310
C. Minero . E. Pelizzetti . M.R. Preston
The recommendations for analytical work are that the analyst should:
1. Calibrate the instrument daily. The heterogeneous nature of marine DOM means that
selection of a suitable material (or materials) for the calibration of DOC analyses is
not straightforward. Glucose has been a common choice, but some fears have been
expressed that glucose solutions are not stable in unsterile solutions (Miller et al.
1993a). Phthalate standards are frequently recommended, because it is readily soluble in both seawater and distilled water and is commercially available in a high purity form. Wangersky (1993) has published a summary of compounds that show virtually complete oxidation by UV or wet chemical oxidation methods. However, a
procedure for calibrating seawater DOC analysers has not yet been universally accepted.
2. Make the appropriate blank correction using carbon-free distilled water. This can
be obtained by a high quality water purifier, such as MilliPore MilliQ Water system,
collection of HTC processed water, prepared by Hz0z/UV oxidation or distilled over
persulfate (Benner and Strom 1993; Cauwet 1994; Peltzer et al. 1996). Instrument
blanks measured with ultrapure water in different laboratories had average values
lO.9 ±1.2 11M C. Conditioning of the catalyst bed in HTC methods is beneficial, as well
as the careful preparation and handling for reducing the potential for errors from
contaminated samples (Sharp et al. 1995).
3. At least once, determine the combustion efficiency for the method either by direct
comparison with a referee technique or by oxidation test performed on recalcitrant
compounds. It is impossible to prove the completeness of oxidation for a method
dealing with natural samples owing to the unknown nature of the material under
analysis. Since the comparisons with STC may be impracticable for routine use, a
suite of recalcitrant compounds to be added to marine water samples has to be identified: leucine and proline are resistant to WCO, as is urea for UV oxidation, and several organic compounds under HTCO conditions (see Fig. 16.3).
4. Certify the accuracy of the method by the regular measurement of reference samples. This is a difficult task to achieve, also because of storage problems. Gershey et al.
(1979) first called attention to deep oceanic waters as a standard for low concentration reference standard. Such a standard has not yet been identified for high-DOC
reference standard.
16.2.5
Comparative Performance of the Different Techniques
The very high level of interest in the Sugimura and Suzuki system and the problems
that it posed, about analytical procedures in general and marine DOC analysis in particular, led to a major international exercise held in Seattle in 1991 that was reported
in a special edition of "Marine Chemistry" in 1993 (Hedges and Lee 1993). Suzuki later
found that there had been a variety of problems with his published data and published
a retraction of the 1988 paper (and an earlier paper on dissolved organic nitrogen,
Suzuki et al. 1993). However, Hedges et al. (1993) strongly resisted any inclinations to
disregard all of the previous work because, by this time, a number of independent
workers had also reported elevated DOC levels. It was clear that although the original
data may have been flawed, Sugimura and Suzuki's work had raised a number of ma-
C. Minero . E. Pelizzetti . M.R. Preston
The recommendations for analytical work are that the analyst should:
1. Calibrate the instrument daily. The heterogeneous nature of marine DOM means that
selection of a suitable material (or materials) for the calibration of DOC analyses is
not straightforward. Glucose has been a common choice, but some fears have been
expressed that glucose solutions are not stable in unsterile solutions (Miller et al.
1993a). Phthalate standards are frequently recommended, because it is readily soluble in both seawater and distilled water and is commercially available in a high purity form. Wangersky (1993) has published a summary of compounds that show virtually complete oxidation by UV or wet chemical oxidation methods. However, a
procedure for calibrating seawater DOC analysers has not yet been universally accepted.
2. Make the appropriate blank correction using carbon-free distilled water. This can
be obtained by a high quality water purifier, such as MilliPore MilliQ Water system,
collection of HTC processed water, prepared by Hz0z/UV oxidation or distilled over
persulfate (Benner and Strom 1993; Cauwet 1994; Peltzer et al. 1996). Instrument
blanks measured with ultrapure water in different laboratories had average values
lO.9 ±1.2 11M C. Conditioning of the catalyst bed in HTC methods is beneficial, as well
as the careful preparation and handling for reducing the potential for errors from
contaminated samples (Sharp et al. 1995).
3. At least once, determine the combustion efficiency for the method either by direct
comparison with a referee technique or by oxidation test performed on recalcitrant
compounds. It is impossible to prove the completeness of oxidation for a method
dealing with natural samples owing to the unknown nature of the material under
analysis. Since the comparisons with STC may be impracticable for routine use, a
suite of recalcitrant compounds to be added to marine water samples has to be identified: leucine and proline are resistant to WCO, as is urea for UV oxidation, and several organic compounds under HTCO conditions (see Fig. 16.3).
4. Certify the accuracy of the method by the regular measurement of reference samples. This is a difficult task to achieve, also because of storage problems. Gershey et al.
(1979) first called attention to deep oceanic waters as a standard for low concentration reference standard. Such a standard has not yet been identified for high-DOC
reference standard.
16.2.5
Comparative Performance of the Different Techniques
The very high level of interest in the Sugimura and Suzuki system and the problems
that it posed, about analytical procedures in general and marine DOC analysis in particular, led to a major international exercise held in Seattle in 1991 that was reported
in a special edition of "Marine Chemistry" in 1993 (Hedges and Lee 1993). Suzuki later
found that there had been a variety of problems with his published data and published
a retraction of the 1988 paper (and an earlier paper on dissolved organic nitrogen,
Suzuki et al. 1993). However, Hedges et al. (1993) strongly resisted any inclinations to
disregard all of the previous work because, by this time, a number of independent
workers had also reported elevated DOC levels. It was clear that although the original
data may have been flawed, Sugimura and Suzuki's work had raised a number of ma-