Spectroscopy: Qualitative & Quantitative Analysis

• Spectroscopy involves the interaction between matter and electromagnetic radiation.
• It also includes interactions between matter and other energy forms like acoustic waves or particle beams.
• Spectroscopy serves for both qualitative and quantitative analyses of matter.

Types of Spectroscopy

• Ultraviolet-Visible Spectroscopy (UV-Vis)
• Infrared Spectroscopy (IR), including Raman Spectroscopy
• Mass Spectroscopy (MS)
• Nuclear Magnetic Resonance (NMR)
• Atomic Absorption Spectroscopy (AAS)
• Fluorescence, Phosphorescence, and Chemiluminescence

Qualitative Analysis

• It relies on the unique interaction of matter with electromagnetic radiation.
• The steps to establish a molecular structure involve various spectroscopic methods, including MS, NMR, IR, and UV.

Quantitative Analysis

• Radiant power is related to the concentration of a sample.
• Spectrochemical methods are categorized by the radiant power they measure, including Emission, Luminescence, Scattering, and Absorption.
• Emission measures emitted radiant power ($P_e$), the concentration is expressed as $P_e = kc$, and it is used in atomic emission.
• Luminescence measures luminescent radiant power ($P_l$), the concentration is expressed as $P_l = kc$, and it is used in atomic and molecular fluorescence, phosphorescence, and chemiluminescence.
• Scattering measures scattered radiant power ($P_{sc}$), the concentration is expressed as $P_{sc} = kc$, and it is used in Raman scattering, turbidimetry, and particle sizing.
• Absorption measures incident ($P_o$) and transmitted ($P$) radiant power, the concentration is expressed as $-log(\frac{P}{P_o}) = kc$, and it is used in atomic and molecular absorption.

Instrumental analysis methods are used for both Qualitative and Quantitative analysis

• Atomic Absorption Spectrometry: Quantitative elemental analysis.
• Atomic Emission Spectrometry: Qualitative and Quantitative elemental analysis.
• Capillary Electrophoresis: Qualitative and Quantitative elemental and molecular analysis.
• Electrochemistry: Qualitative and Quantitative elemental and molecular analysis.
• Gas Chromatography: Qualitative molecular analysis.
• ICP-Mass Spectrometry: Qualitative and Quantitative elemental analysis.
• Infrared Spectroscopy: Qualitative molecular analysis.
• Thermal Analysis: Qualitative molecular analysis.
• UV/Vis Spectrophotometry: Qualitative and Quantitative analysis.

Theory

• Includes concepts and useful equations.

Basic Instrumentation

• Includes the source of radiation, sample holders, monochromator, and detector.

Application

• Interpreting spectra

Electromagnetic Spectrum

• Radiation includes Radio, Microwave, Infrared, Visible, Ultraviolet, X-ray, and Gamma Ray
• The energy of electromagentic radiation can be represented as $E = h\nu = \frac{hc}{\lambda}$
• E is energy
• h is Planck's constant
• $\nu$ (nu) is frequency
• c is the speed of light
• $\lambda$ (lambda) is wave length

Interactions of Radiation with Matter

• Radiation can undergo reflection, scattering, absorption, and transmission.
• Matter can undergo emission

Interactions of Light and Matter

• Absorption and Transmission: Measures Incident and Transmitted light with Atomic and Molecular absorption.
• Absorption then Emission: Measures Emitted light with Atomic/Molecular fluorescence, and Molecular phosphorescence.
• Scattering: Measures Scattered light with Turbidimetry, Nephelometry, and Raman.
• Reflection: Measures Reflected light with Attenuated total reflection and Diffuse reflection IR.
• Emission: Measures Emitted light with Atomic/Molecular emission, and Chemiluminescence.

Interactions of EMR with matter

• Electrons absorb photons and transition to a higher energy level.
• The energy of the absorbed photon should match the energy difference between energy levels.
• This principle applies to both atoms and molecules, atomic absorption spectra are discrete lines or narrow spikes.
• Energy levels for atoms include main energy levels (n=1, 2, 3, etc.) and sub-levels (s, p, d, f, etc.).
• Molecular energy levels include electronic states (based on molecular orbitals), vibrational energy states, and rotational energy states.
• Molecular spectra produce broad absorption bands.

Molecules and Molecular Spectroscopy

• The energy states associated with molecules, like those of atoms, are quantized.
• Absorption spectra are more complex than atomic spectra due to the large number of energy states with rotational, vibrational and electron transitions.
• Rotational energy states are discrete (quantized).
• Absorption of energy causes transitions from lower to higher energy rotational states (molecule rotates faster).
• The rotational energy of a molecule depends on angular velocity and molecular shape/weight distribution.
• Hexane (C6H14) has many possible shapes and rotational energy levels.
• Multiple natural isotopes in a molecule create new rotational energy levels.
• Energies are around 10-24 J per molecule in radiofrequency and microwave regions. Limited to the gas phase, used by radioastronomers to detect chemicals in interstellar clouds.
• Molecules can be visualized as atoms joined by springs (chemical bonds).
• Atoms vibrate or bend, with each vibration has characteristic energy.
• Vibrational energy states are quantized, associated with absorption of radiant energy in the IR region. An increase in vibrational energy means an increased molecular rotation
• Absorption of IR radiation corresponds to changes in rotational and vibrational energies.
• Peaks result in the IR spectrum
• Radiant energy for electronic transitions in molecules lies in the visible and UV regions. Unlike atoms, molecules have rotational and vibrational sublevels, so electron excitation often changes vibrational and rotational energies, with an absorption band resulting.

Types of Spectroscopy Across the Electromagnetic Spectrum

• Radio Waves: Nuclear Magnetic Resonance; Change nuclear orientation
• Microwave: Electron Spin Resonance; Change electron spin orientation
• Infrared: (Pure) Rotational, Vibrational; Change molecular rotational and vibrational states
• Ultraviolet: Electronic; Change atomic or molecular electronic states
• X-Ray: Inner Electronic; Change electronic states or eject electrons
• Gamma Ray: Mössbauer; Changing nuclear energy levels

Emission

• Excitation results from the absorption of radiation, energy transfer due to atomic/molecular collisions, addition of thermal energy, or electrical discharges.

Spectrometer

• Spectrometers utilize unique interactions between light and matter
• Most spectrometers share common elements
• In absorption, a photon is taken in by an atom or molecule.
• In emission, a process produces a photon
• Energy and quantity (intensity) of photons are important.
• Spectrometers are made of similar components, but differ in orientation for emission vs absorption spectroscopy.

Common Components for Emission and Absorption Spectrometers:

• A source of energy or photons: light bulb, lamp, laser, flame, magnetron, or hot ceramic rod.
• A method of differentiation
• A sample that absorbs or emits photons
• A detector: heat absorber, or lightsensitive electronics
• The "method of energy differentiation" is normally performed by a monochromator.
• Prisms, or gratings are used to differentiate the signal

Sources of Radiation

• Must emit radiation over the entire wavelength range
• Intensity must be high enough to avoid extensive signal amplification
• Intensity should not vary significantly at different wavelengths
• Intensity should not fluctuate over long or short time intervals (flicker).
• Continuum sources : emit radiation that changes slowly as a function of wavelength
• Line sources: emit a limited number of lines, or bands of radiation, each of which spans a limited range of wavelengths

Continuum Sources emit a broad range

• UV region: Deuterium lamp is used, discharge causes D2 to dissociate and emit radiation (200-400 nm).
• Visible: Tungsten filament lamp is used, operates at high temperature (3000K), emits radiation in range 320-2500 nm.
• Electric discharge lamps filled with Hg/Xe used in UV-vis spectroscopy.
• Silicon carbide globar and Nernst glower used in OIR(Optical Infrared)

Line Sources:

• Used in AAS, atomic/molecular fluorescence spectroscopy, refractometry, and polarimetry.
• Hollow cathode lamps produce emission lines specific for the element used to construct the cathode.
• A large voltage across anode/cathode ionizes buffer gas, creating a plasma.
• Buffer gas ions accelerate to the cathode, sputtering off atoms from the cathode.
• Sputtered cathode atoms are excited and collide with atoms/particles in plasma.
• Excited atoms decay to lower states and emit photons.
• Electrodeless discharge lamps have the sealed metal of interest in a quartz tube with inert gas.
• RF field is used to excite the gas, which in turn causes the metal to be ionized.
• Lasers are used in IV, VI, and IR with high intensity, coherent light.
• The energy of electromagentic radiation can be represented as $E = h\nu = \frac{hc}{\lambda}$
• E is energy
• h is Planck's constant
• $\nu$ (nu) is frequency
• c is the speed of light
• $\lambda$ (lambda) is wave length
• A narrow bandwidth enhances the sensitivity and selectivity of absorbance measurements.
• Narrow bandwidth is required to obtain a linear relationship between optical signal and concentration.
• Performance improves with narrower bandwidth.

Monochromators

• Angular dispersion of wavelegnths result from refraction for Prisms, diffraction for Gratings
• Components include the entrance slit, colliminating lens or mirror, prism or grating, focusing lens, and exit slit
• Spectral purity is minimum scattered or stray light in exit beam, done with entry/exit windows or light tight housing
• Dispersion ability is to separate WL, differing by Δλ thru difference in ΘR
• Resolving power measures the ability to separate 2 closely spaced peaks with wavelength of $\frac{\lambda}{\Delta \lambda}$
• Light gathering is the efficiency of gathering radiance based on focal length of the collating mirror.
• Filters (absorption and interference) either colored glass and dye or rely on optical interference (caF2 or MgF20
• The grating monochromator produces the angular dispertion results from diffractions. Cheaper to manufacture and better wavelength separation than prism monochromators.

Sample Containers and Optics:

• Cuvettes should be transparent in the wavelength region to be measured.
• Glass and Plastic useful between 380nm - 780nm.
• Fuse Quartz below 380 nm
• Glass, Silica/quartz, and NaCl are used as material components Radiation Transducers convert radiant energy into electrical signals.
• Ideally, they have high sensitivity, high signal-to-noise ratio, a wide wavelength response, and a linear output (S=k·P).
• Radiation Transducers have low dark current (small current when P=0), a compensating circuit is often used.
• Photoelectric Radiation Transducers respond to photons in UV, Vis, near-IR regions
• Thermal Radiation Transducers respond to heat and are used in IR regions.

Photon Transducers:

• Photovoltaic cells generate a current at the interface of a semiconductor and a metal metal-semiconductor-metal sandwiches that produce voltage when irradiated (350-750 nm).
• Phototubes emit radiation and electrons at an angle of 20-100nm
• Photomultiplier tubes (PMT) contain several photo emissive surfaces that emit a cascade of electrons when struck by electrons a series of anodes increase gain to 105-107 electrons for each incident electron
• Photodiote arrays produce create free electrons and no exil slit is required.
• Thermal Detecters: are sensitive to IR and are thermocouples and bolometers