Atomic UV-Visible Spectroscopy

1
Atomic UV-Visible Spectroscopy
Lecture Date January 28th, 2013
2
Electronic Spectroscopy

• Spectroscopy involving energy level transitions
  of the electrons surrounding an atom or a molecule

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 LUMO of benzene)
(The Bohr model for nitrogen)
3
UV-Visible Spectroscopy

• Definition Spectroscopy in the optical
  (UV-Visible) range involving electronic energy
  levels excited by electromagnetic radiation
  (often valence electrons).
• Techniques discussed in this lecture are related
  to the high-energy (non-optical) methods
  covered in the X-ray spectroscopy lecture.
• Methods discussed in this lecture
• Atomic absorption
• Atomic emission
• Laser induced breakdown spectroscopy
• Atomic fluorescence

4
The Electromagnetic Spectrum

• UV-Visible

5
Elemental Analysis

• Elemental analysis qualitative or quantitative
  determination of the elemental composition of a
  sample
• Atomic UV-visible spectroscopic methods are
  heavily used in elemental analysis
• Other elemental analysis methods not discussed
  here
• Mass spectrometry (MS), primarily ICP-MS
• X-ray methods (XRF, SEM/EDXA, Auger spectroscopy,
  XPS, etc)
• Radiochemical or radioisotope methods
• Classical methods (e.g. color tests, titrations)

6
Definitions of Electronic Processes

• Absorption radiation selectively absorbed by
  molecules, ions, or atoms, accompanied by their
  excitation (or promotion) to a more energetic
  state.
• Emission radiation produced by excited
  molecules, ions, or atoms as they relax to lower
  energy levels.

7
The Absorption Process

• 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 moderate energy electronic
  transitions

c the speed of light (3.00 x 108 m/s) ?i
the velocity of the radiation in the medium in
m/s ni the refractive index at the frequency i
8
Absorption and Transmission

• Transmittance
• T P/P0

• Absorbance
• A -log10 T log10 P0/P

b
A is linear vs. b! (A preferred over T)
Graphs from http//teaching.shu.ac.uk/hwb/chemistr
y/tutorials/molspec/beers1.htm
9
The Beer-Lambert Law and Quantitative Analysis

• The Beer-Lambert Law (a.k.a. Beers Law)
• A ebc
• Where the absorbance A has no units, since A
  log10 P0 / P
• e 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)
• Beers 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
• Beers 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

10
Deviations From the Beer-Lambert Law

• Deviations from Beers law (i.e. deviations from
  the linearity of absorbance vs. concentration)
  occur from
• Intermolecular interactions at higher
  concentrations
• Chemical reactions (species having different
  spectra)
• Peak width/polychromatic radiation
• Beers 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-Probabil
ity Model, J. Chem. Educ. 2000, 77, 929.
11
Deviations from the Beer-Lambert Law

• Intermolecular interactions at higher
  concentrations cause deviations, because the
  spectrum changes

Dimers, oligomers
Figure from Chapter 5 of Cazes, Analytical
Instrumentation Handbook 3rd Ed. Marcel-Dekker
2005.
12
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 ?a and ?b

? 1000, 1000
? 1500, 500
? 1750, 250
Absorbance (A)
Concentration (M)
13
Atomic Emission

• Two types of emission spectra
• Continuum
• Line spectra
• Examples
• ICP-OES (inductively-coupled plasma optical
  emission spectroscopy), also known as ICP-AES
  (atomic emission spectroscopy)
• LIBS (laser-induced breakdown spectroscopy)

14
The Emission Process

• 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 hn
Lower energy

• Other process can happen instead of emission,
  such as non-radiative relaxation (e.g. transfer
  of energy by random collisions).

M ? M heat
15
Atomization The Dividing Line for Atomic and
Molecular Optical Electronic Spectroscopy

• Samples used in optical atomic (elemental)
  spectroscopy are usually atomized
• This destroys molecules (if present) and leaves
  just atoms and atomic ions
• The UV-visible spectrum of the atoms is of
  interest, not the molecular spectrum.

16
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/l
ect/light/absorption.html
17
Atomic Electronic Energy Levels

• Term symbols and electronic states used to
  precisely define the state of electrons

spin multiplicity
s total spin quantum number j total
angular momentum quantum number l orbital
quantum number (s,p,d,f) mj state
s,p,d,f,g (l value)
2P
Term
2P3/2
Level
2j1
2P3/2-1/2
State

• Used to denote energy levels, and label Grotrian
  (or term) diagrams for the hydrogen atom

Figure from the Sapphire Electronic Spectroscopy
Software Package, Cavendish Instruments Limited.
18
Energy Levels for Different Atoms

• Atomic absorption and emission are generally
  selective and specific for different elements on
  the periodic table, allowing for qualitative
  identification of elements

Diagrams from http//csep10.phys.utk.edu/astr162/l
ect/light/absorption.html
19
Atomic Electronic Energy Levels

• Term (Grotrian) diagram for the sodium atom
  each transition on the diagram can be linked to a
  peak in the UV-visible 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.
20
The Simulated UV-Visible Spectrum of Na0
From http//www.nist.gov/pml/data/asd.cfm
21
Intensity of 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

E energy of state P number of states
having equal energy at each level N number of
atoms in state

• Key point to get more atoms into excited states,
  you need higher temperatures.

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)
22
Basic Instrument Design for Atomic UV-Visible
Spectrometers

• Atomic absorption

Radiation Source (Selective spectral lines)
Sample (in torch)
Wavelength Selector (can be before sample)
Detector (photoelectric transducer)

• Atomic emission

Source (sample in torch)
Wavelength Selector
Detector (photoelectric transducer)

• Wavelength selector is a mono- or polychromator

23
Sources for Atomic Emission

• History Emission came first (study of sunlight
  by Fraunhofer in 1817, identification of spectral
  lines), studied throughout the 1800s and early
  1900s

• 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 Temperature (C)
Flame 1700-3150
Plasma (e.g. ICP) 4000-8000
Electric arc 4000-5000
Electric spark gt10000

• Today The plasma has become the almost
  universally-preferred method
• History atomic emission placed demands on
  monochromators
• Today Technology has led to polychromators/detect
  ors with sufficient resolution

24
Plasma Torch Sources

• Plasma a low-density gas containing ions and
  electrons, controlled by EM forces

25
Plasma Torch Sources

• In the inductively-coupled plasma (ICP) torch,
  the sample will reside for several milliseconds
  at 4000-8000K.
• Other designs direct current plasma, microwave
  induced plasma

• An argon ICP torch in action

Photo by Steve Kvech, http//www.cee.vt.edu/progr
am_areas/environmental/teach/smprimer/icpms/icpms.
htmArgon20Plasma/Sample20Ionization
26
More on Plasma Torches

• Another view of an argon ICP torch

Diagram from Lagalante, Appl. Spect. Reviews.
34, 191 (1999)
27
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.

28
Designs for Monochromators and Polychromators
Paschen-Runge design, shown as a polychromator
Czerny-Turner design, shown as a monochromator

• Polychromators
• High sample throughput rate
• Spectral interference can be an issue if the
  interfering spectral line is not included on the
  detector array
• Monochromators
• Flexibility to access any wavelength within the
  dimensions of the monochromator
• Good for applications requiring complex
  background corrections
• Less sensitive lower radiation throughput
  (because light blocked by slits)

29
Atomic Emission Diffraction Gratings

• Diffraction gratings are used to select
  wavelengths (in combination with collimating
  lens, and slits)
• Echelle (ladder) gratings high dispersion and
  high resolution (a two-step system with a
  cross-disperser standard grating or prism)
• 1000-1500 grooves/mm typical for UV-Vis work
• Require filters to isolate orders (i.e. n1)

Figure from T. Wang, in J. Cazes, ed, Ewings
Analytical Instrumentation Handbook
30
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
  addressable elements
• These detectors are very sensitive, very
  well-suited to 2D echelle grating polychromators,
  very fast

31
Example Detector Photomultiplier Tubes

• A PMT is a vacuum tube that contains a
  photosensitive material, called the photocathode
• The photocathode ejects electrons when it is
  struck by light. These ejected electrons are
  accelerated towards a dynode which ejects two to
  five secondary electrons for every electron that
  strikes its surface.
• The secondary electrons strike another dynode,
  ejecting more electrons which strike yet another
  dynode, and so on (electron multiplication).
• The electrical current measured at the anode is
  then used as a relative measure of the intensity
  of the radiation reaching the PMT.

32
Modern ICP-OES Spectrometers

• Example system
• Varian Vista PRO
• Features
• 1. Axial flame view
• 2. Echelle grating polychromator (note the design
  is like a Czerny-Turner monochromator)
• 3. CCD detector
• CCD chips are made of sub-arrays matched to
  emission lines.

Figure from Varian Vista PRO sales literature.
33
Detection Limits of ICP-OES

• Typical detection limits (for a Varian Vista MPX)
• Considerations include the number of emission
  lines, spectral overlap
• Linearity can span several orders of magnitude.

34
Atomic Absorption Spectroscopy (AAS)

• 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

35
Atomic Absorption Spectroscopy Instruments

• 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
P0
Sample/Flame
P
Monochromator
Detector
Images are of Aurora AI1200, http//www.spectronic
.co.uk
36
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)
• Single and multi-element lamps are available
• Other AA Sources electrode-less discharge lamp
  (EDL) see Skoog Ch 9B-1

37
Atomic Absorption Monochromators

• The monochromator filters out undesired light in
  AA (typical bandwidths are 1 angstrom/0.1 nm)
• This differs from ICP-OES, where the
  monochromator 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 (combined with a
  cross-disperser) are popular

Figure from T. Wang, in J. Cazes, ed, Ewings
Analytical Instrumentation Handbook
38
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

39
Graphite Furnace and Hydride AAS

• Graphite furnace and electrothermal AAS
• Analyze solutions, solids, slurries, by placing a
  small amount (uL) of sample on a support for
  evaporation and them atomization
• More efficient atomization (entire sample
  atomized at once) leads to smaller sample
  quantity requirements or better sensitivity, but
  reproducibility can be an issue

• Hydride generation AAS
• Efficiently volatilizes hydride forming elements
  (As, Se, Tl, Pb, Bi, Sb, Te) by making their
  hydrides via pre-reaction with sodium borohydride
  and HCl
• Inexpensive method of increasing sensitivity of
  an AAS to ppt levels for these elements
• Mercury cold-vapor AAS (Hg only)

40
Detection Limits of Atomic Absorption Systems

• Detection limits in ppb (µg/L) for a selection of
  elements
• Individual results can vary depending on system,
  matrix, etc

AAS (Flame) AAS (Electrothermal) ICP-OES
Al 30 0.005 2
As 100 0.02 40
Cd 1 0.0001 2
Hg 500 0.1 1
Mg 0.1 0.00002 0.05
Pb 10 0.002 2
Sn 20 0.1 30
Zn 2 0.00005 2
Values from D. A. Skoog, et al., Principles of
Instrumental Analysis, 5th Ed., Orlando,
Harcourt Brace and Co. 1998, pg. 225.
41
How Are Elements Actually Analyzed?

• For AA and ICP-OES, samples are dissolved or
  digested into solution, flowed into the
  flame/plasma and analyzed.
• Two methods for quantitative analysis
  calibration
• Standard calibration the unknown samples
  absorbance/emission is compared with several
  references which bracket the expected
  concentration assuming a 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.
• Speciated analysis may be needed. The analysis of
  atomic species, elements in chemically
  distinguishable environments, usually by
  hyphenation (e.g. ICP-OES coupled to a HPLC, AA
  coupled to a GC) or offline extraction.
• At the end the results yield elements in ppm,
  ppb, mg/mL, or below LOQ or LOD

42
Laser-Induced Breakdown Spectroscopy (LIBS)

• A focused laser can be used to create a plasma
  (usually a pulsed Q-switched NdYAG laser)

• Portable systems capable of standoff analysis are
  now available applications in the detection of
  explosives, chemical warfare agents,
  environmental analysis, etc

Figure from D. A. Cremers, R. C. Chinni,
Laser-induced breakdown spectroscopy -
Capabilities and limitations, Appl. Spectrosc.
Rev., 2009, 44, 457-506, http//dx.doi.org/10.1080
/05704920903058755.
43
A Typical LIBS Spectrum

• The LIBS spectrum of ibuprofen drug substance

• Emission lines used for C, H, O, and N analysis
  were 247.9, 656.3, 777.2 (triplet), and 746.8 nm,
  along with the molecular band of C2 at 516 nm.

Figure from J. Anzano et al., Rapid
characterization of analgesic pills by
laser-induced breakdown spectroscopy (LIBS),
Med. Chem. Res. 2009, 18, 656664.
44
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 that emit a fluorescent photon

Non-radiative
hv
Thermal
hv
Non-radiative
hv
hv
Resonance
Direct Line
Stepwise
Thermally-assisted
45
Atomic Fluorescence

• Basic AF instrument design

Sample
Wavelength Selector
Detector (photoelectric transducer)
Radiation source
(90 angle)

• AF sources include hollow-cathode lamps,
  electrodeless discharge tubes (brighter), and
  lasers (brightest)

Picture of HCT lamps from Perkin-Elmer
46
A Comparison of Atomic Fluorescence with Other
Techniques
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 (100K USD) Low (50K USD) Moderate
47
Further Reading

• Required
• A. F. Lagalante, Atomic absorption spectroscopy
  A tutorial review. Appl. Spectrosc. Rev. 1999,
  34, 173-189.
• A. F. Lagalante, Atomic emission spectroscopy
  A tutorial review. Appl. Spectrosc. Rev. 1999,
  34, 191-207.
• Optional
• J. Cazes, Ed. Ewings Analytical Instrumentation
  Handbook, 3rd Edition, 2005, Marcel Dekker,
  Chapters 3 and 4.
• D. A. Skoog, F. J. Holler and S. R. Crouch,
  Principles of Instrumental Analysis, 6th Edition,
  2006, Brooks-Cole, Chapters 8, 9, and 10.
• N. Lewen, The use of atomic spectroscopy in the
  pharmaceutical industry for the determination of
  trace elements in pharmaceuticals, J. Pharm.
  Biomed. Anal. 2011, 55, 653-661,
  http//dx.doi.org/10.1016/j.jpba.2010.11.030.
• H. A. Strobel and W. R. Heineman, Chemical
  Instrumentation A Systematic Approach, 3rd
  Ed., Wiley (1989).
• D. A. Cremers, R. C. Chinni, Laser-induced
  breakdown spectroscopy - Capabilities and
  limitations, Appl. Spectrosc. Rev., 2009, 44,
  457-506, http//dx.doi.org/10.1080/057049209030587
  55.