CHEM 220 Midterm 3

Spectroscopy

Spectroscopy is the interaction b/w EM radiation & matter

Spectrophotometry (subset of spectroscopy) describes the quantitative study of EM spectra, dealing w/ radiation in the visible, UV, and infrared regions.

In this method, sample absorbs EM radiation (light) from appropriate source & amount of light absorbed is related to [ ] of analyte in sol/sample

EM Spectrum

EM radiation possesses wave and particle particles. it consists of an oscillating electric field component (E) and an oscillating magnetic field component (M). E and M are perpendicular to each other and are perpendicular to the direction of propagation

Wave Motion

Wave motion of radiation vibrate perpendicular to the direction of propagation

• wavelength (λ): distance of one complete cycle

• frequency (v): the # of cycles passing a fixed point per unit of time

• wavenumber (v -): reciprocal of λ & # of waves in a unit length or distance per cycle

λ = c / v and v dash - v / c

Absorption vs Emission

Absorption - photons give their energy to e-

• causes excitation of valence e-

• promotes molecule to higher energy level

• an unstable state & e- will fall to ground state

Emission - excited molecule ejects a photon of energy

• decrease in energy and molecule demoted to lower energy level

Transition

Transition is when a molecular is promoted from one energy level to another. There are 3 possible types of transitions

Rotational < Vibrational < Electronic

Spectrophotometer

Used to measure an adsorption or emission spectra. Requires the following:

1. Light source → need a light source that has detectable output of λ radiation in region specific instrument

   1. no light source can emit a single λ of light but rather multiple λ (polychromic)

2. Wavelength sector → restricts the radiation (or light) being measured to a narrow band that is absorbed or emitted by the analyte

   1. enhances selectivity and sensitivity of SPM

3. Sample cell → sample is usually liquid and contained in cuvette that is transparent, visually and also to the λ region being used

4. Detector → detects how much light has been emitted

5. Readout device

Components of wavelength selector

For good analysis, compounds should be analyzed at a λ for max absorbance. Wavelength selector has a few components:

• Lenses or mirrors → focus on radiation

• Entrance & exit slits → limits unwanted radiation

• Dispersing element → separates λ from polychromic radiation

  • 2 types: prism & diffraction grating

Prisms vs Diffraction Gratings

Prisms: when light passes through a prism, it is refracted — bending of light. Index of refraction depends on λ, short λ are refracted more than longer λ.

Prism monochromator

• Focusing lens: source light is directed to entrance slit

• Collimating lens: makes light parallel

Diffraction Gratings are highly polished surfaces w/ a large # of parallel lines, or grooves, in it

• Incident light hits the groove/gratings & id dispersed

• Resolving power is typically better than prisms

Optical Filters

Optical filters isolate specific λs of light w/ 3 types:

• Narrow-bandpass filters → absorb all radiation except desired λ

• Sharp-cutoff filters → absorbs all radiation up to a specific λ, but passes all radiation at longer λ

• Interference filters → filter designed to have destructive to have destructive interference for all radiation striking it, other than for a narrow band of radiation that it transmits

Band

• No λ selector can produce a single λ of radiation as a group of λ or band

• The nominal λ is selected by the instrument; is the λ within the max intensity passed by the exit slit

• The bandwidth is composed of the λ that passed at half the intensity of the normal λ

Suggestions for Sample Reading

• Always check cuvette to make sure you have clear sides and correct path length in light pass

• Do not transfer sample to cuvette w/ glass Pasteur pipette - can scratch walls

• Before inserting cuvette into SPM, wipe down to remove dust

• Use ‘matched’ cuvettes or put a small dot → ensures optical density of cuvette wall is the same each time

• Read samples in random order

Detector

A photon detector produces an electrical signal when struck by photons. Types:

• Phototubes

• Photomultiplier tubes

• Silicon photodiodes

• Photovoltaic cells

Photo-multipler tube

1. Photons enter an evacuated tube

2. Photons strike the cathode

3. E- are emitted & accelerated towards first dynode

4. After striking, more e- are emitted & accelerated

5. Continues until amplified signal reached anode

6. Creates cascade effect w/ 10^6-7 e- collected for each photon entering the tube

Transmittance vs Absorbance

Transmittance: fraction of light than passes through a sample

• T = P / P₀

• P = power of transmitted radiation

• P₀ = power of incident radiation

• T is b/w 0-1 or %

Absorbance: amount of light not passed through a sample

• A = - log (T)

• Is the opposite of transmission but not a linear relationship

• Most accurate spectroscopy will have reading b/w 0.4-0.9

Beers Law

A = ε b c

• A = absorbance (unitless)
  ε = molar absorptivity (L / mol cm)

• b = path length (cm)

• c = [ ] (M)

Limitations: only accurate for dilute sol.

Can be applied to mixtures that have more than 1 compound that absorbs light, but there can be no interactions b/w components.

SPM Titration

• titration where endpoint is found by monitoring the absorption or emission of light

• can plot absorbance vs V of titrant

• as titration is added, A increases

• once all analyte reacted, A plateaus

• should plot corrected A to account for any dilution effect:

A_corrected = (V_t / V_i) x A_observed

Molecular absorption

• at room temp, molecule is in ground state S0 (e- paired + opposite spin)

• when photon is absorbed, it gets excited and promoted to higher energy singlet excited state (e.g. S1, S2)

  • e- spins are antiparallel

• or e- can go to triplet excited state (e.g. T₁); spins are parallel

• to return to ground state, fluorophone can take nonradiative or radiative pathways

Nonradiative Pathways

Vibrational relaxation: molecule in an excited vibrational energy level loses energy & goes to lower vibration energy level within the SAME electronic state

• same electronic state + same spin

Internal conversion: molecule moves from a higher electronic energy level to a lower energy level

• diff states + same spin (e.g. S₂ → S₁)

Intersystem crossing: e- moves from higher electronic energy level to lower energy e- state that has a diff spin

• diff electronic state + diff spin

External conversion: when absorbed energy from photon is transferred to the solv or part of the sample matrix

• enegery transferred to surr

Note: combo of VR + ICV can convert entire energy of photon to heat, meaning no radiation given off on path block to ground state

Radiative Pathways

Fluorescence: emission of a photon as the fluorophore goes from the lowest vibration energy level of an excited state to a lower energy state

• both states have the same spin (e.g. S1 → S0)

• radiation stops as soon as light source is removed/shut off

Phosphorescence: emission of a photon as the fluorophore goes from lowest vibrational energy of an excited state to a lower energy state

Excitation vs Emission Spectra

Excitation Spectra: fluorescent intensity measured w/ constant emission λ selected & varying excitations λs

Emission Spectra: fluorescent intensity measured w/ constant excitation λ selected & varying excitations λs

Atomic Spectroscopy

• Basic concept: atomize samples by heating to high temps, and then observe either the light that is emitted or absorbed

• elemental e- promoted from the ground state, but ONLY electronic transitions

• Energy absorbed is quantified via λ and intensity monitoring to produce a spectrum - either emission or absorption of radiation

Atomic Emission Spectroscopy (AES)

• Sample is heated enough to break molecular bands and create free atoms

• Valence e- are promoted to hgiehr, unstable energy levels

• Excited e- fall back to lower, or ground, energy states, releasing the absorbed energy in the form of light

• Emitted light passes through a monochromator

• Qualitative Analysis = elements have unique “fingerprints” of specific λ of emitted

• Quantitative analysis = intensity of light corresponds to how much of the element is present

Atomic Absorption Spectroscopy (AAS)

• Sample is atomized to break chemical bonds and create unexcited, ground-state atoms

• Need to install a specific ligth source, such as hollow cathode lamp, that matches the exact element to be measured

• While lamp is on, it will measure that compound and how much light is passed through to detector

• If element is found in 2+ diff forms in sample, AAS will only tell the total amount of compound, not how much was from each species

• Can quantify by using calibration standards

Atomic SPM Components

• Light source (for AAS)

• Atomizer

• Monochromator

• Detector

Flame Atomizer

• Uses a premix burner to combine sample, oxidant, and fuel before going to flame

• Sample drawn into nebulizer by high flow of oxidant (e.g. air)

  • leaves tips as fine mist

• Spray is directed towards glass bead

  • small droplets hit it and become smaller

• Aerosol travels past baffles which promote mixing and blocks large droplets

• Only ~5% of initial sample actually reaches flame

Furnace Atomizer

Steps of Furnace Atomizer

1. Drying → sample heating to ~125C for ~20s to remove any solv

2. Charring → furnace heated to ~1400C for about 60s to destroy any organic matter; known as pyrolysis

3. Atomizations → furnace heated to ~2100C for less than 10s; samples atomized and absorbance measured

4. Vaporization → furnace heated to ~2500C for a few secs to vaporize any remainig residue