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

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 – 20000 nm Far-IR: 20000 nm – 400000 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

IR ABSORPTION BY MOLECULES - 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

IR ABSORPTION BY MOLECULES 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)

PRINCIPAL MODES OF VIBRATION 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)

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

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

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 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

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)

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

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)

DETECTORS Two classes Thermal detectors and photon-sensitive detectors

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

TRANSMISSION (ABSORPTION) TECHNIQUE 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

TRANSMISSION (ABSORPTION) TECHNIQUE 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)

TRANSMISSION (ABSORPTION) TECHNIQUE 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

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)

TRANSMISSION (ABSORPTION) TECHNIQUE 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

INSTRUMENTAL ANALYSIS CHEM 4811