CHAPTER 16 • Oceanic DOC Measurements
303
radation or adsorption. Early methods which involved the use of filtration (pre combusted glass fibre filters) followed by poisoning with HgCl2 or simply freezing in
precombusted glass ampoules have been shown to be as satisfactory as any more
modern alternatives (Tupas et al. 1994). The last researchers suggested that samples
can be conveniently stored at -20 DC in acid-cleaned polypropylene tubes, highdensity polypropylene bottles or combusted glass ampoules even for extended periods (5 months after collection). Preservation at 4 DC needs the addition of 0.025% of
H3P04•
16.2.2
Pretreatment and Removal of DIC
The procedure for removing DIe has not altered much during the history of DOC
measurements. The sample of sea water is acidified to a pH < 3 using H3P04 or HCI. It
is then purged with a CO2-free gas such as N2 or O2, This purging needs to be maintained for a period of time that is proportional to the gas flow rate and the geometry
of the purging vessel. It might typically. range from about 5 minutes for a flow rate of
500 ml min- 1 up to 20 minutes for a flow rate OfI75 ml min- 1 (Peltzer and Brewer 1993).
No effect of the purging was reported for DOC abundance measurements (Tupas et al.
1994).
16.2.3
The Oxidation of DOC
It is the oxidation step in DOC analysis that has probably provoked the greatest difficulties over the years. The various approaches to this problem are briefly reviewed in
this section.
16.2.3.1
Dry Combustion
Dry combustion, often called sealed-tube combustion (STC), methods generally involve the combustion of a dried sample over a catalyst in a sealed ampoule. A catalyst
such as copper oxide and H3P04 (Alperin and Martens 1993; Fry et al. 1993; Chen et al.
1996; Peltzer et al. 1996) or sulfuric acid (Fry et al. 1996) is added. The second catalyst
avoids the problem of CO2 absorption present with copper basic salts (carbonates)
formed during the later combustion. The evaporation of the water is obtained directly
in the ampoule by vacuum distillation, freeze drying or oven drying. After sealing and
a suitable heating period under controlled conditions (typically -580 DC), the ampoule
is opened in such conditions as to quantitatively recover the evolved CO 2 which is then
collected and quantified. A tube cracker under a closed atmosphere is needed. Depending on the measurement technique (such as linear response capacitance manometer),
the CO2 generated in combustion may necessitate purification from traces of water
and other contaminant gases, using vacuum manifolds and sequential sublimation
through liquid nitrogen and dry-ice/ethanol cold traps. Dry combustion has been identified as a reference method (Peltzer et al.1996) because analyses can also be performed
in commercial elemental analysers (MacKinnon 1978).
303
radation or adsorption. Early methods which involved the use of filtration (pre combusted glass fibre filters) followed by poisoning with HgCl2 or simply freezing in
precombusted glass ampoules have been shown to be as satisfactory as any more
modern alternatives (Tupas et al. 1994). The last researchers suggested that samples
can be conveniently stored at -20 DC in acid-cleaned polypropylene tubes, highdensity polypropylene bottles or combusted glass ampoules even for extended periods (5 months after collection). Preservation at 4 DC needs the addition of 0.025% of
H3P04•
16.2.2
Pretreatment and Removal of DIC
The procedure for removing DIe has not altered much during the history of DOC
measurements. The sample of sea water is acidified to a pH < 3 using H3P04 or HCI. It
is then purged with a CO2-free gas such as N2 or O2, This purging needs to be maintained for a period of time that is proportional to the gas flow rate and the geometry
of the purging vessel. It might typically. range from about 5 minutes for a flow rate of
500 ml min- 1 up to 20 minutes for a flow rate OfI75 ml min- 1 (Peltzer and Brewer 1993).
No effect of the purging was reported for DOC abundance measurements (Tupas et al.
1994).
16.2.3
The Oxidation of DOC
It is the oxidation step in DOC analysis that has probably provoked the greatest difficulties over the years. The various approaches to this problem are briefly reviewed in
this section.
16.2.3.1
Dry Combustion
Dry combustion, often called sealed-tube combustion (STC), methods generally involve the combustion of a dried sample over a catalyst in a sealed ampoule. A catalyst
such as copper oxide and H3P04 (Alperin and Martens 1993; Fry et al. 1993; Chen et al.
1996; Peltzer et al. 1996) or sulfuric acid (Fry et al. 1996) is added. The second catalyst
avoids the problem of CO2 absorption present with copper basic salts (carbonates)
formed during the later combustion. The evaporation of the water is obtained directly
in the ampoule by vacuum distillation, freeze drying or oven drying. After sealing and
a suitable heating period under controlled conditions (typically -580 DC), the ampoule
is opened in such conditions as to quantitatively recover the evolved CO 2 which is then
collected and quantified. A tube cracker under a closed atmosphere is needed. Depending on the measurement technique (such as linear response capacitance manometer),
the CO2 generated in combustion may necessitate purification from traces of water
and other contaminant gases, using vacuum manifolds and sequential sublimation
through liquid nitrogen and dry-ice/ethanol cold traps. Dry combustion has been identified as a reference method (Peltzer et al.1996) because analyses can also be performed
in commercial elemental analysers (MacKinnon 1978).