Hawaii Ocean Time-series (HOT)
in the School of Ocean and Earth Science and Technology at the University of Hawai'i
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PARTICULATE CARBON AND NITROGEN
SUMMARY: Seawater samples are collected at discrete depths in 12-liter Niskin bottles. The water samples are prefiltered (202 µm) and transferred to specially designed, precalibrated filtration bottles, pressure filtered through combusted GF/F filters and stored frozen for subsequent analysis. In the laboratory, the filters are dried and analyzed for C and N using a Perkin-Elmer model 2400 analyzer.
Particulate carbon (PC) and particulate nitrogen (PN), including both inorganic and organic forms, are present in seawater primarily as by-products of biological activity. Living organisms also contribute a variable amount (<1% in deep waters to >50% in the euphotic zone) to the total particulate carbon and nitrogen content of seawater.
Both PC and PN can be measured using commercially-available instruments which detect the gaseous by-products of high-temperature combustion. The Perkin-Elmer model 2400 CHN analyzer used in the HOT-GOFS project, combines the classical Pregal and Dumas methods for the determination of carbon and nitrogen, respectively. The samples are combusted in a pure oxygen environment, the gases mixed and separated by frontal chromatography. The separated gases are determined stepwise by a thermal conductivity detector.
Care must be taken to ensure that contamination with PC and PN is avoided. Samples should, therefore, be kept away from paper, wood, food or other carbon-containing compounds. The filtration system that is set up to collect these samples is constructed of plastic. Plastic contains carbon and, therefore, is a potential source of contamination. The carboys and tubing used in this system must be kept clean and free of abrading surfaces.
3. Sampling, Filtration and Storage
Standard procedures for instrument warm-up are followed. Primary PC/PN standards are prepared using acetanilide (C8H9NO; mol. wt. = 135.16). Standards are made to encompass the range 20-500 ug/sample. Blanks are prepared by analyzing combusted GF/F "field filters" and the mean value subtracted from the sample value. Typical ranges in analytical blanks, expressed as percentages of the total signal produced by a sample in the course of a given analysis, are 5-17% for both PC and PN.
5. Data Reduction and Calculations
External standard data are used to prepare a standard curve of C (or N) versus corrected signal counts, and linear regression statistics are calculated. An EXCEL spreadsheet is used to calculate PC and PN (µg l-1) for each sample based on standard curve, corrected signal counts and volume of seawater filtered.
6. Precision and Accuracy
Replicate samples are routinely analyzed to estimate the precision of our PC and PN analyses. The average coefficients of variation for such replicate analyses are 12% and 10%, respectively. Accuracy is estimated from determinations of the C and N contents of reference standards analyzed along with samples during each analytical run. This accuracy, expressed as the mean percent difference between the analytical determination and the known C and N contents of the standards, is found to be 6% for PC and 13% for PN. Note that the estimates of accuracy apply only to standards containing >10 µg N and >70 µg C. Below these levels, we have found the performance of the PE-2400 to be suspect. Therefore sample volumes large enough to keep above this lower limit are used.