Transcript Atomic Emission Spectroscopy Yongsik Lee May 14, 2004 Introduction to AES ► Atomization Emission Sources      ► Flame – still used for metal atoms Electric Spark and Arc Direct current.

Atomic Emission
Spectroscopy
Yongsik Lee
May 14, 2004
Introduction to AES
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Atomization Emission Sources
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Flame – still used for metal atoms
Electric Spark and Arc
Direct current Plasmas
Microwave Induced Plasma
Inductively Coupled Plasma – the most important
technique
Advantages of plasma
 Simultaneous multi-element Analysis – saves
sample amount
 Some non-metal determination (Cl, Br, I, and S)
 Concentration range of several decades (105 –
106)
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Disadvantages of plasma
 very complex Spectra - hundreds to thousands
of lines
 High resolution and expensive optical
components
 Expensive instruments, highly trained personnel
required
10A Plasam Source AES
► Plasma
 an electrically conducting gaseous mixture
containing significant concentrations of
cations and electrons.
► Three
main types
 Inductively Coupled Plasma (ICP)
 Direct Current Plasma (DCP)
 Microwave Induced Plasma (MIP)
ICP
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Inductively Coupled Plasma (ICP)
 Plasma generated in a device called a
Torch
 Torch up to 1" diameter
 Ar cools outer tube, defines plasma
shape
 Rapid tangential flow of argon cools
outer quartz and centers plasma
 Rate of Argon Consumption 5 - 20
L/Min
 Radio frequency (RF) generator 27 or 41
MHz up to 2 kW
 Telsa coil produces initiation spark
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Ions and e- interact with magnetic field
and begin to flow in a circular motion.
Resistance to movement (collisions of
e- and cations with ambient gas) leads
to ohmic heating.
Sample introduction is analogous to
atomic absorption.
Sample introduction
► Nebulizer
► Electrothermal
► Table
vaporizer
8-2 methods of sample introducton
Nebulizer
► convert
solution to
fine spray or aerosol
► Ultrasonic nebulizer
 uses ultrasound waves
to "boil" solution
flowing across disc
► Pneumatic
nebulizer
 uses high pressure
gas to entrain solution
Electro-thermal vaporizer ETV
► Electrothermal
vaporizer (ETV)
 electric current rapidly
heats crucible
containing sample
 sample carried to
atomizer by gas (Ar,
He)
 only for introduction,
not atomization
Plasma structure
► Brilliant
white core
 Ar continuum and lines
► Flame-like
tail
 up to 2 cm
► Transparent
region
 where measurements are
made (no continuum)
Plasma characteristics
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Hotter than flame (10,000 K) more complete atomization/
excitation
Atomized in "inert" atmosphere
Ionization interference small due
to high density of eSample atoms reside in plasma
for ~2 msec and
Plasma chemically inert, little
oxide formation
Temperature profile quite stable
and uniform.
DC plasma
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First reported in 1920s
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
Less sensitive than ICP
Should replace the
carbon anodes in several
hours
Atomic Emission Spectrometer
May be >1,000 visible lines (<1 Å) on continuum
► Need
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higher resolution (<0.1 Å)
higher throughput
low stray light
wide dynamic range (>1,000,000)
precise and accurate wavelength calibration/intensities
stability
computer controlled
Three instrument types:
 sequential (scanning and slew-scanning)
 Multichannel - Measure intensities of a large number of
elements (50-60) simultaneously
 Fourier transform FT-AES
Desirable properties of an AE
spectrometer
Sequential vs. multichannel
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Sequential instrument
 PMT moved behind aperture plate,
 or grating + prism moved to focus new l on exit slit
 Pre-configured exit slits to detect up to 20 lines, slew scan
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characteristics
 Cheaper
 Slower
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Multichannel instrument
 Polychromators (not monochromator) - multiple PMT's
 Array-based system
► charge-injection
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device/charge coupled device
characteristics
 Expensive ( > $80,000)
 Faster
Sequential vs. multichannel
Sequential monochromator
► Slew-scan
spectrometers
 even with many lines, much spectrum
contains no information
 rapidly scanned (slewed) across blank
regions (between atomic emission lines)
►From
165 nm to 800 nm in 20 msec
 slowly scanned across lines
►0.01
to 0.001 nm increment
 computer control/pre-selected lines to scan
Slew scan spectrometer
► Two
slewscan gratings
► Two PMTs for
VIS and UV
► Most use
holographic
grating
Scanning echelle spectrometer
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PMT is moved to monitor signal from slotted aperture.
 About 300 photo-etched slits
 1 second for moving one slit
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Can be used as multi channel spectrometer
Mostly with DC plasma source
AES instrument types
► Three
instrument types:
 sequential (scanning and slew-scanning)
 Multichannel - Measure intensities of a large
number of elements (50-60) simultaneously
 Fourier transform FT-AES
Multichannel polychromator AES
• Rowland circle
• Quantitative det.
20 more elements
Within 5 minutes
In 10 minutes
Applications of AES
► AES
relatively insensitive
 small excited state population at moderate
temperature
► AAS
still used more than AES
 less expensive/less complex instrumentation
 lower operating costs
 greater precision
► 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
Detection power of ICP-AES
ICP/OES INTERFERENCES
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Spectral interferences:
 caused by background emission from continuous or recombination
phenomena,
 stray light from the line emission of high concentration elements,
 overlap of a spectral line from another element,
 or unresolved overlap of molecular band spectra.
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Corrections
 Background emission and stray light compensated for by subtracting
background emission determined by measurements adjacent to the
analyte wavelength peak.
 Correction factors can be applied if interference is well characterized
 Inter-element corrections will vary for the same emission line among
instruments because of differences in resolution, as determined by the
grating, the entrance and exit slit widths, and by the order of
dispersion.
Physical interferences of ICP
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cause
 effects associated with the sample nebulization and transport
processes.
 Changes in viscosity and surface tension can cause significant
inaccuracies,
► especially in samples containing
► or high acid concentrations.
high dissolved solids
 Salt buildup at the tip of the nebulizer, affecting aerosol flow
rate and nebulization.
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Reduction
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by diluting the sample
or by using a peristaltic pump,
by using an internal standard
or by using a high solids nebulizer.
Interferences of ICP
► Chemical
interferences:
 include molecular compound formation,
ionization effects, and solute vaporization
effects.
 Normally, these effects are not significant
with the ICP technique.
 Chemical interferences are highly dependent
on matrix type and the specific analyte
element.
Memory interferences:
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When analytes in a previous sample contribute to the
signals measured in a new sample.
Memory effects can result
 from sample deposition on the uptake tubing to the nebulizer
 from the build up of sample material in the plasma torch and
spray chamber.
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The site where these effects occur is dependent on the
element and can be minimized
 by flushing the system with a rinse blank between samples.
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High salt concentrations can cause analyte signal
suppressions and confuse interference tests.
Typical Calibration ICP curves
Calibration curves of ICP-AES
10B. Arc and Spark AES
► Arc
and Spark Excitation Sources:
 Limited to semi-quantitative/qualitative
analysis (arc flicker)
 Usually performed on solids
 Largely displaced by plasma-AES
► Electric
current flowing between two C
electrodes
Carbon electrodes
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Sample pressed into
electrode or mixed with
Cu powder and pressed
- Briquetting (pelleting)
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
Arc and Spark spectrograph
spectrograph
► Beginning
1930s
► photographic film
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Cheap
Long integration times
Difficult to develop/analyze
Non-linearity of line "darkness“
►Gamma
function
►Plate calibration
Multichannel photoelectric
spectrometer
► multichannel
PMT instruments
 for rapid determinations (<20 lines) but not
versatile
 For routine analysis of solids
►metals,
alloys, ores, rocks, soils
 portable instruments
► Multichannel
charge transfer devices
 Recently on the market
 Orignally developed for plasma sources
Homework
► 10-1,
10-2, 10-5, 10-7