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Lecture 6 ATOMIC SPECTROSCOPY
Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Sample is atomized (atoms/ions)
absorption or emission measured
INTRODUCTION TO ATOMIC SPECTROMETRY
ENERGY LEVEL DIAGRAMS
Every elements has unique set of atomic orbitals
p,d,f... levels split by spin-orbit coupling
Spin (s) and orbital (l) motion create magnetic fields that perturbeach other (couple)if fields parallel - slightly higher energyif fields antiparallel - slightly lower energy
INTRODUCTION TO ATOMIC SPECTROMETRY
INTRODUCTION TO ATOMIC SPECTROMETRY
INTRODUCTION TO ATOMIC SPECTROMETRY
ELECTRONIC TERM SYMBOLS
INTRODUCTION TO ATOMIC SPECTROMETRY
INTRODUCTION TO ATOMIC SPECTROMETRY
• Similar pattern between atoms but different spacing• Spectrum of ion different to atom• Separations measured in electronvolts (eV)
As # of electrons increases, # of levels increasesEmission spectra become more complexLi 30 lines, Cs 645 lines, Cr 2277 lines
INTRODUCTION TO ATOMIC SPECTROMETRY
As # of electrons increases, # of levels increases
Emission spectra become more complex
Li 30 lines, Cs 645 lines, Cr 2277 lines
INTRODUCTION TO ATOMIC SPECTROMETRY
Desire narrow lines for accurate identification
Broadened by(i) uncertainty principle(ii) pressure broadening(iii) Doppler effect(iv) (electric and magnetic fields)
INTRODUCTION TO ATOMIC SPECTROMETRY
(i) Uncertainty Principle:
Quantum mechanical idea states must measure for some minimum time to tell two frequencies apart
ATOMIC LINE WIDTHS
Shows up in lifetime of excited state
• if lifetime infinitely long, E infinitely narrow• if lifetime short, E is broadened
INTRODUCTION TO ATOMIC SPECTROMETRY
Example
Lifetime of Hg*=2x10-8 s. What is uncertainty broadening for 254 nm line?
INTRODUCTION TO ATOMIC SPECTROMETRY
Differentiating with respect to frequency:
sometimes called natural linewidth.
INTRODUCTION TO ATOMIC SPECTROMETRY
INTRODUCTION TO ATOMIC SPECTROMETRY
(ii) Pressure broadening:Collisions with atoms/molecules transfers small quantities of vibrational energy (heat) - ill-defined ground state energy
Effect worse at high pressures• For low pressure hollow cathode lamps (1-10 torr) 10-1-10-2 Å• For high pressure Xe lamps (>10,000 torr) 100-1000 Å (turns lines into continua!)
INTRODUCTION TO ATOMIC SPECTROMETRY
(iii) Doppler broadening:Change in frequency produced by motion relative to detector
INTRODUCTION TO ATOMIC SPECTROMETRY
In gas, broadens line symmetrically
Doppler broadening increases with T• At room T ~10-2-10-3 Å
Total linewidth typically 0.01-0.1 Å
INTRODUCTION TO ATOMIC SPECTROMETRY
Other Effects of T on Atomic Spectrometry:
T changes # of atoms in ground and excited states
Boltzmann equation
INTRODUCTION TO ATOMIC SPECTROMETRY
Important in emission measurements relying on thermal excitation
Na atoms at 2500 K, only 0.02 % atoms in first excited state!
Less important in absorption measurements - 99.98 % atoms in ground state!
INTRODUCTION TO ATOMIC SPECTROMETRY
Methods for Atomizing and Introducing Sample
Sample must be converted to atoms first
INTRODUCTION TO ATOMIC SPECTROMETRY
Must transfer sample to atomizer - easy for gases /solutions but difficult for solids
INTRODUCTION TO ATOMIC SPECTROMETRY
Lecture 7 ATOMIC EMMISION SPECTROSCOPY
Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
ATOMIC EMISSION SPECTROSCOPY (AES)
Identification of elements but not compounds
INTRODUCTION TO ATOMIC SPECTROMETRY
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 species
simultaneously(iv) wide dynamic range
ATOMIC EMISSION SPECTROSCOPY
Flame Excitation Sources:
Primary Combustion ZoneInterzonal RegionSecondary Combustion Zone
ATOMIC EMISSION SPECTROSCOPY
ATOMIC EMISSION SPECTROSCOPY
Laminar Flow Burner:
• Cheap• Simple• Flame stability• Low temperature
ATOMIC EMISSION SPECTROSCOPY
ATOMIC EMISSION SPECTROSCOPY
Arc and Spark Excitation Sources:
• Limited to semiquantitative/qualitative analysis (arc flicker)• Usually performed on solids• Largely displaced by plasma-AES
Electric current flowing between two C electrodes
ATOMIC EMISSION SPECTROSCOPY
Electric current flowing between two C electrodes
ATOMIC EMISSION SPECTROSCOPY
Sample pressed into electrode or mixed with Cu powder and pressed briquetting
Cyanogen bands (CN) 350-420 nm occur with C electrodes in air -He, Ar atmosphere
Arc/spark unstable - each line measured >20 s (needs multichannel detection)
photographic film:• Cheap• Long integration times• Difficult to develop/analyze• Non-linearity of line "darkness"
ATOMIC EMISSION SPECTROSCOPY
ATOMIC EMISSION SPECTROSCOPY
ATOMIC EMISSION SPECTROSCOPY
multichannel PMT instruments:
• for rapid determinations (<20 lines) but not versatile
• routine analysis of solids - metals, alloys, ores, rocks, soils
• portable instruments
ATOMIC EMISSION SPECTROSCOPY
Plasma Excitation Sources: gas containing high proportion of cations and electrons(1) Inductively Coupled Plasma (ICP)
ATOMIC EMISSION SPECTROSCOPY
ATOMIC EMISSION SPECTROSCOPY
• Torch up to 1" diameter
• Ar cools outer tube, defines plasma shape
• Radio-frequency (RF) up to 2 kW
• Ar flow up to 20 L/min
Plasma Structure:• Brilliant white core - Ar continuum and lines• Flame-like tail up to 2 cm• Transparent region - measurements made• Hotter than flame (10,000 K) - more complete atomization/excitation• Atomized in "inert" atmosphere• Little ionization - too many electrons in plasma
ATOMIC EMISSION SPECTROSCOPY
(2) Direct Current (DC) Plasma• DC current (10-15 A) flows between C anodes and W cathode
• Plasma core at 10,000 K, viewing region at ~5,000 K
• Simpler, less Ar than ICP - less expensive
ATOMIC EMISSION SPECTROSCOPY
Atomic Emission SpectrometersMay be >1,000 visible lines (<1 Å) on continuumNeed• high resolution (<0.1 Å)• high throughput• low stray light• wide dynamic range (>106)• precise and accurate wavelength calibration/intensities• stability• computer controlled
ATOMIC EMISSION SPECTROSCOPY
Three instrument types:
sequential (scanning and slew-scanning)
Multichannel
(Fourier transform FT-AES)
ATOMIC EMISSION SPECTROSCOPY
Sequential monochromators:Slew-scan spectrometers - even with many lines, much spectrum contains no information
• rapidly scanned (slewed) across blank regions
• slowly scanned across lines
• computer control/preselected lines to scan
ATOMIC EMISSION SPECTROSCOPY
Multichannel AES:
ATOMIC EMISSION SPECTROSCOPY
Sequential instrument - PMT moved behind aperture plate, or grating+prism moved to focus new on exit slitCheaperSlowerPre-configured exit slits to detect up to 20 lines, slew
scanMultichannel instrument - multiple PMT'sExpensiveFaster
ATOMIC EMISSION SPECTROSCOPY
Solution Sample Introduction:(1) Electrothermal vaporizer* (ETV)electric current rapidly heats crucible containing
samplesample carried to atomizer by gas (Ar, He)only for introduction, not atomization
ATOMIC EMISSION SPECTROSCOPY
ATOMIC EMISSION SPECTROSCOPY
(2) Nebulizer - convert solution to fine spray or aerosol
a) Ultrasonic nebulizer uses ultrasound waves to "boil" solution flowing across disc
b) Pneumatic nebulizer uses high pressure gas to entrain solution
ATOMIC EMISSION SPECTROSCOPY
Cross-flow Nebulizer
ATOMIC EMISSION SPECTROSCOPY
Solid Sample Introduction:(1) Electrothermal vaporizer*(2) Direct Insertion(*) uses powder placed inside flame, plasma, arc or spark atomizer (atomizer acts as vaporizer)Coating on electrode in atomizer(3) Ablation uses coating of electrodes in discharge cell and sample entrained in Ar or He gas
Laser ablation uses laser to vaporize sample
ATOMIC EMISSION SPECTROSCOPY
APPLICATION OF AESAES relatively insensitive (small excited state population at moderate temperature)
AAS still used more than AES(i) less expensive/complex instrumentation(ii) lower operating costs(iii) greater precision
ATOMIC EMISSION SPECTROSCOPY
In practice ~60 elements detectable10 ppb range most metalsLi, K, Rb, Cs strongest lines in IRLarge # of lines, increase chance of overlap
ATOMIC EMISSION SPECTROSCOPY
ATOMIC EMISSION SPECTROSCOPY
Lecture 8 ATOMIC ABSORPTION SPECTROSCOPY
Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
ATOMIC ABSORPTION SPECTROSCOPY (AAS)
ATOMIC ABSORPTION SPECTROSCOPY (AAS)
ATOMIC ABSORPTION SPECTROSCOPY (AAS)
ATOMIC ABSORPTION SPECTROSCOPY (AAS)
ATOMIC ABSORPTION SPECTROSCOPY (AAS)
ATOMIC ABSORPTION SPECTROSCOPY (AAS)
ATOMIC ABSORPTION SPECTROSCOPY (AAS)
ATOMIC ABSORPTION SPECTROSCOPY (AAS)
ATOMIC ABSORPTION SPECTROSCOPY (AAS)
ATOMIC ABSORPTION SPECTROSCOPY (AAS)
ATOMIC ABSORPTION SPECTROSCOPY (AAS)
ATOMIC ABSORPTION SPECTROSCOPY (AAS)
ATOMIC ABSORPTION SPECTROSCOPY (AAS)
ATOMIC ABSORPTION SPECTROSCOPY (AAS)
ATOMIC ABSORPTION SPECTROSCOPY (AAS)
ATOMIC ABSORPTION SPECTROSCOPY (AAS)
ATOMIC ABSORPTION SPECTROSCOPY (AAS)
ATOMIC ABSORPTION SPECTROSCOPY (AAS)
AAS intrinsically more sensitive than AES
similar atomization techniques to AES
addition of radiation source
high temperature for atomization necessary flame and electrothermal atomization
very high temperature for excitation not necessary / generally no plasma/arc/spark AAS
ATOMIC ABSORPTION SPECTROSCOPY (AAS)
FLAME AAS:simplest atomization of gas/solution/solid
laminar flow burner - stable "sheet" of flame
flame atomization best for reproducibility (precision) (<1%)
relatively insensitive - incomplete volatilization, short time in beam
ATOMIC ABSORPTION SPECTROSCOPY (AAS)
ATOMIC ABSORPTION SPECTROSCOPY
Primary combustion zone - initial decomposition, molecular fragments, cool
Interzonal region - hottest, most atomic fragments, used for emission/fluorescence Secondary combustion zone - cooler, conversion of atoms to stable molecules, oxides
element rapidly oxidizes - largest [atom] near burner element poorly oxidizes - largest [atom] away from burner
ATOMIC ABSORPTION SPECTROSCOPY
most sensitive part of flame for AAS varies with analyte
ATOMIC ABSORPTION SPECTROSCOPY
Consequences?
sensitivity varies with element
must maximize burner position
makes multielement detection difficult
Electrothermal Atomizers:
entire sample atomized short time (2000-3000 °C)
sample spends up to 1 s in analysis volume
superior sensitivity (10-10-10-13 g analyte)
less reproducible (5-10 %)
ATOMIC EMISSION SPECTROSCOPY
Graphite furnace ETA
ATOMIC ABSORPTION SPECTROSCOPY
external Ar gas prevents tube destructioninternal Ar gas circulates gaseous analyte
ATOMIC ABSORPTION SPECTROSCOPY
Three step sample preparation for graphite furnace: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 analyte (1 s)
ATOMIC ABSORPTION SPECTROSCOPY
ATOMIC ABSORPTION SPECTROSCOPY
Atomic Absorption Instrumentation:
AAS should be very selective - each element has different set of energy levels and lines very narrow
BUT for linear calibration curve (Beers' Law) need bandwidth of absorbing species to be broader than that of light source difficult with ordinary monochromator
ATOMIC ABSORPTION SPECTROSCOPY
Solved by using very narrow line radiation sources
minimize Doppler broadening
pressure broadening
lower P and T than atomizerand using resonant absorption
Na emission 3p2s at 589.6 nm used to probe Na in analyte
ATOMIC ABSORPTION SPECTROSCOPY
Hollow Cathode Lamp:
ATOMIC ABSORPTION SPECTROSCOPY
300 V applied between anode (+) and metal cathode (-)
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 cathode restricts light direction
ATOMIC ABSORPTION SPECTROSCOPY
ELECTRODELESS DISCHARGE LAMP
ATOMIC ABSORPTION SPECTROSCOPY
AAS Spectrophotometers:
ATOMIC ABSORPTION SPECTROSCOPY
Signal at one wavelength often contains luminescence from interferents in flame
Chemical interference:(i) reverses atomization equilibria(ii) reacts with analyte to form low volatility compound
releasing agent - cations that react preferentially withinterferent - Sr acts as releasing agent for Ca with
phosphateprotecting agent - form stable but volatile compounds with
analyte (metal-EDTA formation constants)
ATOMIC ABSORPTION SPECTROSCOPY
IONIZATION
ATOMIC ABSORPTION SPECTROSCOPY
ATOMIC ABSORPTION SPECTROSCOPY
hotter atomization means:
more ionization
emission from interferents
ATOMIC ABSORPTION SPECTROSCOPY
Spectral interference - emission or absorption from interferent overlaps analyte
ATOMIC ABSORPTION SPECTROSCOPY
Beam usually chopped or modulated at known frequency
Signal then contains constant (background) and dynamic (time varying) signals
ATOMIC ABSORPTION SPECTROSCOPY
DETECTION LIMITS for AAS/AES?
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)
ATOMIC ABSORPTION SPECTROSCOPY
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ATOMIC ABSORPTION SPECTROSCOPY
ATOMIC ABSORPTION SPECTROSCOPY
ATOMIC ABSORPTION SPECTROSCOPY
ATOMIC ABSORPTION SPECTROSCOPY
ATOMIC ABSORPTION SPECTROSCOPY
ATOMIC ABSORPTION SPECTROSCOPY
ATOMIC ABSORPTION SPECTROSCOPY
ATOMIC ABSORPTION SPECTROSCOPY
ATOMIC ABSORPTION SPECTROSCOPY
ATOMIC ABSORPTION SPECTROSCOPY
ATOMIC ABSORPTION SPECTROSCOPY
ATOMIC ABSORPTION SPECTROSCOPY
ATOMIC ABSORPTION SPECTROSCOPY
ATOMIC ABSORPTION SPECTROSCOPY
ATOMIC ABSORPTION SPECTROSCOPY
ATOMIC EMISSION SPECTROSCOPY
INTRODUCTION TO ATOMIC SPECTROMETRY
INTRODUCTION TO ATOMIC SPECTROMETRY
INTRODUCTION TO ATOMIC SPECTROMETRY
INTRODUCTION TO ATOMIC SPECTROMETRY
INTRODUCTION TO ATOMIC SPECTROMETRY
INTRODUCTION TO ATOMIC SPECTROMETRY
INTRODUCTION TO ATOMIC SPECTROMETRY
INTRODUCTION TO ATOMIC SPECTROMETRY
INTRODUCTION TO ATOMIC SPECTROMETRY
INTRODUCTION TO ATOMIC SPECTROMETRY
INTRODUCTION TO ATOMIC SPECTROMETRY
INTRODUCTION TO ATOMIC SPECTROMETRY
INTRODUCTION TO ATOMIC SPECTROMETRY
INTRODUCTION TO ATOMIC SPECTROMETRY
INTRODUCTION TO ATOMIC SPECTROMETRY
INTRODUCTION TO ATOMIC SPECTROMETRY
INTRODUCTION TO ATOMIC SPECTROMETRY
INTRODUCTION TO ATOMIC SPECTROMETRY