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Open-File Report 2007–1267

Prepared in cooperation with the U.S. Environmental Protection Agency

Phosphorus Concentrations in Stream-Water and Reference SamplesAn Assessment of Laboratory Comparability

By Michael R. McHale and Dennis McChesney

U.S. Geological Survey Open-File Report 2007–1267


This Report is available Online Only


Cover - PDF format (269 KB)
Full Report - PDF format (2,165 KB)


In 2003, a study was conducted to evaluate the accuracy and precision of 10 laboratories that analyze water-quality samples for phosphorus concentrations in the Catskill Mountain region of New York State. Many environmental studies in this region rely on data from these different laboratories for water-quality analyses, and the data may be used in watershed modeling and management decisions. Therefore, it is important to determine whether the data reported by these laboratories are of comparable accuracy and precision. Each laboratory was sent 12 samples for triplicate analysis for total phosphorus, total dissolved phosphorus, and soluble reactive phosphorus. Eight of these laboratories reported results that met comparability criteria for all samples; the remaining two laboratories met comparability criteria for only about half of the analyses. Neither the analytical method used nor the sample concentration ranges appeared to affect the comparability of results. The laboratories whose results were comparable gave consistently comparable results throughout the concentration range analyzed, and the differences among methods did not diminish comparability. All laboratories had high data precision as indicated by sample triplicate results. In addition, the laboratories consistently reported total phosphorus values greater than total dissolved phosphorus values, and total dissolved phosphorus values greater than soluble reactive phosphorus values, as would be expected. The results of this study emphasize the importance of regular laboratory participation in sample-exchange programs.





Purpose and Scope


Sample Preparation

Statistical Methods

Data Comparability



References Cited


1–3. Graphs showing—

  1. Concentrations of total phosphorus, total dissolved phosphorus, and soluble reactive phosphorus (orthophosphate) in 10-laboratory comparison of stream-water and reference samples, 2003.
  2. The coefficient of variation (CV) for total phosphorus, total dissolved phosphorus, and soluble reactive phosphorus (orthophosphate) for all stream-water and reference sample triplicates reported by the 10 participating laboratories.
  3. Difference between phosphorus fractions in 10-laboratory comparison of phosphorus concentration in stream-water and reference samples, 2003.


  1. Laboratories participating in 2003 study of laboratory comparability, U.S. Environmental Protection Agency methods used, and method titles.
  2. Samples included in the interlaboratory comparison.
  3. Concentrations of total phosphorus, total dissolved phosphorus, and soluble reactive phosphorus (orthophosphate) for stream-water and reference samples, z-values, and ratings for the 10 laboratories involved in the 2003 comparability study.

Suggested Citation

McHale, M.R., and McChesney, Dennis, 2007, Phosphorus concentrations in stream-water and reference samples—An assessment of laboratory comparability: U.S. Geological Survey Open-File Report 2007–1267, 25 p. Online Only

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