INSTRUMENTAL ANALYSIS CHEM 4811

Name: INSTRUMENTAL ANALYSIS CHEM 4811
Uploaded: 2017-06-29T16:15:06+00:00
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Description: INSTRUMENTAL ANALYSIS CHEM 4811

INSTRUMENTAL ANALYSIS CHEM 4811
CHAPTER 4 DR. AUGUSTINE OFORI AGYEMAN Assistant professor of chemistry Department of natural sciences Clayton state university

INFRARED SPECTROSCOPY
CHAPTER 4 INFRARED SPECTROSCOPY (IR)

IR SPECTROSCOPY - Used for qualitative identification of organic and
inorganic 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 and higher energy than microwave The IR region is divided into Near-IR (NIR): 750 nm – 2500 nm Mid-IR: 2500 nm – nm Far-IR: nm – nm

IR ABSORPTION BY MOLECULES
- Molecules with covalent bonds may absorb IR radiation - Absorption is quantized - 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 to higher vibrational states - Radiation higher than 100 µm will cause excitation to higher rotational states

- Absorption spectrum is composed of broad vibrational absorption bands - Molecules absorb radiation when a bond in the molecule vibrates at the same frequency as the incident radiant energy - Molecules vibrate at higher amplitude after absorption - A molecule must have a change in dipole moment during vibration in order to absorb IR radiation

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 the bond 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 absorb IR radiation

DIPOLE MOMENT (µ) - The repetitive changes in µ makes it possible for polar molecules to absorb IR radiation - Symmetrical molecules do not absorb IR radiation since they do 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 be IR active depending on whether they have permanent net dipole moment

PRINCIPAL MODES OF VIBRATION
Stretching - Change in bond length resulting from change in interatomic distance (r) Two stretching modes - Symmetrical and asymmetrical stretching - Symmetrical stretching is IR-inactive (no change in µ)

Bending - Change in bond angle or change in the position of a group of atoms 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 Twisting Out-of-plane (oop) bending modes (atoms move out of plane)

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

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 V3 V2 V1 Vo

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-inactive vibrations, 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 new vibrational frequency Combinational Band ν1 + ν2 = ν3 Difference Band ν1 - ν2 = ν3

VIBRATIONAL MOTION - Consider a bond as a spring
c = speed of light (cm/s) f = force constant (dyn/cm; proportional to bond strength) f for a double bond = 2f for a single bond f for a triple bond = 3f for a single bond - M1 and M2 are masses of vibrating atoms connecting the bond

INSTRUMENTATION 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

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

RADIATION SOURCES 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

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 filament increasing stability - Output range is between 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

RADIATION SOURCES IR Laser Sources
- Excellent light source for measuring gases Two types Gas-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 such that 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 are 180o out of phase - An FT is used to convert the time-domain spectrum obtained into a 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 due to the absence of slits (throughput or Jacquinot’s advantage)

Focusing Mirrors of Interferometer
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

Beam Splitter of Interferometer
FT SPECTROMETERS Beam Splitter of Interferometer - Made of Ge or Si coated onto a highly polished IR-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)

photon-sensitive detectors
Two classes Thermal detectors and photon-sensitive detectors

- Pyroelectric devices
DETECTORS 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

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

DETECTORS Thermal Detectors Thermistors
- Made of fused mixture of metal oxides - Increasing temperature decreases electrical resistance which 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 material which 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

RESPONSE TIME - The length of time that a detector takes to reach a steady signal when radiation falls on it - Response time for older dispersive IR spectrometers were ~ 15 minutes - Response time is very slow in thermal detectors due to slow change in temperature near equilibrium

RESPONSE TIME Semiconductors
- Are insulators when no radiation falls on them but conductors when radiation falls on them - There is no temperature change involved - Measured signal is due to change in resistance upon exposure to IR radiation ( is rapid) - Response time is the time required to change the semiconductor from insulator to conductor (~ 1 ns)

SATURATION - Very high levels of light can saturate detector
- Linearity is very important especially in the 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 are soluble in water

Solid Samples Three sampling techniques Mulling, 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 techniques Mulling, 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 techniques Mulling, 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 not good for quantitative analysis

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)

Gas Samples - Gas cells have longer pathlengths to compensate for very low concentrations of gas samples - Generally about 10 cm (commercial ones are up to 120 cm) - Temperature and pressure are controlled if quantitative analysis 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)

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-sample interface, 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 evanescent wave is absorbed - IR absorption spectrum is obtained from the interactions between 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

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)

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

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

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

NEAR-IR (NIR) SPECTROSCOPY Primary absorption bands seen in NIR
C−H Bands 2100 – 2450 nm and 1600 – 1800 nm N−H Bands 1450 – 1550 nm and 2800 – 3000 nm O−H Bands 1390 – 1450 nm and 2700 – 2900 nm

NEAR-IR (NIR) SPECTROSCOPY
- Used for quantitative analysis of solid and liquid samples containing 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 be drugs are analyzed without destroying the wrappers

RAMAN SPECTROSCOPY - Light scattering technique for studying molecular vibrations - Change in polarization is necessary for a vibration to be seen in Raman spectrum - Implies change in distribution of electron cloud around vibrating 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 when the 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

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

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

APPLICATIONS OF IR SPECTROSCOPY
Quantitative - Extent of absorption and Beer’s law can be used to determine concentration of unknown analytes in sample - Absorption band unique to the analyte molecule should be used for measurements - Generally performed with samples in solutions - Light scattering may occur with pellets which deviates from Beer’s law

Quantitative - Measure absorption intensities of standard solutions and unknown 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 concentration of unknown - Not as accurate as using UV-VIS spectroscopy

Quantitative - Determination of impurities in raw materials (quality control) - Analysis of contaminations from oil or grease - Determination of reaction rates of slow reactions

Qualitative - Identification of unknown samples by matching the absorption spectra 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 strong absorption peaks - Identify the compound as aromatic or aliphatic - Subtract the FW of all functional groups identified from the given molecular 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

INTERPRETATION OF IR SPECTRA
Hydrocarbons - Absorption bands are due to the stretching or bending of C−H and C−C bonds - C−C stretching vibrations are distributed across the fingerprint 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

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−H and C=C bonds - C=C band will not appear if there is symmetrical substitution about the C=C bond

INTERPRETATION OF IR SPECTRA Aromatic Hydrocarbons
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 around 1400 – 1600 cm-1 appears as doublet - Aromatic C↓H oop band around 690 – 900 cm-1 - Overtones around 1660 – 2000 cm-1

Alcohols - OH band in neat aliphatic alcohols is a broad band centered at ~ 3300 cm-1 due to hydrogen bonding (3100 – 3600 cm-1) - OH band in dilute solutions of aliphatic alcohols is a sharp peak ~ 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

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

Aliphatic Acids - Broad OH band around 2900 cm-1 - C−H stretching bands from CH3 and CH2 stick out at the bottom of the broad OH band - C=O stretch ~ 1710 cm-1 - In-plane C −O−H bend ~ 1410 cm-1 and oop C −O−H bend ~ 930 cm-1 - C −C−O stretch dimer at ~ 1280 cm-1

Carboxylic Acids, Esters, Ketones, Aldehydes - Characterized by very strong carbonyl (C=O) stretching band 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 low frequency wagging mode - 2o amines (NH) only have wagging mode (cannot scissor) - 3o amines have no NH band and are characterized by C−N stretching modes ~ 1000 – 1200 cm-1 and 700 – 900 cm-1 - 1o, 2o, 3o amides are similar to their amine counterparts but have additional C=O stretching band

INTERPRETATION OF IR SPECTRA Nitrogen-Containing Compounds
- C=O stretching called amide I in 1o and 2o amides and amide 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

INTERPRETATION OF IR SPECTRA Halogenated Compounds
- C→X strong absorption bands in the fingerprint and aromatic regions - More halogens on the same C results in an increase in intensity and 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

INSTRUMENTAL ANALYSIS CHEM 4811

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