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Metals Analysis of Soils and Sediments

by Flame Atomic Absorption Spectrophotometry

Last revised July 22, 1997

Page contents (click to skip down):

* [Available analytes]
* [Overview of technique]
* [Considerations for soils and sediments]
* [Sample requirements]

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Available analytes

- soil Calcium:
* [double-acid extraction]
* [Flame-AA assay for Ca]
- soil Magnesium:
* [double-acid extraction]
* [Flame-AA assay for Mg]
- soil Potassium:
* [double-acid extraction]
* [Flame-AA assay for K]

Overview of technique

In flame atomic absorption spectroscopy a liquid sample is aspirated and mixed as an aerosol with combustible gasses (acetylene and air or acetylene and nitrous oxide.) The mixture is ignited in a flame of temperature ranging from 2100 to 2800 degrees C (depending on the fuel gas used.) During combustion, atoms of the element of interest in the sample are reduced to the atomic state. A light beam from a lamp whose cathode is made of the element being determined is passed through the flame into a monochronometer and detector. Free, unexcited ground state atoms of the element absorb light at characteristic wavelengths; this reduction of the light energy at the analytical wavelength is a measure of the amount of the element in the sample.

Considerations for soil and sediments

Samples must be in liquid form to be aspirated by the instrument. Therefore, solid material must be liquified by means of some form of extract or digest protocol. Procedures have been devised that make the total amount of an element in the sample available for assay or that use some particular property to extract that portion of the element which exists in some chemical forms but not in others.

For ecological purposes there is more interest in measures of extractable or labile soil constituents than in total elemental content. Certain partitions of the total soil content of a given element are operationally defined by an extraction procedure, and arguments are usually offered that these partitions, so defined, correspond to different levels of biological availability or activity.

The HCl/H2SO4 double acid extraction solution, also referred to as North Carolina and Mehlich-1, is widely used to determine bioavailable Ca, K, Mg, Mn, P, and Zn in sandy acid soils characteristic of the eastern and southeastern United States

Sample requirements
The [double-acid extraction] procedure calls for an amount of sample soil that yields 5 gm. (~4 ml. volume) after drying and passing through a 2 mm sieve. Investigators should consult the standard literature for considerations and cautions regarding soil sample collection, processing, and storage.
Baker, Dale E. and Norman H Shur. 1982.
Atomic absorption and flame emission spectrometry. pp. 13-27. In A. L. Page et al., eds., Methods of Soil Analysis: Part 2, Chemical and Microbiological Properties. Agronomy, A Series of Monographs, no.9 pt.2, Soil Science Society of America, Madison, Wisconsin USA.

Mehlich, A. 1953.
Determination of P, K, Na, Ca, Mg and NH4. Soil Test Division Mimeo, North Carolina Department of Agriculture, Raleigh, NC USA.

U. S. Environmental Protection Agency. 1983.
Metals (atomic absorption methods). pp. 55-72. In Methods for Chemical Analysis of Water and Wastes, EPA-600/4-79-020. U.S.E.P.A., Cincinnati, Ohio, USA.

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