Transcript Optical Electronic Spectroscopy 1
Optical Electronic Spectroscopy 1
Lecture Date: January 23 rd , 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, …)
From http://education.jlab.org
(The Bohr model for nitrogen) (The LUMO of benzene)
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*
Higher energy
E
=
h
n
Lower energy
Other process can be active, such as “non-radiative” relaxation (e.g. transfer of energy by random collisions).
M* M + heat 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:
n i
c
n
i c
n
i n i
= the speed of light (~3.00 x 10 8 m/s) = the velocity of the radiation in the medium in m/s = the refractive index at the frequency
i
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.
Absorption and Transmission
Transmittance: T = P/P 0 P 0 Absorbance: A = -log 10 T = log 10 P 0 /P b 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 = e bc Where the absorbance A has no units, since A = log 10 e is the molar absorbtivity with units of L mol -1 cm -1 P 0 / P 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: Dimers, oligomers Figure from Chapter 5 of Cazes, Analytical Instrumentation Handbook 3 rd Ed. Marcel-Dekker 2005.
Deviations from the Beer-Lambert Law
Deviations caused by use of polychromatic light on a spectrum in which e changes a lot over the bandwidth of the light. Consider two wavelengths a and b with e a and e b e = 1000, 1000 1
A a
b
log
P
0
a
10 e
P
0
a a bc
P
0
b P
0
b
10 e
b bc
e = 1500, 500 0.8
e = 1750, 250 0.6
0.4
0.2
1 0.2
0.4
0.6
Concentration (M)
0.8
Absorption:
Basic Instrument Layout for Optical Spectroscopy
Radiation Source Sample Wavelength Selector Detector (photoelectric transducer)
Fluorescence, Phosphorescence and Scattering:
Sample Wavelength Selector
(90 ° angle)
Radiation source
Emission and chemi-luminescence
Detector (photoelectric transducer) Sample (source) Wavelength Selector Detector (photoelectric transducer)
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
Term symbols and electronic states: used to precisely define the state of electrons
spin multiplicity
2
s
1
l j m j j l s
= total spin quantum number = total angular momentum quantum number = orbital quantum number (s,p,d,f…)
m j
= state
Term: 2 P s,p,d,f,g (l value) Level: 2 P 3/2 2j+1 State: 2 P 3/2 -1/2
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.
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:
N excited N ground
P excited P ground
exp
E excited
E ground kT
E
= energy of state
P N
= number of states having equal energy at each level = 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)
N e /N g
at 2000 K
N e /N g
at 3000 K Na 589.0
Ca 422.7
Zn 213.8
9.9 x 10 -6 1.2 x 10 -7 5.9 x 10 3.7 x 10 -4 -5 7.3 x 10 -15 5.4 x 10 -10 (Values from Cazes pg 79, Table 1)
N e /N g
at 10000 K 2.6 x 10 -1 1.0 x 10 -2 3.6 x 10 -3
Atomic Electronic Energy Levels
The simulated spectrum for the sodium atom
100000 80000 60000 40000 20000 0 0 5000 10000 Wavelength / nm 15000 20000
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 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 Atomizer/ Emission Source Flame Plasma (e.g. ICP) Electric arc Electric spark Temperature ( °C) 1700-3150 4000-8000 4000-5000 >10000 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 An argon ICP torch in action: Photo by
Steve Kvech
, http://www.cee.vt.edu/program_areas/environmental/teach/smprimer/icpms/icpms.htm#Argon%20Plasma/Sample%20Ionization
More on Plasma Torches
Another view of an argon ICP torch: Diagram from Lagalante, Appl. Spect. Reviews. 34, 191 (1999)
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 10 9 – 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.
Typical detection limits (Varian Vista MPX):
Detection Limits of ICP-OES
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.
Na Ni P Pb Rb S Sb Se Sr Sn Ti Tl V Zn
Element Wavelength (nm)
Ag 328.068
Al 396.152
As As Ba Ba Ba Be 188.98
193.696
233.527
455.403
455.403
313.107
Ca Ca Cd Co Cr Cu Fe K Li Mg Mg Mn Mo 396.847
317.933
214.439
238.892
267.716
327.395
238.204
766.491
670.783
279.55
279.8
257.61
202.03
589.59
231.6
177.43
220.35
780.03
181.972
206.83
196.03
407.77
189.93
336.12
190.79
292.4
213.86
Detection Limit axial (ug/L)
0.5
0.9
3 4 0.1
0.03
0.03
0.05
0.01
0.8
0.2
0.4
0.5
0.9
0.3
0.3
0.06
0.05
1.5
0.1
0.5
0.2
0.7
4 1.5
1 4 3 4 0.02
2 0.5
2 0.7
0.2
Detection limit radial (ug/L)
1 4 12 11 0.7
0.15
0.15
0.15
0.3
6.5
0.5
1.2
1 1.5
0.9
4 1 0.1
10 0.133
2 1.5
2.1
25 8 5 13 16 16 0.1
8 1 13 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/H 2 /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 Monochromator Detector P 0 P Sample/Flame 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 (0.02
Å) 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:
hv
Resonance
hv Non-radiative Thermal hv hv Non-radiative
Direct Line Stepwise Thermally-assisted
Atomic Fluorescence
Instrumentation
Sample Wavelength Selector Detector (photoelectric transducer)
(90 ° angle)
Radiation source
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:
Fiber optic Figure from US Army/Ames
Elemental Analysis with Optical Spectroscopy
A comparison of the techniques – the choice is not always clear!
Dynamic Range Qualitative Analysis Multielement Scan?
Plasma Emission (ICP-OES) Wide Good Good AA (Flame) Limited Poor Poor Atomic Fluorescence Wide Poor Poor Trace Analysis Small samples Matrix interferences Spectral interferences Good Good Low High Good Good High Low Good Good 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 Problems Optical Electronic Spectroscopy
Chapter 8: Problem 8-9 Chapter 10: Problem 10-2
Further Reading
Review Skoog et al. Chapters 6-10 Review Cazes Chapters 3-4 Optical Electronic Spectroscopy
H. A. Strobel and W. R. Heineman, “Chemical Instrumentation: A Systematic Approach”, 3 rd Ed., Wiley (1989).