Lecture 6 ATOMIC SPECTROSCOPY

Report
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 energy
 if 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 increases
Emission spectra become more complex
Li 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) 101-10-2 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 Zone
Interzonal Region
Secondary 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
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
ATOMIC EMISSION SPECTROSCOPY
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
• 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
ATOMIC EMISSION SPECTROSCOPY
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 Spectrometers
May be >1,000 visible lines (<1 Å) on continuum
Need
• 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
slit
 Cheaper
 Slower
 Pre-configured exit slits to detect up to 20 lines, slew
scan
Multichannel instrument - multiple PMT's
 Expensive
 Faster
ATOMIC EMISSION SPECTROSCOPY
Solution Sample Introduction:
(1) Electrothermal vaporizer* (ETV)
 electric current rapidly heats crucible containing
sample
 sample 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 AES
AES 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 detectable
 10 ppb range most metals
 Li, K, Rb, Cs strongest lines in IR
 Large # of lines, increase chance of overlap
ATOMIC EMISSION SPECTROSCOPY
ATOMIC EMISSION SPECTROSCOPY
Lecture 8
Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
ATOMIC ABSORPTION
SPECTROSCOPY
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
Consequences?
sensitivity varies with element
must maximize burner position
makes multielement detection
difficult
ATOMIC ABSORPTION SPECTROSCOPY
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 destruction
internal 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 3p2s at 589.6 nm used to probe Na in
analyte
ATOMIC ABSORPTION SPECTROSCOPY
Hollow Cathode Lamp:
ATOMIC ABSORPTION SPECTROSCOPY
 300
 Ar
V applied between anode (+) and metal cathode (-)
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 with
interferent - Sr acts as releasing agent for Ca with
phosphate
protecting 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
Beam usually chopped
or modulated at known
frequency
Signal then contains
constant (background)
and dynamic (time
varying) signals
ATOMIC ABSORPTION SPECTROSCOPY
ATOMIC ABSORPTION SPECTROSCOPY
DETECTION LIMITS for AAS/AES?
AA/AE comparable (ppb in flame)
AAS less suitable for
 weak
absorbers (forbidden transitions)
 metalloids
 metals
and non-metals (absorb in UV)
with low IP (alkali metals)
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 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

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