USGS Inorganic Blind Sample Project (IBSP)
Monitoring and Evaluating Laboratory Analytical Quality

(Based on Fact Sheet FS-136-97)

he U.S. Geological Survey (USGS) collects and disseminates information about the Nation's water resources. Surface- and ground-water samples are collected and sent to USGS laboratories for chemical analyses. The laboratories identify and quantify the constituents in the water samples. Random and systematic errors occur during sample handling, chemical analysis, and data processing. Although all errors cannot be eliminated from measurements, the magnitude of this uncertainty can be estimated and tracked over time. Since 1981, the USGS has operated an independent, external, quality-assurance project called the Inorganic Blind Sample Project (IBSP). The purpose of the IBSP is to monitor and evaluate the quality of laboratory analytical results through the use of double-blind quality-control (QC) samples. The information provided by the IBSP assists the laboratories in detecting and correcting problems in the analytical procedures. The information also can aid laboratory users in estimating the extent that laboratory errors contribute to the overall errors in their environmental data.

Standard reference samples (Farrar and Long, 1997) are used to make the QC samples for the IBSP. They are used diluted with deionized water, undiluted, and mixed in varying proportions with other standard reference samples. This sample-mixing procedure produces a large number of unique samples available for quality-assurance purposes. The standard reference samples are usually natural matrix samples collected from different sources such as snowmelt, streams, and ground water.

The IBSP samples are made to appear as much like environmental samples as possible and are subjected to identical laboratory handling, processing, and analytical procedures. After the laboratories analyze the samples, IBSP personnel compile and review the analytical results. The resulting data are stored in the USGS National Water Information System data base.

Analytical errors fall into two major categories: bias and variability. Bias is systematic error that causes consistently positive or negative deviation in the results from the most probable value. Variability is random error that affects the ability to reproduce results. Repeated measurements of the IBSP samples over time provide estimates of both systematic bias and random variability in the laboratory analytical procedures. The IBSP uses a variety of graphical and statistical tools to evaluate laboratory analyses of the blind QC samples. These tools include control charts, relative standard deviation charts, box plots of error distributions, Wilcoxon signed-rank test for bias, binomial-probability distribution test for variability, and statistical summaries.

Control charts are produced for each analytical method as a review of the laboratory performance. They are a graphical display of the analytical deviation from the most probable value with respect to time. Control charts show if analytical results are within the expected control limits. The IBSP sets control limits at ±2 standard deviations of the most probable value. These charts can illustrate a systematic or sudden shift in bias or variability. The control chart (fig. 1) for whole water-recoverable iron analyses indicates a change from a positive bias in February 1996 to a negative bias in September 1996.

Another graphical tool used by the IBSP is the relative standard deviation chart, which displays analytical variability with respect to concentration. These charts allow a data reviewer to estimate analytical variability at a given concentration. The relative standard deviation chart (fig. 2) for dissolved zinc analyses is typical in showing that variability decreases with increasing concentrations.

Inorganic Blind Sample Project Chief  (struzesk@usgs.gov)
U.S. Geological Survey
Denver Federal Center
Mail Stop 401, Box 25046
Denver, CO 80225
303.236.1872