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Inductively Coupled Plasma-Optical Emission Spectrometry (ICP-OES)
What is ICP-OES?
Calibration
Projects using ICP-OES
Inductively Coupled Plasma Optical Emission
Spectrometry (ICP-OES) is a fast, multi-element
technique used to measure trace metals such as lead
(Pb), copper (Cu), nickel (Ni) and zinc (Zn) and major
cations such as calcium (Ca), magnesium (Mg) and
sodium (Na). Inductively Coupled Plasma techniques
operate by decomposing a liquid sample by intense
heat into a cloud of hot gases with an inductive coupled
plasma (a state of matter containing electrons and
ionised atoms of Argon). The plasma reaches
temperatures of around 10,000°C
A calibration is necessary for quantitative analysis (figure 2). By
comparing the intensity of light emitted by solutions of known
metal concentrations with unknown sample solutions, metal
concentration can be determined.
We use ICP-OES to measure trace metals and major
cations in a range of environmental samples from
urban canals to remote lake sediments. Recent third
year IGS projects have included analysis of Pb in
vegetables grown on heavily polluted urban soils and
analysis of Cu, Zn, Cd and Ni in soils close to a major
road.
Examples of research at QMUL
Miriam Reid and Kate Spencer use ICP-OES to
investigate heavy metal contamination in estuarine
sediments. Toxic metals have been released by
industry and sewage outflow and deposited in
sediments. Figure 3 shows Ni concentrations in
Medway Estuary sediments.
The high temperature causes excitation and ionisation
of the sample atoms. Once the atoms or ions are in their
excited energy states, they can decay to lower energy
states whilst emitting light of specific wavelengths
depending of the elements in the solution. In OES, the
intensity of the light emitted at specific wavelengths is
measured and used to determine the concentrations of
the elements of interest.
Figure 2. Calibration curve for Cu: intensity measured from standards of
0. 1. 2. 4 and 10 mg l-1
Advantages and limitations of ICP-OES
ICP-OES is a moderately sensitive techniques that can analyse a
wide range of elements simultaneously. Under optimum conditions
it can analyse over 100 samples per day. It is important, however,
to be aware of the limitations of the method. These include:
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Figure 1. The Varian Vista-PRO CCD Simultaneous ICP-OES in
the Physical Geography laboratories
References
Jose Luis Todol, Luis Gras, Vicente Hernandis and Juan Mora (2002).
Elemental matrix effects in ICP-AES. J. Anal. At. Spectrom., 17, 142–169
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Spectral interference between different elements. The
wavelength of one element's light emission can sometimes
be close enough to that of another element to cause
problems.
Matrix effects caused by high concentrations of an element in
the sample, (most commonly the easily ionisable Na, K, Mg
or Ca) can change the way the sample is introduced to the
flame or the thermal characteristics of the plasma and lead to
over or underestimation of sample concentration.
Optimum conditions for analysis occur for different elements
under different conditions, therefore sensitivity can be
compromised when running for multi-element analysis.
Figure 3. Nickel contamination of sediments in the Medway
Estuary (Miriam Reid).
Other recent projects include:
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Analysis of arsenic and lead contamination in soils
around a new development for Harrow Borough
Council.
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Analysis of lake sediment samples from the Peak
and Lake Districts for Cu, Zn, Cd and Pb in
collaboration with researchers at York University.
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Analysis of base cations in storm flows from
streams on Exmoor in collaboration with
researchers from the University of Plymouth.