Inductively Coupled Plasma-Optical Emission Spectrometry (ICP-OES)

Figure 1. The Varian Vista-PRO CCD Simultaneous ICP-OES in the Physical Geography laboratories

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.

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:

• 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.

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

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

70000 72000 74000 76000 78000 80000 82000 84000 86000 88000 90000 92000

64000

66000

68000

70000

72000

74000

76000 N i m g/kg

14

16

18

20

22

24

26

28

30

32

34

36

38

40

42

44

46

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.

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.

Projects using ICP-OES Calibration What is ICP-OES?

Examples of research at QMUL

Figure 3. Nickel contamination of sediments in the Medway Estuary (Miriam Reid).

Advantages and limitations of ICP-OES

Other recent projects include:

• Analysis of arsenic and lead contamination in soils around a new development for Harrow Borough Council.

• Analysis of lake sediment samples from the Peak and Lake Districts for Cu, Zn, Cd and Pb in collaboration with researchers at York University.

• Analysis of base cations in storm flows from streams on Exmoor in collaboration with researchers from the University of Plymouth.