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Lecture 6 ATOMIC SPECTR
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Sample is atomized (atoms/ions)
absorption or emission measured
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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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ELECTRONIC TERM SYMBOLS
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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
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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
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Flame Excitation Sources:
Primary Combustion Zone
Interzonal RegionSecondary Combustion Zone
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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
ATOMIC EMISSION SPECTR
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
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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
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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
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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
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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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ATOMIC ABSORPTION SPECTROSCO
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
ATOMIC ABSORPTION SPECTROSCO
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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most sensitive part of flame for AAS varies with
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ATOMIC ABSORPTION SPECTR
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 %)
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Graphite furnace ETA
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external Ar gas prevents tube destructioninternal Ar gas circulates gaseous analyte
ATOMIC ABSORPTION SPECTR
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
ATOMIC ABSORPTION SPECTR
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
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Hollow Cathode Lamp:
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ATOMIC ABSORPTION SPECTR
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
ATOMIC ABSORPTION SPECTR
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)
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IONIZATION
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hotter atomization means:
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more ionization
emission from interferents
ATOMIC ABSORPTION SPECTR
Spectral interference - emission or absorption interferent overlaps analyte
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ATOMIC ABSORPTION SPECTR
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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