Atomic spectroscopyAtomic spectroscopy

Elemental compositionElemental composition

Atoms have a number of excited energy levels accessible by visible-UV optical methods

Atoms have a number of excited energy levels accessible by visible-UV optical methods

Must have atoms (break up molecules)Must have atoms (break up molecules)Optically transparent sample of neutral Optically transparent sample of neutral

atoms (flames, electrical discharges, atoms (flames, electrical discharges, plasmas)plasmas)

Metals accessible by UV-Vis, non-Metals accessible by UV-Vis, non-metals generally less than 200nm metals generally less than 200nm where vacuum UV needed)where vacuum UV needed)

Atomic spectraAtomic spectra

Outer shell electrons excited to higher energy Outer shell electrons excited to higher energy levelslevels

Many lines per atom (50 for small metals over Many lines per atom (50 for small metals over 5000 for larger metals)5000 for larger metals)

Lines very sharp (inherent linewidth of Lines very sharp (inherent linewidth of 0.00001 nm)0.00001 nm)

Collisional and Doppler broadening (0.003 nm)Collisional and Doppler broadening (0.003 nm) Strong characteristic transitionsStrong characteristic transitions

Atomic Emission SchematicAtomic Emission Schematic

Atomic spectroscopy for analysisAtomic spectroscopy for analysis

Flame emission - heated atoms emit Flame emission - heated atoms emit characteristic lightcharacteristic light

Electrical or discharge emission - higher Electrical or discharge emission - higher energy sources with more lines energy sources with more lines

Atomic absorption - light absorbed by Atomic absorption - light absorbed by neutral atomsneutral atoms

Atomic fluorescence - light used to excite Atomic fluorescence - light used to excite atom then similar to FESatom then similar to FES

Flame Sources - remove solvent, free atoms, excite atomsFlame Sources - remove solvent, free atoms, excite atoms

Nebulizer or direct injectionNebulizer or direct injection Dry solvent, form and dissociate saltDry solvent, form and dissociate salt T= 1700-3200 *C gives some neutral atomsT= 1700-3200 *C gives some neutral atoms Thermal or light induced excitationThermal or light induced excitation Neutrals can react (refractory cpd)Neutrals can react (refractory cpd) Molecular emission from gas give broad Molecular emission from gas give broad

emission interferences)emission interferences)

General issues with flamesGeneral issues with flames

Turbulence / stability / reproducibility Turbulence / stability / reproducibility Fuel rich mixtures more reducing to Fuel rich mixtures more reducing to

prevent refractory formationprevent refractory formationHigh temperature reduces oxide High temperature reduces oxide

interferences but decreases ground interferences but decreases ground state population of neutrals state population of neutrals (fluctuations are critical)(fluctuations are critical)

Chemical interferences - FESChemical interferences - FES

Refractory compounds like oxides and Refractory compounds like oxides and phosphates (depends on matrix)phosphates (depends on matrix)

Reduce refractory formation by higher temp., Reduce refractory formation by higher temp., add releasing agent (La) to complex anion, or add releasing agent (La) to complex anion, or complex cation (EDTA)complex cation (EDTA)

Ionization (electrons in flame depend on matrix)Ionization (electrons in flame depend on matrix) Keep electrons high and constant with easily Keep electrons high and constant with easily

ionizes metal (LiCl)ionizes metal (LiCl)

High energy sourcesHigh energy sources

Reduce chemical interferencesReduce chemical interferences Simultaneous multielement analysisSimultaneous multielement analysis Introduction of solidsIntroduction of solidsElectrical arcs and sparks (the first general Electrical arcs and sparks (the first general

elemental technique)elemental technique) Plasma sources eliminate many problems Plasma sources eliminate many problems

with electrical arcs etc but require solutionswith electrical arcs etc but require solutions

Atomic emission from spark or arcAtomic emission from spark or arc

Electrical ARC - sustained discharge between 2 electrodesElectrical ARC - sustained discharge between 2 electrodes

T=4000-6000*CT=4000-6000*C Poor precision due to Poor precision due to

wanderwander Metal or graphite Metal or graphite

electrodes can be formedelectrodes can be formed Different materials Different materials

volatilized at different volatilized at different rates so quantitization rates so quantitization difficultdifficult

Electrical SPARK (AC)Electrical SPARK (AC)

More reproducible as there are multiple More reproducible as there are multiple discrete electrical breakdowns in gasdiscrete electrical breakdowns in gas

T= up to 40,000*KT= up to 40,000*KHigh precision but limited sensitivity High precision but limited sensitivity

(0.01% level)(0.01% level)Lots of electrical noiseLots of electrical noiseMust integrate emissions over timeMust integrate emissions over time

Multielement analysisMultielement analysis

Simultaneous emission of many Simultaneous emission of many lines requires very high resolutionlines requires very high resolution

Gratings have capability to resolve Gratings have capability to resolve if distances are great and if distances are great and overlapping orders are addressedoverlapping orders are addressed

Measuring emission linesMeasuring emission lines

Photographic (simple and inexpensive)Photographic (simple and inexpensive) Sequential (scan through wavelengths with Sequential (scan through wavelengths with

only a few seconds per line S/N)only a few seconds per line S/N)Advantages of being inexpensive & simple, Advantages of being inexpensive & simple, but slow and irreproduciblebut slow and irreproducible

Simultaneous (direct readout using PM tube Simultaneous (direct readout using PM tube at each exit slit)at each exit slit)Fast (20-60 elements), precise, but expensiveFast (20-60 elements), precise, but expensive

Issues and tradeoffsIssues and tradeoffs

Molecular interferencesMolecular interferencesRelative vs absolute sensitivityRelative vs absolute sensitivityResolution vs S/N or limit of detectionResolution vs S/N or limit of detectionStandard addition vs calibration curveStandard addition vs calibration curveEmission vs AA or fluorescenceEmission vs AA or fluorescence

DC coupled plasma emissionDC coupled plasma emission

Inductively Coupled PlasmaInductively Coupled Plasma

Inductively Coupled PlasmaInductively Coupled Plasma

AA Instrument SchematicAA Instrument Schematic

Atomic AbsorptionAtomic Absorption

AA instrumentationAA instrumentation

Radiation source (hollow cathode lamps)Radiation source (hollow cathode lamps) Optics (get light through ground state atoms Optics (get light through ground state atoms

and into monochromator)and into monochromator) Ground state reservoir (flame or Ground state reservoir (flame or

electrothermal)electrothermal) MonochromatorMonochromator Detector , signal manipulation and readout Detector , signal manipulation and readout

devicedevice

Hollow Cathode Lamp

Emission is from elements in cathode that have been sputtered off into gas phase

Hollow Cathode Lamp

Emission is from elements in cathode that have been sputtered off into gas phase

Light Source

Hollow Cathode LampHollow Cathode Lamp - - seldom used, expensive, low seldom used, expensive, low intensityintensity

Electrodeless Discharge LampElectrodeless Discharge Lamp - - most used most used source, but hard to produce, so its use has declinedsource, but hard to produce, so its use has declined

Xenon Arc LampXenon Arc Lamp - - used in multielement analysisused in multielement analysis

LasersLasers - - high intensity, narrow spectral bandwidth, less high intensity, narrow spectral bandwidth, less scatter, can excite down to 220 nm wavelengths, but expensivescatter, can excite down to 220 nm wavelengths, but expensive

Atomizers

Flame AtomizersFlame Atomizers - rate at which - rate at which sample is introduced into flame and sample is introduced into flame and where the sample is introduced are where the sample is introduced are importantimportant

AA - Flame atomizationAA - Flame atomization

Use liquids and nebulizerUse liquids and nebulizerSlot burners to get large optical pathSlot burners to get large optical pathFlame temperatures varied by gas Flame temperatures varied by gas

compositioncompositionMolecular emission background Molecular emission background

(correction devices )(correction devices )

Sources of errorSources of error

solvent viscositysolvent viscosity temperature and solvent evaporationtemperature and solvent evaporation formation of refractory compoundsformation of refractory compounds chemical (ionization, vaporization)chemical (ionization, vaporization) salts scatter lightsalts scatter light molecular absorptionmolecular absorption spectral lines overlapspectral lines overlap background emissionbackground emission

Atomizers

Flame AtomizersFlame Atomizers - rate at which sample is - rate at which sample is introduced into flame and where the sample is introduced into flame and where the sample is introduced is importantintroduced is important

Graphite Furnace AtomizersGraphite Furnace Atomizers - used if sample is too - used if sample is too small for atomization, provides reducing environment small for atomization, provides reducing environment

for oxidizing agents for oxidizing agents - - small volume of sample is small volume of sample is evaporated at low temperature and then ashed at higher evaporated at low temperature and then ashed at higher temperature in an electrically heated graphite cup. temperature in an electrically heated graphite cup. After ashing, the current is increased and the sample is After ashing, the current is increased and the sample is atomizedatomized

Electrothermal atomizationElectrothermal atomization

Graphite furnace (rod or tube)Graphite furnace (rod or tube)Small volumes measured, solvent Small volumes measured, solvent

evaporated, ash, sample flash evaporated, ash, sample flash volatilized into flowing gasvolatilized into flowing gas

Pyrolitic graphite to reduce memory Pyrolitic graphite to reduce memory effecteffect

Hydride generatorHydride generator

Graphite Furnace AAGraphite Furnace AA

Closeup of graphite furnaceCloseup of graphite furnace

Controls for graphite furnaceControls for graphite furnace

Detector

Photomultiplier Tube Photomultiplier Tube has an active surface which is capable of absorbing has an active surface which is capable of absorbing

radiation radiation absorbed energy causes emission of electrons and absorbed energy causes emission of electrons and

development of a photocurrentdevelopment of a photocurrent encased in glass which absorbs lightencased in glass which absorbs light

Charge Coupled DeviceCharge Coupled Device made up of semiconductor capacitors on a silicon made up of semiconductor capacitors on a silicon

chip, expensivechip, expensive

Background correctionsBackground corrections

Two lines (for flame)Two lines (for flame)Deuterium lampDeuterium lampSmith-Hieftje (increase current to Smith-Hieftje (increase current to

broaden line)broaden line)Zeeman effect (splitting of lines in Zeeman effect (splitting of lines in

a strong magnetic field)a strong magnetic field)

Problems with Technique

Precision and accuracy are highly Precision and accuracy are highly dependent on the atomization stepdependent on the atomization step

Light sourceLight source molecules, atoms, and ions are all in heated molecules, atoms, and ions are all in heated

medium thus producing three different medium thus producing three different atomic emission spectraatomic emission spectra

Problems continued Line broadening occurs due to the uncertainty principleLine broadening occurs due to the uncertainty principle

limit to measurement of exact lifetime and frequency, or limit to measurement of exact lifetime and frequency, or exact position and momentumexact position and momentum

Temperature Temperature increases the efficiency and the total number of atoms in the increases the efficiency and the total number of atoms in the

vaporvapor but also increases line broadening since the atomic particles but also increases line broadening since the atomic particles

move faster.move faster. increases the total amount of ions in the gas and thus increases the total amount of ions in the gas and thus

changes the concentration of the unionized atom changes the concentration of the unionized atom

Interferences

If the matrix emission overlaps or lies too close to the If the matrix emission overlaps or lies too close to the emission of the sample, problems occur (decrease in emission of the sample, problems occur (decrease in resolution)resolution)

This type of matrix effect is rare in hollow cathode This type of matrix effect is rare in hollow cathode sources since the intensity is so lowsources since the intensity is so low

Oxides exhibit broad band absorptions and can scatter Oxides exhibit broad band absorptions and can scatter radiation thus interfering with signal detectionradiation thus interfering with signal detection

If the sample contains organic solvents, scattering If the sample contains organic solvents, scattering occurs due to the carbonaceous particles left from the occurs due to the carbonaceous particles left from the organic matrixorganic matrix

Interferences continued

Gas laserGas laser

Dye laserDye laser

Diode laserDiode laser