Transcript Atomic UV-Visible Spectroscopy Lecture Date: January 28 , 2013 th

Atomic UV-Visible Spectroscopy
Lecture Date: January 28th, 2013
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 Bohr model for nitrogen)
(The LUMO of benzene)
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
The Electromagnetic Spectrum
 UV-Visible
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)
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.
Continuous spectrum
Emission spectrum
Absorbance spectrum
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:
ni 
c
i
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
 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
Absorption and Transmission
 Transmittance:
T = P/P0
P0
P
 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/chemistry/tutorials/molspec/beers1.htm
The Beer-Lambert Law and Quantitative Analysis
 The Beer-Lambert Law (a.k.a. Beer’s 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)
 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) occur from:
– 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 cause
deviations, because the spectrum changes
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 ea and eb
1
Aa b
Absorbance (A)
0.8
a
b


P

P
0
0


 log a e abc
b
e b bc 
 P 10
 P0 10
 0

e = 1000, 1000
e = 1500, 500
e = 1750, 250
0.6
0.4
0.2
0.2
0.4
0.6
Concentration (M)
0.8
1
Atomic Emission
 Two types of emission
Continuous spectrum
spectra:
– Continuum
– Line spectra
Emission spectrum
 Examples:
– ICP-OES (inductivelycoupled plasma optical
emission spectroscopy), also
known as ICP-AES (atomic
emission spectroscopy)
– LIBS (laser-induced
breakdown spectroscopy)
Absorbance spectrum
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 = h
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
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.
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
2 s 1 m j
spin
lj
multiplicity
s = total spin quantum number
j = total angular momentum quantum number
l = orbital quantum number (s,p,d,f…)
mj = state
Term:
2P
Level:
2P
3/2
State:
2P -1/2
3/2
s,p,d,f,g
(l value)
2j+1
 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.
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/lect/light/absorption.html
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 transitionmetal 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.
The Simulated UV-Visible Spectrum of Na0
Intensity / Arbitrary Units
100000
80000
60000
40000
20000
0
0
5000
10000
Wavelength / nm
From http://www.nist.gov/pml/data/asd.cfm
15000
20000
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:
 Eexcited  Eground 
N excited
Pexcited


exp  
N ground
Pground
kT


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)
Basic Instrument Design for
Atomic UV-Visible Spectrometers
 Atomic absorption:
Radiation
Source
Sample
(in torch)
Wavelength
Selector
Detector
(photoelectric transducer)
 Atomic emission
Source
(sample in
torch)
 Wavelength selector
Wavelength
Selector
Detector
(photoelectric transducer)
is a mono- or polychromator
Sources for 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
Temperature
(°C)
Flame
1700-3150
Plasma (e.g.
ICP)
4000-8000
Electric arc
4000-5000
Electric spark
>10000
Today: The plasma has become the almost universallypreferred method
History: atomic emission placed demands on monochromators
Today: Technology has led to polychromators/detectors with
sufficient resolution
Plasma Torch Sources
 Plasma:
a low-density gas
containing ions and electrons,
controlled by EM forces
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/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.
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)
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. n=1)
Figure from T. Wang, in J. Cazes, ed, “Ewing’s Analytical Instrumentation Handbook”
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
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.
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 subarrays matched to
emission lines.
Figure from Varian Vista PRO sales literature.
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.
Detection Limit
Element Wavelength (nm)
axial (ug/L)
Ag
328.068
0.5
Al
396.152
0.9
As
188.98
3
As
193.696
4
Ba
233.527
0.1
Ba
455.403
0.03
Ba
455.403
0.03
Be
313.107
0.05
Ca
396.847
0.01
Ca
317.933
0.8
Cd
214.439
0.2
Co
238.892
0.4
Cr
267.716
0.5
Cu
327.395
0.9
Fe
238.204
0.3
K
766.491
0.3
Li
670.783
0.06
Mg
279.55
0.05
Mg
279.8
1.5
Mn
257.61
0.1
Mo
202.03
0.5
Na
589.59
0.2
Ni
231.6
0.7
P
177.43
4
Pb
220.35
1.5
Rb
780.03
1
S
181.972
4
Sb
206.83
3
Se
196.03
4
Sr
407.77
0.02
Sn
189.93
2
Ti
336.12
0.5
Tl
190.79
2
V
292.4
0.7
Zn
213.86
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 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
Atomic Absorption Spectroscopy: Instruments
 Atomic absorption spectrometry
is one of the most widely used
methods for elemental analysis.
Source
 Basic principles of AA:
P0
– 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
Sample/Flame
P
Monochromator
Detector
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)
 Single and multi-element lamps are available
 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)
 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, “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
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)
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
AAS
ICP-OES
(Flame)
(Electrothermal)
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,” 5 th Ed., Orlando, Harcourt Brace and Co. 1998, pg. 225.
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 sample’s 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
Laser-Induced Breakdown Spectroscopy (LIBS)
 A focused laser can be used to create a plasma (usually a pulsed Qswitched Nd:YAG 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.
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, 656–664.
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:
hv
Non-radiative
Thermal
hv
Resonance
Non-radiative
Direct Line
hv
Stepwise
hv
Thermally-assisted
Atomic Fluorescence
 Basic AF instrument design:
Sample
Wavelength
Selector
(90° angle)
Radiation
source
 AF sources include hollowcathode lamps,
electrodeless discharge
tubes (brighter), and lasers
(brightest)
Picture of HCT lamps from Perkin-Elmer
Detector
(photoelectric transducer)
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
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, 191207.
Optional:
J. Cazes, Ed. Ewing’s 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/10.1080/05704920903058755.