Lecture 6 Atomic Spectroscopy

7/28/2019 Lecture 6 Atomic Spectroscopy

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Lecture 6 ATOMIC SPECTR

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Sample is atomized (atoms/ions)

absorption or emission measured

INTRODUCTION TO ATOMIC SPECTROMETR


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ENERGY LEVEL DIAGRAMS

Every elements has unique set of atomic orbital

p,d,f... levels split by spin-orbit coupling

Spin (s) and orbital (l) motion create magnetic

that perturbeach other (couple) if fields parallel - slightly higher energy if fields antiparallel - slightly lower energy



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5/130INTRODUCTION TO ATOMIC SPECTROMETR


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ELECTRONIC TERM SYMBOLS



7/130INTRODUCTION TO ATOMIC SPECTROMETR


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Similar pattern between atoms but different s Spectrum of ion different to atom Separations measured in electronvolts (eV)

As # of electrons increases, # of levels increaseEmission spectra become more complexLi 30 lines, Cs 645 lines, Cr 2277 lines



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As # of electrons increases, # of levels increase

Emission spectra become more complex

Li 30 lines, Cs 645 lines, Cr 2277 lines



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Desire narrow lines for accurate identification

Broadened by

(i) uncertainty principle(ii) pressure broadening(iii) Doppler effect(iv) (electric and magnetic fields)



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(i) Uncertainty Principle:

Quantum mechanical idea states must measure

some minimum time to tell two frequencies apa

ATOMIC LINE WIDTHS


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Shows up in lifetime of excited state

if lifetime infinitely long, E infinitely narrow

if lifetime short, E is broadened



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Example

Lifetime of Hg*=2x10-8 s. What is uncertainty

broadening for 254 nm line?



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Differentiating with respect to frequency:

sometimes called natural linewidth.



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(ii) Pressure broadening:Collisions with atoms/molecules transfers smallquantities of vibrational energy (heat) - ill-defin

ground state energy

Effect worse at high pressures For low pressure hollow cathode lamps (1-10 1-10-2 For high pressure Xe lamps (>10,000 torr) 10 (turns lines into continua!)



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(iii) Doppler broadening:Change in frequency produced by motion relatdetector



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Other Effects of T on Atomic Spectrometry

T changes # of atoms in ground and excited sta

Boltzmann equation



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Important in emission measurements relying onexcitation

Na atoms at 2500 K, only 0.02 % atoms in firststate!

Less important in absorption measurements - 9atoms in ground state!



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Methods for Atomizingand Introducing Sample

Sample must be convertedto atoms first



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Must transfer sampleto atomizer - easy forgases /solutions but

difficult for solids



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Lecture 7 ATOMIC EMMISION SPECTR



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Excitation and Atomization:

Traditionally based on flame but

arc and spark plasma

excitation offers(i) increased atomization/excitation

(ii) wider range of elements(iii) emission from multiple speciessimultaneously

(iv) wide dynamic range

ATOMIC EMISSION SPECTR


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Flame Excitation Sources:

Primary Combustion Zone

Interzonal RegionSecondary Combustion Zone

ATOMIC EMISSION SPECTROSCOPY


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Arc and Spark Excitation Sources:

Limited to semiquantitative/qualitative analys

flicker) Usually performed on solids Largely displaced by plasma-AES

Electric current flowing between two C electrode



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Electric current flowing between two C electrode


Sample pressed into

electrode or mixed withCu powder and pressed

briquetting

Cyanogen bands (CN)

350-420 nm occur with

C electrodes in air -

He, Ar atmosphere


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Arc/spark unstable - each line measured >20 s multichannel detection)

photographic film: Cheap Long integration times Difficult to develop/analyze Non-linearity of line "darkness"



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Plasma Excitation Sources:

gas containing high proportion of cations and el(1) Inductively Coupled Plasma (ICP)



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Plasma Structure: Brilliant white core - Arcontinuum and lines Flame-like tail up to 2 cm Transparent region -measurements made Hotter than flame (10,000K) - more completeatomization/excitation

Atomized in "inert"atmosphere Little ionization - too manyelectrons in plasma



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(2) Direct Current (DC)Plasma DC current (10-15 A)flows between C anodesand W cathode

Plasma core at 10,000 K,viewing region at ~5,000K

Simpler, less Ar than ICP- less expensive



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Three instrument types:

sequential (scanning and slew-scanning)

Multichannel

(Fourier transform FT-AES)



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S i l i PMT d b hi d


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Sequential instrument - PMT moved behind apeplate, or grating+prism moved to focus new oslit

Cheaper Slower Pre-configured exit slits to detect up to 20 line

scanMultichannel instrument - multiple PMT'sExpensiveFaster


S l ti S l I t d ti


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Solution Sample Introduction:(1) Electrothermal vaporizer* (ETV) electric current rapidly heats crucible containin

sample sample carried to atomizer by gas (Ar, He) only for introduction, not atomization



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(2) N b li t l ti t fi


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(2) Nebulizer - convert solution to fine spray or

a) Ultrasonic nebulizer uses ultrasound waves t

solution flowing across disc

b) Pneumatic nebulizer uses high pressure gas entrain solution


Cross flow Nebulizer


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Cross-flow Nebulizer

ATOMIC EMISSION SPECTROSCOPY

Solid Sample Introduction:


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Solid Sample Introduction:

(1) Electrothermal vaporizer*(2) Direct Insertion(*) uses powder placed insid

plasma, arc or spark atomizer (atomizer acts asvaporizer)Coating on electrode in atomizer(3) Ablation uses coating of electrodes in dischaand sample entrained in Ar or He gas

Laser ablation uses laser to vaporize sample


APPLICATION OF AES


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APPLICATION OF AES

AES relatively insensitive (small excited state poat moderate temperature)

AAS still used more than AES(i) less expensive/complex instrumentation(ii) lower operating costs(iii) greater precision


In practice ~60 elements detectable


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In practice ~60 elements detectable 10 ppb range most metals Li, K, Rb, Cs strongest lines in IR

Large # of lines, increase chance of overlap



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Lecture 8 ATOMIC ABSOSPECTR



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ATOMIC ABSORPTION SPECTROSCO


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AAS intrinsically more sensitive than AES


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similar atomization techniques to AES

addition ofradiation source

high temperature for atomization necessaryflame and electrothermal atomization

very high temperature for excitation not necessary/ gno plasma/arc/spark AAS


FLAME AAS:i l t t i ti f / l ti / lid


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simplest atomization of gas/solution/solid

laminar flow burner - stable "sheet" of flame

flame atomization best for reproducibility (pre(


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ATOMIC ABSORPTION SPECTR


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most sensitive part of flame for AAS varies with


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Consequences?

sensitivity varies with ele

must maximize burner po

makes multielement dete

difficult

Electrothermal Atomizers:


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entire sample atomized short time (2000-3000

sample spends up to 1 s in analysis volume

superior sensitivity (10-10-10-13 g analyte)

less reproducible (5-10 %)


Graphite furnace ETA


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external Ar gas prevents tube destructioninternal Ar gas circulates gaseous analyte


Three step sample preparation for graphite furn1) Dry - evaporation of solvents (10->100 s)


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1) Dry - evaporation of solvents (10->100 s)2) Ash - removal of volatile hydroxides, sulfates

carbonates (10-100 s)3) Fire/Atomize - atomization of remaining anal



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ATOMIC ABS

SPECTR

Atomic Absorption Instrumentation:


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AAS should be very selective - each element hdifferent set of energy levels and lines very na

BUT for linear calibration curve (Beers' Law) nbandwidth of absorbing species to be broader that of light source difficult with ordinary

monochromator


Solved by using very narrow line radiation sour

i i i l b d i


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minimize Doppler broadening

pressure broadening

lower P and T than atomizerand using resonanabsorption

Na emission 3p2s at 589.6 nm used to probe

analyte


Hollow Cathode Lamp:


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300 V applied between anode (+) and metal cathode

Ar ions bombard cathode and sputter cathode atoms


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Ar ions bombard cathode and sputter cathode atoms

Fraction of sputtered atoms excited, then fluoresce

Cathode made of metal of interest (Na, Ca, K, Fe...)different lamp for each element

restricts multielement detection

Hollow cathode to

maximize probability of redeposition on cathoderestricts light direction


ELECTRODELESS DISCHARGE LAMP


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Signal at one wavelength often contains luminescence finterferents in flame


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Chemical interference:

(i) reverses atomization equilibria(ii) reacts with analyte to form low volatility compoundreleasing agent- cations that react preferentially interferent - Sr acts as releasing agent for Ca withphosphate

protecting agent- form stable but volatile compoanalyte (metal-EDTA formation constants)


IONIZATION


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hotter atomization means:


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more ionization

emission from interferents


Spectral interference - emission or absorption interferent overlaps analyte


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Beam usuall

ormodulated

frequency

Signal then c

constant (ba

and dynamic

varying) sign


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DETECTION LIMITS for AAS/AES?


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AA/AE comparable (ppb in flame)

AAS less suitable forweak absorbers (forbidden transitions)

metalloids and non-metals (absorb in UV)

metals with low IP (alkali metals)



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Lecture 6 Atomic Spectroscopy

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