Application of Zonal Centrifugation in
Environmental Studies
W. P. BONNER, Research Scientist
C. W. FRANCIS, Research Scientist
TSUNEO TAMURA, Research Scientist
Health Physics Division
Oak Ridge National Laboratory
Oak Ridge, Tennessee
INTRODUCTION
To determine the fate of pollutants in the environment, it is necessary to
understand the processes which occur therein. One way to aid in the understanding
of the processes is to separate the components and to study them separately, as well
as in selected combinations, simulating the varied and many environmental reactions.
In classical centrifugation techniques the components are separated by
centrifuging the sample from a solution of constant density (water), thus permitting
separation based upon particle size and sedimentation rate. In most cases the
separated fractions contain a mixed suite of minerals. Frequently, strong chemical
treatments are used to remove cementing agents and to disperse the sample prior to
separation by centrifugation. These treatments destroy the organic components and
could chemically alter a component of interest. Zonal centrifugation, developed
primarily for the separation of biological components, has been used to separate
environmental media into many of its organic and inorganic components according
to density. To minimize alteration of the sample, strong chemical pretreatment has
been avoided.
The purpose of this paper is to briefly discuss the technique of zonal
centrifugation and some of its applications in the investigation of environmental
media. Although the technique has many potential applications, a few typical
applications are discussed. Emphasis is placed on results from our Laboratory,
especially results from the separation and analyses of components of soils and
bottom sediments.
PRINCIPLE OF ZONAL CENTRIFUGATION
Separation by zonal centrifugation involves the preparation of a continuous
liquid density gradient in a tube, layering the sample to be separated onto the
gradient solution and centrifuging until the component or components of interest are
sufficiently separated. Separation is achieved by either of two methods, rate or
isopycnic zonal cetrifugation. Rate separations are effective when a component of
interest differs from the other components in its rate of sedimentation either in a
liquid density gradient or a solution of uniform density. Isopycnic separations are
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