Metals Analysis by Flame Atomic Absorption Spectrophotometry

Last revised July 22, 1997

Page contents (click to skip down):

* [Available analytes]
* [Overview of technique]
* [Specific sample considerations]
* [Sample requirements]

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Available analytes

- Calcium:
* Plant [dry ash/double-acid extraction]
* Water [Ca dissolved/total partitioning]
* Soil [double-acid extraction]
* [Flame-AA assay for Ca]

- Magnesium:
* Plant [dry ash/double-acid extraction]
* Water [Mg dissolved/total partitioning]
* Soil [double-acid extraction]
* [Flame-AA assay for Mg]

- Potassium:
* Plant [dry ash/double-acid extraction]
* Water [K dissolved/total partitioning]
* Soil [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.

Specific sample considerations

Plant: solid 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. The [plant dry ash/double acid] extraction method determines the total element content of the sample.

Soil: for ecological purposes there is more interest in measures of extractable or labile soil constituents than in total element 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 method, 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

Water: aquatic samples of course need no liquification step, but researchers must still decide which analyte partition (dissolved, suspended, total) is of interest. Differing treatments of each sample partition are detailed in the U.S. E.P.A.'s discussion of [Content partitioning] of water samples.

Sample requirements
Plant:The [dry ash/double-acid extraction] procedure calls for an amount of material that yields 0.5 gm. after drying and grinding.

Soil: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.

Water:To allow for repeat measurements of dissolved element, the investigator should collect 20 ml of sample for each desired analyte. If a digest for total element is desired, considerably more sample must be collected. Please refer to the E.P.A.'s particulars for [Total element digests] of water samples. The investigator should also refer to the E.P.A.'s table of recommended [Sample collection/preservation] procedures for specific details of aquatic sampling.

Allen, S. E., et al. 1974.
Chemical Analysis of Ecological Materials. John Wiley and Sons, New York.

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.

Jones, J. B. Jr., B. Wolf and H. A. Mills. 1990.
Organic matter destruction procedures. pp.195-6. In Plant Analysis Handbook. Micro-Macro Publishing, Inc., Athens, GA.

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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