Optical Optical AtomicAtomic SpectroscopySpectroscopy

Optical SpectrometryOptical Spectrometry AbsorptionAbsorption EmissionEmission FluorescenceFluorescence

Mass SpectrometryMass Spectrometry X-Ray SpectrometryX-Ray Spectrometry

Optical Atomic Optical Atomic SpectroscopySpectroscopy

Atomic spectra: single external electronAtomic spectra: single external electron

Slightly different in energy

Atomic spectrum MgAtomic spectrum Mg

Singlet ground state Triplet excited stateSinglet excited state

Spins are pairedNo split

Spins are unpairedEnergy splitting

Atomic spectroscopyAtomic spectroscopy

EmissionEmission

AbsorptionAbsorption

FluorescenceFluorescence

Line Broadening Line Broadening Uncertainty EffectsUncertainty Effects

Heisenberg uncertainty principle: Heisenberg uncertainty principle: The nature of the matter places limits The nature of the matter places limits on the precision with which certain on the precision with which certain pairs of physical measurements can be pairs of physical measurements can be made.made.

One of the important forms One of the important forms Heisenberg Heisenberg uncertainty principle:uncertainty principle:

tt ≥ 1 ≥ 1 p156p156

To determineTo determine with negligibly small uncertainty, a huge with negligibly small uncertainty, a huge measurement time is required.measurement time is required.

Natural line widthNatural line width

7Douglas A. Skoog, Douglas A. Skoog, et al.et al. Principles of Instrumental Analysis, Principles of Instrumental Analysis, ThomsonThomson, , 20072007

Superposition of tw sinusoidal wave of different frequencies but identical amplitudes.

n Should be

Line Broadening Line Broadening

Doppler broadeningDoppler broadening Doppler shift: Doppler shift:

The wavelength of radiation emitted The wavelength of radiation emitted or absorbed by a rapidly moving atom or absorbed by a rapidly moving atom decreases if the motion is toward a decreases if the motion is toward a transducer, and increases if the transducer, and increases if the motion is receding from the motion is receding from the transducer.transducer.

In flame, Doppler broadening is In flame, Doppler broadening is much larger than natural line much larger than natural line widthwidth

Line Broadening Line Broadening Doppler broadeningDoppler broadening

Line Broadening Line Broadening

Pressure broadeningPressure broadening

Caused by collisions of the Caused by collisions of the emitting or absorbing species emitting or absorbing species with other ions or atomswith other ions or atoms

High pressure Hg and xenon High pressure Hg and xenon lamps, lamps, continuum spectracontinuum spectra

Temperature Effects Temperature Effects

Bolzmann equationBolzmann equation

Effects on AAS, AFS, and Effects on AAS, AFS, and AESAES

)exp(00 kT

E

g

g

N

N jjj

)exp(00 kT

E

g

g

N

N jjj

Atomic spectroscopyAtomic spectroscopy

Interaction of an Interaction of an atomatom in the in the gas phasegas phase with EMR with EMR

Samples are solids, liquids Samples are solids, liquids and gases but usually not and gases but usually not ATOMS!ATOMS!

Atomic Atomic SpectroscopySpectroscopySample IntroductionSample IntroductionFlameFlameFurnaceFurnaceICPICP

Sources for Atomic Sources for Atomic Absorption/FluorescenceAbsorption/Fluorescence

Hollow Cathode LamsHollow Cathode LamsSources for Atomic EmissionSources for Atomic Emission

FlamesFlamesPlasmasPlasmas

Wavelength Separators + Slits Wavelength Separators + Slits +Detectors+Detectors

How to get things to How to get things to atomize?atomize?

How to get samples into How to get samples into the instruments?the instruments?

Sample IntroductionSample Introduction

Pneumatic NebulizersPneumatic Nebulizers Break the sample solution into small Break the sample solution into small

droplets.droplets. Solvent evaporates from many of the Solvent evaporates from many of the

droplets.droplets. Most (>99%) are collected as wasteMost (>99%) are collected as waste The small fraction that reach the The small fraction that reach the

plasma have been de-solvated to a plasma have been de-solvated to a great extent.great extent.

What is a nebulizer?What is a nebulizer?

SAMPLE AEROSOL

Concentric Tube

Cross-flow

Fritted-disk

Babington

What happens inside the What happens inside the flame?flame?

FLAMESFLAMES

Rich in free atoms

FLAMESFLAMES

T E

GOOD AND BAD GOOD AND BAD THINGSTHINGS

oxidation

(E/RT)-

oo

e ) g

*g(

N

*N

Boltzmann Equation: Relates Boltzmann Equation: Relates Excited State Population/Ground Excited State Population/Ground

State Population Ratios to Energy, State Population Ratios to Energy, Temperature and DegeneracyTemperature and Degeneracy

Samples are nebulized (broken into small droplets) Samples are nebulized (broken into small droplets) as they enter the spray chamber via a wire capillaryas they enter the spray chamber via a wire capillary

Only about 5% reach the flameOnly about 5% reach the flame

Larger droplets are collectedLarger droplets are collected

Some of the solvent evaporatesSome of the solvent evaporates

–Flow spoilersFlow spoilers

»Cheaper, somewhat more ruggedCheaper, somewhat more rugged

–Impact beadsImpact beads

»Generally greater sensitivityGenerally greater sensitivity

Flame AAS/AESFlame AAS/AES Spray Spray Chamber/Burner ConfigurationsChamber/Burner Configurations

ElectroThermal AAS (ETAAS or ElectroThermal AAS (ETAAS or GFAAS)GFAAS)

The sample is contained in a heated, graphite The sample is contained in a heated, graphite furnace.furnace.

The furnace is heated by passing an electrical The furnace is heated by passing an electrical current through it (thus, it is electro thermal). current through it (thus, it is electro thermal).

To prevent oxidation of the furnace, it is sheathed in To prevent oxidation of the furnace, it is sheathed in gas (Ar usually)gas (Ar usually)

There is no nebulziation, etc. The sample is There is no nebulziation, etc. The sample is introduced as a drop (usually 5-20 uL), slurry or introduced as a drop (usually 5-20 uL), slurry or solid particle (rare)solid particle (rare)

The furnace goes through several steps…The furnace goes through several steps…

Drying (usually just above 110 deg. C.)Drying (usually just above 110 deg. C.) Ashing (up to 1000 deg. C)Ashing (up to 1000 deg. C) Atomization (Up to 2000-3000 C)Atomization (Up to 2000-3000 C) Cleanout (quick ramp up to 3500 C or so). Waste is blown Cleanout (quick ramp up to 3500 C or so). Waste is blown

out with a blast of Ar.out with a blast of Ar.

The light from the source (HCL) passes through the The light from the source (HCL) passes through the furnace and absorption during the atomization step furnace and absorption during the atomization step is recorded over several seconds. This makes is recorded over several seconds. This makes ETAAS more sensitive than FAAS for most ETAAS more sensitive than FAAS for most elements.elements.

ElectroThermal AAS (ETAAS or ElectroThermal AAS (ETAAS or GFAAS)GFAAS)

Hollow Cathode LampHollow Cathode Lamp

Conventional HCLConventional HCL

Radiation Sources for Radiation Sources for AASAAS

Ne or Ar at 1-5 Torr

Hollow Cathode Lamp (Cont’d)Hollow Cathode Lamp (Cont’d)

a tungsten anode and a cylindrical cathodeneon or argon at a pressure of 1 to 5 torrThe cathode is constructed of the metal whose spectrum is desired or served to support a layer of that metal

Ionize the inert gas at a potential of ~ 300 VGenerate a current of ~ 5 to 15 mA as ions and electrons migrate to the electrodes.

The gaseous cations acquire enough kinetic energy to dislodge some of the metal atoms from the cathode surface and produce an atomic cloud.A portion of sputtered metal atoms is in excited states and thus emits their characteristic radiation as they return to the ground sateEventually, the metal atoms diffuse back to the cathode surface or to the glass walls of the tube and are re-deposited

Hollow Cathode Lamp (Cont’d)Hollow Cathode Lamp (Cont’d)

High potential, and thus high currents High potential, and thus high currents lead to greater intensitieslead to greater intensities

Doppler broadening Doppler broadening of the emission of the emission lines from the lamplines from the lamp

Self-absorption:Self-absorption: the greater currents the greater currents produce an increased number of produce an increased number of unexcitedunexcited atoms in the cloud. The atoms in the cloud. The unexcited atoms, in turn, are capable unexcited atoms, in turn, are capable of absorbing the radiation emitted by of absorbing the radiation emitted by the the excitedexcited ones. This self-absorption ones. This self-absorption leads to lowered intensities, particular leads to lowered intensities, particular at the center of the emission bandat the center of the emission band

Doppler broadeningDoppler broadening ? ?

Improvement…….Improvement……. Most direct method of obtaining Most direct method of obtaining

improved lamps for the emission of more improved lamps for the emission of more intense atomic resonance lines is to intense atomic resonance lines is to separate the two functions involving the separate the two functions involving the production and excitation of atomic production and excitation of atomic vapor vapor

Boosted discharge hollow-cathode lamp Boosted discharge hollow-cathode lamp (BDHCL) is introduced as an AFS (BDHCL) is introduced as an AFS excitation source by Sullivan and Walsh.excitation source by Sullivan and Walsh.

It has received a great deal of attention It has received a great deal of attention and a number of modifications to this and a number of modifications to this type of source have been conducted. type of source have been conducted.

Boosted discharge hollow-cathode lamp Boosted discharge hollow-cathode lamp (BDHCL)(BDHCL)

Operation principle of BDHCLOperation principle of BDHCL

A secondary discharge (boost) is struck A secondary discharge (boost) is struck between an efficient electron emitter between an efficient electron emitter and the anode, passing through the and the anode, passing through the primary atom cloud.primary atom cloud.

The second discharge The second discharge does not produce does not produce too much extra atom vaportoo much extra atom vapor by by sputtering the walls of the hollow sputtering the walls of the hollow cathode, but does increase significantly cathode, but does increase significantly the the efficiency in the excitationefficiency in the excitation of of sputtered atom vapor. sputtered atom vapor.

This greatly reduces the self-This greatly reduces the self-absorption resulting from simply absorption resulting from simply increasing the operating potential increasing the operating potential (increase Doppler broadening and self-(increase Doppler broadening and self-absorption) to the primary anode and absorption) to the primary anode and cylindrical cathode. cylindrical cathode.

Electrodeless Discharge Lamps Electrodeless Discharge Lamps (EDL)(EDL)

Electrodeless discharge lamps (EDL) Electrodeless discharge lamps (EDL)

Constructed from a sealed quartz tube Constructed from a sealed quartz tube containing a few torr of an inert gas such as containing a few torr of an inert gas such as argon and a small quantity of the metal of argon and a small quantity of the metal of interest (or its salt).interest (or its salt).

The lamp does not contain an electrode but The lamp does not contain an electrode but instead is energized by an instead is energized by an intense field of intense field of radio-frequency or microwave radiationradio-frequency or microwave radiation. .

Radiant intensities usually one or two orders Radiant intensities usually one or two orders of magnitude greater than the normal HCLs.of magnitude greater than the normal HCLs.

The main drawbacks: their performance does The main drawbacks: their performance does not appear to be as reliable as that of the HCL not appear to be as reliable as that of the HCL lamps (signal instability with time) and they lamps (signal instability with time) and they are only commercially available for some are only commercially available for some

elements.elements.

Single-beam design

DOUBLE BEAM FAA DOUBLE BEAM FAA SPECTROMETERSPECTROMETER

Note: the Ref bean does not passthrough the flame thus does not correct for the interferences from the flame!

synchronized

Spectral InterferencesSpectral Interferences OverlappingOverlapping Broadening absorption for air/fuel Broadening absorption for air/fuel

mixturemixture Scattering or absorption by sample Scattering or absorption by sample

matrixmatrix

Interferences in AAS Interferences in AAS and AFSand AFS

Background CorrectionBackground Correction

Two-line Correction (like Internal Two-line Correction (like Internal Standard)Standard)

Continuum-Source CorrectionContinuum-Source Correction Zeeman EffectZeeman Effect Source Self-Reversal (Smith –Source Self-Reversal (Smith –

Hieftje) Hieftje)

Continuum-Source Continuum-Source CorrectionCorrection

Continuum-Source Continuum-Source CorrectionCorrection

0.04 nm

A B

The light from the HCL is absorbed by both the sample and the background, but the light from the D2 lamp is absorbed almost entirely by the background

A: HCL lamp, the shaded portion shows the light absorbed from the HCL. The emission has a much narrower line width than the absorption line.

B: D2 lamp, the shaded portion shows the light absorbed by D2 lamp. The lamp emission is much broader than the sample absorption, and an averaged absorbance taken over the whole band pass of the monochromator.

(The draw is not to scale)

Zeeman Effect Background Zeeman Effect Background Correction:Correction:

Source Self-Reversal (Smith –Source Self-Reversal (Smith –Hieftje) Hieftje)

A relative new technique

Self-absorptionLine broadening

Source Self-Reversal (Smith Source Self-Reversal (Smith –Hieftje) –Hieftje)

Vandecasteele and Block, 1997, p126

Absorbed by sample and background

Absorbed by sample reduced, not complete eliminate!But the background absorbs the same portion of light.

Chemical InterferencesChemical Interferences Formation of compounds of low Formation of compounds of low

volatilityvolatilityCalcium analysis in the presence of Sulfate or Calcium analysis in the presence of Sulfate or

phosphatephosphate

SolutionsSolutions Higher temperatureHigher temperature Releasing agents: cations that react Releasing agents: cations that react

preferntially with the interference ions.preferntially with the interference ions. Protection agents: form stable but volatile Protection agents: form stable but volatile

species with the analytes (i.e. species with the analytes (i.e. EDTA,APDC….)EDTA,APDC….)

Interferences in AAS Interferences in AAS and AFSand AFS

Chemical InterferencesChemical Interferences Atom ionizationAtom ionization

M ↔ MM ↔ M++ + e + e

Atomic Fluorescence SpectrometryAtomic Fluorescence Spectrometry

Commercial AFS instruments are on the market!

Learn more in CHM 6157