Elemental Spectroscopy
ICP-OES
Content: ICP-OES
• Fundamentals of ICP-OES
• Instrument Components
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Theory of Inductively Coupled Plasma
Optical Emission Spectroscopy
ICP Basics
ICP is shorthand for ICP-AES or ICP-OES.
What is ICP-AES? It is:
Inductively Coupled Plasma Atomic Emission Spectrometer.
What is ICP-OES? It is:
Inductively Coupled Plasma Optical Emission Spectrometer.
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Atomic Emission Theory
• Atomic emission spectroscopy (AES or OES) uses quantitative measurement of the
optical emission from excited atoms to determine analyte concentration
• Analyte atoms in solution are aspirated into the excitation region where they are
desolvated, vaporized, and atomised by a plasma
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Atomic Emission Theory
Inductively Coupled Plasma Atomic Emission Spectrometer
Plasma
6
Polychromator
Detector
Excitation
Excitation
E
Relaxation
Ground State
Excited State
Electrons can be in their ground state (unexcited) or
enter one of the upper level orbitals when energy is
applied to them. This is the excited state
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Atomic Emission
A photon of light is emitted when an electron falls from
its excited state to its ground state
+ hv
Excited State
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Ground State
Photon
Element Wavelengths
• Each element has a unique set of wavelengths that it
can emit
<-- uv -->
180nm

9
1
<-- visible -->
400nm

2

3

4
800nm

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Atomic Emission explained
• Atomic Emission – the wavelength regions
Spectral Region
Vacuum UV
160
Ultra-Violet
190
Visible
360
Wavelength = nm
Near IR
760
900
Lower wavelengths are shorter and have more energy,
higher wavelengths e.g. in the Visible region, are longer
and have less energy
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Effect of Temperature on Emission
Wavelength increasing ->
200
300
400
600
800
5000 k
As Pb
Mn Mg Cu
Ca
Ba
Na
Li
K
Mg Cu
Ca
Ba
Na
Li
K
Na
Li
K
3000 k
2000 k
Ca
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Emission sources
• Flames
• Arcs / Sparks
• Direct Current Plasmas (DCP)
• Inductively Coupled Plasmas (ICP)
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Inductively Coupled Plasma (ICP)
– source, plasma formation, plasma zones
• Quartz torch surrounded by induction coil
• Magnetic coupling to ionized gas
• High temperature – equivalent to 10,000k
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Plasma Advantages
• High Temperature – allows for full dissociation of sample
components
• Argon is Inert – non reactive with sample
• Linearity – analysis of samples from ppb to ppm range in the same
method
• Matrix tolerance – robust and flexible design with Duo and Radial
options
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Plasma Torch
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Plasma Zones
Plasma Zones
observation
region (mm)
25
6000 k
20
6500 k
15
7000 k
RESIDENCE TIME ~ 2MS
8000 k
0
10000 k
sample
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TEMPERATURE ~ 2X
NITROUS OXIDE
ACETYLENE FLAME
Instrument Components
There are six basic
components to an ICP
1. Sample Introduction
2. Energy Source
3. Spectrometer
4. Detector
5. Electronics
6. Computer and Software
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Instrument Components
1.
Sample
Introduction
2.
Energy
Source
3.
6.
Computer and
Software
5.
Spectrometer
Electronics
4.
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Detector
Instrument Components
1. Sample
Introduction
The sample solution
cannot be put into the
energy source
directly. The solution
must first be
converted to an
aerosol.
The function of the
sample introduction
system is to produce
a steady aerosol of
very fine droplets.
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Instrument Components
1. Sample
Introduction
There are three basic
parts to the sample
introduction system.
i. the Peristaltic pump
draws up sample
solution and delivers it to
ii.the Nebulizer
which converts the solution
to an aerosol that is sent to
iii. the Spray chamber
which filters out the large,
uneven droplets from the
aerosol.
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Instrument Components
1. Sample
Introduction
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i.
the Peristaltic
pump
ii.
the Nebulizer
iii.
the Spray
chamber
Concentric Nebuliser
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Instrument Components
2. Energy
Source
= plasma
The sample aerosol
is directed into the
center of the plasma.
The energy of the
plasma is transferred
to the aerosol.
The main function of
the energy source is
to get atoms
sufficiently energized
such that they emit
light.
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Instrument Components
2. Energy
Source
There are three basic parts to the
energy source.
i. the Radio frequency generator
which generates an oscillating electomagnetic field at a frequency of 27.12
million cycles per second. This radiation
is directed to
ii.the Load coil
which delivers the radiation to
iii. the Torch
which has argon flowing through it which
will form a plasma in the RF field.
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Instrument Components
2. Energy
Source
i. the Radio
Frequency
generator
ii. the Load coil
iii. the Torch
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Plasma Configuration
• Axial
• Radial
• Axial and Radial
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Radial or Axial Configuration
• Radial design – Robust, fewer interferences
• Petrochemical
• Metallurgy
• Axial design – best sensitivity,
lowest detection limits
• Environmental
• Chemical
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Axial Advantage
• Much more light available. This gives you the opportunity to
achieve Lower Detection Limits than Radial Plasma
• BUT- unfortunately, you also get...
• More Matrix Interferences
• Slightly Reduced Dynamic Range
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Duo viewing
• Axial view plasma looks down the central channel of the plasma,
this provides the best sensitivity and detection limits
• DUO – this is an axially configured plasma that also allows for radial
view through a hole in the side of the axial torch
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Dual View Optics
Radial view
Axial view
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Instrument Components
3. Spectrometer
Once the atoms in a
sample have been
energized by the
plasma, they will emit
light at specific
wavelengths. No two
elements will emit
light at the same
wavelengths.
The function of the
spectrometer is to
diffract the white light
from the plasma into
wavelengths.
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Simultaneous Optics – Echelle Spectrometer
Detector
Grating
Prism
ICP-Source
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Instrument Components
3. Spectrometer
There are several
types of
spectrometers
used for ICP.
Regardless of type,
all of them use a
diffraction grating.
CID Detector
Focusing
Mirror
Echelle
grating
Prism
Shutter
Collimating
Mirror
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Slit
(dual)
For the iCAP, an
echelle
spectrometer is
used. The
components in this
spectrometer are
shown at left.
iCAP Optics - Polychromator
• High resolution
• 7pm @ 200nm
• High image quality & low stray light
• aberration compensation over whole CID
• High energy throughput
• double pass prism
• All lines on chip
• anamorphic magnification
• Stable
• thermal insulation & heater control to 0.10C
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Instrument Components
4. Detector
Now that there are
individual wavelengths,
their intensities can be
measured using a
detector. The intensity
of a given wavelength
is proportional to the
concentration of the
element.
The function of the
detector is to measure
the intensity of the
wavelengths.
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Charge Injection Device Array Detector
•
>291,600 addressable silicon-based
photo detectors
•
Full Spectrum Imaging
•
Random Access Integration (RAI)
•
Inherently Anti-blooming
– Non Destructive Readout (NDRO),
allows the S/N ratio to be improved by
repeatedly reading each pixel
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Instrument Components
4. Detector
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The detector is a
silicon chip that is
composed of many
individual photoactive sections called
“picture elements”.
These picture
elements, or pixels,
will build up charge
as photons impinge
on them. Individual
pixels are of a size
such that they can be
used to measure
individual
wavelengths.
Emission lines appear as points of light
800 nm
178 nm
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740 nm
177 nm
Readout Subarray - CID
Intensit
y
28 by 28 mm
detector element
Wavelength
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What you get
Full, continuous
wavelength coverage;
never miss an analyte
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Power and flexibility
• Rapid qualitative analysis
• Ability to analyze for elements in the
future without rerunning samples
• Fingerprinting
• Matrix or spectral subtraction
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Instrument Components
5. Electronics
The output from the
detector is processed
by a set of
electronics. The
electronics control the
detector as well as
collect the readings
from the pixels
The function of the
electronics is to
measure and process
the output of the
detector.
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Instrument Components
6. Computer and Software
The software, via a
computer, controls and
runs the instrument. Not
only are measurements
made but the other five
components of the
instrument are controlled
and monitored by the
computer and software,
The function of the
computer and software is
to operate, monitor, and
collect data from the
instrument.
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ICP Basics
ICP Performance
• Typical analysis time for ICP is ~2-3 minutes. This includes
flush time, multiple repeats, printing, etc. (Analysis time is
independent of the number of elements being determined)
• Typical precision, amongst repeats within an analysis, is
~0.5%
• Typical drift is ≤ 2% per hour
• Typical detection limits are ~ 1-10 parts per billion
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