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ICP-OES- Inductively Coupled Plasma Optical Emission Spectrometry
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Inductively Coupled Plasma Atomic Emission Spectrometry(ICP-AES)
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Atomic spectrometry
Atomic Absorption
Mass Spectrometry
Atomic Emission
Light of specific characteristic wavelength is
absorbed by promoting an electron to a higher
energy level (excitation)
Light absorption is proportional to
elemental concentration
Light of specific wavelength
from Hollow Cathode Lamp (HCL)
Light and heat energy from high
intensity source (flame or
plasma)
Light and heat energy from high
intensity source (plasma)
High energy (light and heat) promotes an
electron to a higher energy level (excitation).
Electron falls back and emits light at
characteristic wavelength
Light emission is proportional to
elemental concentration
-
-
-
- -
-
-
High energy (light and heat) ejects electronfrom shell (ionization). Result is free electron
and atom with positive charge (Ion)
Ions are extracted and measured directly in
mass spectrometer
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ICP-MS
ICP-AES
GF-AAS
Ranges< ppt
100 ppb0.1 ppb
10 ppm1 ppb
100 ppb
Merit
Demerit
Detection mass emission absorbance
high sensitivity
multielement
multielement
damage from
high salinity
relatively low
sensitivity
high sensitivity
monoelement
Characteristics of instruments
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i. Need to break sample into atoms to observe atomic spectra
ii. Basic steps:a) nebulization solution sample, get into fine droplets by spraying through thin nozzle
b) desolvation - heat droplets to evaporate off solvent just leaving analyte and other
matrix compounds
c) volatilization convert solid analyte/matrix particles into gas phase
d) dissociation break-up molecules in gas phase into atoms.
e) excitation and ionization
with light, heat, etc. for spectra measurement.->cause theatoms to become charged
Atomization
Evaporation/ Vapouration /
Dissociation
Excitation and ionization/
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Inductively Coupled Plasma
Inductively Coupled Plasma (ICP)
Plasma generated in a device called a 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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Torch
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Magnetic field
Ions forced to flow in closed
path, Resistance to flow
causes heating
Ar charges
by Tesla coil(high voltages at high frequency)
Temperature Regions
in Plasma Torch
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Plasma characteristics
Hotter than flame (10,000 K) -more complete atomization/excitation
Atomized in "inert" atmosphere
Ionization interference due to high
density of e- is very small Sample atoms reside in plasma for
~2 msec
Plasma chemically inert, little oxideformation
Temperature profile quite stableand uniform.
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Instrumentation of ICP-AES
1. Radio Frequency (RF) Generator
2. Sample Introduction System
3. Torch
4. Spectrometer (Polychromators)
5. Detector
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Schematic diagram of ICP-AES instrument
Radiofrequencygenerator
Spray chamber
To drain
Injectorgas
Samplesolution
Peristalticpurge
Sample capillaryNebulizer
Coating gas
Auxiliary gas
Coolant gas Torch
SpectrometerInduction cell
Fireball
Tail flame
PMT
ComputerComputer
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A. RF Generator
Radio-Frequency (RF) Generator is a device that is used toprovide the power required for the generation and sustaining of
the plasma discharge.
The power required for ICP-AES measurements ranges
between 600 and 2500 W
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B. Sample Introduction System
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Peristaltic Pump
(1) Peristaltic Pump
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(2) Spray Chamber
Scott chamber- traditional equipment
- large volume
Cyclonic chamber
more aerosolsmaller volume
better wash out
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Why we need Spray Chambers?
Separation of small droplets from large ones
- small droplets to the plasma
- large droplets to the drain
Compensate pulsation of the pump
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(3) Nebu lizer
Transform a liquid sample into an aerosol
- direct introduction of a liquid would extinguish plasma
- two types of nebulizers are commonly used
(a) pneumatic nebulizers
(b) ultrasonic nebulizers- mostly use of a peristaltic pump to transport
(a) sample to the nebulizers
(b) aerosol to the plasma
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Torch
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Plasma Torches
The plasma torch consists of three concentric quartz tubesthrough which the gas (normally argon) flows:
- The outer tubecontains the coolant gas flow, whichspirally flows tangentially through the torch at a high
velocity. This assists in cooling the torch and henceprevents damage.
- The middle tubecontains the auxiliary gas flow tokeep the plasma discharge away from the auxiliaryand nebulizer tubes
- The innermost tubehas the nebulizer gas flow whichcarries the sample aerosol to the plasma.
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4 S t t (P l h t )
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4. Spectrometer (Polychromator)
The role of a spectrometer is to isolate the analytical wavelengths of interest
from the light emitted by the plasma source.
The advantage of polychromators is:- capable of determining several analytes simultaneously.
- high sample throughput
- lower running cost
Echelle Polychromator
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5. Detectors
Commonly used detectors:
Photomultiplier tubes (PMT)
Solid-state detectors: Charge-coupled devices (CCD)arrays detector
Silicon photodiodes with thousands of individual
elements
Very sensitive, very well-suited to echelle gratingpolychromators, very fast
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Photomultiplier tubes (PMT)
Schematic diagram of
an end-on PMT
PMT consists of a photo-sensitive cathodeand a series of dynodes, which are set at successively
more positive potentials until an anode is reached. Light comes from the plasma, passes through
the transparent casing of the multiplier and strikes cathode. This then emits electrons, which are
accelerated down the dynode chain. Each time an electron impacts with a dynode, a number of
secondary electrons are emitted.
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CCD
Examples of the wavelength ranges covered
by individual CCD chips:
Chip 1 127,000 - 142,125 nm
Chip 2 141,285 - 160,681 nm
Chip 3 160,040 - 179,618 nm Chip 4 178,704 - 198,477 nm
Chip 5 197,023 - 216,947 nm
Chip 6 215,903 - 235,897 nm
Chip 7 234,792 - 254,788 nm
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K 766nm
Virtual Entrance Slit
Second Grating2400 l/mm
First Grating2924 l/mm
Entrance Slit
125nm
460nm
Li 670nm
Na 589nm
Multi CCD
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Detection Limits of ICP-OES
Typical detection
limits (Varian VistaMPX):
Considerations
include the number
of emission lines,
spectral overlap
Linearity can span
several orders of
magnitude.
Element Wavelength (nm)
Detection Limit
axial (ug/L)
Detection limit
radial (ug/L)
Ag 328.068 0.5 1
Al 396.152 0.9 4
As 188.98 3 12
As 193.696 4 11Ba 233.527 0.1 0.7
Ba 455.403 0.03 0.15
Ba 455.403 0.03 0.15
Be 313.107 0.05 0.15
Ca 396.847 0.01 0.3
Ca 317.933 0.8 6.5
Cd 214.439 0.2 0.5
Co 238.892 0.4 1.2
Cr 267.716 0.5 1
Cu 327.395 0.9 1.5Fe 238.204 0.3 0.9
K 766.491 0.3 4
Li 670.783 0.06 1
Mg 279.55 0.05 0.1
Mg 279.8 1.5 10
Mn 257.61 0.1 0.133
Mo 202.03 0.5 2
Na 589.59 0.2 1.5
Ni 231.6 0.7 2.1
P 177.43 4 25
Pb 220.35 1.5 8
Rb 780.03 1 5
S 181.972 4 13
Sb 206.83 3 16
Se 196.03 4 16
Sr 407.77 0.02 0.1
Sn 189.93 2 8
Ti 336.12 0.5 1
Tl 190.79 2 13
V 292.4 0.7 2
Zn 213.86 0.2 0.8
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Sampling and sample preparation
Are the samples representative of what you are trying to
measure?
Will any elements volatilize during sample preparation?
How much contamination can the sample tolerate during
preparation?
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Sample preparation
For many applications, the sample analyzed by ICP-AES will not be insuitable form . In order to transform solid samples into suitable form, sample pre-treatment is required.
The pre-treatment method used will be depend on the natureof the sample and the element which are to be determined.
Those methods commonly used are:- Dry-ash,
- Acid digestion,
- Fusion,
- Solubilization
- Microwave digestion.
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Analysis using ICP Sample preparation
Dry Ashing
samplecrucible
Mufflefurnace
Dry at 105 -100
Ash at 200-800
Dissolve the ashed sample inacids, usually HCl, H2SO4,
HNO3and HCl/ HNO3
Acid Digestion
Preparation is simple and widely applicable but sample lossesthrough volatilization and retention
Use of strongly oxidizing mineral acid, suchas HNO3, HF, H2SO4and HClO4 to oxidizethe resistant components, with gentle heat.
It can be carried out in closed or openreaction systems. Closed systems minimize
the losses in volatilization.
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Advantages:
Disadvantages:
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Analysis using ICP Sample preparation
Salt Fusion
Sample is mixed in a platinum crucible with a fluxwhich attacks all the major rock-forming silicates.
Fused in a furnace.
Cooled to room temperature.
Dissolved in HNO3.
Fusion methods are commonly used with geological samples. Sample + Fluxcrucible
Mufflefurnace
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Advantages:
Disadvantages:
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Analysis using ICP Sample preparation
Microwave digestion
Microwave sample preparationuses microwave power to heat
several samples at once, whichcan speed up a digestionprocesses.
Advantages: improved detectionlimits, low acid concentrationand a reduced need for dryashing or fusion.
Reaction pressure, temperatureand time are computer controlled
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Plug
Safety Disk
SealScrew Cap
Liner
Bomb Jacket
Vessel Base Plate
Supporting Vessel
Pressure vessel (TFM)
Microwave sample preparation system
MULTIWAVE (Anton Paar GmbH)
Acid digestion apparatus: microwave system
Preparation of Solid Sample
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Analysis using ICP Sample preparationMicrowave digestion
USEPA method 3051: MICROWAVE ASSISTED ACID DIGESTION OF SEDIMENTS,SLUDGES, SOILS, AND OILS
Digestion vessels carefully acid washed and rinsed with water before use.
Sample up to 0.5 g and 10 ml of concentrated HNO3are placed in microwave vessel.
(For soils, sediments, and sludge use no more than 0.50 g. For oils use no more than 0.25 g.) Sample vessel equipped with a single-port cap and a pressure relief valve.
The vessels are capped and heated by a suitable laboratory microwave unit.
The vessel contents are filtered, centrifuged, or allowed to settle and then
diluted to volume and analyzed.
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Advantages:
Disadvantages:
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ICP/AES INTERFERENCES
Spectral Interferences
Physical Interferences
Chemical Interferences
Memory Effect
S t l i t f
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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.
Corrections
Background emission and stray light compensated for by subtractingbackground 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.
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The Example for Choosing Wavelength in ICP-AES
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The Example for Choosing Wavelength in ICP AES
Element wavelength
(nm)Element wavelength
(nm)
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Physical interferences
Cause effects associated with the sample nebulization and transport
processes.
Changes in viscosity and surface tension can cause significantinaccuracies,
especially in samples containing high dissolved solids
or high acid concentrations.
Salt build up at the tip of the nebulizer, affecting aerosol flow rateand nebulization.
Reduction by diluting the sample or by using a peristaltic pump,
by using an internal standard
or by using a high solids nebulizer.
Ph i l i t f
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Physical interferences
Wavelength calibration
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Chemical interferences:
include molecular compound formation
Normally, this effect is not significant with the
ICP technique.
Chemical interferences are highly dependent
on matrix type and the specific analyteelement.
C bi d Eff t
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Combined Effects
Compensation : (a) matrix of standards should be closely matched with that
of the samples (matrix-matched calibration)
(b) Matrix removal
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Memory interferences:
When analytes in a previous sample contribute to thesignals 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 andspray chamber.
The site where these effects occur is dependent on theelement and can be minimized by flushing the system with a rinse blank between samples.
High sal t concentrat ions can cause analyte signalsuppressions and confuse interference tests.
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