is
produced by a helium-neon
laser beam travelling through
the interferometer into a
special detector
Laser interferogram is a nearly
ideal cosine wave
Laser interferogram tells the
position of moving mirror with
excellent accuracy
A
IR-interferogram
950
1950
2950
3950
Laser-interferogram
OPD
x =632.8 nm
Introduction to FTIR
02 January 2006
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Laser interferogram signal
is used to digitize the IR
interferogram
Single mode HeNe-laser
provides a constant
wavelength output at 632.8
nm
Accurate and precise
digitization interval
provides high wavelength
accuracy in the spectrum
The data points for IR
interferogram are recorded
every time the mirror has
moved forward by one
HeNe laser wavelength
Infrared source
Helium-neon laser
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The digitized IR
interferogram (an XY
table) is transmitted to
computer where the Fast
Fourier Transform (FFT)
algorithm computes the
spectrum
X (nm)
-2531,2
-1898,4
-1265,6
-632,8
0
632,8
1265,6
1898,4
2531,2
Y (Volt)
4,2
2,1
-1,2
3,6
7,2
3,6
-1,2
2,1
4,2
Infrared source
Infrared source
Helium-Neon laser
Helium-neon laser
-L
Optical path
0 difference
Because the analyst requires a frequency spectrum (a plot of the intensity
at each individual frequency) in order to make an identification, the
measured interferogram signal can not be interpreted directly. A means of
“decoding” the individual frequencies is required. This can be
accomplished via a well-known mathematical technique called the Fourier
transformation. This transformation is performed by the computer which
then presents the user with the desired spectral information for analysis.
1. The Source: Infrared energy is emitted from a glowing black-body source.
This beam passes through an aperture which controls the amount of energy
presented to the sample (and, ultimately, to the detector).
2. The Interferometer: The beam enters the interferometer where the “spectral
encoding” takes place. The resulting interferogram signal then exits the
interferometer.
3. The Sample: The beam enters the sample compartment where it is
transmitted through or reflected off of the surface of the sample, depending on
the type of analysis being accomplished. This is where specific frequencies of
energy, which are uniquely characteristic of the sample, are absorbed.
4. The Detector: The beam finally passes to the detector for final measurement.
The detectors used are specially designed to measure the special interferogram
signal.
5. The Computer: The measured signal is digitized and sent to the computer
where the Fourier transformation takes place. The final infrared spectrum is
then presented to the user for interpretation and any further manipulation.
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Because there needs to be a relative scale for the
absorption intensity, a background spectrum must also be
measured. This is normally a measurement with no sample
in the beam.
This can be compared to the measurement with the
sample in the beam to determine the “percent
transmittance.”
This technique results in a spectrum which has all of the
instrumental characteristics removed.
Thus, all spectral features which are present are strictly
due to the sample. A single background measurement can
be used for many sample measurements because this
spectrum is characteristic of the instrument itself.
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In recent years, the technique of dynamic light
scattering (DLS) -- also called quasi-elastic light
scattering (QELS) or photon correlation spectroscopy
(PCS) -- has proven to be an invaluable -analytical
tool for characterizing the size distribution of
particles suspended in a solvent (usually water).

DLS-based sizing instruments have been used
extensively to characterize a wide range of
particulate systems, including synthetic polymers
(e.g. latexes, PVCs, etc.), oil-in-water and waterin-oil emulsions, vesicles, micelles, biological
macromolecules, pigments, dyes, silicas, metallic
sols, ceramics and numerous other colloidal
suspensions and dispersions.
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Particles suspended in a liquid are subject to
Brownian motion.
Small particles diffuse"faster".
Large particles diffuse "slower".
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The suspended particles are not stationary;
rather, they move about, or diffuse, in
random-walk fashion by the process known
as Brownian motion (caused by collisions of
neighboring solvent molecules).
Brownian motion is indirectly proportional to
size
◦ Larger particles diffuse slower than smaller
particles
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Temperature and viscosity must be known
Temperature stability is necessary
◦ Convection currents induce particle movement
that interferes with size determination
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Temperature is proportional to diffusion
rate
◦ Increasing temperature increases Brownian
motion
Random movement of particles due to
bombardment of solvent molecules
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A beam of laser light is focused in the sample.
Particles scatter light in all directions.
The scattered photons are measured by a
photomultiplier tube
The time variation of the scattered intensity is
analyzed by examining their auto-correlation.
From this a diffusion coefficient can be derived.
From the measured diffusion coefficient particle
size is calculated.
kT
dH 
3D
dH = hydrodynamic diameter (m)
k = Boltzmann constant
(J/K=kg·m2/s2·K)
T = temperature (K)
η = solvent viscosity (kg/m·s)
D = diffusion coefficient (m2/s)
Particle diameter

Hydrodynamic diameter
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The diameter
measured by DLS
correlates to the
effective particle
movement within a
liquid
Particle diameter +
electrical double layer
Affected by surface
bound species which
slows diffusion
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Light from a laser is focused into the sample
containing a dilute suspension of particles.
The temperature of this scattering cell is held
constant.
Each of the particles illuminated by the incident
laser beam scatters light in all directions.
The intensity of light scattered by a single, isolated
particle depends on its molecular weight and
overall size and shape, and also on the difference
in refractive indices of the particle and the
surrounding solvent.
I
A digital correlator is used to compute the
autocorrelation function.
Each monodisperse population of particle sizes produces its own unique
autocorrelation function - a single exponential decay.
I
“deconvolve” C(t’) and thereby extract the
distribution of D values (and hence of
particle diameters) from the detailed shape
of C(t’)
http://www.nytimes.com/imagepages/2005/02/21/science/20050222_NANO
1_GRAPHIC.html