An html version of the abstract is shown below. This report is available as a 4.13 MB pdf file: WRIR 99-4072
The U.S. Geological Survey operated four external quality-assurance
programs for the National Atmospheric Deposition Program/National Trends
Network (NADP/NTN) in 1995 and 1996: the intersite-comparison program, the
blind-audit program, the interlaboratory- comparison program, and the
collocated-sampler program. The intersite-comparison program assessed the
precision and bias of pH and specific-conductance determinations made by NADP/NTN
site operators. The analytical bias introduced during routine handling,
processing, and shipping of wet-deposition samples and precision of analyte
values was estimated using a blind-audit program. An
interlaboratory-comparison program was used to evaluate differences between
analytical results and to estimate the analytical precision of five North
American laboratories that routinely analyzed wet deposition. A
collocated-sampler program estimated the precision of the overall
precipitation collection and analysis system from initial sample collection
through final storage of the data.
Results of two intersite-comparison studies completed in 1995 indicated 94.6 and 94.4 percent of the onsite pH determinations met the NADP/NTN accuracy goals, whereas 97.2 and 98.3 percent of the specific-conductance determinations were within the established limits. The percentages of onsite determinations that met the accuracy goals in 1996 were slightly less for both pH and specific-conductance than in 1995. In 1996, 93.2 and 87.5 percent of onsite pH determinations met the accuracy goals, whereas the percentage of onsite specific-conductance measurements that met the goals was 93.9 and 94.9 percent.
The blind audit program utilizes a paired sample design to
evaluate the effects of routine sample handling, processing and shipping on
the chemistry of weekly precipitation samples. The portion of the blind audit
sample subject to all of the normal onsite handling and processing steps of
a regular weekly precipitation sample is referred to as the bucket portion,
whereas the portion receiving only minimal handling is referred to as the bottle
portion. Throughout the report, the term positive bias in regard to blind-audit
results indicates that the bucket portion had a higher concentration than the
bottle portion. The paired t-test of 1995 blind-audit data indicated that routine
sample handling, processing, and shipping introduced a very small positive bias
(a=0.05) for hydrogen ion and specific conductance and a slight negative bias
(a =0.05) for ammonium and sodium. In 1995, the median paired differences between
the bucket and bottle portions ranged from -0.02 milligram per liter for both
ammonium and nitrate to +0.002 milligram per liter for calcium. Although the
paired t-test indicated a very small positive bias for hydrogen ion, the median
paired difference between the bucket and bottle portions was 0.00 microequivalents
per liter, whereas for specific conductance, the median paired difference between
the bucket and bottle portions was 0.200 microsiemens per centimeter in 1995.
The paired t-test of blind-audit results in 1996 indicated statistically significant
bias for 6 of the 10 analytes. Only chloride, nitrate, hydrogen ion, and specific
conductance were not biased in 1996. However, the magnitude of the bias in 1996
was very small and only of limited importance from the viewpoint of an analytical
chemist or data user. The median paired differences between the bucket and bottle
portions ranged from -0.02 milligram per liter for both ammonium and chloride
to +0.006 milligram per liter for calcium. For hydrogen ion, the median paired
difference between the bucket and bottle portions was ‑0.357 microequivalent
per liter; for specific conductance, the median paired difference between the
bucket and bottle portions was 0.00 microsiemens per centimeter in 1996.
Surface-chemistry
effects due to different amounts of precipitation contacting the sample collection
and shipping container surfaces were studied in the blind-audit program using
three different sample volumes. The results of a hypothesis test of the relation
between hydrogen-ion differences and sample volume were not statistically significant
in either 1995 or 1996. This supports the premise that the chemical reactions
between the 13-liter bucket shipping container and the sample, which resulted
in an increasing loss of hydrogen ion with increasing volume prior to 1994,
have been eliminated by the new 1-liter bottle sample-shipping protocol.
In
the interlaboratory-comparison program in 1995, results of the Friedman test
indicated significant (a =0.05) differences in analyte measurements for calcium,
potassium, nitrate, sulfate, and hydrogen ion among the five laboratories. In
1996, the Friedman test indicated significant (a=0.05) differences in analyte
measurements for potassium, ammonium, chloride, sulfate, nitrate, and hydrogen
ion. Intralaboratory bias was indicated for most laboratories in tests of certified
analyte concentrations from standard reference material samples and from Ultrapure
deionized-water samples. Variability among laboratories was assesed by comparing
precision estimates. Precision estimates for the cations at the 50th
percentile exhibited less variability than precision estimates at the 50th
percentile for the anions, pH, and specific conductance for the five laboratories
that participated in the interlaboratory comparison program.
Results from the collocated-sampler program indicated the median relative error calculated from deposition amounts exceeded 20 percent for sodium and potassium at all of the 1995 collocated sites. In 1996 the median relative error calculated from deposition amounts also exceeded 20 percent at two of the four sites for sodium and potassium. In contrast, the median relative error for sulfate and nitrate deposition was less than 20 percent at all sites in 1995 and 1996, and at four of the eight sites, sulfate and nitrate median relative error was less than 10 percent. The median relative error for hydrogen-ion concentration and deposition ranged from 4.6 to 26.3 percent at the eight sites and was inversely proportional to the acidity of the precipitation at a given site. In those cases where the median laboratory error was greater then zero, median collocated-sampling error estimates in units of concentration were typically at two to four times larger than median laboratory error estimates for most analytes. In fact, the median laboratory error estimates for the laboratory that analyzes the samples for he NADP/NTN was 0.00 milligrams per liter for 4 of 10 analytes.
Abstract
Introduction
Statistical Approach
Intersite-Comparison Program
Results for Intersite-Comparison Studies 33 Through 36
Intersite-Comparison Study Followup Program
Blind-Audit Program
Data Analysis
Interlaboratory-Comparison Program
Laboratory Precision
Interlaboratory Bias
Intralaboratory Bias
National Institute of Standards and Technology
Standard Reference Samples
Ultrapure Deionized-Water Samples
Collocated-Sampler Program
Comparison of Laboratory and Network Error
Summary
References Cited
1 jgordon@usgs.gov, U.S. Geological Survey, Denver, CO
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