Transcript Part_VII_-Introduction_to_Raman_Spectroscopy

Vibrational Spectroscopy for
Pharmaceutical Analysis
Part VII. Introduction to Raman Spectroscopy
Rodolfo J. Romañach, Ph.D.
ENGINEERING RESEARCH CENTER FOR
STRUCTURED ORGANIC PARTICULATE SYSTEMS
RUTGERS UNIVERSITY
PURDUE UNIVERSITY
NEW JERSEY INSTITUTE OF TECHNOLOGY
UNIVERSITY OF PUERTO RICO AT MAYAGÜEZ
10/11/2005
1
Scattering
• Mid-IR and NIR require absorption of radiation
from a ground level to an excited state, requires
matching of radiation from source with difference
in energy states.
• Raman spectroscopy involves scattering of
radiation (matching of radiation is not required).
E. Smith and G. Dent, “Modern Raman
Spectroscopy. A Practical Approach.”, Wiley
2005, pages 3 – 5.
2
Raman Spectroscopy
• A single frequency of radiation irradiates the
molecule and the radiation distorts (polarizes)
the cloud of electrons surrounding the nuclei to
form a short-lived state called a “virtual state”.
This state is not stable and the photon is quickly
re-radiated.
E. Smith and G. Dent, “Modern Raman
Spectroscopy. A Practical Approach.”, Wiley 2005,
pages 3 – 5.
3
What is Raman Spectroscopy?
Raman is a scattering technique
LASER
Rayleigh scattering:
1400
Elastic scatter
1200
1000
Inelastic scatter
Stokes
Raman Intensity
Raman :
Anti-Stokes
800
600
400
200
400
200
0
-200
-400
Raman Shift (cm-1)
Slide courtesy Kaiser Optical Systems.
4
Raman Scattering
from Molecular Vibrations
=
½
()
1 k
2c 
 = Vibrational frequency
k = Spring force constant
 = Reduced mass of atoms, m1m2/(m1+m2)
Higher vibrational frequency with stronger chemical bond and lighter atoms
Stokes – Photon has less energy
Rayleigh – Elastic
Strongest Component
Only one in 106 or
108 photons is
Raman scattered.
Anti-Stokes –
Photon Gains Energy
Adapted from Kaiser Optical Systems slide
5
Quantum Mechanical Model
of Raman Scattering
E1
Virtual state
hvex
hvex
h(vex-vv)
E0
Stokes
hvex
hvex
h(vex+vv)
v=3
v=2
v=
v=0
1
Rayleigh Anti-Stokes
Courtesy Kaiser Optical Systems.
6
Raman Scattering
• The difference in wavelength between the incident and
scattered visible radiation corresponds to wavelengths in
the mid-infrared region.
• An Indian physicist C.V. Raman discovered this effect in
1928.
• This has been considered an experimentally difficult
technique for many years; but in recent years a number of
advances in instrumentation has made it more available to
non-specialized labs.
Courtesy Kaiser Optical Systems.
7
Raman Scattering
• Sample is irradiated with intense monochromatic
radiation usually in the visible or NIR region of the
spectrum.
• The wavelength is well away from any absorption peaks
of the analyte.
• The abscissa in the spectra are in terms of wavenumber
shift Δυ between the observe radiation and that of the
source, and we speak of Raman shift instead of
frequency of absorption.
Skoog Holler Niemann, p. 429-433, 435 – 441.
8
Raman Scattering and Polarizability
Electric field of radiation:
E = E0cos (2πυext)
When it interacts with an electron cloud of an analyte
bond, it induces a dipole moment m in the bond that
is given by:
m = αE = αE0cos (2πυext)
Where α is a proportionality constant called the
polarizability of the bond.
9
Stokes Scattering
• Stokes scattering is, by convention, positive-shifted
Raman scatter. Most Analytical work is done in this
region.
• Represents inelastic scattering to a region of lower
energy. This means that the energy of the detected
radiation is higher in wavelength relative to the
laser.
• The scattered spectrum appears similar to an IR
spectrum and is interpreted similar IR spectrum.
Adapted from Kaiser Optical Systems slide
10
Raman Scattering
• C=C, and C≡C, C≡N bonds are strong scatterers, bonds
undergo polarization.
• Symmetric stretches undergo greater changes in
polarization, and are stronger in Raman than asymmetric
stretches.
E. Smith and G. Dent, Wiley 2005, page 6.
11
Advantages of Raman Spectroscopy –
Chemical Information
• Raman bands can provide structural information
(presence of functional groups).
• Raman spectroscopy can be used to measure bands of
symmetric linkages which are weak in an infrared
spectrum (e.g. -S-S-, -C-S-, -C=C-).
• The standard spectral range reaches well below 400 cm1, making the technique ideal for both organic and
inorganic species.
12
Advantages of Raman Spectroscopy – Ease of Use for
Process Measurements
•
•
•
•
•
Fiber optics (up to 100's of meters in length) can be used for remote
analyses.
Purging of sample chamber is unnecessary since Water and CO2 vapors
are very weak scatterers.
Little or no sample preparation is required
Water is a weak scatterer - no special accessories are needed for
measuring aqueous solutions
Inexpensive glass sample holders, non-invasive probes and immersion
probes are ideal in most cases
13
Disadvantages of Raman Spectroscopy
• Inherently not sensitive (need ~ 1 million incident
photons to generate 1 Raman scattered photon)
• Fluorescence is a common background issue
• Typical detection limits in the parts per thousand
range
• Fluorescence Probability versus Probability of
Raman Scatter ( 1 in 103-105 vs 1 in 107-1010)
• Requires expensive lasers, detectors and filters.
• Small sample volume can make it difficult to obtain
a representative sample.
14
Complementary Nature of IR and Raman Spectroscopy
• IR absorption intensities are proportional to the
change in dipole moment as the molecule
vibrates.
• Raman scattering intensities are proportional to
the change in molecular polarizabilities upon
vibrational excitation.
• For molecules with a center of inversion IR and
Raman and mutually exclusive.
15
• Need to emphasize complementarity with more
specific examples.
16
Placzek's Equation
for Raman Scattering Intensity
4
243 hILN(0-)
2
2
[45(
')
+7(
')
]
IR =
a
a
-h/kT
2
4
)
(45)(3 )c (1-e
Where
• IR proportional to IL
• IR proportional to N
• IR stronger at shorter
wavelength
• Statistical factor: (1-eh/kT
)
c = speed of light
h = Planck's constant
IL = laser intensity
N = number of scattering molecules
 = molecular vibrational frequency in Hz
L = laser excitation frequency, in Hz
 = reduced mass of the vibrating atoms
k = Boltzmann's constant
T = absolute temperature
a' = mean value invariant of the polarizability te
a' = anisotropy invariant of the polarizability te
courtesy Kaiser Optical Systems
17
Analytical Raman
Spectroscopy
Il=sLCI
Sample
Il = Raman intensity
s = Raman cross section
L = Pathlength
C = Concentration
I = Instrument parameters
courtesy Kaiser Optical Systems
18
Raman Scattering is Stronger from Some Vibrations than from
Others
• 3N-6 vibrations possible, many have no Raman bands
• Change in polarizability during a molecular vibration leads
to Raman scattering.
– Covalent bonds more polarizable than ionic bonds
– Intensity from stretching vibration increases with bond
order
– Intensity tends to increase with increasing atomic
number
– Symmetry-forbidden vibrations
Adapted from Kaiser Optical Systems slide
19
Raman Scattering is Stronger from Some Vibrations than from
Others
• Stretching bands often stronger than bending ones
• Symmetric bands often stronger than anti-symmetric ones
• Crystalline materials often have stronger Raman bands than noncrystalline materials
Adapted from Kaiser Optical Systems slide
20
SNV Raman Intensity
5.5
animal source A
animal source B
Vegetable source
3.5
1.5
-0.5
3000
3.75
2.75
2950
2900
2850
2800
animal source A
animal source B
Vegetable source
C-H Rocking
1.75
0.75
-0.25
1500 1300 1100 900
700
Wavenumber (cm-1)
500
300
21
Vector Normalization
0.9
0.5
3000
CaHPO4
0.13% MgSt
0.25% MgSt
0% MgSt
0.5% MgSt
2950
2900
-1
Wavenumber(cm )
2850
2800
22
Kaiser Optical Systems Rxn-1-785 nm Raman Spectrometer.
23
Fluorescence
•
•
Properties
– Very efficient conversion of laser photons into unwanted light
– Emission spectrum usually changes little, if at all, with changing
laser wavelength
– Fluorescence lifetime typically
Fluorescence Energy
1 to 10 nanoseconds
Level Diagram
Sources
– impurities
– additives
Elimination
– Near-infrared wavelength excitation
– Far-UV wavelength excitation
– Photobleaching
– Spectral subtraction methods
– Time-resolved detection
Energy
•
Stokes
Slide Courtesy of Kaiser Optical Systems
AntiStokes
Two
photon
24
Example of a Raman Spectrum
Intensity in detected photons x10-5
4 component mixture: omp-xylene and ethylbenzene
m, e
6
4
o
m, e
m
2
p, e
p
m
p
e
o
p
o
o
omp
ompe
m
ep
0
400
600
800
1000
1200
1400
1600
1800
Raman shift in wavenumbers from the laser line
Slide Courtesy of Kaiser Optical Systems
25
Glass/Amorphous Materials
• Stress on molecular
groups from local
environment
changes vibrational
energy.
• Discrete peaks
become broad
bands.
Slide Courtesy of Kaiser Optical Systems
26
Raman Scattering from Crystals
• Periodicity of a crystalline lattice
reduces the number of vibrations
that Raman observes.
• Spectrum consists of narrow peaks.
• Spectrum effected by orientation
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
Polarizability
changes
add together
X
Polarizability
changes
cancel out
Slide Courtesy of Kaiser Optical Systems
27
Theophylline Anhydrous vs. Monohydrate
anhydrous
monohydrate
1941.9
1800
1700
1600
1500
1400
1300
1200
1100
1000
900
793.5
Raman Shift / cm-1
Slide Courtesy of Kaiser Optical Systems, work by Lynne Taylor’s group,.
Industrial & Physical Pharmacy, Purdue University
28
Theophylline Phase Stability as a
Function of Temperature
48°C
Intensity
54°C
58°C
64°C
69°C
1816.1
1700
1600
1500
1400
1300
Raman Shift /
1200
1100
1000
877.4
cm-1
Literature Transition temperature for hydrate to anhydate is around 60°C
Slide Courtesy of Kaiser Optical Systems
29
Components of a Raman
Spectrograph
•
•
•
•
•
Laser
Fiber optic sampling device
Notch filter
Grating
CCD Detector
Slide Courtesy of Kaiser Optical Systems
30
RamanRxn1 Schematic
Overview
TE Cooled
CCD
Detector
Control
Electronics
ProbeHead
Imaging Spectrograph
HoloPlex
ProbeHead
Slit
Notch
Invictus NIR Laser
Invictus NIR Laser
Filtering Universal ProbeHead
Immersion and Non-Contact
Sampling Optics
Axial Transmissive Spectrograph
HoloPlex Grating
TE Cooled CCD Detector
Slide Courtesy of Kaiser Optical Systems
31
Axial Transmissive Design
Output Plane
HoloPlex Advantages: Quantitative Raman!
 Full Simultaneous Spectral Coverage
Multi-element
Lenses
 High Throughput
 High Spectral Resolution
 No Moving Parts
Low 1.8 f/# means Higher Optical
Throughput (~4X)
Improved Thermal Stability (5X)
Holographic
Transmission
Grating
Rugged Compact Design
Entrance
Slit
Innovative all refractive design!
Slide Courtesy of Kaiser Optical Systems
32
Lasers Commonly used for Raman
• 1064 nm
– Nd:YAG laser
– FT Instrumentation
– Out of range of CCD, must use InGaAs or Ge
• 830 nm
– Not common but could help avoid fluorescence
• 785 nm
– Diode laser
– Most common laser used for Raman work
– Good compromise between fluorescence and Raman
efficiency…makes it somewhat universal
– Stable to environment
– Electronically efficient
Slide Courtesy of Kaiser Optical Systems
33
Lasers Commonly used for Raman
•
•
•
•
•
633 nm
– He-Ne laser
– Longer lifetime
532 nm
– Frequency doubled Nd:YAG laser
– Good efficiency, low power
– Watch out for fluorescence!
– Sensitive to temperature
514 nm
– Ar-ion laser
488 nm
– Ar-ion laser
UV lasers
– Resonance Raman
– EXPENSIVE
Slide Courtesy of Kaiser Optical Systems
34
Sampling Options
Stream
Stream
Immersion Probe
Pilot Plant Trial
Non-Contact
Optic
Production Installation
Slide Courtesy of Kaiser Optical Systems
532 nm excitation
35
Purpose of Notch Filters
• Filter out non-informative radiation
• In the case of Raman instrumentation,
this means filtering the Rayleigh
scattered energy
• Holographic notch filters are the most
common…in nearly every Raman
instrument you will find a Kaiser notch
filter
Slides by courtesy of: Mark Kemper, [email protected]
36
Properties of Holographic Notch
Filters
High attenuation
Narrow bandwidth
Sharp spectral edges
Good transmission
High damage
threshold
• Environmentally
stable
80
% Transmission
•
•
•
•
•
40
0
700 720 740 760 780 800 820 840
Wavelength (nm)
Center = 785 nm
FWHM at 50%T = 12 nm
0.3 to 4.0 OD edge = 7.1 nm
Slide Courtesy of Kaiser Optical Systems
37
CCD Detector
•
•
•
•
•
Multi element silicon detector (1024 x 128)
Maintained at low temperature (-40ºC)
Key reason for lack of moving parts
High sensitivity
Detection range 400 – 1050 nm
Slides by courtesy of: Mark Kemper, [email protected]
38
To learn more about Raman
Spectroscopy:
• E. Smith and G. Dent, “Modern Raman
Spectroscopy A Practical Approach”, John Wiley &
Sons Ltd; (Chichester, United Kingdom), 2005.
39
Comparing FT-IR and Raman
FT-IR
Raman



















Absorption
Fundamental information
Sample preparation
Process measurements
difficult
High spectral density
Organics
Dipoles
O-H, C=O, N-H
Water a problem
Emission
Fundamental information
No sample preparation
Process measurements
High spectral density
Sampling challenges
Organics and inorganics
Polarizability
Aromatics, C=C
Water no problem
Slide courtesy Kaiser Optical Systems.
40