304
C. Minero . E. Pelizzetti . M.R. Preston
The method is handicapped by the difficulties in handling the large quantities of
water involved in the analysis of low- Toe samples and is it is more suitable for the
analysis of fresh water, because the salt present in sea water makes this approach difficult (Cauwet 1994). Troubles with blanks when drying low carbon offshore DOC
samples using a freeze-dryer has been reported (Fry et al. 1993; Peterson et al. 1996),
mainly due to back-streaming of vacuum pump oil vapours. Oven drying is therefore
preferred (Fry et al. 1996) not only because of this problem but also because the salty
brine solution formed during freeze-drying is increasingly resistant to the removal of
the last traces of water.
16.2.3.2
Wet Chemical Oxidation (WCO) and UV-Assisted WCO
Wet chemical methods employ an oxidizing agent (typically K2S20 S (PS» and heating
in a closed vessel, followed by a similar quantitative step as for the dry combustion
(Van Hall et al. 1965; Miller et al. 1993b). Similarly, the use of UV photooxidation has
sometimes relied on irradiation in a sealed WV transparent) container followed by
release and quantitation (Armstrong et al. 1966). The efficiency of the UV oxidation
procedure depends to a considerable degree on both the type and the age of the source
lamp. The type of the lamp influences the spectral characteristics of the emitted radiation and the intensity of the irradiation decreases with age. Most systems use high
power (1 kW) medium pressure mercury lamps as suggested by Collins and Williams
(1977) for 20 min to 1 h treatment time.
The advantages of both the chemical and UV oxidation procedures are that they
lend themselves to applications in continuous flow systems. A typical, combined
PS/UV system has been described by Mantoura and Woodward (1983) and is shown
diagrammatically in Fig. 16.1. As well as the potassium persulfate reagent, this unit uses
a borate buffer to stabilize the pH and hydroxylamine hydrochloride to inhibit corrosion of the peristaltic pump tubes by the free chlorine generated during the oxidation
step. Such systems give a total system blank of around 20 11M C with a precision for
real samples of ±6 11M C at a concentration of 145 11M C (Miller 1996), or about 5% on
marine samples (Sharp et al. 1995).
The systems are robust enough for use at sea though some early designs of detectors were more susceptible to vibration than others, and care needs to be taken to ensure that vibration does not cause analytical problems.
The drawbacks are in the possibility that the compound may not be transformed
into CO 2 , For example, 14C-Iabelled DOC produced by some species of phytoplankton
grown in the presence of 14C bicarbonate resists both persulfate and UV oxidation
(Ridal and Moore 1993). Resistance to oxidation was also reported recently for compounds in which the carbon is in its higher oxidation state (like CCl4 and cyanuric
acid for which oxidation is not allowed), and thermal hydrolysis is slow (Calza et al.
1997b). To overcame the difficulties associated with the possible incomplete oxidation
by persulfate of the original material (or volatile compounds produced in the oxidation process), the wet oxidation step may be combined with a catalytic oxidation in a
furnace at 900°C over CuO (Fung et al. 1996). The volatile compounds are distilled
from the acidified sample containing persulfate under a flux of oxygen at about
2-3 ml min -I. Finally the CO 2 formed in both steps is collected in alkaline solution and
C. Minero . E. Pelizzetti . M.R. Preston
The method is handicapped by the difficulties in handling the large quantities of
water involved in the analysis of low- Toe samples and is it is more suitable for the
analysis of fresh water, because the salt present in sea water makes this approach difficult (Cauwet 1994). Troubles with blanks when drying low carbon offshore DOC
samples using a freeze-dryer has been reported (Fry et al. 1993; Peterson et al. 1996),
mainly due to back-streaming of vacuum pump oil vapours. Oven drying is therefore
preferred (Fry et al. 1996) not only because of this problem but also because the salty
brine solution formed during freeze-drying is increasingly resistant to the removal of
the last traces of water.
16.2.3.2
Wet Chemical Oxidation (WCO) and UV-Assisted WCO
Wet chemical methods employ an oxidizing agent (typically K2S20 S (PS» and heating
in a closed vessel, followed by a similar quantitative step as for the dry combustion
(Van Hall et al. 1965; Miller et al. 1993b). Similarly, the use of UV photooxidation has
sometimes relied on irradiation in a sealed WV transparent) container followed by
release and quantitation (Armstrong et al. 1966). The efficiency of the UV oxidation
procedure depends to a considerable degree on both the type and the age of the source
lamp. The type of the lamp influences the spectral characteristics of the emitted radiation and the intensity of the irradiation decreases with age. Most systems use high
power (1 kW) medium pressure mercury lamps as suggested by Collins and Williams
(1977) for 20 min to 1 h treatment time.
The advantages of both the chemical and UV oxidation procedures are that they
lend themselves to applications in continuous flow systems. A typical, combined
PS/UV system has been described by Mantoura and Woodward (1983) and is shown
diagrammatically in Fig. 16.1. As well as the potassium persulfate reagent, this unit uses
a borate buffer to stabilize the pH and hydroxylamine hydrochloride to inhibit corrosion of the peristaltic pump tubes by the free chlorine generated during the oxidation
step. Such systems give a total system blank of around 20 11M C with a precision for
real samples of ±6 11M C at a concentration of 145 11M C (Miller 1996), or about 5% on
marine samples (Sharp et al. 1995).
The systems are robust enough for use at sea though some early designs of detectors were more susceptible to vibration than others, and care needs to be taken to ensure that vibration does not cause analytical problems.
The drawbacks are in the possibility that the compound may not be transformed
into CO 2 , For example, 14C-Iabelled DOC produced by some species of phytoplankton
grown in the presence of 14C bicarbonate resists both persulfate and UV oxidation
(Ridal and Moore 1993). Resistance to oxidation was also reported recently for compounds in which the carbon is in its higher oxidation state (like CCl4 and cyanuric
acid for which oxidation is not allowed), and thermal hydrolysis is slow (Calza et al.
1997b). To overcame the difficulties associated with the possible incomplete oxidation
by persulfate of the original material (or volatile compounds produced in the oxidation process), the wet oxidation step may be combined with a catalytic oxidation in a
furnace at 900°C over CuO (Fung et al. 1996). The volatile compounds are distilled
from the acidified sample containing persulfate under a flux of oxygen at about
2-3 ml min -I. Finally the CO 2 formed in both steps is collected in alkaline solution and