Atomic Spectroscopy• Atomic spectroscopy refers to measurement of

elemental concentrations via optical processesof absorption, emission or fluorescence asexemplified by AAS, AES, and AFS.

• Atoms change states when electrons move todifferent orbitals.

• Mass Spectroscopic techniques such asinductively coupled plasma-mass spectrometryor glow discharge-mass spectrometry are alsoclassified as atomic spectroscopy

Atomic Absorption Process

• A neutral atom in the gaseous state can absorbradiation and transfer an electron to an excitedstate.

• Simple electronic transitions possible with novibrational and rotational energy levels possible.Bandwidth much narrower!

• Occur at discreet • Na(g) 3s 3p and 3p 5s as well as other

transitions are possible at the correspondingphoton energies for transition.

Atomic Absorption Transitions

Atomic Line WidthsAtomic line widths areaffected by:

•natural broadening dueto the uncertainty effect

•doppler broadening

•pressure or collisionalbroadening

Atomic Line Widths cont

• Theoretically, atomic lines will have 0 linewidth, but

• Line broadening caused by– uncertainty principle (10-4 Å)

• natural line width - caused by finite lifetime ofexcited state

– pressure effects• collisions with other atoms cause changes in ground

state energy– electric and magnetic field effects

Atomic Line Widths cont

– Doppler broadening• if an atom emits radiation while moving toward the

detector, the waves will be compressed, and thewavelength will be shorter

• if an atom emits radiation while moving away fromthe detector, the waves will stretched out, and thewavelength will be longer

• related effect for absorption

Temperature Effects• Determines the number of ground state and

excited state atoms•

– N - number of atoms in state• o - ground; j - excited

– P - statistical factor– E - energy difference between states– k - Boltzmann constant

• k=1.38 x 10-23 J/K=1.38 x 10-16 erg/K– T - Temperature (K)

)exp(kT

EP

P

N

N

o

j

o

j u

o

u

o

Ekt

NN

=gg

eex

Temperature Dependence inBoltzman Equation

• 3 steps required before measurements are possible in an A.A. experiment: 1. vaporization 2.reduction to the elemental state and then 3. exposure to radiation.

• The first two steps are accomplished by a flame.• Effect of flame temperature: Since flame is at high temperature might have an effect on fraction of

atoms in excited state.• Boltzmann's equation describes effect of flame temperature: where

• N = # of atoms in a given state;• g = statistical factor for a given level and measures the number of possible electrons in each level;• g = 2J + 1 where J = Russel-Saunders coupling constant and is given by J = L + S or L S where L =

orbital angular momentum quantum # (=0,1,2,3 for s, p, d, f respectively) and S = spin = ±½.• E.g. For the Na transition

– 3s½ 3p3/2 gu = 2(L+S) + 1 = 2(1 + ½) + 1 = 4 andgo = 2(0 + ½) + 1 = 2.

– 3s½ 3p½ go = 2 and gu = 2(1½) + 1 = 2.• Overall population of both of these states: since they are only separated by 5Å, let's use average of

their wavelengths and add population for the two excited states:• g = 4 + 2 = 6 and go = 2 (as before); lave 5892Å.

u

o

u

o

Ekt

NN

=g

ge

ex

hc

E 6.626x10 erg 2.998x10 cm /

5892X cm= erg

27 10

sec sec .

103 37 10

812

The Flame and Excited States

• Assume Air-acetylene flame (2400°C):Temperatures for different flames used in AA arelisted in another slide

• T = (2400 + 273)K = 2673K;• Substituting into the Boltzmann equation:

= 3.23x104

• Very small fraction of the atoms in the flame areexcited to this excited state.

e26

NN

2673KK 1erg10 161.38x

erg10 123.37x

o

u

Relative population of higherenergy transitions

• 3p 5s transition is also possible and has = 6161Å (E = 3.22x1012 erg.• The fraction of 3p electrons excited to the 5 s orbital is calculated as before:

= 5.34x10-5

• Fraction involved in this transition even smaller.• Finally, we can estimate the fraction of electrons in the 5s state relative to the

3s state:

= 5.34x105×3.23x104 = 1.72x108

• Only very small proportion of the absorbing species is in the excited statefrom excitation by flame; higher energy transitions much less likely than thelower energy transitions.

e62

NN

2673KK 1erg10 161.38x

erg10 123.22x

o

u

5s

3s

3p

3s

5s

3p

NN

N

NNN

MEASURING ATOMICABSORPTION

• Recall Beer's Law (A = log = bC ) is obeyed when line widthsmall compared to absorption band.

• Atoms or molecules absorb radiation at discrete wavelengths.• Broadband radiation contains photons of several wavelengths, some of

which may be useful but many of which will not. This will make Po (=Pusable + Puseless) larger and the absorbance smaller than would beexpected with only the usable portion of the light available forabsorption.

• Besides the Pusable can be composed of wavelengths with differentabsorptivities i.e. the sample does not absorb all radiation to the samedegree.

• Non-linear behavior observed when range of excitation source isgreater than range of absorber; bandwidth of excitation source mustbe narrower than bandwidth of absorber.

oPP

Linewidth of Atomic Transitions

• Natural linewidth of an absorption spectrum is very small(104Å) but is broadened by– Doppler broadening: Random thermal motions of atoms relative to

the detector– Pressure broadening: In the atomic absorption experiment the

pressure is large enough that atoms can undergo some interatomiccollisions which cause small changes in the ground state levels.

• Normal line width of excitation lines much greater thanthis line width

• Monochromator cannot be used to select range in AA(bandwidth few tenths of a nm).

FLAME CHEMISTRY

SOURCES• Solution to line width problem: Use atomic source of same material.• e.g. For Na analysis Na vapor is used.• Atoms are excited by electrical discharge; the excited atoms emit a characteristic .

The bandwidth of the source << sample linewidth since it is generated underconditions where there is less broadening.

• Hollow Cathode Tube : Hollow cathode made of the material needed is vaporizedand emits radiation of the characteristic wavelength.

• The ion current to the cathode controls photon intensity; Increasing the voltagebetween the anode and cathode will control the current and thus total photon flux.

• Optimum current for each lamp (1-20ma).

Hollow Cathode Lamps• Tungsten anode• Cylindrical cathode - made of the element

of interest• Sealed glass tube - Ar or Ne at 1-5 torr• 300 V, 5-15 mAmps

FORMATION OF ATOMICVAPOR

Four methods used to vaporize sample from solution:• Ovens: Sample placed in an oven; after evaporating

solvent, sample vaporized into irradiation area by rapidlyincreasing temperature.

• Electric arc or spark: Sample subjected to high current orhigh potential A.C. spark.

• Ion bombardment: Sample placed on cathode andbombarded with + ions (Ar+). Sputtering processdislodges them from cathode and directs them toirradiation region.

• Flame atomization: Sample sprayed into flame where itundergoes atomization and irradiation.

Flame Atomization

Sample Introduction

• Solutions– Pneumatic nebulization– Ultrasound nebulization– Electrothermal vaporization– Hydride generation

• Solids– Electrothermal vaporization– Direct insertion– Laser ablation– Spark or arc ablation– Glow discharge sputtering

Nebulizers• Convert liquid into a fine mist (aerosol)

INSTRUMENTATIONNebulizers:

Burgener nebulizer

FLAME ATOMIZERS• Total consumption burner: Separate

channels bring sample, fuel, andoxidant to combustion area. All of thesample, that is carried into the burner,is burned;

• Sensitivity is greater than in a burnerwhere the sample is not completelyburned.

• extra turbulence in the flame fromvariations in droplet size increasenoise. Undergraduate Instrumental Analysis,

Robinson, p. 267.

Premix Laminar Flow Burner

• Sample, fuel, and oxidant mixed prior to entering flame.• Turbulence drastically reduced by removing larger

droplets.• Mixing baffles insure only fine mist makes it through to

burner.

Instrumental Methods of Chemical Analysis, Ewing, p. 110.

Laminar Flow Burner

Laminar Flow Burners

• Advantages– stable flame– long pathlength– reproducible signal

• ~1 % RSD

• Disadvantages– flashback danger (if gas flow is too slow)– low sampling efficiency

• atoms spend only ~10-4 s in flame• most of the sample goes to waste

FUELS/OXIDANTS• Low T flames : easily reduced elements (Cu,

Pb, Zn, Cd)• High T flames: difficult to reduce elements

(e.g. alkaline earths).• Fuels: natural gas, propane, butane, H2, and

acetylene;• Oxidants- Air and O2 (low temperature

flames). N2O (high temperature flames).• Flame characteristics:• Sample enters flame, is vaporized, reduced and

eventually oxidized.• Exact region of flame in which each of these

occurs depends upon:• flow rate,• drop size, and• oxidizability of sample.• Optimum position for flame for many metals.

Properties of Some Fuel-Oxidant Mixtures

33421100-2480C2H2-O2

3150285C2H2-N2O3094370-390C3H8-02

3080900-1400H2-O2

2540158-266C2H2-air2380300-440H2-air226739-43C3H8-air

StoichiometricTemp, oC

TheoreticalVelocity, cm/s

BurningMixture

Dependence of AAS Signalupon Height Above Burner

• Cr + O CrO• MgCl2 Mg MgO• AgCl Ag

MEASUREMENT OF AA• Ideally, the amount of light reaching the detector is given by Beers Law: P = Po×10bC .• several interferences can change this to:• P = Po×10bC + Pemission Pbackground Pscattering.• Pemission is due to analyte emission in the flame• eliminated from the absorbance by modulation of the light source: measures only AC

levels; emission DC level.• Pbackground, Pscattering: due to absorption by the flame or are induced by sample matrix and

are independent of the analyte.• Broad band in nature.• Flame interferences nulled by comparing a blank with sample

– Sample matrix is a problem. Caused by, for example, high salt content (e.g. NaCl or KI).These have broad band absorption spectrum in flame since they are not reduced by it. Mostcommon approach uses secondary continuum source (e.g. D2 lamp):

– Each lamp (D2 and HCT) modulated but are 180° out of phase with each other.– Detection system measures difference between two absorbance signals: AHCT = Asample + Abrdband

while Acontinuum source = Abrd band. will be absorbance of sample.

D2 Source Elimination of Background

Why a line source and not acontinuous source?

• Continuous source with monochromator:

• Line source:MonochromatorLine

AbsorptionLine

AbsorptionLine

LineSource

Why a line source and not acontinuous source?

• With a monochromator band from a continuoussource, the relative amount of incident lightabsorbed is much smaller. This gives lowersensitivity, and a curved calibration curve.– absorption linewidth

• ~ 10-3 nm from both pressure and Doppler broadening

– monochromator linewidth• ~ 10-2 nm

– hollow cathode lamp linewidth• ~ 10-4 nm

MONOCHROMATOR

• Needed to choose one ofseveral possible emissionlines (emitted) associatedwith HCT.

• Since they are usuallyreasonably well separatedfrom the line of interest, it isstraightforward to use amonochromator to eliminatethis interference.

ANALYTICALTECHNIQUES

• Beer's law, A = k×C, not always true making a calibrationcurve necessary.

• Standard addition method is used to minimize the effectsfrom the matrix

• Anion- height of the absorbance peak is influenced by typeand concentration of anion. It can reduce the number ofatoms made. An unknown matrix is thus hard to correctfor

• Cation: The presence of a second cation sometimes causesstable compounds to form with the cation being analyzed.e.g. Al + Mg produces low results for Mg due to theformation of an Al/Mg oxide.

Sample Problem• The nickel content in river water

was determined by AA analysisafter 5.00 L was trapped by ionexchange. Rinsing the columnwith 25.0 mL of a salt solutionreleased all of the nickel and thewash volume was adjusted to75.00 mL; 10.00 mL aliquots ofthis solution were analyzed byAA after adding a volume of0.0700 g Ni/mL to each. Aplot of the results are shownbelow. Determine theconcentration of the Ni in theriver water.

Determination of NickelContent by AA

y = 5.6x + 20

0

40

80

120

0 5 10 15

Volume of Nickel Added(mL)

Abs

orba

nce

Un

its

Energy sources for atomization

40000 oCHigh Voltage Spark (HVS)

4000-5000DC Arc (DCA)

6000-10000Direct Current (DCP)

5000-7000Microwave Induced (MIP,CMP)

6000-8000Inductively Coupled (ICP)

1200-3000Furnace (electrothermal)

1700-3150 oCFlame

METHODS OF SAMPLE INTRODUCTIONIN ATOMIC SPECTROSCOPY

conducting solidGlow Discharge Sputtering

conducting solidSpark or Arc Ablationsolid, metalLaser Ablationsolid, powderDirect Insertion

solution of certain elementsHydride Generation

solid, liquid, solutionElectrothermal Vaporization

solutionUltrasonic Nebulization

solution or slurryPneumatic Nebulization

Low excitation energy

7800200

3900400

2600 K600 nm

Temperature for 0.1 %Excited State

Transition

Atomization Methods• Flame (1700 - 3200 oC)• Electrothermal vaporization (ETV) (1200 - 3000 oC)• Inductively coupled plasma (ICP) (4000 - 6000 oC)• Direct current plasma (DCP) (4000 - 6000 oC)• Microwave-induced plasma (MIP) (2000 - 3000 oC)• Glow discharge plasma (GD) (nonthermal)• Electric arc (4000 - 5000 oC)• Electric spark (?)

Atomic AbsorptionSpectrometry

• Atomization Techniques (sometimes combinedwith sample introduction techniques)– *flame– *electrothermal– glow discharge– hydride generation– cold-vapor (Hg)

ELECTROTHERMALATOMIZATION

• all of the sample used is atomized in furnace(electrothermal) atomizer.

• detection limit is 100-1000x lower than withaspiration techniques.

• only a few mL of solution is used.• Basic Principle:

– sample container resistively heated to vaporizethe metal atoms.

– sample dried (evaporate solvent) at 110°C;– ash sample called "burn off" (200-300°C);– atomization.(2000-3000°C)– comparison with flame atomization:

• interaction with sample matrix and electrode• poorer reproducibility• detection limits of 1010-1012g (or 1ppb) are

possible.Instrumental Methods of Analysis, Willard,Merritt, Dean andSettle, p. 147

Graphite Furnace

Graphite Furnace

Dry - removesolvent

20 sec 125 oC

Ash - destroyorganic matter

60 sec 1600 oC

Atomize 10 sec 2100 oC

Clean 3 sec 2500 oC

• Example temperature program:

• Atom signal:– ~ 1-10 sec

Graphite Furnaces

• Advantages– small sample volumes

• 0.5 - 10 L

– good detection limits• 10-10 - 10-13 g

• Disadvantages– poor reproducibility

• ~ 5 - 10 % RSD

– slow– limited linear range

Spectrometer Designs

Spectral Interferences• Atomic interferences

– Vanadium @ 3082.11 Å– Aluminum @ 3082.15 Å

• Broadband interferences– scattering from particles– molecular absorption– correction approaches

• two line correction (reference line from Ne or Ar)• Continuum source background correction (D2 lamp)• Zeeman background correction

D2 Lamp Correction• Subtract absorption of D2 radiation from absorption

of HCL• Monochromator slit width large, so that atomic

absorption of D2 radiation is negligible• Disadvantages

– degrades S/N– lamp beams must be aligned– not good for visible region

Zeeman Correction

Chemical Interferencs• formation low volatility compounds

– increase temperature– releasing agent

• Sr + CaPO4 SrPO4 + Ca

– protective agent• CaSO4 + EDTA CaEDTA + SO4

• dissociation equilibria• MO M + O• NaCl Na + Cl

• ionization equilibria• M M+ + e-

• add easily ionized atom to shift equilibrium of analyte to left(ionization suppressor)

Sample Preparation• For flame AAS, sample must be in solution

– hot mineral acids– oxidation (wet ashing)– combustion– fusion with with boric oxide sodium carbonate

• For GFAAS, sample can be solid, butcalibration can be difficult

Atomic Emission

• Flame AES– good for easily excited elements - Li, Na, K– often used for determination of Na and K in

blood serum, with Li as an internal standard

Plasma EmissionSpectroscopy (PES)

• Plasma - electrically conducting gas mixturecontaining cations and electrons

• Inductively Coupled Plasma (ICP) - argonplasma - Ar+ and e-’s

• Direct Current Plasma (DCP)• Microwave Induced Plasma (MIP)

Inductively Coupled PlasmaSource

Inductively Coupled PlasmaSource cont

• Sample injected into the core of annular plasma• rf field (0.5 -2 kW power) - induces an oscillating

magnetic field• Ar is the most common gas used to support the

plasma– 5 - 20 L/min

• Temperatures of 6000 K achieved - produces A*species

Echelle Spectrometer• Echelle grating diffracts wave-lengths in two

dimensions• Charge-coupled device (CCD) detector takes

picture of absorption pattern

ICP Polychromator

Inductively Coupled Plasma (ICP)

• Advantages– good detection limits– minimal chemical interferences– multielement determinations– reproducible

• Disadvantages– spectral interferences– expensive operating costs– requires solution samples

MICROWAVE PLASMA DETECTOR

HV SPARK DC PLASMA