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Optical Electronic Spectroscopy 1
Lecture Date: January 23rd, 2008
The Electromagnetic Spectrum
UV-Visible
X-ray
What is Electronic Spectroscopy?
Spectroscopy of the electrons surrounding an atom or a molecule: electron energy-level transitions
Atoms: electrons are in hydrogen-like orbitals
(s, p, d, f)
Molecules: electrons are in molecular orbitals (HOMO,
LUMO, …)
(The LUMO of benzene)(The Bohr model for nitrogen)
From http://education.jlab.org
Optical Electronic Spectroscopy
Definition: Spectroscopy in the optical (UV-Visible) range involving electronic energy levels excited by electromagnetic radiation (often valence electrons).
This lecture is related to the “high-energy” (“non-optical”) electron spectroscopy covered in the X-ray lecture
Methods:– Atomic absorption
– Atomic emission (e.g ICP-OES)
– Molecular UV-Visible absorption
– Luminescence, Fluorescence, Phosphorescence
Definitions of Electronic Processes
Emission: radiation produced by excited molecules, ions, or atoms as they relax to lower energy levels.
Absorption: radiation selectively absorbed by molecules, ions, or atoms, accompanied by their excitation (or promotion) to a more energetic state.
Luminescence: radiation produced by a chemical reaction or internal electronic process, possibly following absorption.
More Electronic Processes
Fluorescence: absorption of radiation to an excited state, followed by emission of radiation to a lower state of the same multiplicity
– Occurs about 10-5 to 10-8 seconds after photon absorption
Phosphorescence: absorption of radiation to an excited state, followed by emission of radiation to a lower state of different multiplicity
– Occurs about 10 to 10-5 seconds after photon absorption
What is Emission?
Atoms/molecules are driven to excited states (in this case electronic states), which can relax by emission of radiation.
M + heat M*
Other process can be active, such as “non-radiative” relaxation (e.g. transfer of energy by random collisions).
M* M + heat
E = h
Higher energy
Lower energy
OES = Optical Emission Spectroscopy
What is Absorption?
Electromagnetic radiation travels fastest in a vacuum. – When EM radiation travels through a substance, it can be slowed
by propagation “interactions” that do not cause frequency (energy) changes:
Absorption does involve frequency/energy changes, since the energy of EM radiation is transferred to a substance, usually at specific frequencies corresponding to natural atomic or molecular energies
– Absorption occurring at optical frequencies involves low to mid-energy electronic transitions.
ii
c n
c = the speed of light (~3.00 x 108 m/s) i = the velocity of the radiation in the medium in m/sni = the refractive index at the frequency i
Absorption and Transmission
Transmittance:
T = P/P0
b
P0 P
Absorbance:
A = -log10 T = log10 P0/P
A is linear vs. b!(A preferred over T)
Graphs from http://teaching.shu.ac.uk/hwb/chemistry/tutorials/molspec/beers1.htm
The Beer-Lambert Law
The Beer-Lambert Law (a.k.a. Beer’s Law):
A = bcWhere the absorbance A has no units, since A = log10 P0 / P
is the molar absorbtivity with units of L mol-1 cm-1
b is the path length of the sample in cm
c is the concentration of the compound in solution, expressed in mol L-1 (or M, molarity)
Beer’s law can be derived from a model that considers infinitesimal portions of a “block” absorbing photons in their cross-sections, and integration over the entire block
– Beer’s law is derived under the assumption that the fraction of the light absorbed by each thin cross-section of solution is the same
– See pp. 302-303 of Skoog, et al. for details
Deviations From the Beer-Lambert Law
Deviations from Beer’s law (i.e. deviations from the linearity of absorbance vs. concentration):
– Intermolecular interactions at higher concentrations
– Chemical reactions (species having different spectra)
– Peak width/polychromatic radiation Beer’s law is only strictly valid with single-frequency radiation Not significant if the bandwidth of the monochromator is less
than 1/10 of the half-width of the absorption peak at half-height.
For an alternative view, see: Bare, William D. A More Pedagogically Sound Treatment of Beer's Law: A Derivation Based on a Corpuscular-Probability Model, J. Chem. Educ. 2000, 77, 929.
Deviations from the Beer-Lambert Law
Intermolecular interactions at higher concentrations:
Figure from Chapter 5 of Cazes, Analytical Instrumentation Handbook 3rd Ed. Marcel-Dekker 2005.
Dimers,oligomers
Deviations from the Beer-Lambert Law
Deviations caused by use of polychromatic light on a spectrum in which changes a lot over the bandwidth of the light.
Consider two wavelengths a and b with a and b
= 1500, 500
0.2 0.4 0.6 0.8 1
0.2
0.4
0.6
0.8
1
= 1750, 250
Concentration (M)
Ab
sorb
ance
(A
)
bcbbca
ba
ba ba
PP
PP A
1010log
00
00
= 1000, 1000
Basic Instrument Layout for Optical Spectroscopy
Absorption:
RadiationSource
SampleWavelength
SelectorDetector
(photoelectric transducer)
Fluorescence, Phosphorescence and Scattering:
SampleWavelength
SelectorDetector
(photoelectric transducer)
Radiationsource
Emission and chemi-luminescence
Sample(source)
WavelengthSelector
Detector(photoelectric transducer)
(90° angle)
Atomization: The Dividing Line for Atomic/Molecular
Samples used in optical atomic (elemental) spectroscopy are usually atomized
This destroys molecules (if present) and leaves the atoms
The UV-visible spectrum of the atoms is of interest, not the molecular spectrum.
Elemental Analysis
Elemental analysis – qualitative or quantitative determination of the elemental composition of a sample
Optical electronic methods are heavily used in elemental analysis
Other elemental analysis methods not discussed here:– Mass spectrometry (MS), e.g. ICP-MS
– X-ray methods
– Other methods (radiochemical)
– Classical
Atomic Electronic Energy Levels
Electronic energy level transitions in hydrogen – the simplest of all!
Balmer series (visible)– Transitions start
(absorption) or end (emission) with the first excited state of hydrogen
Lyman series (UV)– Transitions start
(absorption) or end (emission) with the ground state of hydrogen
Diagrams from http://csep10.phys.utk.edu/astr162/lect/light/absorption.html
Atomic Electronic Energy Levels
Used to denote energy levels, and label term (Grotrian) diagrams for the hydrogen atom
Figure from the Sapphire Electronic Spectroscopy Software Package, Cavendish Instruments Limited.
Term symbols and electronic states: used to precisely define the state of electrons
2P3/2
s,p,d,f,g (l value)
2P3/2-1/2
jmj
s l12 spin multiplicity
2j+1
s = total spin quantum numberj = total angular momentum quantum numberl = orbital quantum number (s,p,d,f…)mj = state
2PTerm:
Level:
State:
Atomic Electronic Energy Levels
Term symbol (Grotrian) diagram for the sodium atom
Each transition on the diagram can be linked to a peak in the spectrum
The number of lines can approach 5000 for transition-metal elements.
Line broadening can be caused by:
– Doppler effects
– pressure broadening (collisions)
– Lifetime of state (uncertainty)Figure from H. A. Strobel and W. R Heineman, Chemical Instrumentation: A Systematic Approach, Wiley, 1989.
Atomic Electronic Energy Levels The population of energy levels partly determines the
intensity of an emission peak
The Boltzmann distribution relates the energy difference between the levels, temperature, and population:
kT
EE
P
P
N
N groundexcited
ground
excited
ground
excited exp
E = energy of stateP = number of states having equal energy at each levelN = number of atoms in state
Key point: to get more atoms into excited states, you need higher temperatures. (See example 8-2, problem 8-9)
Element/Line (nm) Ne/Ng at 2000 K Ne/Ng at 3000 K Ne/Ng at 10000 K
Na 589.0 9.9 x 10-6 5.9 x 10-4 2.6 x 10-1
Ca 422.7 1.2 x 10-7 3.7 x 10-5 1.0 x 10-2
Zn 213.8 7.3 x 10-15 5.4 x 10-10 3.6 x 10-3
(Values from Cazes pg 79, Table 1)
Atomic Electronic Energy Levels
0 5000 10000 15000 20000
0
20000
40000
60000
80000
100000
Wavelength / nm
Inte
nsity
/ A
rbitr
ary
Uni
tsThe simulated spectrum for the sodium atom
Atomic Emission
Two types of emission spectra:– Continuum
– Line spectra
Examples:– ICP-OES (inductively-coupled
plasma optical emission spectroscopy), also known as ICP-AES
– LIBS (laser-induced breakdown spectroscopy)
Torches and Atomic Emission History: Emission came first (study of sunlight by Fraunhofer in
1817, identification of spectral “lines”), studied throughout the 1800’s and early 1900’s
Atomizer/
Emission Source
Temperature (°C)
Flame 1700-3150
Plasma (e.g. ICP)
4000-8000
Electric arc 4000-5000
Electric spark >10000
Before the use of the plasma for OES in 1964, the flame/gas torch (or arc/spark, etc…) had the following problems:– Temperature instability
– Not hot enough to excite/decompose all materials
Today: The plasma has become the almost universally-preferred method
History: atomic emission placed demands on monochromators
Today: Technology has led to polychromators/detectors with sufficient resolution
Plasma Torches
Plasma: a low-density gas containing ions and electrons, controlled by EM forces
Plasma Torches In the inductively-coupled
plasma (ICP) torch, the sample will reside for several milliseconds at 4000-8000K.
Other torches – direct current plasma
Microwave induced plasma
Photo by Steve Kvech, http://www.cee.vt.edu/program_areas/environmental/teach/smprimer/icpms/icpms.htm#Argon%20Plasma/Sample%20Ionization
An argon ICP torch in action:
More on Plasma Torches
Diagram from Lagalante, Appl. Spect. Reviews. 34, 191 (1999)
Another view of an argon ICP torch:
Arc and Spark Sources for Atomic Emission
Arc and spark sources – used for qualitative analysis of organic and geological samples
– Only semi-quantitative because of source instability
– Spark sources achieve higher energies
Several mg of solid sample is packed between electrodes, 1-30 A of current is passed achieving several hundred volts potential.
Applications include metals analysis or cases where solids must be analyzed.
Atomic Emission: Mono- and Polychromators
Diffraction gratings are used to select wavelengths (in combination with collimating lens, and slits)
Echelle (ladder) gratings: high dispersion and high resolution
– ~1000-1500 grooves/mm typical for UV-Vis work
– Require filters to isolate “orders” (i.e. n=1)
m = d(sin i + sin r)
Atomic Emission: Detectors
At the end of the spectrometer, photons are detected.
Commonly used detectors:
– Photomultiplier tubes (PMT) – dynamic range 109
– Solid-state detectors: Charge-coupled devices (CCD) – 1D or 2D arrays
(charge readout or “transfer” devices) Silicon photodiodes with thousands of individual
elements Very sensitive, very well-suited to echelle grating
polychromators, very fast
Modern ICP-OES Spectrometers
Example system:Varian Vista PRO
Features:1. Axial flame view
2. Echelle grating polychromator
3. CCD detector
CCD chips are often made of sub-arrays matched to emission lines.
Figure from Varian Vista PRO sales literature.
Detection Limits of ICP-OES
Typical detection limits (Varian Vista MPX):
Considerations include the number of emission lines, spectral overlap
Linearity can span several orders of magnitude.
See also Figure 10-13 in Skoog, et al.
Element Wavelength (nm)Detection Limit
axial (ug/L)Detection limit
radial (ug/L)Ag 328.068 0.5 1Al 396.152 0.9 4As 188.98 3 12As 193.696 4 11Ba 233.527 0.1 0.7Ba 455.403 0.03 0.15Ba 455.403 0.03 0.15Be 313.107 0.05 0.15Ca 396.847 0.01 0.3Ca 317.933 0.8 6.5Cd 214.439 0.2 0.5Co 238.892 0.4 1.2Cr 267.716 0.5 1Cu 327.395 0.9 1.5Fe 238.204 0.3 0.9K 766.491 0.3 4Li 670.783 0.06 1
Mg 279.55 0.05 0.1Mg 279.8 1.5 10Mn 257.61 0.1 0.133Mo 202.03 0.5 2Na 589.59 0.2 1.5Ni 231.6 0.7 2.1P 177.43 4 25Pb 220.35 1.5 8Rb 780.03 1 5S 181.972 4 13Sb 206.83 3 16Se 196.03 4 16Sr 407.77 0.02 0.1Sn 189.93 2 8Ti 336.12 0.5 1Tl 190.79 2 13V 292.4 0.7 2Zn 213.86 0.2 0.8
Atomic Absorption – Early History
In the beginning – atomic emission was the only way to do elemental analysis via optical spectroscopy
Bunsen and Kirchhoff (1861) – invented a non-luminous flame to study emission. Showed that alkali elements in the flame removed lines from a continuous source.
Walsh (1955) – notices that molecular spectra are often obtained in absorption (e.g. UV-Vis and IR), but atomic spectra are always obtained in emission. Proposes to use atomic absorption (AA or AAS) for elemental analysis
– Advantages over emission – far less interference, avoids problems with flame temperature
Atomic Absorption (AA) and Elemental Analysis
Atomic absorption spectrometry is one of the most widely used methods for elemental analysis.
Basic principles of AA:– The sample is atomized via:
A flame (methane/H2/acetylene and air/oxygen)
An electrothermal atomizer (an electrically-heated graphite tube or cup)
– UV-Visible light is projected through the flame
– The atoms absorb light (electronic excitation), reducing the beam
– The difference in intensity is measured by the spectrometer
Source
Detector
Sample/Flame
Monochromator
P0
P
Images are of Aurora AI1200, http://www.spectronic.co.uk
Atomic Absorption: Sources
Hollow cathode lamps – sputtering of an element of interest, generating a line emission spectrum:
Typical linewidths of 0.002 nm (0.02Å)
Other AA Sources: electrode-less discharge lamp (EDL) – see Skoog Ch 9B-1
Atomic Absorption: Monochromators The monochromator filters out undesired light in AA
(typical bandwidths are 1 angstrom/0.1 nm)
Unlike ICP-OES, where the mono- or polychromator actually analyzes the frequency.
– In other words – there is no need to scan the grating, just set (aimed through a slit) and run
Echelle (ladder) gratings are popular:
Figure from T. Wang, in J. Cazes, ed, “Ewing’s Analytical Instrumentation Handbook”
Other Features of Atomic Absorption Systems
Sample nebulizers: Produces aerosols of samples to introduce into the flame (oxyacetylene is the hottest)
Detectors: Common examples are photomultiplier tubes, CCD (charge-coupled devices), and many more.
Monochromator: removes emissions from the flame (flame is often kept cool just to avoid emission)
Modulated source (chopper): also removes the remaining emissions from the flame. The signal of interest is given an AC modulation and passed through a high-pass filter.
Spectral interferences:– Absorption from other things (besides the element of interest) –
other flame components, particulates, etc… Scattering can cause similar problems
– Background correction can help
Detection Limits of Atomic Absorption Systems
How Are Elements Actually Analyzed?
For AA and ICP-OES, samples are dissolved or digested into solution.
Samples are flowed into the flame/plasma and analyzed.
Two methods for quantitative analysis:– Standard calibration: the unknown sample’s
absorbance/emission is compared with several references which “bracket” the expected concentration. (Linear relationship)
– Standard addition: the unknown sample is divided into several portions. One portion is directly analyzed, the others have the reference material added in varying amounts. The linear relationship is determined, and the intercept is used to calculate the real concentration of the unknown
At the end: the results yield elements in ppm, ppb, mg/mL, etc…
Atomic Fluorescence
Developed as an alternative to AA and ICP-OES, with potentially greater sensitivity.
– Has not yet achieved widespread use but cheaper tunable lasers may change this.
Laser – stimulated emission (coherent emission from an excited state induced by a second photon)
Processes:
Resonance Direct Line
hv
hv
Non-radiative
Stepwise Thermally-assisted
Non-radiative
hv
Thermal
hv
Atomic Fluorescence
Instrumentation
SampleWavelength
SelectorDetector
(photoelectric transducer)
Radiationsource
(90° angle)
Sources include hollow-cathode lamps, electrodeless discharge tubes (brighter), and lasers (brightest)
Picture from Perkin-Elmer
Laser-Induced Breakdown Spectroscopy (LIBS)
Just like ICP-OES, except a focussed laser creates the plasma:
Figure from US Army/Ames
Fiber optic
Elemental Analysis with Optical Spectroscopy A comparison of the techniques – the choice is not always clear!
Plasma Emission (ICP-OES)
AA (Flame) Atomic Fluorescence
Dynamic Range Wide Limited Wide
Qualitative Analysis Good Poor Poor
Multielement Scan? Good Poor Poor
Trace Analysis Good Good Good
Small samples Good Good Good
Matrix interferences Low High Low
Spectral interferences
High Low Low
Cost Moderate Low Moderate
Speciated analysis: The analysis of atomic “species”, elements in chemically distinguishable environments.
Examples of hyphenation to add “speciation”:
– ICP-OES coupled to a HPLC
– AA coupled to a GC
Homework ProblemsOptical Electronic Spectroscopy
Chapter 8:Problem 8-9
Chapter 10:Problem 10-2
Further Reading
Review Skoog et al. Chapters 6-10Review Cazes Chapters 3-4
Optical Electronic SpectroscopyH. A. Strobel and W. R. Heineman, “Chemical
Instrumentation: A Systematic Approach”, 3rd Ed., Wiley (1989).