ICP – OES

Inductively Coupled Plasma – Optical Emission Spectrometry

Also called ICP-AES (Atomic Emission Spectrometry)

Many different soil extracts and digests can be analyzed by ICP-OES. The instrument can analyze for multiple elements at the same time. This is very convenient as it means that only one set of multi-element standards needs to be made up. This also allows one to obtain a large number of data from running the sample just once. An ICP-OES instrument is quite sensitive, hence it is possible to measure very low concentrations. Detection limits can be very low depending on the element and sample matrix (e.g., when analyzing the solutions from an aqua regia digestion, the detection limit for Ni might be 0.01 ppm).

Samples are introduced to the instrument as an aerosol, and as the sample passes through the plasma it becomes ionized. Excited atoms emit energy as they come back to ground state; the detector records the wavelengths and intensity of these emissions. Based on the standards that have been run, the software will then interpret this output as a concentration for each element.

Key advantages of ICP-OES:

• Low detection limits
• Can analyze for many elements at once/li>
• Fairly simple to run the instrument/li>

One disadvantage of ICP-OES is that the instrument is expensive to run (high purity argon gas is required)

Atomic Absorption Spectrophotometry (AAS)

AAS can be used to analyze a variety of soil extracts and digests for metal concentrations.

The sample is aspirated into a flame, where it becomes an atomic vapor. A radiation source (hollow cathode lamps are the most common) is directed at the sample and the atoms in the sample absorb light, each element absorbing specific wavelengths. The detector measures the change in radiation intensity on the other side, after it has passed through the sample. Based on the Beer-Lambert law, the amount of absorbed light is proportional to the concentration of the analyte.

Advantages of AAS compared to ICP-OES:

• Less expensive
• Fairly simple to run and maintain

Disadvantages of AAS compared to ICP-OES:

• Detection limits are not as low
• Analyzes for one element at a time

Resources and References

• Calderon, V. No date. Rapid measurement of major, minor and trace levels in soils using the Varian 730-ES. ICP-OES Application Note Number 35. Varian, Inc.
• L’vov, B.V. 1997. Forty years of electrothermal atomic absorption spectrometry. Advances and problems in theory. Spectrochimica Acta Part B: Atomic Spectroscopy 52:1239-1245.
• Nham, T.T. No date. Analysis of soil extracts using the Varian 725-ES. ICP-OES Application Note Number 34. Varian, Inc.
• Soltanpour, P.N., G.W. Johnson, S.M. Workman, J.B. Jones Jr, R.O. Miller. 1996. Inductively Coupled Plasma Emission Spectrometry. In Sparks, D.L. (ed). Methods of Soil Analysis: chemical methods. Part 3. Soil Sci. Soc. Am. Book Series No. 5. ASA-SSSA, Madison, WI.
• Tan, K.H. 2005. Soil Sampling, Preparation, and Analysis. 2nd edition. CRC Press, Taylor & Francis Group. Boca Raton, FL.
• Watson, M.E., and R.A. Isaac. 1990. Analytical Instruments for Soil and Plant Analysis. In Westerman, R.L. (ed.) Soil Testing and Plant Analysis. 3rd edition. ASA-SSSA, Madison, WI.
• Wright, R.J., and T.I. Stuczynski. 1996. Atomic Absorption and Flame Emission Spectrometry. In Sparks, D.L. (ed). Methods of Soil Analysis: chemical methods. Part 3. Soil Sci. Soc. Am. Book Series No. 5. ASA-SSSA, Madison, WI.