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, …)

(The LUMO of benzene)(The Bohr model for nitrogen)

From http://education.jlab.org

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:

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

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 and Quantitative Analysis

The Beer-Lambert Law (a.k.a. Beer’s Law):A = bc

Where 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 cmc 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

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)

Abs

orba

nce

(A)

bcbbca

ba

ba ba

PPPP A

1010log

00

00

= 1000, 1000

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)

Continuous spectrum

Emission spectrum

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*

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

M* M + heat

E = h

Higher energy

Lower energy

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

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.

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:

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 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.

The Simulated UV-Visible Spectrum of Na0

0 5000 10000 15000 20000

0

20000

40000

60000

80000

100000

Wavelength / nm

Inte

nsity

/ Ar

bitra

ry U

nits

From http://www.nist.gov/pml/data/asd.cfm

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:

kTEE

PP

NN 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.

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:

RadiationSource

(Selective spectral lines)

Sample(in torch)

WavelengthSelector

(can be beforesample)

Detector(photoelectric transducer)

Atomic emission

Source(sample in

torch)

WavelengthSelector

Detector(photoelectric transducer)

Wavelength selector 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

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 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

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.

Designs for Monochromators and Polychromators

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)

Paschen-Runge design, shown as a polychromator

Czerny-Turner design, shown as a monochromator

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 view2. 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.

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.

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 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.

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) 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

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

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

etc…

Values from D. A. Skoog, et al., “Principles of Instrumental Analysis,” 5th 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 Q-

switched 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

Figure from J. Anzano et al., “Rapid characterization of analgesic pills by laser-induced breakdown spectroscopy (LIBS),” Med. Chem. Res. 2009, 18, 656–664.

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.

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:

Resonance Direct Line

hv

hv

Non-radiative

Stepwise Thermally-assisted

Non-radiative

hv

Thermal

hv

Atomic Fluorescence Basic AF instrument design:

Sample WavelengthSelector

Detector(photoelectric transducer)

Radiationsource

(90° angle)

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

Picture of HCT lamps from Perkin-Elmer

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 ReadingRequired: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. 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.