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INSTRUMENTAL ANALYSISCHEM 4811
CHAPTER
DR. Neelam KumariAssOCIATE professor
DEPARTMENT of chemistryMeerut college meerut
U.P.
CHAPTER 4
INFRARED SPECTROSCOPY
(IR)
CHAPTER 4
INFRARED SPECTROSCOPY
(IR)
IR SPECTROSCOPY
- Used for qualitative identification of organic andinorganic compounds
- Used for checking the presence of functional groups in molecules
- Can also be used for quantitative measurements of compounds
- Each compound has its unique IR absorption pattern
- Wavenumber with units of cm-1 is commonly used
- Wavenumber = number of waves of radiation per centimeter
- Used for qualitative identification of organic andinorganic compounds
- Used for checking the presence of functional groups in molecules
- Can also be used for quantitative measurements of compounds
- Each compound has its unique IR absorption pattern
- Wavenumber with units of cm-1 is commonly used
- Wavenumber = number of waves of radiation per centimeter
IR SPECTROSCOPY
- The IR region has lower energy than visible radiation andhigher energy than microwave
The IR region is divided into
Near-IR (NIR): 750 nm – 2500 nm
Mid-IR: 2500 nm – 20000 nm
Far-IR: 20000 nm – 400000 nm
- The IR region has lower energy than visible radiation andhigher energy than microwave
The IR region is divided into
Near-IR (NIR): 750 nm – 2500 nm
Mid-IR: 2500 nm – 20000 nm
Far-IR: 20000 nm – 400000 nm
IR ABSORPTION BY MOLECULES
- Molecules with covalent bonds may absorb IR radiation
- Absorption is quantized- Molecule shows change in dipolemoment during vibrational transitions
- Molecules move to a higher energy state
- IR radiation is sufficient enough to cause rotation and vibration
- Radiation between 1 and 100 µm will cause excitation tohigher vibrational states
- Radiation higher than 100 µm will cause excitation tohigher rotational states
- Molecules with covalent bonds may absorb IR radiation
- Absorption is quantized- Molecule shows change in dipolemoment during vibrational transitions
- Molecules move to a higher energy state
- IR radiation is sufficient enough to cause rotation and vibration
- Radiation between 1 and 100 µm will cause excitation tohigher vibrational states
- Radiation higher than 100 µm will cause excitation tohigher rotational states
IR ABSORPTION BY MOLECULES
- Absorption spectrum is composed of broad vibrationalabsorption bands
- Molecules absorb radiation when a bond in the molecule vibratesat the same frequency as the incident radiant energy
- Molecules vibrate at higher amplitude after absorption
- A molecule must have a change in dipole moment duringvibration in order to absorb IR radiation
- Absorption spectrum is composed of broad vibrationalabsorption bands
- Molecules absorb radiation when a bond in the molecule vibratesat the same frequency as the incident radiant energy
- Molecules vibrate at higher amplitude after absorption
- A molecule must have a change in dipole moment duringvibration in order to absorb IR radiation
IR ABSORPTION BY MOLECULES
Absorption frequency depends on
- Masses of atoms in the bonds
- Geometry of the molecule
- Strength of bond
- Other contributing factors
Absorption frequency depends on
- Masses of atoms in the bonds
- Geometry of the molecule
- Strength of bond
- Other contributing factors
DIPOLE MOMENT (µ)
µ = Q x r
Q = charge and r = distance between charges
- Asymmetrical distribution of electrons in a bond renders thebond polar
- A result of electronegativity difference
- µ changes upon vibration due to changes in r
- Change in µ with time is necessary for a molecule to absorbIR radiation
µ = Q x r
Q = charge and r = distance between charges
- Asymmetrical distribution of electrons in a bond renders thebond polar
- A result of electronegativity difference
- µ changes upon vibration due to changes in r
- Change in µ with time is necessary for a molecule to absorbIR radiation
DIPOLE MOMENT (µ)
- The repetitive changes in µ makes it possible for polar moleculesto absorb IR radiation
- Symmetrical molecules do not absorb IR radiation since theydo not have dipole moment (O2, F2, H2, Cl2)
- Diatomic molecules with dipole moment are IR-active(HCl, HF, CO, HI)
- Molecules with more than two atoms may or may not beIR active depending on whether they have permanent
net dipole moment
- The repetitive changes in µ makes it possible for polar moleculesto absorb IR radiation
- Symmetrical molecules do not absorb IR radiation since theydo not have dipole moment (O2, F2, H2, Cl2)
- Diatomic molecules with dipole moment are IR-active(HCl, HF, CO, HI)
- Molecules with more than two atoms may or may not beIR active depending on whether they have permanent
net dipole moment
PRINCIPAL MODES OF VIBRATION
Stretching
- Change in bond length resulting from change ininteratomic distance (r)
Two stretching modes- Symmetrical and asymmetrical stretching
- Symmetrical stretching is IR-inactive (no change in µ)
Stretching
- Change in bond length resulting from change ininteratomic distance (r)
Two stretching modes- Symmetrical and asymmetrical stretching
- Symmetrical stretching is IR-inactive (no change in µ)
PRINCIPAL MODES OF VIBRATION
Bending
- Change in bond angle or change in the position of a group ofatoms with respect to the rest of the molecule
Bending Modes- Scissoring and Rocking
- In-plane bending modes (atoms remain in the same plane)
- Wagging and TwistingOut-of-plane (oop) bending modes (atoms move out of plane)
Bending
- Change in bond angle or change in the position of a group ofatoms with respect to the rest of the molecule
Bending Modes- Scissoring and Rocking
- In-plane bending modes (atoms remain in the same plane)
- Wagging and TwistingOut-of-plane (oop) bending modes (atoms move out of plane)
PRINCIPAL MODES OF VIBRATION
3N-6 possible normal modes of vibration
N = number of atoms in a molecule
Degrees of freedom = 3N
H2O for example- 3 atoms
- Degrees of freedom = 3 x 3 = 9- Normal modes of vibration = 9-6 = 3
3N-6 possible normal modes of vibration
N = number of atoms in a molecule
Degrees of freedom = 3N
H2O for example- 3 atoms
- Degrees of freedom = 3 x 3 = 9- Normal modes of vibration = 9-6 = 3
PRINCIPAL MODES OF VIBRATION
Linear Molecules
- Cannot rotate about the bond axis
- Only 2 degrees of freedom describe rotation
3N-5 possible normal modes of vibration
CO2 for example- 3 atoms
- Normal modes of vibration = 9-5 = 4
Linear Molecules
- Cannot rotate about the bond axis
- Only 2 degrees of freedom describe rotation
3N-5 possible normal modes of vibration
CO2 for example- 3 atoms
- Normal modes of vibration = 9-5 = 4
TRANSITIONS
Fundamental
- Excitation from the ground state Vo to the first excited state V1
- The most likely transition and have strong absorption bands
Fundamental
- Excitation from the ground state Vo to the first excited state V1
- The most likely transition and have strong absorption bands
Vo
V1
V2
V3
FUNDAMENTAL TRANSITIONS
Overtone
- Excitation from ground state to higher energy states V2, V3, ….
- Result in overtone bands that are weaker than fundamental
- Frequencies are integral multiples of fundamental absorption
- Fewer peaks are seen than predicted on spectra due to IR-inactivevibrations, degenerate vibrations, weak vibrations
- Additional peaks may be seen due to overtones
Overtone
- Excitation from ground state to higher energy states V2, V3, ….
- Result in overtone bands that are weaker than fundamental
- Frequencies are integral multiples of fundamental absorption
- Fewer peaks are seen than predicted on spectra due to IR-inactivevibrations, degenerate vibrations, weak vibrations
- Additional peaks may be seen due to overtones
COUPLING
- The interaction between vibrational modes
- Two vibrational frequencies may couple to produce a newvibrational frequency
Combinational Bandν1 + ν2 = ν3
Difference Bandν1 - ν2 = ν3
- The interaction between vibrational modes
- Two vibrational frequencies may couple to produce a newvibrational frequency
Combinational Bandν1 + ν2 = ν3
Difference Bandν1 - ν2 = ν3
VIBRATIONAL MOTION
- Consider a bond as a spring
μf
2π1ν~
c
c = speed of light (cm/s)f = force constant (dyn/cm; proportional to bond strength)
f for a double bond = 2f for a single bondf for a triple bond = 3f for a single bond
c = speed of light (cm/s)f = force constant (dyn/cm; proportional to bond strength)
f for a double bond = 2f for a single bondf for a triple bond = 3f for a single bond
- M1 and M2 are masses of vibrating atoms connecting the bond
21
21
MM
MMgramsinmassreducedμ
INSTRUMENTATION
Components
- Radiation source
- Sample holder
- Monochromator
- Detector
- Computer
Components
- Radiation source
- Sample holder
- Monochromator
- Detector
- Computer
RADIATION SOURCES
Intensity of radiation should- Be continuous over the λ range and cover a wide λ range
- Constant over long periods of time- Have normal operating temperatures between 1100 and 1500 K
- Have maximum intensity between 4000 and 400 cm-1
(mid-IR region)
- Modern sources include furnace ignitors, diesel engine heaters
- Sources are usually enclosed in an insulator to reduce noise
Intensity of radiation should- Be continuous over the λ range and cover a wide λ range
- Constant over long periods of time- Have normal operating temperatures between 1100 and 1500 K
- Have maximum intensity between 4000 and 400 cm-1
(mid-IR region)
- Modern sources include furnace ignitors, diesel engine heaters
- Sources are usually enclosed in an insulator to reduce noise
RADIATION SOURCES
Mid-IR Sources
Nernst Glowers- Heated ceramic rods
- Cylindrical bar- Made of zirconium oxide, cerium oxide, thorium oxide
- Heated electrically to 1500 K – 2000 K- Resistance decreases as temperature increases
- Can overheat and burn out
Mid-IR Sources
Nernst Glowers- Heated ceramic rods
- Cylindrical bar- Made of zirconium oxide, cerium oxide, thorium oxide
- Heated electrically to 1500 K – 2000 K- Resistance decreases as temperature increases
- Can overheat and burn out
RADIATION SOURCES
Mid-IR Sources
Globar- Silicon carbide bar
- Heated electrically to emit continuous IR radiation
- More sensitive than the Nernst glower
Mid-IR Sources
Globar- Silicon carbide bar
- Heated electrically to emit continuous IR radiation
- More sensitive than the Nernst glower
RADIATION SOURCES
Mid-IR Sources
Heated Wire Coils- Heated electrically to ~ 1100 oC
- Similar in shape to incandescent light bulb filament- Nichrome wire is commonly used
- Rhodium is also used- May easily fracture and burn out
Mid-IR Sources
Heated Wire Coils- Heated electrically to ~ 1100 oC
- Similar in shape to incandescent light bulb filament- Nichrome wire is commonly used
- Rhodium is also used- May easily fracture and burn out
RADIATION SOURCES
Near-IR Sources
Quartz halogen lamp- Contains tungsten wire filament
- Also contains iodine vapor sealed in a quartz bulb
- Evaporated tungsten is redeposited on the filamentincreasing stability
- Output range is between 25000 and 2000 cm-1
Near-IR Sources
Quartz halogen lamp- Contains tungsten wire filament
- Also contains iodine vapor sealed in a quartz bulb
- Evaporated tungsten is redeposited on the filamentincreasing stability
- Output range is between 25000 and 2000 cm-1
RADIATION SOURCES
Far-IR Sources
High pressure Hg discharge lamp- Made of quartz bulb containing elemental Hg
- Also contains inert gas and two electrodes
Far-IR Sources
High pressure Hg discharge lamp- Made of quartz bulb containing elemental Hg
- Also contains inert gas and two electrodes
RADIATION SOURCES
IR Laser Sources
- Excellent light source for measuring gases
Two typesGas-phase (tunable CO2) laser and solid-state laser
Laser- Light source that emits monochromatic radiation
IR Laser Sources
- Excellent light source for measuring gases
Two typesGas-phase (tunable CO2) laser and solid-state laser
Laser- Light source that emits monochromatic radiation
MONOCHROMATORS
- Salt prisms and metal gratings are used as dispersion devices
- Mirrors made of metal with polished front surface
- Spectrum is recorded by moving prism or grating suchthat different radiation frequencies pass through
the exit slit to the detector
- Spectrum obtained is %T verses wavenumber (or frequency)
- Salt prisms and metal gratings are used as dispersion devices
- Mirrors made of metal with polished front surface
- Spectrum is recorded by moving prism or grating suchthat different radiation frequencies pass through
the exit slit to the detector
- Spectrum obtained is %T verses wavenumber (or frequency)
FT SPECTROMETERS
- Based on Michelson interferometer
- Employs constructive and destructive interferences
- Destructive interference is a maximum when two beams are180o out of phase
- An FT is used to convert the time-domain spectrum obtained intoa frequency-domain spectrum
- The system is called FTIR
- Based on Michelson interferometer
- Employs constructive and destructive interferences
- Destructive interference is a maximum when two beams are180o out of phase
- An FT is used to convert the time-domain spectrum obtained intoa frequency-domain spectrum
- The system is called FTIR
FT SPECTROMETERS
- Has higher signal-to-noise ratio
- More accurate and precise than dispersive monochromators(Conne’s advantage)
- Much greater radiation intensity falls on the detector dueto the absence of slits
(throughput or Jacquinot’s advantage)
- Has higher signal-to-noise ratio
- More accurate and precise than dispersive monochromators(Conne’s advantage)
- Much greater radiation intensity falls on the detector dueto the absence of slits
(throughput or Jacquinot’s advantage)
FT SPECTROMETERS
Focusing Mirrors of Interferometer
- Made of metal and polished on the front surface
- Gold coated to prevent corrosion
- Focus source radiation onto the beam splitter
- Focus light emitting from sample onto detector
Focusing Mirrors of Interferometer
- Made of metal and polished on the front surface
- Gold coated to prevent corrosion
- Focus source radiation onto the beam splitter
- Focus light emitting from sample onto detector
FT SPECTROMETERS
Beam Splitter of Interferometer
- Made of Ge or Si coated onto a highly polishedIR-transparent substrate
- Ge coated on KBr is used for mid-IR
- Si coated on quartz is used for NIR
- Mylar film is used for far-IR
- Transmits and reflects 50% each of beam (ideally)
Beam Splitter of Interferometer
- Made of Ge or Si coated onto a highly polishedIR-transparent substrate
- Ge coated on KBr is used for mid-IR
- Si coated on quartz is used for NIR
- Mylar film is used for far-IR
- Transmits and reflects 50% each of beam (ideally)
DETECTORS
Two classes
Thermal detectors
and
photon-sensitive detectors
Two classes
Thermal detectors
and
photon-sensitive detectors
DETECTORS
Thermal Detectors
- Bolometers
- Thermocouples
- Thermistors
- Pyroelectric devices
Thermal Detectors
- Bolometers
- Thermocouples
- Thermistors
- Pyroelectric devices
DETECTORS
Thermal Detectors
Bolometers- Very sensitive electrical resistance thermometer
- Older ones were made of Pt wire(resistance change = 0.4% per oC)
- Modern ones are made of silicon placed in a wheatstone bridge
- Is a few micrometers in diameter
- Fast response time
Thermal Detectors
Bolometers- Very sensitive electrical resistance thermometer
- Older ones were made of Pt wire(resistance change = 0.4% per oC)
- Modern ones are made of silicon placed in a wheatstone bridge
- Is a few micrometers in diameter
- Fast response time
DETECTORS
Thermal Detectors
Thermocouples- Made up of two wires welded together at both ends
- Two wires are from different metals- One welded joint (hot junction) is exposed to IR radiation
- The other joint (cold junction) is kept at constant temperature- Hot junction becomes hotter than cold junction
- Potential difference is a function of IR radiation- Slow response time
- Cannot be used for FTIR
Thermal Detectors
Thermocouples- Made up of two wires welded together at both ends
- Two wires are from different metals- One welded joint (hot junction) is exposed to IR radiation
- The other joint (cold junction) is kept at constant temperature- Hot junction becomes hotter than cold junction
- Potential difference is a function of IR radiation- Slow response time
- Cannot be used for FTIR
DETECTORS
Thermal Detectors
Thermistors- Made of fused mixture of metal oxides
- Increasing temperature decreases electrical resistancewhich is a function of IR radiation
- Resistance change is about 5% per oC
- Slow response time
Thermal Detectors
Thermistors- Made of fused mixture of metal oxides
- Increasing temperature decreases electrical resistancewhich is a function of IR radiation
- Resistance change is about 5% per oC
- Slow response time
DETECTORS
Thermal Detectors
Pyroelectric Detectors- Made of dielectric materials (insulators), ferroelectric materials,
or semiconductors
- A thin crystal of the material is placed between two electrodes
- Change in temperature changes polarization of materialwhich is a function of IR radiation exposed
- The electrodes measure the change in polarization
Thermal Detectors
Pyroelectric Detectors- Made of dielectric materials (insulators), ferroelectric materials,
or semiconductors
- A thin crystal of the material is placed between two electrodes
- Change in temperature changes polarization of materialwhich is a function of IR radiation exposed
- The electrodes measure the change in polarization
DETECTORS
Photon Detectors
- Made of materials such as lead selenide (PbSe),indium gallium arsenide (InGaSa), indium antimonide (InSb)
- Materials are semiconductors
- S/N increases as temperature decreases (cooling is necessary)
- Very sensitive and very fast response time
- Good for FTIR and coupled techniques
Photon Detectors
- Made of materials such as lead selenide (PbSe),indium gallium arsenide (InGaSa), indium antimonide (InSb)
- Materials are semiconductors
- S/N increases as temperature decreases (cooling is necessary)
- Very sensitive and very fast response time
- Good for FTIR and coupled techniques
RESPONSE TIME
- The length of time that a detector takes to reach a steadysignal when radiation falls on it
- Response time for older dispersive IR spectrometerswere ~ 15 minutes
- Response time is very slow in thermal detectors due to slowchange in temperature near equilibrium
- The length of time that a detector takes to reach a steadysignal when radiation falls on it
- Response time for older dispersive IR spectrometerswere ~ 15 minutes
- Response time is very slow in thermal detectors due to slowchange in temperature near equilibrium
RESPONSE TIME
Semiconductors
- Are insulators when no radiation falls on them but conductorswhen radiation falls on them
- There is no temperature change involved
- Measured signal is due to change in resistance uponexposure to IR radiation ( is rapid)
- Response time is the time required to change the semiconductorfrom insulator to conductor (~ 1 ns)
Semiconductors
- Are insulators when no radiation falls on them but conductorswhen radiation falls on them
- There is no temperature change involved
- Measured signal is due to change in resistance uponexposure to IR radiation ( is rapid)
- Response time is the time required to change the semiconductorfrom insulator to conductor (~ 1 ns)
SATURATION
- Very high levels of light can saturate detector
- Linearity is very important especially inthe mid-IR region
- Very high levels of light can saturate detector
- Linearity is very important especially inthe mid-IR region
TRANSMISSION (ABSORPTION) TECHNIQUE
- Can be used for both qualitative and quantitative analysis
- Provides very high sensitivity at relatively low cost
- Sample cell material must be transparent to IR radiation(NaCl, KBr)
- Samples should not contain water since materials aresoluble in water
- Can be used for both qualitative and quantitative analysis
- Provides very high sensitivity at relatively low cost
- Sample cell material must be transparent to IR radiation(NaCl, KBr)
- Samples should not contain water since materials aresoluble in water
TRANSMISSION (ABSORPTION) TECHNIQUE
Solid Samples
Three sampling techniquesMulling, pelleting, thin film
Mulling- Samples are ground to power with a few drops of viscous
liquid like Nujol (size < 2 µm)
- 2-4 mg sample is made into a mull (thick slurry)
- Mull is pressed between salt plates to form a thin film
Solid Samples
Three sampling techniquesMulling, pelleting, thin film
Mulling- Samples are ground to power with a few drops of viscous
liquid like Nujol (size < 2 µm)
- 2-4 mg sample is made into a mull (thick slurry)
- Mull is pressed between salt plates to form a thin film
TRANSMISSION (ABSORPTION) TECHNIQUE
Solid Samples
Three sampling techniquesMulling, pelleting, thin film
Pelleting- 1 mg ground sample is mixed with 100 mg of dry KBr powder
- Mixture is compressed under very high pressure
- Small disk with very smooth surfaces forms (looks like glass)
Solid Samples
Three sampling techniquesMulling, pelleting, thin film
Pelleting- 1 mg ground sample is mixed with 100 mg of dry KBr powder
- Mixture is compressed under very high pressure
- Small disk with very smooth surfaces forms (looks like glass)
TRANSMISSION (ABSORPTION) TECHNIQUE
Solid Samples
Three sampling techniquesMulling, pelleting, thin film
Thin Film- Molten sample is deposited on the surface of KBr or NaCl plates
- Sample is allowed to dry to form a thin film on substrate
- Good for qualitative identification of polymers but notgood for quantitative analysis
Solid Samples
Three sampling techniquesMulling, pelleting, thin film
Thin Film- Molten sample is deposited on the surface of KBr or NaCl plates
- Sample is allowed to dry to form a thin film on substrate
- Good for qualitative identification of polymers but notgood for quantitative analysis
TRANSMISSION (ABSORPTION) TECHNIQUE
Liquid Samples
- Many liquid samples are analyzed as is
- Dilution with the appropriate solvent may be necessary(CCl4, CS2, CH3Cl)
- Solvent must be transparent in the region of interest
- Salt plates are hygroscopic and water soluble (avoid water)
- Special cells are used for water containing samples (BaF2, AgCl)
Liquid Samples
- Many liquid samples are analyzed as is
- Dilution with the appropriate solvent may be necessary(CCl4, CS2, CH3Cl)
- Solvent must be transparent in the region of interest
- Salt plates are hygroscopic and water soluble (avoid water)
- Special cells are used for water containing samples (BaF2, AgCl)
TRANSMISSION (ABSORPTION) TECHNIQUE
Gas Samples
- Gas cells have longer pathlengths to compensate for verylow concentrations of gas samples
- Generally about 10 cm(commercial ones are up to 120 cm)
- Temperature and pressure are controlled if quantitativeanalysis is required
Gas Samples
- Gas cells have longer pathlengths to compensate for verylow concentrations of gas samples
- Generally about 10 cm(commercial ones are up to 120 cm)
- Temperature and pressure are controlled if quantitativeanalysis is required
TRANSMISSION (ABSORPTION) TECHNIQUE
Background Absorption
- Absorption from material other than sample
Two sources- Solvent
- Air in the light path
- Background spectrum is subtracted from sample spectrum
- CO2 and H2O absorb IR radiation(sealed desiccants are used to reduce CO2 and H2O absorption)
Background Absorption
- Absorption from material other than sample
Two sources- Solvent
- Air in the light path
- Background spectrum is subtracted from sample spectrum
- CO2 and H2O absorb IR radiation(sealed desiccants are used to reduce CO2 and H2O absorption)
ATTENUATED TOTAL REFLECTION(ATR) TECHNIQUE
- Uses high refractive index optical element
- Called internal reflection element (IRE) or ATR crystal(germanium, ZnSe, silicon, diamond)
- Sample has lower refractive index than the ATR crystal
- Sample is placed on the ATR crystal
- When light travelling in the crystal hits the crystal-sampleinterface, total internal reflection occurs if the angle
of incidence is greater than the critical angle
- Uses high refractive index optical element
- Called internal reflection element (IRE) or ATR crystal(germanium, ZnSe, silicon, diamond)
- Sample has lower refractive index than the ATR crystal
- Sample is placed on the ATR crystal
- When light travelling in the crystal hits the crystal-sampleinterface, total internal reflection occurs if the angle
of incidence is greater than the critical angle
ATTENUATED TOTAL REFLECTION(ATR) TECHNIQUE
- The beam of light penetrates the sample a very short distance
- This is known as the evanescent wave
- Light beam is attenuated (reduced in intensity) if the evanescentwave is absorbed
- IR absorption spectrum is obtained from the interactionsbetween the evanescent wave and the sample
- The beam of light penetrates the sample a very short distance
- This is known as the evanescent wave
- Light beam is attenuated (reduced in intensity) if the evanescentwave is absorbed
- IR absorption spectrum is obtained from the interactionsbetween the evanescent wave and the sample
SPECULAR REFLECTANCE TECHNIQUE
- Occurs when angle of reflected light equals angle of incident light(45o angle of incidence is typically used)
- Occurs when light bounces off a smooth surface
- Nondestructive method for studying thin films
- Beam passes through thin film of sample where absorption occurs
- Beam is reflected from a polished surface below the sample
- Reflected beam passes through sample again
- Occurs when angle of reflected light equals angle of incident light(45o angle of incidence is typically used)
- Occurs when light bounces off a smooth surface
- Nondestructive method for studying thin films
- Beam passes through thin film of sample where absorption occurs
- Beam is reflected from a polished surface below the sample
- Reflected beam passes through sample again
DIFFUSE REFLECTANCE TECHNIQUE
- DR OR DRIFTS
- Used to obtain IR or NIR spectrum from a rough surface
- Beam penetrates sample to about 100 µm
- Reflected light is scattered at many angles
- Scattered radiation is collected with a large mirror
- Good for powdered samples (sample is mixed with KBr powder)
- DR OR DRIFTS
- Used to obtain IR or NIR spectrum from a rough surface
- Beam penetrates sample to about 100 µm
- Reflected light is scattered at many angles
- Scattered radiation is collected with a large mirror
- Good for powdered samples (sample is mixed with KBr powder)
IR EMISSION
- Certain samples are characterized using emission spectrum
- Sample molecules are heated to occupy excited vibrational states
- Radiation is emitted upon returning to the ground state
- Emitted IR radiation is directed into the spectrometer
- Radiation source is not needed (sample is the source)
- Method is nondestructive
- Emitted radiation is used to identify sample
- Certain samples are characterized using emission spectrum
- Sample molecules are heated to occupy excited vibrational states
- Radiation is emitted upon returning to the ground state
- Emitted IR radiation is directed into the spectrometer
- Radiation source is not needed (sample is the source)
- Method is nondestructive
- Emitted radiation is used to identify sample
FTIR MICROSCOPY
- For forensic analysis
- Analysis of paint chips from cars (hit-and-run accidents)
- Examination of fibers, drugs, explosives
- Characterization of pharmaceuticals, adhesives and gemstones
- For forensic analysis
- Analysis of paint chips from cars (hit-and-run accidents)
- Examination of fibers, drugs, explosives
- Characterization of pharmaceuticals, adhesives and gemstones
NEAR-IR (NIR) SPECTROSCOPY
- Region covers 750 nm – 2500 nm (13000 cm-1 – 4000 cm-1)
- Long wavelength end of IR region
- Bands occurring in this region are due to OH, NH, and CH bonds
- Bands are primarily overtone and combination bands
- Light source is tungsten-halogen lamp
- Detector is lead sulfide photodetector
- Quartz or fused silica sample cells with long pathlengths are used
- Region covers 750 nm – 2500 nm (13000 cm-1 – 4000 cm-1)
- Long wavelength end of IR region
- Bands occurring in this region are due to OH, NH, and CH bonds
- Bands are primarily overtone and combination bands
- Light source is tungsten-halogen lamp
- Detector is lead sulfide photodetector
- Quartz or fused silica sample cells with long pathlengths are used
NEAR-IR (NIR) SPECTROSCOPY
Primary absorption bands seen in NIR
C−H Bands2100 – 2450 nm and 1600 – 1800 nm
N−H Bands1450 – 1550 nm and 2800 – 3000 nm
O−H Bands1390 – 1450 nm and 2700 – 2900 nm
Primary absorption bands seen in NIR
C−H Bands2100 – 2450 nm and 1600 – 1800 nm
N−H Bands1450 – 1550 nm and 2800 – 3000 nm
O−H Bands1390 – 1450 nm and 2700 – 2900 nm
NEAR-IR (NIR) SPECTROSCOPY
- Used for quantitative analysis of solid and liquid samplescontaining OH, NH, CH bonds
- For quantitative characterization of polymers, food,proteins, agricultural products
- Pharmaceutical tablets can be analyzed nondestructively
- Forensic analysis of unknown wrapped powders believed to bedrugs are analyzed without destroying the wrappers
- Used for quantitative analysis of solid and liquid samplescontaining OH, NH, CH bonds
- For quantitative characterization of polymers, food,proteins, agricultural products
- Pharmaceutical tablets can be analyzed nondestructively
- Forensic analysis of unknown wrapped powders believed to bedrugs are analyzed without destroying the wrappers
RAMAN SPECTROSCOPY
- Light scattering technique for studying molecular vibrations
- Change in polarization is necessary for a vibration to beseen in Raman spectrum
- Implies change in distribution of electron cloud aroundvibrating atoms
- Polarization is easier for long bonds than for short bonds
- IR-inactive radiations are Raman-active
- Light scattering technique for studying molecular vibrations
- Change in polarization is necessary for a vibration to beseen in Raman spectrum
- Implies change in distribution of electron cloud aroundvibrating atoms
- Polarization is easier for long bonds than for short bonds
- IR-inactive radiations are Raman-active
RAMAN SPECTROSCOPY
Principles
- Part of the radiation is scattered by molecules whenthe radiation passes through sample
Three types of scattering occurs- Rayleigh scattering- Stokes scattering
- Anti-Stokes scattering
Principles
- Part of the radiation is scattered by molecules whenthe radiation passes through sample
Three types of scattering occurs- Rayleigh scattering- Stokes scattering
- Anti-Stokes scattering
RAMAN SPECTROSCOPY
Principles
Rayleigh scattering- Result of elastic collisions between photons and sample molecules
- Energy is same as incident radiation
Stokes scattering- Scattered photons have less energy than the incident radiation
- Results in spectral lines called Raman lines
Anti-Stokes scattering- Raman lines result from photons scattered with more energy
Principles
Rayleigh scattering- Result of elastic collisions between photons and sample molecules
- Energy is same as incident radiation
Stokes scattering- Scattered photons have less energy than the incident radiation
- Results in spectral lines called Raman lines
Anti-Stokes scattering- Raman lines result from photons scattered with more energy
RAMAN SPECTROSCOPY
Principles
- Stokes and anti-stokes are due to inelastic collisions
- The process is not quantized
- Raman lines are shifted in frequency from Rayleigh frequency
- Radiation measured is visible or NIR
Principles
- Stokes and anti-stokes are due to inelastic collisions
- The process is not quantized
- Raman lines are shifted in frequency from Rayleigh frequency
- Radiation measured is visible or NIR
RAMAN SPECTROSCOPY
Advantages over IR
- Aqueous solutions can be analyzed
- Fewer and much sharper lines so better for quantitative analysis
Techniques- Resonance Raman Spectroscopy
- Surface-Enhanced Raman Spectroscopy (SERS)- Raman Microscopy
Advantages over IR
- Aqueous solutions can be analyzed
- Fewer and much sharper lines so better for quantitative analysis
Techniques- Resonance Raman Spectroscopy
- Surface-Enhanced Raman Spectroscopy (SERS)- Raman Microscopy
APPLICATIONS OF IR SPECTROSCOPY
Quantitative
- Extent of absorption and Beer’s law can be used to determineconcentration of unknown analytes in sample
- Absorption band unique to the analyte molecule should beused for measurements
- Generally performed with samples in solutions
- Light scattering may occur with pellets whichdeviates from Beer’s law
Quantitative
- Extent of absorption and Beer’s law can be used to determineconcentration of unknown analytes in sample
- Absorption band unique to the analyte molecule should beused for measurements
- Generally performed with samples in solutions
- Light scattering may occur with pellets whichdeviates from Beer’s law
APPLICATIONS OF IR SPECTROSCOPY
Quantitative
- Measure absorption intensities of standard solutions andunknown at exactly the same wavenumber
- All measurements must be made from the same baseline
- Plot a calibration curve
- Use the relationship obtained to determine the concentrationof unknown
- Not as accurate as using UV-VIS spectroscopy
Quantitative
- Measure absorption intensities of standard solutions andunknown at exactly the same wavenumber
- All measurements must be made from the same baseline
- Plot a calibration curve
- Use the relationship obtained to determine the concentrationof unknown
- Not as accurate as using UV-VIS spectroscopy
APPLICATIONS OF IR SPECTROSCOPY
Quantitative
- Determination of impurities in raw materials (quality control)
- Analysis of contaminations from oil or grease
- Determination of reaction rates of slow reactions
Quantitative
- Determination of impurities in raw materials (quality control)
- Analysis of contaminations from oil or grease
- Determination of reaction rates of slow reactions
APPLICATIONS OF IR SPECTROSCOPY
Qualitative
- Identification of unknown samples by matching the absorptionspectra with that of known compounds
- Identification of functional groups present in a sample(classification of unknowns)
Qualitative
- Identification of unknown samples by matching the absorptionspectra with that of known compounds
- Identification of functional groups present in a sample(classification of unknowns)
PREDICTING STRUCTURE OF UNKNOWN
- Identify the major functional groups from the strongabsorption peaks
- Identify the compound as aromatic or aliphatic
- Subtract the FW of all functional groups identified from the givenmolecular weight of the compound
- Look for C≡C and C=C stretching bands
- Look for other unique CH bands (e.g. aldehyde)
- Use the difference obtained to deduce the structure
- Identify the major functional groups from the strongabsorption peaks
- Identify the compound as aromatic or aliphatic
- Subtract the FW of all functional groups identified from the givenmolecular weight of the compound
- Look for C≡C and C=C stretching bands
- Look for other unique CH bands (e.g. aldehyde)
- Use the difference obtained to deduce the structure
INTERPRETATION OF IR SPECTRA
Functional Group Region- Strong absorptions due to stretching from hydroxyl, amine,
carbonyl, CHx
4000 – 1300 cm-1
Fingerprint Region- Result of interactions between vibrations
1300 – 910 cm-1
Functional Group Region- Strong absorptions due to stretching from hydroxyl, amine,
carbonyl, CHx
4000 – 1300 cm-1
Fingerprint Region- Result of interactions between vibrations
1300 – 910 cm-1
INTERPRETATION OF IR SPECTRA
Hydrocarbons
- Absorption bands are due to the stretching or bending ofC−H and C−C bonds
- C−C stretching vibrations are distributed across thefingerprint region (not useful for identification)
- C−C bending vibrations occur below 500 cm-1
(not useful for identification)
- Observed bands are due to C−H stretching or bending
Hydrocarbons
- Absorption bands are due to the stretching or bending ofC−H and C−C bonds
- C−C stretching vibrations are distributed across thefingerprint region (not useful for identification)
- C−C bending vibrations occur below 500 cm-1
(not useful for identification)
- Observed bands are due to C−H stretching or bending
INTERPRETATION OF IR SPECTRA
Cyclic Alkanes- No peak around 1375 cm-1 due to absence of methyl groups
- Two peaks at ~ 900 cm-1 and 860 cm-1 due to ring deformation
Alkenes- Contain many more peaks than alkanes
- Peaks of interest are due to stretching and bending of C−Hand C=C bonds
- C=C band will not appear if there is symmetrical substitutionabout the C=C bond
Cyclic Alkanes- No peak around 1375 cm-1 due to absence of methyl groups
- Two peaks at ~ 900 cm-1 and 860 cm-1 due to ring deformation
Alkenes- Contain many more peaks than alkanes
- Peaks of interest are due to stretching and bending of C−Hand C=C bonds
- C=C band will not appear if there is symmetrical substitutionabout the C=C bond
INTERPRETATION OF IR SPECTRA
Alkynes- C≡C peak appears around 2100 – 2200 cm-1
- Terminal alkyne ≡C−H stretch occurs near 3300 cm-1
Aromatic Hydrocarbons- C→H absorption occurs above 3000 cm-1
- Aromatic C=C ring stretching absorption around1400 – 1600 cm-1 appears as doublet
- Aromatic C↓H oop band around 690 – 900 cm-1
- Overtones around 1660 – 2000 cm-1
Alkynes- C≡C peak appears around 2100 – 2200 cm-1
- Terminal alkyne ≡C−H stretch occurs near 3300 cm-1
Aromatic Hydrocarbons- C→H absorption occurs above 3000 cm-1
- Aromatic C=C ring stretching absorption around1400 – 1600 cm-1 appears as doublet
- Aromatic C↓H oop band around 690 – 900 cm-1
- Overtones around 1660 – 2000 cm-1
INTERPRETATION OF IR SPECTRA
Alcohols
- OH band in neat aliphatic alcohols is a broad band centeredat ~ 3300 cm-1 due to hydrogen bonding (3100 – 3600 cm-1)
- OH band in dilute solutions of aliphatic alcohols is a sharppeak ~ 3600 cm-1
- C−C−O stretch ~ 1048 cm-1 for primary alcohols
- Decreasing frequency by 10 cm-1 in the order 1o>2o>3o
- Methyl bending vibrations at ~ 1200 – 1500 cm-1
Alcohols
- OH band in neat aliphatic alcohols is a broad band centeredat ~ 3300 cm-1 due to hydrogen bonding (3100 – 3600 cm-1)
- OH band in dilute solutions of aliphatic alcohols is a sharppeak ~ 3600 cm-1
- C−C−O stretch ~ 1048 cm-1 for primary alcohols
- Decreasing frequency by 10 cm-1 in the order 1o>2o>3o
- Methyl bending vibrations at ~ 1200 – 1500 cm-1
INTERPRETATION OF IR SPECTRA
Phenol
- CO→H stretch is broad band
- C→H stretch ~ 3050 cm-1
- C−C→O band ~ 1225 cm-1
- C −O−H bend ~ 1350 cm-1
- Aromatic ring C stretching between 1450 – 1600 cm-1
- Monosubstituted bands ~ 745 – 895 cm-1 and 1650 – 2000 cm-1
Phenol
- CO→H stretch is broad band
- C→H stretch ~ 3050 cm-1
- C−C→O band ~ 1225 cm-1
- C −O−H bend ~ 1350 cm-1
- Aromatic ring C stretching between 1450 – 1600 cm-1
- Monosubstituted bands ~ 745 – 895 cm-1 and 1650 – 2000 cm-1
INTERPRETATION OF IR SPECTRA
Aliphatic Acids
- Broad OH band around 2900 cm-1
- C−H stretching bands from CH3 and CH2 stick out at thebottom of the broad OH band
- C=O stretch ~ 1710 cm-1
- In-plane C −O−H bend ~ 1410 cm-1 andoop C −O−H bend ~ 930 cm-1
- C −C−O stretch dimer at ~ 1280 cm-1
Aliphatic Acids
- Broad OH band around 2900 cm-1
- C−H stretching bands from CH3 and CH2 stick out at thebottom of the broad OH band
- C=O stretch ~ 1710 cm-1
- In-plane C −O−H bend ~ 1410 cm-1 andoop C −O−H bend ~ 930 cm-1
- C −C−O stretch dimer at ~ 1280 cm-1
INTERPRETATION OF IR SPECTRA
Carboxylic Acids, Esters, Ketones, Aldehydes
- Characterized by very strong carbonyl (C=O) stretchingband between 1650 cm-1 and 1850 cm-1
- Fermi resonance seen in aldehydes(doublet due to resonance with an overtone of the aldehydic
C−H bend at 1390 cm-1)
Carboxylic Acids, Esters, Ketones, Aldehydes
- Characterized by very strong carbonyl (C=O) stretchingband between 1650 cm-1 and 1850 cm-1
- Fermi resonance seen in aldehydes(doublet due to resonance with an overtone of the aldehydic
C−H bend at 1390 cm-1)
INTERPRETATION OF IR SPECTRA
Nitrogen-Containing Compounds
- 1o amines (NH2) have scissoring mode and lowfrequency wagging mode
- 2o amines (NH) only have wagging mode (cannot scissor)
- 3o amines have no NH band and are characterized by C−Nstretching modes ~ 1000 – 1200 cm-1 and 700 – 900 cm-1
- 1o, 2o, 3o amides are similar to their amine counterpartsbut have additional C=O stretching band
Nitrogen-Containing Compounds
- 1o amines (NH2) have scissoring mode and lowfrequency wagging mode
- 2o amines (NH) only have wagging mode (cannot scissor)
- 3o amines have no NH band and are characterized by C−Nstretching modes ~ 1000 – 1200 cm-1 and 700 – 900 cm-1
- 1o, 2o, 3o amides are similar to their amine counterpartsbut have additional C=O stretching band
INTERPRETATION OF IR SPECTRA
Nitrogen-Containing Compounds
- C=O stretching called amide I in 1o and 2o amides andamide II in 3o amides
- N−H stretch doublet ~ 3370 – 3291 cm-1 for 1o amines
- 1o N−H bend at ~ 1610 cm-1 and 800 cm-1
- Single N−H stretch ~ 3293 cm-1 for 2o but absent in 3o amine
- C−N stretch weak band ~ 1100 cm-1
Nitrogen-Containing Compounds
- C=O stretching called amide I in 1o and 2o amides andamide II in 3o amides
- N−H stretch doublet ~ 3370 – 3291 cm-1 for 1o amines
- 1o N−H bend at ~ 1610 cm-1 and 800 cm-1
- Single N−H stretch ~ 3293 cm-1 for 2o but absent in 3o amine
- C−N stretch weak band ~ 1100 cm-1
INTERPRETATION OF IR SPECTRA
Amino Acids [RCH(NH2)COOH]
- IR spectrum is related to salts of amines and salts of acids
- Broad CH bands that overlap with each other
- Broad band ~ 2100 cm-1
- NH band ~ 1500 cm-1
- Carboxylate ion stretch ~ 1600 cm-1
Amino Acids [RCH(NH2)COOH]
- IR spectrum is related to salts of amines and salts of acids
- Broad CH bands that overlap with each other
- Broad band ~ 2100 cm-1
- NH band ~ 1500 cm-1
- Carboxylate ion stretch ~ 1600 cm-1
INTERPRETATION OF IR SPECTRA
Halogenated Compounds
- C→X strong absorption bands in the fingerprint andaromatic regions
- More halogens on the same C results in an increase in intensityand a shift to higher wavenumbers
- Absorption due to C−Cl and C−Br occurs below 800 cm-1
Halogenated Compounds
- C→X strong absorption bands in the fingerprint andaromatic regions
- More halogens on the same C results in an increase in intensityand a shift to higher wavenumbers
- Absorption due to C−Cl and C−Br occurs below 800 cm-1
LIMITATIONS OF IR SPECTROSCOPY
- Short pathlength
- Pathlength may vary from sample to sample
- Sample cells are soluble in water
- Short pathlength
- Pathlength may vary from sample to sample
- Sample cells are soluble in water