Peter A. Tanner: bhtancityu.edu.hk

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• Peter A. Tanner bhtan_at_cityu.edu.hk
• IR Sampling Techniques
• Conventional techniques for gases, solids and
  liquids
  
• IR beam condenser
• IR microscope.
• Specular reflectance
• DRIFTS
• ATR
• IR emission
• Photoacoustic spectroscopy
• 2-dimensional IR
• Some problems with spectral quality 

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Transmission of solvents in the infrared
Water has strong absorptions and attacks alkali
halides
Horizontal lines show useful regions
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Conventional Techniques use IR transmission
Gases Introduce into long-pathlength gas cell
Liquids (i) place as a film between halide
plates (ii) use a fixed pathlength cell. Determi
ne pathlength, b, when empty by counting
interference fringes.
Teflon spacers from 0.015 to 1 mm
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Cells for liquids
Fixed pathlength
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(iii) use a variable pathlength cell
Exploded view
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Solids (i) make a mull with nujol, fluorolube
and/or hexachlorobutadiene, so that mulling agent
bands do not overlap sample bands.
(ii) Make a KBr disc (1-3 mg sample in 250-300 mg
KBr). This may present artifacts.
CaCO3 in KBr, showing the mean diameters of the
absorbing particles

55?
2?
5?
15?
40?
23?
(a) increase of light loss from reflection and
scattering by large particles.
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(b)matching of sample and medium RI to prevent
scattering. e.g. PVC (nD 1.548) dispersed in KB
r (nD 1.56), KCl and KI.
KBr
KCl
KI
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(c) Chemical and physical factors such as
chemical reaction with the halide, or adsorption.
Spectra of benzoic acid in alkali halide discs.
NaCl spectrum is similar to benzoic acid monomer
forming hydrogen bonds to dioxan NaI spectrum is
similar to free benzoic acid molecules
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Beam condenser reflecting or transmitting beam
condensers can reduce source image x6. Normal
FTIR instrument can analyze samples 0.5 mm
diameter. With beam condenser, samples 25-50 ?m
can be analyzed
Example of use of beam condenser
25 ?m polystyrene sample
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• IR Microscope
• Fit into sample compartment and use normal
  detector
  
• Bolt on to exterior and use high sensitivity MCT
  detector.
  
• analysis of samples 5-10 ?m x 5-10 ?m.
• Examples of analysis

Film thickness (100 Å) of fluorine system
lubricant on Si wafer by transmission

Polystyrene 5?m pinhole
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Transmission measurement
Cross-section of food packaging multilayer film
cut to 10 ?m thickness by microtome.
(1) 60 ?m
(2) 8 ?m
(3) 40 ?m
(1)
(1), (3) are PE and PP H2O barriers
(2) Is ethylene vinyl alcohol copolymer, O2
barrier
(2)
(3)
Microscopic IR spectra of layers
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Area analysis of uneven coloring in colored
acrylic resin
Acrylic 1250, 960 cm-1
CO32- 875 cm-1
Optical micrograph of resin
Acrylic 1250 cm-1
IR spectral map of 15 points on surface, step 50
?m
CO32- (875 cm-1)
Microscopic spectra of even and uneven colored
parts
Contour plot/3D isometric plot of ratios of
carbonate/acrylate peak areas
Quality of carbonate additive poor
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Analysis of matter adhering to microswitch
contacts
A measured by microreflection B matter removed an
d measured by transmission
A
B
Optical micrograph of foreign matter on
microswitch
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angle i angle r
angle i ? angle r
 Reflection occurs from (solid) sample surface,
or from underlying reflective substrate.
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SPECULAR mirrorlike reflectance from a surface
well defined angle of reflection.
Analysis of films or coatings on reflective
surfaces e.g. polymer coatings on food contain
ers. Can obtain qualitative analysis of film, and
its thickness (smaller angle i gives longer
sample pathlength). Pure specular reflectance sp
ectrum largely shows how RI changes with
wavelength, and is transformed to transmittance
using Kramers-Krønig relation. Specular
reflection through surface coatings is double
transmittance.

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Applications of specular reflectance
Microanalysis 10-50 ?m -analysis of adhesio
ns on microswitch -detection of foreign matter on
polymer film -detection of adhesions to IC subst
rates or colored sections
Colored/Discolored PVC sections
A colored section
B discolored section
A
B
A-B difference
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DIFFUSE (DRIFTS) reflected radiant energy that
has been partially absorbed, transmitted and
partially scattered by a surface, with no defined
angle of reflection. Applications strongly abso
rbing samples, e.g. coal, pharmaceuticals, pla
stics... Small, irregular samples, powders. A
dvantages Minimal sample preparation sample n
ot destroyed
Example of micro-DRIFTS analysis of calculus
Cholesterol
Calcium oxalate and calcium phosphate
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Principle of DRIFTS Measure intensity of refl
ected radiation from sample surface (I),
generally reported as percent reflectance (R)
and compared with intensity of radiation
reflected from some standard nonabsorbing,
reflecting surface (Io) R100 I/I0.
Kulbelka-Munk (KM) units are proportional to
concentration (just like A) AKM1-(S/R)2/2(R/
S), where Rnonabsorbing reference, Sdeep
sample single beam response.
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A
Comparison of absorption and DRIFTS spectrum of
carbazole
KM
Construction of DRIFTS accessory
Sample placed in cup. Integrating sphere permits
collection of diffusely-reflected light, blocking
specular component. Definite fraction reflected
to exit slit and detector. Reference is KBr, Al2
O3, MgO.
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DRIFTS accessory fits into sample compartment
To detector
Beam in
Specular component is blocked
DRIFTS normally carried out on well-ground
diluted samples (in nonabsorbing KBr matrix) to
obtain transmission rather than specular
reflection from sample.
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Total internal reflection, TIR
Radiation strikes an interface with a medium of
lower RI, with an angle ?c.

Penetration of a sample is independent of its
thickness Interference and scattering do not
occur in a sample Absorbance in a sample is
independent of direction.
Radiation actually penetrates sample and is
partially absorbed
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Effect of Refractive Index (RI) and Angle of
Incidence RI of a substance changes with frequenc
y, especially where absorption occurs. Changes in
sample RI in the region of intense absorption
bands can sometimes change the value of ?c for a
particular crystal/sample combination. If the new
value of ?c becomes than angle of incidence,
then TIR no longer occurs, and the absorption
band becomes distorted usually a high degree of
peak asymmetry and baseline drift occur. These
distortions may be removed by (a) increase angle
of incidence (b) use a crystal of higher RI.
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Hirschfield and Wilks empirical rule optimum
angle of incidence is 3o greater than calculated
?c,
where, sin ?c (ns0.2)/np.
Can vary angle i in some accessories
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ATR-FTIR of China Clay Filled Polyester Film
30o
Effect of Angle of Incidence d, 1/e depth of pene
tration (? ?) d ? / 2 ?np sin2? - (ns / np)21
/2  np RI of crystal ns RI of sample ? angle
of incidence varying from 30o - 60o decreases d
by factor 10. Can depth profile by changing ?.
Smaller angle, deeper penetration
45o
60o
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26
ATR with aqueous solutions Axiom Tunnel Cells
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ATR-FTIR at electrode surfaces
Incident ray is totally reflected at
electrolyte-ATR element (Ge) interface. Part is
absorbed by electrolyte. 1. Ge ATR element, 2. P
t-counter electrode, 3. Reference electrode, 4.
PMMA main body of cell, 5. Electrolyte.
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Applications of ATR Surface analysis of solids
(coatings on paper, ink on cardboard).
Spectra from strongly absorbing samples
(textiles, fibres, foods, rubbers, minerals,
adhesive tapes, paint). Viscous liquids, or aqueo
us solutions.
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IR emission Thermal-induced emission spectra prov
ide rapid qualitative analysis of
irregular/opaque samples thin layers on
substrates anodized metal surfaces.
Turn off IR source. Insert emission accessory.
Heat sample to about 100 oC. Scan 1 min. Usually
a MCT detector used.
Polysiloxane coating on a Si wafer at 50 oC
Silicone grease on Al at 100 oC
Polyimide coating on Si wafer at 100 oC
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Spectral search results identifying the emission
spectrum of a thin film deposited on a copper
substrate as phenacetin
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Microemission attachment
Paraboloid collector
Ellipsoidal collector
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Photoacoustic spectroscopy IR or electronic
spectra of gas or solid samples. Sample acts as
periodic heat source.
Depth profiling by changing modulation frequency
Source modulated at audiofrequency
UV-vis spectra of Cr2O3
PAS
http//www.photoacoustics.hu/fsub_page_others.htm
http//www.photoacoustics.hu/
Detects sound waves
Alternating compression/rarefaction
Trans
DRIFTS
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(No Transcript)
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2-dimensional ir spectroscopy   FEATURES and USE
Simplify complex ir spectra with many overlapp
ing peaks Enhancement of spectral resolution by
spreading peaks over the second dimension.
Establishing unambiguous assignments through
correlation analysis of bands selectively coupled
by various interaction mechanisms.
http//www.amolf.nl/research/2d/research.html
http//www.chem.uga.edu/dluhy/pages/2DIR_1P.htm
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2D IR correlation spectrum of atactic polystyrene
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Some problems with spectra
Asymmetric, sloping bands. Badly ground.
Sample does not cover beam.
Also for air bubble in liquid cell polymer film
with hole or crack
Sample (mull) too thick
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Liquid evaporated between KBr plates
Wet sample. Sloping to high energy. Water bands.
Sample too thin
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Make sure that reference background does not
change before and after running sample.
Otherwise, the background bands will not subtract
out Sample background(1) background(2)
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KBr disc problems Problem Reason Clear disc
becomes cloudy No vacuum used when
pressing the disc. H2O vapour
entrained. Disc is cloudy in centre Anvil
faces not flat or parallel.
Water of crystallization bands (Partial)
dehydration of sample have variable intensity oc
curs on from one spectrum to another pressin
g disc.