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).