82
Marine Sediments
gation. Jacobs and Ewing [705] used continuous centrifugation to collect total suspended matter in the oceans, and Lammers [706] discussed the possible uses of the
method. The biggest drawback seems to be that the separation is governed by particle
density, rather than particle size. However, the method holds considerable promise
for the collection of colloidal material as a separate fraction.
4.7.3
Fractionation by Filtration
Some work has been done on size fractionation of particulate matter in water samples
by the use of graduated filters. Since the fIlters in common use do not display a sharp
cut-off in particle size retention, interpretation of the results is difficult. Repeated
fIltration of a single sample through fIlters of different pore size does not divide the
particulate matter into definite size classes, since each fIlter retains particles smaller
than the nominal pore size. The results of the fIltration of separate aliquots through a
series of fIlters can only be reported in terms of "particles smaller than" the nominal
pore size and are equally difficult to interpret. Although such size fractionation has
been reported (Mullin [707]), the conclusions can only be accepted in the broadest
possible sense. Particle size distributions based on filtration should be supported by
Coulter counter data before any conclusions can be drawn.
Once a sample of dissolved organic matter has been isolated, it is still seldom in a
form which permits simple analysis. In most cases there are far too many compounds
present, and some form of fractionation must take place in order to remove interferences and simplity analytical procedures.
One could devise many different bases for the fractionation of organic materials,
and functional groups, degree of saturation, presence or absence of aromatic groups,
and degree of polarity have all been used. The approach most often used is a fractionation by size. At the upper end of the size range, we are dealing with particles
consisting of many discrete molecules. Fractionation is accomplished by differential
filtration, using fIlters and screens of decreasing pore size.
Particles of smaller sizes, from the colloidal to the micromolecular, are separated by
membrane fIlters. The most familiar of these is the Amicon Diflo fIlter, although several
other companies now manufacture similar products. Separations in the same size range
can also be achieved with hollow polymeric fibres. At the upper end of their size range,
these fIlters cen be used to separate different size classes of material we would normally
consider as colloidal. At the smaller end, the separation is made on the basis of
molecular size. The results are presented in terms of molecular weight, but the molecular weight calibration is done with spherical molecules. The results are therefore given
as equivalent spheres rather than as true molecular weights. The techniques have been
applied to coastal sea water. Ultraflltration as a fractionation method gave recoveries of
80-100 % when the carbon present in each fraction is summed.
In ultrafiltration techniques employing membrane fIlters and those using hollow
fibres, both worked well for the concentration and desalting of humic and fulvic acids.
However, the high priming volume needed for the hollow fibre apparatus restricts it
in large volume applications. This is not likely to be a problem in marine work, where
large volumes are required because of the low concentrations of organic materials.
Marine Sediments
gation. Jacobs and Ewing [705] used continuous centrifugation to collect total suspended matter in the oceans, and Lammers [706] discussed the possible uses of the
method. The biggest drawback seems to be that the separation is governed by particle
density, rather than particle size. However, the method holds considerable promise
for the collection of colloidal material as a separate fraction.
4.7.3
Fractionation by Filtration
Some work has been done on size fractionation of particulate matter in water samples
by the use of graduated filters. Since the fIlters in common use do not display a sharp
cut-off in particle size retention, interpretation of the results is difficult. Repeated
fIltration of a single sample through fIlters of different pore size does not divide the
particulate matter into definite size classes, since each fIlter retains particles smaller
than the nominal pore size. The results of the fIltration of separate aliquots through a
series of fIlters can only be reported in terms of "particles smaller than" the nominal
pore size and are equally difficult to interpret. Although such size fractionation has
been reported (Mullin [707]), the conclusions can only be accepted in the broadest
possible sense. Particle size distributions based on filtration should be supported by
Coulter counter data before any conclusions can be drawn.
Once a sample of dissolved organic matter has been isolated, it is still seldom in a
form which permits simple analysis. In most cases there are far too many compounds
present, and some form of fractionation must take place in order to remove interferences and simplity analytical procedures.
One could devise many different bases for the fractionation of organic materials,
and functional groups, degree of saturation, presence or absence of aromatic groups,
and degree of polarity have all been used. The approach most often used is a fractionation by size. At the upper end of the size range, we are dealing with particles
consisting of many discrete molecules. Fractionation is accomplished by differential
filtration, using fIlters and screens of decreasing pore size.
Particles of smaller sizes, from the colloidal to the micromolecular, are separated by
membrane fIlters. The most familiar of these is the Amicon Diflo fIlter, although several
other companies now manufacture similar products. Separations in the same size range
can also be achieved with hollow polymeric fibres. At the upper end of their size range,
these fIlters cen be used to separate different size classes of material we would normally
consider as colloidal. At the smaller end, the separation is made on the basis of
molecular size. The results are presented in terms of molecular weight, but the molecular weight calibration is done with spherical molecules. The results are therefore given
as equivalent spheres rather than as true molecular weights. The techniques have been
applied to coastal sea water. Ultraflltration as a fractionation method gave recoveries of
80-100 % when the carbon present in each fraction is summed.
In ultrafiltration techniques employing membrane fIlters and those using hollow
fibres, both worked well for the concentration and desalting of humic and fulvic acids.
However, the high priming volume needed for the hollow fibre apparatus restricts it
in large volume applications. This is not likely to be a problem in marine work, where
large volumes are required because of the low concentrations of organic materials.