CHAPTER 16 • Oceanic DOC Measurements
311
jor analytical issues of international importance. The careful utilization of HTCO
methods has demonstrated the importance of blank measurements and the control
of several parameters. One of the major outcomes of the Seattle meeting was a realization that the determination of appropriate blanks for the analytical system is a nontrivial exercise.
A variety of both home made and commercially produced clones of the original
system were produced but little consensus could be reached about the better performance of some methods. As stated by Peltzer et al. (1996), there is no generic persulfate,
HTC, commercial or home-made methods, but rather there are various techniques
practiced by individual analysts, each of which requires calibration and verification.
Various checks on oxidation efficiency have been made by using comparisons of
high temperature catalytic oxidation and sealed tube combustion of the same samples
(Hedges and Farrington 1993; Sharp et al. 1995). Some earlier intercomparision and
field studies were inconclusive (Miller et al. 1993a,b), but later comparison showed that
comparable results (±7.5%) were produced using five different HTC instruments and
WCO methods (Sharp et al. 1995). Measurements carried out by Peltzer et al. (1996)
on a limited number of samples showed that there was agreement among HTC (three
methods), PS and STC methods, but yields for all methods decreased compared to the
STC technique at concentrations higher than 400 flM C. However, these researchers
outlined that the intercomparison of various methods needs a proper evaluation of
instrument blanks, which can be reduced by proper conditioning of the catalyst bed
in HTC methods. Contamination problems were also reported. As recommended by
Gershey et al. (1979), a regression analysis would provide important insights on the
importance of blanks. When comparing two methods, if the intercept of the linear
regression analysis is different from zero, the difference between the pair of methods
might be due to a blank problem. In a similar manner, the slope of the fitted line gives
information about the relative oxidation efficiency. For the best statistical approach
for doing this regression analysis, see Peltzer et al.(1996).
Ogawa and Ogura (1992) subjected to replicate DOC determinations using both
WCO and HTCO methods and marine dissolved organic matter fractionated by ultrafiltration. Three fractions have been collected,,!; < 1000,1000 < f2 < 10 ooo,h > 10000,
respectively. There was a close agreement between WCO and HTCO results for fraction h (accounting for 3-22% of the total DOC) and fraction fi (accounting for 32-56%
of the total DOC). HTCO results were somewhat higher than those from WCO for fraction fl' The mean difference was reported in the range 15-25%, supporting the conclusion that a portion of DOC is missed in WCO methods. Other experiments agreed that
the difference between HTCO and WCO represents no more than 10-15% (Cauwet
1994). Since naturally occurring volatile substances are a minor part of DOC, this portion has to be attributed to organic species with low boiling points, formed during the
PS oxidation. The WCO system proposed by Fung et al. (1996) mentioned above would
overcome this effect.
16.3
New Concepts for DOC Determination
The detection of TOC in a sample can be achieved theoretically with all the techniques
able to sense the amount of carbon in the sample, independent of the type of substances
311
jor analytical issues of international importance. The careful utilization of HTCO
methods has demonstrated the importance of blank measurements and the control
of several parameters. One of the major outcomes of the Seattle meeting was a realization that the determination of appropriate blanks for the analytical system is a nontrivial exercise.
A variety of both home made and commercially produced clones of the original
system were produced but little consensus could be reached about the better performance of some methods. As stated by Peltzer et al. (1996), there is no generic persulfate,
HTC, commercial or home-made methods, but rather there are various techniques
practiced by individual analysts, each of which requires calibration and verification.
Various checks on oxidation efficiency have been made by using comparisons of
high temperature catalytic oxidation and sealed tube combustion of the same samples
(Hedges and Farrington 1993; Sharp et al. 1995). Some earlier intercomparision and
field studies were inconclusive (Miller et al. 1993a,b), but later comparison showed that
comparable results (±7.5%) were produced using five different HTC instruments and
WCO methods (Sharp et al. 1995). Measurements carried out by Peltzer et al. (1996)
on a limited number of samples showed that there was agreement among HTC (three
methods), PS and STC methods, but yields for all methods decreased compared to the
STC technique at concentrations higher than 400 flM C. However, these researchers
outlined that the intercomparison of various methods needs a proper evaluation of
instrument blanks, which can be reduced by proper conditioning of the catalyst bed
in HTC methods. Contamination problems were also reported. As recommended by
Gershey et al. (1979), a regression analysis would provide important insights on the
importance of blanks. When comparing two methods, if the intercept of the linear
regression analysis is different from zero, the difference between the pair of methods
might be due to a blank problem. In a similar manner, the slope of the fitted line gives
information about the relative oxidation efficiency. For the best statistical approach
for doing this regression analysis, see Peltzer et al.(1996).
Ogawa and Ogura (1992) subjected to replicate DOC determinations using both
WCO and HTCO methods and marine dissolved organic matter fractionated by ultrafiltration. Three fractions have been collected,,!; < 1000,1000 < f2 < 10 ooo,h > 10000,
respectively. There was a close agreement between WCO and HTCO results for fraction h (accounting for 3-22% of the total DOC) and fraction fi (accounting for 32-56%
of the total DOC). HTCO results were somewhat higher than those from WCO for fraction fl' The mean difference was reported in the range 15-25%, supporting the conclusion that a portion of DOC is missed in WCO methods. Other experiments agreed that
the difference between HTCO and WCO represents no more than 10-15% (Cauwet
1994). Since naturally occurring volatile substances are a minor part of DOC, this portion has to be attributed to organic species with low boiling points, formed during the
PS oxidation. The WCO system proposed by Fung et al. (1996) mentioned above would
overcome this effect.
16.3
New Concepts for DOC Determination
The detection of TOC in a sample can be achieved theoretically with all the techniques
able to sense the amount of carbon in the sample, independent of the type of substances