Metals Analysis of Soils and Sediments
by Flame Atomic Absorption Spectrophotometry
Last revised July 22, 1997
Page contents (click to skip down):
[Overview of technique]
[Considerations for soils and sediments]
[Flame-AA assay for Ca]
[Flame-AA assay for Mg]
[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
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,
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.