Analyses of Dissolved Organic Carbon in Seawater:
the JGOFS EqPac Methods Comparison

Jonathan H. Sharp
College of Marine Studies
University of Delaware
Lewes, DE 19958

Ronald Benner
Marine Science Institute
University of Texas at Austin
Port Aransas, TX 78373

Lenore Bennett
College of Marine Studies
University of Delaware
Lewes, DE 19958

Craig A. Carlson
Horn Point Environmental Laboratory
University of Maryland Center for Estuarine and Environmental Studies
Cambridge, MD 21613

Steve E. Fitzwater
Moss Landing Marine Laboratories
Moss Landing, CA 95039

Edward T. Peltzer
Marine Chemistry and Geochemistry Department
Woods Hole Oceanographic Institutiton
Woods Hole, MA 02543

Luis M. Tupas
Department of Oceanography
University of Hawaii
Honolulu, HI 96822

Marine Chemistry (1995) 48: 91-108.

Received: 9 December 1993.
Accepted: 14 July 1994.

ABSTRACT

Results of a dissolved organic carbon (DOC) methods comparison are presented here in which five high temperature combustion (HTC) instruemnts and a wet chemical oxidation (WCO) method were used on a series of oceanic samples. The samples were collected during the US JFGOFS Equatorial Pacific Ocean cruises (EqPac) and most of the authors were involved with DOC analyses for the EqPac Program. Samples were collected with a "clean" protocol and were immediately quick frozen in replicate sample bottles. They were distributed by the first author to the other authors for "blind" analyses later on land on the stored samples.

Comparable results (± 7.5%) were found by three HTC instruments and the WCO method. There were difficulties with the other two HTC methods for which explanations and improvements are offered. The single most critical element for comparable DOC values appears to be assessment and subtraction of the total instrument blank (or reagent and handling blank for WCO methods). A "zero" carbon (very low C) water sample assisted in having all analysts achieve a uniform assessment of individual instrument blanks. "Conditioning" of the catalyst bed in the combustion tube is critical to achieve consistent low instrument blanks. Failure to thoroughly condition the catalyst bed may be a significant error that can give erroneously high DOC values for oceanic samples. Reference standards available to all analysts also allowed comparison of instrument and methods performance. Contamination problems were demonstrated and it was shown that careful preparation and handling can reduce the potential for errors from contaminated samples.

Results indicate that Equatorial Pacific oceanic DOC values in near surface waters are on the order of 60-70 µMC and deep water values on the order of 35-40 µMC. Since the "zero" carbon water contained a small, but measurable amount of DOC, the sample values reported here may be slightly low. Because the lowest instrument blanks were equivalent to about 10 µMC, it is suggested that even if there were no instrument blank at all and all of this "blank" were in the "zero" carbon water, the oceanic sample concentrations could not be underestimated by more than 10 µMC.

Acknowledgements

We thank H. W. Ducklow for assistance to SEF and ETP in sample collection during the spring cruises. D. C. Miller and H. L. MacIntyre helped with statistical analyses and interpretation. Extensive suggestions prior to the analyses, on data interpretation, and on the manuscript have been offered by a steering committee consisting of Neil Andersen, John Hedges, Charles Hopkinson, Cindy Lee, and Tony Knap. We appreciate this assistance.

This research was partially supported by National Science Foundation grants OCE 92-00970 to JHS (with a contribution from the Department of Energy), OCE 91-02407 to RB, OCE 91-16304 and OCE 90-15888 to HWD and CAC, OCE 91-15201 to ETP, OCE 93-01368 to David M. Karl, LMT and D. V. Hebel and by ONR grant N-000-14-84-C-0619 to the late John H. Martin. Some of the logistical support came from the US JGOFS Steering Committee grant (OCE 92-08261 to H. D. Livingston). This is contribution number 8581 from the Woods Hole Oceanographic Institution, contribution UMCEES #2508 from the University of Maryland, contribution number 908 from the University of Texas, and US-JGOFS contribution number 50.

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