Extraction of organic compounds from sediments 125
5.2.1 Filtration methods
It should be mentioned at this point that the acceptance of 0.45 µm as the dividing line
is purely nominal, since few workers in the field actually use filters with this pore size.
The glass fibre filters used by many workers have pore sizes which are considerably
larger, ranging from 0.7 µm for Whatman GF/F to 1.32 µm for GF/C. With these
filters, all particles larger than the nominal pore size are retained, but many smaller
particles are also trapped. The silver filters, and most particularly the 0.4 µm size,
contain relatively large and variable amounts of carbon, which must be removed by
combustion. After this combustion, the pore sizes are considerably enlarged, with the
0.45 µm filter approaching 0.8 µm in pore size. The nominal 0.8 µm pore size filter
is used by many investigators because the pore size changes very little under heat
treatment. Thus, although 0.45 µm has been accepted as the minimum size particulate
matter by definition, the filters actually used have a somewhat larger pore size, and
retain particles which are considerably smaller than the nominal cut-off size (Sheldon
and Sutcliffe [10]).
The choice of filter can determine the amount of material considered as particulate,
sometimes with unexpected results. Thus, the Whatman GF/C filter with its larger pore
size actually retains about three times as much particulate organic carbon as does the
0.8 µm silver filter. Presumably the difference results from the larger number of small
particles retained by the glass fibre filters.
The method of calculation of the blank can also influence the determined sediment
content. If surface sea water is filtered through a pad consisting of two or more filters,
either glass fibre or silver, the bottom filter will often contain a small amount of
sediment above the blank value. Some workers have maintained that this is due to
the adsorption of dissolved organic matter on to the filter and that this value should
therefore be subtracted from the weight of sediment found on the top filter (Banoub
and Williams et al [11], Menzel et al [12]). Other workers feel that the material caught
in the second filter is largely composed of smaller particles passing through the first
filter. Depending on the way in which the particulate fraction is defined, the material
caught by the second filter should either be added to that collected on the first filter
(Bishop and Edmond et al [13]) or ignored (Gordon and Sutcliffe [14], Sharp [15],
Wangersky [16]). It can easily be seen that the choice of blank calculation can cause a
considerable difference to the final values given for sediment content, at least in surface
waters,. As far as the particulate fraction is defined, not in terms of particle size, but
in terms of material caught on a specific filter, it is recommended that only one filter,
rather than a pad of two or more, be used, since the material caught on subsequent
filters is irrelevant by definition.
5.2.2 Separation by centrifugation
A method for removing particles which is not limited in volume sampled, and
which suffers less from problems of overlapping classification is continuous-flow centrifugation. Separation into density classes can be achieved by choice of speed of
centrifugation [18].
Centrifugation a dry sediment in tetrabromoethane provided several fractions
based on density (Table 5.1). The mass balance of lead, cadmium and zinc in each
5.2.1 Filtration methods
It should be mentioned at this point that the acceptance of 0.45 µm as the dividing line
is purely nominal, since few workers in the field actually use filters with this pore size.
The glass fibre filters used by many workers have pore sizes which are considerably
larger, ranging from 0.7 µm for Whatman GF/F to 1.32 µm for GF/C. With these
filters, all particles larger than the nominal pore size are retained, but many smaller
particles are also trapped. The silver filters, and most particularly the 0.4 µm size,
contain relatively large and variable amounts of carbon, which must be removed by
combustion. After this combustion, the pore sizes are considerably enlarged, with the
0.45 µm filter approaching 0.8 µm in pore size. The nominal 0.8 µm pore size filter
is used by many investigators because the pore size changes very little under heat
treatment. Thus, although 0.45 µm has been accepted as the minimum size particulate
matter by definition, the filters actually used have a somewhat larger pore size, and
retain particles which are considerably smaller than the nominal cut-off size (Sheldon
and Sutcliffe [10]).
The choice of filter can determine the amount of material considered as particulate,
sometimes with unexpected results. Thus, the Whatman GF/C filter with its larger pore
size actually retains about three times as much particulate organic carbon as does the
0.8 µm silver filter. Presumably the difference results from the larger number of small
particles retained by the glass fibre filters.
The method of calculation of the blank can also influence the determined sediment
content. If surface sea water is filtered through a pad consisting of two or more filters,
either glass fibre or silver, the bottom filter will often contain a small amount of
sediment above the blank value. Some workers have maintained that this is due to
the adsorption of dissolved organic matter on to the filter and that this value should
therefore be subtracted from the weight of sediment found on the top filter (Banoub
and Williams et al [11], Menzel et al [12]). Other workers feel that the material caught
in the second filter is largely composed of smaller particles passing through the first
filter. Depending on the way in which the particulate fraction is defined, the material
caught by the second filter should either be added to that collected on the first filter
(Bishop and Edmond et al [13]) or ignored (Gordon and Sutcliffe [14], Sharp [15],
Wangersky [16]). It can easily be seen that the choice of blank calculation can cause a
considerable difference to the final values given for sediment content, at least in surface
waters,. As far as the particulate fraction is defined, not in terms of particle size, but
in terms of material caught on a specific filter, it is recommended that only one filter,
rather than a pad of two or more, be used, since the material caught on subsequent
filters is irrelevant by definition.
5.2.2 Separation by centrifugation
A method for removing particles which is not limited in volume sampled, and
which suffers less from problems of overlapping classification is continuous-flow centrifugation. Separation into density classes can be achieved by choice of speed of
centrifugation [18].
Centrifugation a dry sediment in tetrabromoethane provided several fractions
based on density (Table 5.1). The mass balance of lead, cadmium and zinc in each