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c. Minero . E. Pelizzetti . M.R. Preston
of the DOC in sea water (Wells and Goldberg 1991, 1992, 1993; Ogawa and Ogura 1992).
The particulate retained on the filter is less than 5% of OC in the ocean and may be
usually neglected. However, the particulate peaks around 10% of OC during phytoplankton blooms (Wangersky 1993).
The high ratio of dissolved inorganic carbon (DIC) to DOC in sea water (-5411)
means that it is first necessary to remove the DIC by acidification and purging. Volatile organic carbon (VOG) belongs to TOC, and part of it is lost during the stripping
of inorganic carbon. Most commercial analysers neglect the amount of lost VOC. Marine VOC concentrations may rise locally to 10% of DOC, either due to anthropogenic
input or to intense biogenic production (Juttner and Henatsch 1986; Bianchi and Varney
1992). Normally VOC is a minor contribution to TOC (::;5 jlM C) and comparable to
the precision of contemporary TOC analyses (±1-2jlM C). Inorganic compounds like
cyanates, isocyanates and thiocyanates are determined in the DOC fraction as TOC.
The CO2 that is evolved can then be directly measured using a suitable detector
(e.g. a non -dispersive infrared gas analyser, NDIR) or converted to methane which may
then be determined using a conventional flame ionisation detector (FlO). The NDIR
has proved to be the more popular and easily applicable unit, though its usage has not
been without problems.
The analytical steps may then be broken down into the following stages:
i. sample collection and storage
ii. pretreatment and removal of DIC
iii. the oxidation of DOC
iv. the removal of interfering species
v. the detection and quantitation
The issue of instrumental and methodological blank ( vi.) pertains to all of the steps
and strongly influences analytical precision and accuracy.
16.2.1
Sample Collection and Storage
An examination of much of the early literature regarding DOC measurements reveals
a considerable paucity of details regarding sample collection and storage. Many of the
major improvements of analytical data quality in recent years have come about because of a greatly increased awareness of the importance of these aspects. Inappropriate sampling procedures can result in sample contamination by the ship's gear, the
sampling bottles themselves and, perhaps most importantly, by the surface microlayer.
All lead to erroneously elevated values. Detailed sampling procedures have been described by Statham and Williams (1983), Peltzer and Brewer (1993) and Sharp and
Peltzer (1993).
Collected samples have often had to be stored rather than subjected to immediate
analysis. The main problem appears to derive from microbial activity, and this is a
particular problem in highly productive waters (Chen et al. 1996). Studies of different
storage procedures conducted in recent years have shown that even the sign of the
change in measured DOC concentrations may not be consistent when different methods are used. Concentrations may be elevated by contamination or decreased by deg-
c. Minero . E. Pelizzetti . M.R. Preston
of the DOC in sea water (Wells and Goldberg 1991, 1992, 1993; Ogawa and Ogura 1992).
The particulate retained on the filter is less than 5% of OC in the ocean and may be
usually neglected. However, the particulate peaks around 10% of OC during phytoplankton blooms (Wangersky 1993).
The high ratio of dissolved inorganic carbon (DIC) to DOC in sea water (-5411)
means that it is first necessary to remove the DIC by acidification and purging. Volatile organic carbon (VOG) belongs to TOC, and part of it is lost during the stripping
of inorganic carbon. Most commercial analysers neglect the amount of lost VOC. Marine VOC concentrations may rise locally to 10% of DOC, either due to anthropogenic
input or to intense biogenic production (Juttner and Henatsch 1986; Bianchi and Varney
1992). Normally VOC is a minor contribution to TOC (::;5 jlM C) and comparable to
the precision of contemporary TOC analyses (±1-2jlM C). Inorganic compounds like
cyanates, isocyanates and thiocyanates are determined in the DOC fraction as TOC.
The CO2 that is evolved can then be directly measured using a suitable detector
(e.g. a non -dispersive infrared gas analyser, NDIR) or converted to methane which may
then be determined using a conventional flame ionisation detector (FlO). The NDIR
has proved to be the more popular and easily applicable unit, though its usage has not
been without problems.
The analytical steps may then be broken down into the following stages:
i. sample collection and storage
ii. pretreatment and removal of DIC
iii. the oxidation of DOC
iv. the removal of interfering species
v. the detection and quantitation
The issue of instrumental and methodological blank ( vi.) pertains to all of the steps
and strongly influences analytical precision and accuracy.
16.2.1
Sample Collection and Storage
An examination of much of the early literature regarding DOC measurements reveals
a considerable paucity of details regarding sample collection and storage. Many of the
major improvements of analytical data quality in recent years have come about because of a greatly increased awareness of the importance of these aspects. Inappropriate sampling procedures can result in sample contamination by the ship's gear, the
sampling bottles themselves and, perhaps most importantly, by the surface microlayer.
All lead to erroneously elevated values. Detailed sampling procedures have been described by Statham and Williams (1983), Peltzer and Brewer (1993) and Sharp and
Peltzer (1993).
Collected samples have often had to be stored rather than subjected to immediate
analysis. The main problem appears to derive from microbial activity, and this is a
particular problem in highly productive waters (Chen et al. 1996). Studies of different
storage procedures conducted in recent years have shown that even the sign of the
change in measured DOC concentrations may not be consistent when different methods are used. Concentrations may be elevated by contamination or decreased by deg-