Analytical - Spectroscopy

laser

* Based on simulated emission of radiation * Need a population inversion 1. More population in excited state 2. Can't achieve in a two-level system 3. Need three or four quantum states with appropriate lifetimes to get a laser to work 4. Gives high intensity, monochromatic, coherent light

AAS - Flame

* In order to have atomization efficiency must use more volatile elements 1. Low mass, pre Transition Metals, and post TM 2. Do no use elements in groups 1 and 2 because excitation efficiency 3. Do not use groups 1 and 2 and rows 4, 5, and 6 because of ionization efficiency 4. Don't use early TM and main group metals because of oxide formation

Hypsochromic Shift

* shift in λmax to the blue (lower λ, higher E) - Occurs by stabilization of the ground state - Occurs when molecule is placed in a polar solvent - Remember, absorption peak broadens in polar solvents as well - The more polar the solvent, the bigger the effects

Radiation Source - Line Source

- Atomic Emission ONLY TYPES - Low pressure metal vapor lamps * Hg * Na - Hollow cathode lamp (HCL) * Element specific - Laser * Based on simulated emission of radiation * Need a population inversion

Atoms - Absorption lines

- Atoms have only electronic states - Spacing between energy levels is large (UV-Vis) - Atomic spectra are line spectra in the gas phase

Radiation Source - Continuous Source

- Emits over a large range of wavelengths - Examples: 1. IR: Globar and Nichrome wire 2. Visible: W filament lamp 3. UV: Deuterium lamp and Xe arc-lamp 4. High pressure: Hg arc lamp

AAS - Flame/furnace

- Flame/furnace needed to nebulize and atomize solid or liquid sample

Molecules - Absorption lines

- Molecules can also vibrate and rotate - Many more transition available

Monochromator

- Need to select a single wavelength of light from broad-band light sources - Separate light frequencies through dispersion - 3 types: 1. Filters 2. Prism 3. Grating

Simultaneous ICP-AES Detector

- Photodiode Array - PMT - CCD camera

Monochromator - Filters

- absorb unwanted light 1. Polychromator 2. High or low pass * Low pass filter absorbs all wavelengths above cut-off * High pass filter absorbs all wavelengths below cut-off * Notch filter has high and low wavelength cut-offs

Photomultiplier tube (PMT) Detector

- advanced form of phototube; has a series of anodes with amplification at each step

Auxochrome

- auxiliary group that can be added to a chromophore to modify the ability of that chromophore to absorb - when adding an auxochrome, the λmax of the absorbing chromophore is shifted (results from stabilizing p* orbital)

Refraction

- bending of light after passing between two different media 1. Prism monochromators

Diffraction

- bending of light after passing through a small opening 1. X-ray diffraction 2. Grating monochromators

ICP-AES - Atomic Emission Spectroscopy

- colored flame, multiple elements at once

p to p*

- compounds with multiple bonds - λmax ~ 150 - 250 nm - very intense but often E too high (λ too low)

ICP - AES Source

- flame or plasma torch i. Nebulizes sample ii. Metals are atomized and excited in plasma

AAS - Atomic Absorption Spectroscopy

- gas phase atoms - Atomic transitions have a natural linewidth - very narrow (can be affected by doppler and pressure broadening) - Can be affected by Doppler broadening i. Molecules are in constant random motion ii. KE of motion couples to ΔE of transition iii. Blue shift - shift to lower λ iv. Red shift - shift to higher λ v. Peaks get broader - Pressure broadening can occur i. Molecules interact with each other and affect the quantum energy levels

Reflection

- light bounces off of hard surface 1. Mirrors, prisms often used in optics of instruments 2. Attenuated total reflection can be used as spectroscopy technique

n to p*

- lone pair and multiple bonds - λmax ~ 150 - 250 nm - less intense as p to p*, but more accessible

Radiative

- molecule (atom) emits a photon with energy matching the energy gap

Non-radiative

- molecule (atom) relaxes without the emission of a photon - Collisional deactivation - Release energy as heat to solvent

Heat Detectors for IR

- need to measure IR photons 1. Thermocouple 2. Golay Cell

Photodiode Array Detector

- only allows current flow upon hv excitation, no exit slit, measures all wavelengths at once

Chromophore

- part of a molecule that can absorb light and is responsible for color

Detectors - Spectrometry (light)

- photoelectric detectors that convert light to electrons (current) can be measured 1. Photovoltaic 2. Phototube 3. Photomultiplier Tube (PMT) 4. Photodiode Array

Photovoltaic Detector

- photon creates an electric hole pair in the semi conductor and therefore creates a current

Phototube Detector

- powered so that a photon causes the ejection of electrons which are collected at the anode

Golay Cell IR Detector

- pressure measure as temperature changes

n to s*

- saturated organics with lone pairs (H2O, ROH, R2O, RNH2, etc.) - λmax ~ 150 - 250 nm - these molecules are often used as solvents - provides lower limit on wavelength range for analyte (solvent cutoff)

Bathochromic Shift

- shift of λmax to the red (higher λ, lower E) - These shifts occur due to conjugation and resonance (stabilization of p*) - More conjugated double bonds, the redder the absorption - Charges and radicals also cause bathochromic shifts

s to s*

- very high energy (vac, uv), ex. CH4 = 125 nm - not terribly useful analytically

Thermocouple IR Detector

- voltage measure as temperature changes

Monochromator - Prism

- works through refraction of light 1. Separates light by dispersion 2. Angular dispersion through refraction of light - Dispersion * Want all wavelengths to be dispersed equally in space, but prisms don't do this * Separation better at short wavelengths * Fixed slit causes different bandwidths - span of wavelengths exiting the monochromator 4. Materials * UV - quartz * Visible - glass * IR - NaCl or KBr

Radiative emission types - Molecular Emission

1. Absorption 2. Fluorescence - from the same multiplicity 3. Phosphorescence - different multiplicity

Monochromator - Grating

1. Dispersion via diffraction and interference - Diffraction occurs off of a finely spaced grating - Different wavelengths will be in phase at different points along the focal plane - Rotate grating to change which wavelength hits the slit - Normal dispersion (not wavelength dependent)

Non-radiative emission types - Molecular Emission

1. ISC - between states of different multiplicity 2. IC - between states of the same multiplicity 3. VR

UV-Vis Major types of instruments

1. Photometer: no wavelength dispersion 2. Spectrophotomer: wavelength discussion - Single beam - Measure Po before and maintain same Po 3. Double beam Spectrophotometer - One splits the source light in 2 and constantly measures the Po and P separately either with 2 detectors or with a chopper system

Polarization

1. Unpolarized light has propagation vector in random directions 2. Polarized light has all propagation vectors heading in the same direction and the crests/troughs line up

UV-Vis Allowed Transitions

1. s to s* 2. n to s* 3. p to p* 4. n to p*

emission

Excited states relax back down to the ground state

UV-Vis

M + hv : fluorescence and phosphorescence

Scanning ICP-AES Detector

PMT

Molecular Emission Spectroscopy - Quantitative Analysis

a. Beer's Law: Abs = -log(P/P0) = ebc - If one increases P0 one also increases P, so Abs does not change b. With a flurorescence experiment: - F = k'(P0 - P) - k' related to quantum efficiency - F depends on the total number of excited states generated - More P0 means more excited states, means more F

AAS - Furnace

b. Graphite Furnace Atomizer * No nebulizer

Properties of EM Radiation

i. Light travels very fast - 3 X 108 m/s in vacuum ii. Light has properties of both waves and particles - Wave properties: C = λv - Particle properties: E = hv = hc/λ = hcv * V in hcv is wavenumber * V = 1/λ

ICP-AES - Monochromator

i. Scanning - Exit slit used - Computer controls the monochromator ii. Simultaneous detection - No exit slit - Detects multiple wavelengths at once

Molecular Emission Spectroscopy

measures fluoresence more common than phosphorescence a. Typically one or the other C. Aromatic molecules often fluoresce a. p to p* excitations favored b. Polycyclic aromatic hydrocarbons (PAH) fluoresce readily D. The more rigid (symmetric) the molecule, the more it will fluoresce a. Fewer different vibrational modes due to symmetry b. Less chance of accidental overlap with solvent (low kEC)

what are the two types of emission?

radiative and non-radiative