Precipitation Chemistry Quality Assurance Project (PCQA)

National Atmospheric Deposition Program / Mercury Deposition Network

Interlaboratory Comparison Program Description

The U.S. Geological Survey (USGS) initiated the interlaboratory comparison program in 2004 as part of the external quality assurance activities conducted for the National Atmospheric Deposition Program (NADP) . The objectives of the interlaboratory comparison program are: (1) to verify the quality of chemical analyses of precipitation samples determined by the Frontier Global Sciences Inc. , (2) to estimate the analytical precision of participating laboratories, and (3) to determine if statistically significant differences exist among the analytical results of participating laboratories. Nine laboratories are currently participating in the program:

  1. Frontier Global Sciences Inc. (FGS) in Seattle, Washington

  2. ACZ Laboratories (ACZ) in Steamboat Springs, Colorado

  3. IVL Swedish Environmental Research Institute (IVL) in Stockholm, Sweden

  4. North Shore Analytical, Inc. (NSA) in Duluth, Minnesota

  5. U.S. Geological Survey, Water Resources Discipline (WML) in Middleton, Wisconsin

  6. Northern Lake Service, Inc. (NLS) in Crandon, Wisconsin

  7. Flett Research, Ltd. (FRL) in Winnipeg, Manatoba, Canada

  8. Flemish Institute for Technological Research (VITO) in Mol, Belgium

  9. Quebec Laboratory of Environmental Testing (LEEQ) in Montreal, Quebec, Canada

  10. Atlantic Laboratory of Environmental Technology (ALET) in Moncton, New Brunswick, Canada (Discontinued in 2010)

Samples from the following sources are used in the interlaboratory comparison program: (1) Ultrapure deionized-water samples prepared by the USGS; (2) standard reference samples with certified U.S. National Institute of Standards and Technology (NIST) traceable values, prepared by High Purity Standards, diluted and spiked with mercury by the USGS; and (3) excess natural wet-deposition samples collected at NADP sites and prepared by the USGS.

USGS ships interlaboratory comparison program to the participating laboratories every month. The FGS, NSA, and NLS labs receive four samples per month, and ACZ, IVL, WML, LEEQ, FRL, and VITO labs receive two samples per month. Samples are labeled with a sample number to ensure that laboratory personnel cannot determine the sample type or the target analyte concentrations until the chemical analyses are performed. A flowchart of the interlaboratory comparison program outlines the program's components. Participating laboratories provide analytical results to the USGS on a regular basis. The data are presented in control charts and tables on this website. Each laboratory's data are only accessible by the individual laboratory and the USGS. The control charts are used to identify potential systematic error that might be affecting the quality of a laboratory's data. Consistent, acceptable laboratory performance is indicated when the data remain within the control limits on the control charts.

Bias of the MDN Mercury Analytical Laboratory (HAL at FGS) results relative to other participating laboratories is illustrated in Z-value plots beginning in 2008. The Z-value is a non-parametric analogue of the Z-score. Computation of the Z-values uses the f-pseudosigma (instead of the standard deviation) for all reported values for each solution within a given year.

Where: Z-value = [Laboratory's Median Reported Value - Most Probable Value] / [f-pseudosigma for all laboratories combined],

and f-pseudosigma = [Interquartile Range] / 1.349.

Z-values are caclulated for each laboratory's median reported value for each test solution, and then the median reported values are plotted by their associated Z-values for each laboratory. These plots show how the laboratories compare to each other, but only the HAL data are identified in the plots to preserve the anonymity of the other participating laboratories. The HAL's laboratory identification number for these plots is 1, denoted by a solid purple diamond.


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