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National Water Quality Laboratory

Friday December 15, 2017

U.S. GEOLOGICAL SURVEY Open-File Report 92-480

Methods of Analysis by the U.S. Geological Survey National Water Quality Laboratory - Determination of Dissolved Organic Carbon by uv-promoted Persulfate Oxidation and Infrared Spectrometry

By Ronald W. Brenton and Tony L. Arnett

Water samples can be analyzed for dissolved organic carbon by either an Oceanographic International (OI) carbon analyzer or a Dohrmann carbon analyzer. Thirty natural-water samples were analyzed by both techniques, and then precision between the two techniques was evaluated. Linear regression analysis was used to evaluate bias and correlation. A paired difference test using the t statistic was used to evaluate the significance of any bias.

Distilled-water spike samples were prepared by dissolving a known amount of sodium benzoate in an exact volume of distilled water. Nine samples were analyzed by both techniques. Again, the precision between the two techniques was compared, and analyte recovery was evaluated.

The results from the spike-sample analyses of distilled water showed that the analyte recovery averaged 103 percent for the OI technique and 100 percent for the Dohrmann technique. The average absolute difference between both sets of results was 1.8 percent from the mean of both datasets.

The linear regression analysis of data from the analysis of the natural-water samples indicated that there may be a slight bias between the two techniques. The correlation coefficient for the analysis was 0.99. The paired-difference test showed that the results of the two techniques are comparable at the 99-percent confidence level. The average absolute difference between the datasets was 8.4 percent from the mean. The analysis indicates that the two techniques are equivalent within the realm of experimental error.

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