Water-Resources Investigations Report 99-4072

External Quality-Assurance Results for the

National Atmospheric Deposition Program/

National Trends Network, 1995-96

U.S. Department of the Interior

U.S. Geological Survey
Front cover photographs are of selected National Atmospheric Deposition Program/National Trends Network Sites used in the 

collocated project, 1995-96 (clockwise from top right):

Little Bighorn Battlefield National Monument, Montana, site MT00/00MT

Dixson Springs Agricultural Research Center, Illinois, site IL63/63IL

Dixson Springs Agricultural Research Center, Illinois, site IL63/63IL

Underhill, Vermont, site VT99/99VT

Little Bighorn Battlefield National Monument, Montana, site MT00/00MT

All photographs were taken by the U.S. Geological Survey.

Much appreciation is extended to the following U.S. Geological Survey employees for ther contributions to this report:

Members of the Tallahassee Reports Production Unit (Teresa Embry, Patsy Mixson and Jim Tomberlin)

for final production of the report; Mary Kidd and Martha Crawford from the Colorado district for their

editorial and verification reviews, respectively.

Appreciation is also extended to the technical reviewers of the report:

Jane Rothert, Illinois State Water Survey and Dr. Luther Smith, ManTech Environmental Technology, Inc.
U.S. GEOLOGICAL SURVEY

Water-Resources Investigations Report 99-4072

External Quality-Assurance Results for the

National Atmospheric Deposition Program/

National Trends Network, 1995-96

By John D. Gordon

Denver, Colorado

1999
U.S. DEPARTMENT OF THE INTERIOR

BRUCE BABBITT, Secretary

U.S. GEOLOGICAL SURVEY

Charles G. Groat, Director

Copies of this report can be 

purchased from:

U.S. Geological Survey

Information Services

Box 25286

Federal Center

Denver, CO 80225

888-ASK-USGS

The use of trade, product, industry, or firm names is for descriptive purposes only and does not 

imply endorsement by the U.S. Government.

For additional information

write to:

Chief, Branch of Quality Systems

U.S. Geological Survey

Box 25046, MS 401

Denver Federal Center

Denver, CO 80225-0046
Contents iii

CONTENTS

Abstract.................................................................................................................................................................................. 1

Introduction ........................................................................................................................................................................... 3

Statistical Approach............................................................................................................................................................... 3

External Quality-Assurance Results...................................................................................................................................... 4

Intersite-Comparison Program .................................................................................................................................... 4

Results for Intersite-Comparison Study Numbers 35-38 .................................................................................. 4

Intersite-Comparison Study Follow-up Program .............................................................................................. 12

Blind-Audit Program ................................................................................................................................................... 15

Assessing Analytical Bias ................................................................................................................................. 18

Paired Blind-Audit Sample Differences ............................................................................................................ 18

Statistical Significance ............................................................................................................................ 22

Analyte Concentration Effects................................................................................................................. 22

Ultrapure Sample Analyses ..................................................................................................................... 22

Percent Differences.................................................................................................................................. 29

Sample Volume Effects............................................................................................................................ 30

Interlaboratory-Comparison Program.......................................................................................................................... 37

Laboratory Precision.......................................................................................................................................... 39

Interlaboratory Bias ........................................................................................................................................... 40

Intralaboratory Bias ........................................................................................................................................... 41

National Institute of Standards and Technology Standard Reference Samples....................................... 41

Ultrapure Deionized-Water Samples ....................................................................................................... 53

Collocated-Sampler Program ...................................................................................................................................... 53

Comparison of Laboratory and Network Error ..................................................................................................................... 64

Summary................................................................................................................................................................................ 67

References Cited.................................................................................................................................................................... 68

FIGURES

1. Flowchart showing the intersite-comparison program of the U.S. Geological Survey QualityAssurance Program ................................................................................................................................................. 5

2-3. Graphs showing:

2. Distribution of pH and specific-conductance values for intersite-comparison study numbers 35 and 36, 

completed in 1995 .......................................................................................................................................... 9

3. Distribution of pH and specific-conductance values for intersite-comparison study numbers 37 and 38, 

completed in 1996 .......................................................................................................................................... 10

4. Diagrams showing percentiles for pH and specific conductance from intersite-comparison study numbers 

(a) 35 and 36 and (b) 37 and 38 .............................................................................................................................. 11

5-6. Graphs showing:

5. Follow-up study results for intersite-comparison study numbers 35 and 36.................................................. 13

6. Follow-up study results for intersite-comparison study numbers 37 and 38.................................................. 14

7. Flowchart showing blind-audit program of the U.S. Geological Survey................................................................ 16

8-17. Graphs Showing:

8. Paired bucket-sample concentrations minus bottle-sample concentrations in the blind-audit 

program for 1995............................................................................................................................................ 20

9. Paired bucket-sample concentrations minus bottle-sample concentrations in the blind-audit 

program for 1996............................................................................................................................................ 21

10. Relation between paired blind-audit differences for major ions and the analyte concentrations of

solutions used in the blind-audit program in 1995 ......................................................................................... 24
iv Contents

11. Relation between paired blind-audit differences for major ions and the analyte concentrations of 

solutions used in the blind-audit program in 1996......................................................................................... 25

12. Relation between paired blind-audit differences for hydrogen-ion and specific conductance and 

the analyte concentrations of solutions used in the blind-audit program in 1995 ......................................... 26

13. Relation between paired blind-audit differences for hydrogen-ion and specific conductance and 

the analyte concentrations of solutions used in the blind-audit program in 1996 ......................................... 27

14. Relation between paired blind-audit differences for the major ions and sample volume for the 

250-, 500-, and 1,000-milliliter U.S. Geological Survey solution samples in 1995...................................... 31

15. Relation between paired blind-audit differences for the major ions and sample volume for the 

250-, 500-, and 1,000-milliliter U.S. Geological Survey solution samples in 1996...................................... 32

16. Relation between paired blind-audit differences and sample volume for pH and specific conductance 

for the 250-, 500-, and 1,000-milliliter U.S. Geological Survey solution samples in 1995 .......................... 33

17. Relation between paired blind-audit differences for pH and specific conductance and sample volume 

for the 250-, 500-, and 1,000-milliliter U.S. Geological Survey solution samples in 1996 .......................... 34

18. Flowchart showing interlaboratory-comparison program of the U.S. Geological Survey..................................... 38

19-28. Graphs showing:

19. Difference between the measured value reported by the Atmospheric Environment Service and the 

median value calculated for all participating laboratories in the interlaboratory-comparison program 

during 1995 .................................................................................................................................................... 43

20. Difference between the measured value reported by the Atmospheric Environment Service and the 

median value calculated for all participating laboratories in the interlaboratory-comparison program 

during 1996 .................................................................................................................................................... 44

21. Difference between the measured value reported by the Illinois State Water Survey, Central Analytical 

Laboratory and the median value calculated for all participating laboratories in the interlaboratorycomparison program during 1995.................................................................................................................. 45

22. Difference between the measured value reported by the Illinois State Water Survey, Central Analytical 

Laboratory and the median value calculated for all participating laboratories in the interlaboratorycomparison program during 1996.................................................................................................................. 46

23. Difference between the measured value reported by QST Environmental and the median value 

calculated for all participating laboratories in the interlaboratory-comparison program during 1995.......... 47

24. Difference between the measured value reported by QST Environmental and the median value calculated 

for all participating laboratories in the interlaboratory-comparison program during 1996 ........................... 48

25. Difference between the measured value reported by the Global Geochemistry Corportation and the 

median value calculated for all participating laboratories in the interlaboratory-comparison program

during 1995 .................................................................................................................................................... 49

26. Difference between the measured value reported by the Global Geochemistry Corporation/Shepard 

Analytical Services and the median value calculated for all participating laboratories in the interlaboratory-comparison program during 1996 ................................................................................................ 50

27. Difference between the measured value reported by the Ontario Ministry of the Environment, 

Water Quality Section, and the median value calculated for all participating laboratories in the 

interlaboratory-comparison program during 1995......................................................................................... 51

28. Difference between the measured value reported by the Ontario Ministry of the Environment, 

Water Quality Section, and the median value calculated for all participating laboratories in the 

interlaboratory-comparison program during 1996......................................................................................... 52

29. Map showing location of National Atmospheric Deposition Program/National Trends Network sites with 

collocated samplers in 1995-96 .............................................................................................................................. 54

30-37. Graphs showing:

30. Median sample chemistry for selected analytes at four National Atmospheric Deposition Program/National 

Trends Network sites with collocated samplers in 1995 ........................................................................................ 56

31. Median sample chemistry for selected analytes at four National Atmospheric Deposition Program/National 

Trends Network sites with collocated samplers in 1996 ........................................................................................ 57

32. Median hydrogen-ion concentration, specific conductance, sample volume, and precipitation depth for 

four National Atmospheric Deposition Program/National Trends Network sites with collocated samplers 

in 1995 .................................................................................................................................................................... 58
Contents v

33. Median hydrogen-ion concentration, specific conductance, sample volume, and precipitation depth 

for four National Atmospheric Deposition Program/National Trends Network sites with collocated 

samplers in 1996 ............................................................................................................................................ 59

34. Median relative error for analyte concentration, deposition, and other physical parameters for weekly 

samples from collocated wet-dry precipitation collectors and precipitation depth from collocated rain 

gages in 1995 ................................................................................................................................................. 61

35. Median relative error for analyte concentration, deposition, and other physical parameters for weekly 

samples from collocated wet-dry precipitation collectors and precipitation depth from collocated rain 

gages in 1996 ................................................................................................................................................. 62

36. Bias for analyte concentration, deposition, and other physical parameters for weekly samples from 

collocated wet-dry precipitation collectors and precipitation depth from collocated rain gages in 1995 ..... 65

37. Bias for analyte concentration, deposition, and other physical parameters for weekly samples from 

collocated wet-dry precipitation collectors and precipitation depth from collocated rain gages in 1996 ..... 66

TABLES

1. Site-operator responses and summary statistics for the 1995 intersite-comparison program ................................ 6

2. Site-operator responses and summary statistics for the 1996 intersite-comparison program ................................ 7

3. Solutions used in the 1995-96 blind-audit and interlaboratory-comparison programs .......................................... 15

4. Target values for solutions used in the 1995-96 U.S. Geological Survey blind-audit program and 

interlaboratory-comparison program ...................................................................................................................... 17

5. Selected statistics for the paired bucket-sample concentration minus bottle-sample concentration differences 

in the blind-audit program during 1995.................................................................................................................. 19

6. Selected statistics for the paired bucket-sample concentration minus bottle-sample concentration differences

in the blind-audit program during 1996.................................................................................................................. 19

7. Results of the tests for bias in the blind-audit program during 1995, using the paired t-test and a Wilcoxon 

signed-rank test....................................................................................................................................................... 22

8. Results of the tests for bias in the blind-audit program during 1996, using the paired t-test and a Wilcoxon 

signed-rank test....................................................................................................................................................... 23

9. Results of the Kruskal-Wallis analysis of variance test to determine the relation between paired blind-audit 

sample differences and the target concentrations used in the blind-audit program in 1995 and 1996................... 28

10. Number of determinations exceeding the method detection limit for the Ultrapure deionized water samples

submitted annually as part of the blind-audit program, 1995-1996........................................................................ 28

11. Relative and absolute bucket minus bottle differences calculated as a percentage of the target bottle 

concentration for each analyte in 1995................................................................................................................... 29

12. Relative and absolute bucket minus bottle differences calculated as a percentage of the target bottle 

concentration for each analyte in 1996................................................................................................................... 30

13. Results of the 1995 Kruskal-Wallis analysis of variance tests to determine if bucket minus bottle differences 

for the 250-, 500-, and 1,000-mL samples of the USGS solution used in the blind-audit program have equivalent

distributions ............................................................................................................................................................ 35

14. Results of the 1996 Kruskal-Wallis analysis of variance tests to determine if bucket minus bottle differences for

the 250-, 500-, and 1,000-mL samples of the USGS solution used in the blind-audit program have equivalent

 distributions ........................................................................................................................................................... 36

15. Fiftieth and ninetieth percentile absolute differences for analyses of replicate samples determined by 

five laboratories participating in the 1995 interlaboratory-comparison program................................................... 39

16. Fiftieth and ninetieth percentile absolute differences for analyses of replicate samples determined by 

five laboratories participating in the 1996 interlaboratory-comparison program................................................... 40

17. Summary of laboratory median values per analyte in 1995 and the analyte range reported by the National 

Institute of Standards and Technology for standard reference materials 2694A-I and 2694A-II .......................... 42

18. Summary of laboratory median values per analyte in 1996 and the analyte range reported by the National 

Institute of Standards and Technology for standard reference materials 2694A-I and 2694A-II .......................... 42

19. Number of analyte determinations greater than the largest minimum reporting limit used by any participating

laboratory for each ion for the Ultrapure deionized-water samples during 1995................................................... 53

20. Number of analyte determinations greater than the largest minimum reporting limit used by any participating 

laboratory for each ion for the Ultrapure deionized-water samples during 1996................................................... 53
vi Contents

21. Median absolute error for analyte concentration and deposition values determined from weekly collocated 

precipitation samples and replicate samples measured by the Illinois State Water Survey, Central Analytical

Laboratory in 1995 ............................................................................................................. .................................... 63

22. Median absolute error for analyte concentration and deposition values determined from weekly collocated 

precipitation samples and replicate samples measured by the Illinois State Water Survey, Central Analytical 

Laboratory in 1996 ............................................................................................................. .................................... 64

CONVERSION FACTORS

Temperature can be converted from degree Celsius (°C) to degree Fahrenheit (°F) by using the following equation:

°F = 9/5(°C) + 32

ADDITIONAL TERMS AND ABBREVIATIONS

microsiemens per centimeter at 25 degrees Celsius (μS/cm)

megohm (MΩ)

microequivalent per liter (μeq/L)

milligram per liter (mg/L)

kilogram per hectare (kg/ha)

microgram (μg)

absolute value of x = |x|, where x takes the form of numerical values or 

 algebraic expressions

maximum probability of rejecting the null hypothesis when it is true (α)

Multiply By To obtain

centimeter (cm) 0.3937 inch

liter (L) 1.057 quart

milliliter (mL) 0.03381 ounce, fluid
Abstract 1

External Quality-Assurance Results for the 

National Atmospheric Deposition Program/National 

Trends Network, 1995-96

By John D. Gordon 

Abstract 

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 collocatedsampler 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 by 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 collocatedsampler program was used to estimate 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 that about 

95 and 94 percent of the onsite pH determinations met the NADP/NTN accuracy goals, 

whereas about 97 and 98 percent of the specificconductance determinations were within the 

established limits. The percentages of onsite 

determinations that met the accuracy goals in 

1996 were slightly less for pH and specificconductance than in 1995. In 1996, about 94 and 

88 percent of onsite pH determinations met the 

accuracy goals, whereas the percentage of 

onsite specific-conductance measurements that 

met the goals was about 94 and 95 percent.

The blind-audit program requires 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 [probability level 
2 External Quality-Assurance Results for the National Atmospheric Deposition Program/National Trends Network, 1995-96

(α)=0.05] for hydrogen ion and specific conductance and a slight negative bias (α=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 microequivalent 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 did not show bias 

in 1996. The magnitude of the bias in 1996, 

however, was small and only of limited importance from the viewpoint of an analytical chemist or data use. 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 samplecollection and shipping-container surfaces were 

studied in the blind-audit program by 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 before1994, have been 

eliminated by the new 1-liter bottle sampleshipping protocol.

In the interlaboratory-comparison program in 1995, results of the Friedman test 

indicated significant (α=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 (α=0.05) differences in 

analyte measurements for potassium, ammonium, chloride, nitrate, sulfate, 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 

assessed 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. 

The median laboratory error estimates for the 

laboratory that analyzes the samples for the 

NADP/NTN was 0.00 milligram per liter for 4 

of 10 analytes.
Introduction 3

INTRODUCTION

The National Atmospheric Deposition Program/National Trends Network (NADP/NTN) investigates the occurrence and effects of wet deposition 

across the United States. The NADP/NTN data are 

used to monitor spatial and temporal trends in the 

chemical composition of wet deposition (Robertson 

and Wilson, 1985; Peden, 1986). Research scientists 

use NADP/NTN data to study the effects of acidic 

deposition on human health and the environment. All 

operators of NADP/NTN sites use the same type of 

wet-deposition collectors, which are described by 

Bigelow and Dossett (1988). Sample-handling and 

shipping protocols were extensively revised in January of 1994 in an effort to reduce sample contamination. These protocol changes have been delineated in 

a revised edition of the NADP/NTN instruction manual, which is currently available to site operators in a 

draft form but has yet to be published (Scott Dossett, 

Illinois State Water Survey, written commun., 1999). 

All site operators used the same sample-handling protocols and sent their samples for chemical analysis to 

the Illinois State Water Survey, Central Analytical 

Laboratory (CAL).

This report describes the results of the external 

quality-assurance (QA) programs operated by the 

U.S. Geological Survey (USGS) in support of the 

NADP/NTN during 1995 and 1996. These programs 

are designed to: (1) assess the precision and accuracy 

of onsite determinations of pH and specific conductance (intersite-comparison program); (2) evaluate 

potential contamination of samples due to handling, 

processing, and shipping of samples collected within 

the NADP/NTN (blind-audit program); (3) estimate 

the comparability, bias, and precision of analytical 

results obtained by separate laboratories that routinely 

measure wet deposition when portions of common 

samples are sent to the participating laboratories 

(interlaboratory-comparison program); and (4) estimate the overall precision of the monitoring network, 

from the point of sample collection through storage of 

the data in the NADP/NTN data base, by the analysis 

of paired samples from collocated samplers at 

selected sites in the network (collocated-sampler program). A fifth external QA program, the field-blank 

and reference-sample program, was begun in late 

1996 as a pilot study on a limited basis. A protocol 

report providing detailed information on the procedures and analytical methods used in these four QA 

programs is available (See and others, 1990). For the 

purposes of this report, the term “major ions” refers to 

calcium, magnesium, sodium, potassium, ammonium, 

chloride, nitrate, and sulfate.

STATISTICAL APPROACH

Nonparametric hypothesis-testing techniques 

(for a non-normal data distribution) for differences in 

external QA results were used extensively instead of 

traditional parametric hypothesis-testing techniques 

for a normal data distribution in this report. The nonparametric techniques included the Wilcoxon signedranks test, the Kruskal-Wallis test, and the Friedman 

test. The paired t-test (parametric) was included in 

this report in the analysis of paired blind-audit results. 

The paired t-test is based on blind-audit samples that 

are paired analyses, whereas the nonparametric alternative to the paired t-test, the Wilcoxon signed-rank 

test, is based on ranks instead of the actual data 

(avoiding the requirement of a normal distribution) 

but has the limitation of assuming that the data consist 

of random, independent samples (Conover, 1980). 

The Wilcoxon signed-ranks test is, therefore, less 

powerful for paired samples than the paired t-test 

(Kanji, 1993). Nonparametric statistical tests were 

commonly used in the analyses for this report because 

the data sets (and all water-quality data sets for that 

matter) do not satisfy normal distribution requirements of parametric statistical techniques. The use of 

nonparametric tests also eliminated problems associated with data transformations that commonly are 

used to compensate for non-normal data, such as the 

difficulty of converting the results of statistical tests 

on transformed data back to the original scale of the 

data (Berthouex and Brown, 1995). All of the hypothesis tests were based on two-tailed rather than onetailed alternatives. The reader should refer to any 

introductory statistics text for an in-depth discussion 

of the difference between two-tailed and one-tailed 

hypothesis testing. Huntsberger and Billingsley 

(1981) for example, devote several chapters of their 

text to a detailed explanation of two-tailed and onetailed hypothesis testing. Concise graphical displays, 

such as boxplots, were used to depict data distribu-
4 External Quality-Assurance Results for the National Atmospheric Deposition Program/National Trends Network, 1995-96

tions and to provide visual representations of 

NADP/NTN data quality. Tukey’s “schematic plot” 

version of the notched boxplot (Chambers and others, 

1983) was used in this report. Boxplots with notches 

in their sides help guide the assessment of relative 

locations. The notches provide an informal 95-percent 

test of the null hypotheses that the true medians of 

side by side boxplots are equal: if the notches overlap, 

the null hypothesis is not rejected with approximate 

95-percent confidence (Chambers and others, 1983). 

If the length of the notch exceeds the length of the box 

portion of the boxplot, the plot will have the appearance of doubling back on itself. Therefore, the notch 

was not displayed when the notch length exceeded the 

length of the box portion of the boxplot. Extending 

from the box portion of the boxplot are thin lines 

called “whiskers.” Using Tukey’s definition, the 

whiskers extend only to the last observation within a 

distance of 1.5 times the interquartile range from the 

top or bottom of the box. For all of the boxplots, 

values outside the whiskers are shown individually as 

an asterisk (Helsel and Hirsch, 1992). The magnitude 

of measurement bias was quantified in several different ways for the convenience of the reader, including 

units of concentration, mass, and percent differences.

EXTERNAL QUALITY-ASSURANCE 

RESULTS

Intersite-Comparison Program

NADP/NTN site operators measure pH and 

specific conductance on weekly precipitation samples, 

as long as sample volume considerations are satisfied. 

These measurements are performed on precipitation 

collected at sampling sites. These onsite measurements 

are useful and provide the most accurate assessment of 

pH and specific conductance. Because of the low ionic 

strength of precipitation, minor changes in pH may 

occur between the sample collection and laboratory 

analysis (Bigelow and others, 1989). Many authors 

have determined that onsite measurements (of pH in 

particular) are more representative of precipitation than 

subsequent laboratory determinations (Hem, 1985). To 

assess the accuracy of onsite measurements, an intersite-comparison study is performed on a semiannual 

basis. If measurements are not accurate, the site operators are notified of their results in order to troubleshoot 

their methods and equipment.

Intersite-comparison studies require 

NADP/NTN site operators to determine the pH and 

specific conductance of synthetic precipitation-check 

samples by using standard protocols that are identical 

to those used on weekly sample measurements 

(Gordon and others, 1991; Bigelow and Dossett, 1988). 

The synthetic precipitation-check samples that are prepared by the USGS have ranges of pH and specific conductance similar to natural wet deposition samples that 

are collected in the NADP/NTN network. The pH is 

adjusted to a specific value within the range of 3.9 to 

5.3 by adding a small amount of nitric acid. The final 

specific conductance of the solution is either just a 

function of the nitric acid that was added, or it is 

increased with small amounts of potassium chloride 

(KCl). The target values of the solutions are validated 

through nitrate analysis and pH and specific-conductance measurements.

Results for Intersite-Comparison 

Study Numbers 35-38

A flowchart that depicts the chronological order 

of the intersite-comparison program is shown in 

figure 1. Samples for intersite study numbers 35 and 36 

were mailed to the site operators in May and October 

1995, respectively. The samples for study numbers 37 

and 38 were mailed in April and November 1996. Site 

operators are allowed 45 days to perform the field measurements. Sites are not included in the percentage of 

sites that achieved the specified goals if: (1) they 

respond late; (2) the field equipment is completely 

inoperable; (3) the site is not in operation at the time of 

the study; or (4) no field chemistry determinations are 

made at the site. (Beginning in 1995, accuracy goals 

that take into consideration the fact that the difficulty of 

measuring pH increases as the hydrogen-ion concentration decreases were used.) Accuracy goals are based on 

a multiple-regression equation that incorporates the 

solution’s hydrogenion concentration and the results 

from past intersite studies. The accuracy goals are symmetrical in units of hydrogen ion and, therefore, are 

asymmetrical in units of pH. A summary of the results 

for study numbers 35 and 36 is given in table 1, and the 

results for study numbers 37 and 38 are summarized in 

table 2. The accuracy goals used in each study also are 

listed in tables 1 and 2.
External Quality-Assurance Results 5

Figure 20.--Bias for analyte concentration, deposition, and other physical 

parameters for weekly samples from collocated wet-dry precipitation 

collectors and precipitation depth from collocated rain gages. All data in 

percent.

Intersite-comparison study samples

prepared by U.S. Geological Survey

Samples analyzed for pH and specific

conductance by site operators

List of nonresponding 

site operators sent to Program Office

Site liaison from the Program 

Office contacts nonresponding 

site operators

Final data base compiled

Results sent to

site operators

Results sent to

Program Office Results presented to the

National Atmospheric Deposition Program/National Trends Network Operations Subcommittee

accuracy goals

Did site

operator have

difficulty meeting

in the previous

two studies?

NO

Reports and 

publications

Site operator included

 in follow-up program

YES

Site operator results

during the previous 

two studies analyzed

Did site operator 

YES

NO

Response cards completed and mailed

to U.S. Geological Survey

Preliminary data 

base compiled

meet measurement

accuracy goals?

Figure 1. Intersite-comparison program of the U.S. Geological Survey Quality-Assurance Program.
6 External Quality-Assurance Results for the National Atmospheric Deposition Program/National Trends Network, 1995-96

Table 1. Site-operator responses and summary statistics for the 1995 intersite-comparison program

Site-operator responses Intersite-comparison study number

35 36

Number of site operators receiving samples 189 190

Number of site operators submitting pH values by closing date of study 186 180

Number of site operators submitting specific-conductance values by 

closing date of study

184 181

Site operators responding late 0 0

Number of nonresponding site operators 3 6

Sites that were not in operation 0 2

Site operators reporting equipment problems:

pH meter/electrode completely inoperable 0 2

pH meter/electrode problems 5 1

Specific-conductance probe/meter completely inoperable 2 1

Specific-conductance probe/meter problems 3 0

Median pH, target pH 4.95, 4.94 4.92, 4.94

Number of responding sites that met the pH accuracy goals 176 170

Accuracy goals for pH: lower and upper acceptable values 4.76, 5.14 4.73, 5.10

Percentage of responding sites that met the pH accuracy goals 94.6 94.4

F-pseudosigma for pH 0.074 0.082

Median specific conductance, target specific conductance, in microsiemens per centimeter at 25 degrees Celsius (μS/cm)

15.7, 15.01

1

Sample was spiked with potassium chloride to increase the target specific conductance from 5.3 to 15.0 microsiemens per centimeter at 25 degrees Celsius.

5.3, 4.9

Number of responding sites that met the specific-conductance accuracy 

goals

179 178

Accuracy goals for specific conductance: lower and upper acceptable 

values

11.7, 19.7 3.3, 7.3

Percentage of responding sites that met the specific-conductance 

accuracy goals.

97.2 98.3

F-pseudosigma for specific conductance 0.519 0.297
External Quality-Assurance Results 7

Table 2. Site-operator responses and summary statistics for the 1996 intersite-comparison program

Site-operator responses Intersite-comparison study number

37 38

Number of site operators receiving samples 190 187

Number of site operators submitting pH values by closing date of study 179 176

Number of site operators submitting specific-conductance values by 

closing date of study

181 177

Site operators responding late 1 0

Number of nonresponding site operators 3 2

Sites that were not in operation 5 7

Site operators reporting equipment problems:

pH meter/electrode completely inoperable 2 2

pH meter/electrode problems 7 5

Specific-conductance probe/meter completely inoperable 0 1

Specific-conductance probe/meter problems 6 1

Median pH, target pH 4.47, 4.47 4.22, 4.22

Number of responding sites that met the pH accuracy goals 167 154

Accuracy goals for pH: lower and upper acceptable values 4.35, 4.57 4.12, 4.29

Percentage of responding sites that met the pH accuracy goals 93.2 87.5

F-pseudosigma for pH 0.037 0.037

Median specific conductance, in microsiemens per centimeter at 

25 degrees Celsius (μS/cm)

26.3, 26.31

1

Sample was spiked with potassium chloride to increase the target specific conductance from 15.3 to 26.3 microsiemens per centimeter at 25 degrees Celsius.

26.8, 26.8

Number of responding sites that met the specific-conductance accuracy 

goals

170 168

Accuracy goals for specific conductance: lower and upper acceptable 

values

22.3, 30.3 22.8, 30.8

Percentage of responding sites that met the specific-conductance 

accuracy goals.

93.9 94.9

F-pseudosigma for specific conductance 0.889 1.07
8 External Quality-Assurance Results for the National Atmospheric Deposition Program/National Trends Network, 1995-96

The target pH for the 1995 intersite-comparison study number 35 was 4.94; the resulting 

median pH produced by site operators responding 

by the closing date of the study was 4.95. The target specific conductance of study number 35 was 

increased by the addition of KCl to 15.0 μS/cm; the 

median value determined from all site-operator 

measurements was 15.7 μS/cm. As in past years 

(Gordon and others, 1997; See and others, 1989), 

the median values from all responding site operators were used as the most probable value for the 

intersite solutions in 1995 and 1996 on the premise 

that the median values from about 200 site-operator 

measurements were a better representation of the 

most likely values for the intersite-comparison 

study solutions than either a few in-house measurements or the theoretical values. Previous studies 

have found no appreciable deterioration of intersite 

solutions over the length of the studies, further 

supporting the use of the median site-operator 

values as the most probable values. The stability of 

hydrogen ion and specific conductance in filtered 

wet-deposition samples that are stored at ambient 

temperatures was investigated (Gordon and others, 

1995) and found to be satisfactory for short-term 

(45-60 days) studies, such as intersite-comparison 

studies. More than 94 percent (176 out of 186) of 

the site operators met the pH upper and lower accuracy goals of 4.76 to 5.14 (table 1). The 

NADP/NTN accuracy goals for specific conductance for study number 35 was ±4 μS/cm of the 

median value of 15.7. By use of this criterion, 

97.2 percent of responding site operators met the 

goals for specific-conductance measurements. The 

criterion for specific conductance used in 1995 and 

1996 varied with the specific conductance. If the 

most probable specific conductance was 10 μS/cm 

or less, the criterion was ±2 μS/cm. If the most 

probable specific conductance was greater than 

10 μS/cm but less than or equal to 60 μS/cm, the 

criterion was ±4 μS/cm.

The reference solution used in intersite-comparison study number 36 had a target pH of 4.94 

and a target specific conductance of 4.9 μS/cm 

(table 1). A total of 170 of the site operators, or 

94.4 percent, submitting pH values by the closing date 

of the study met the pH accuracy goals of 4.73 to 5.10. 

The median specific conductance was 5.3, with 

98.3 percent of site operators that reported on time 

achieving the ±2 μS/cm of the median value accuracy 

goal.

For intersite-comparison study number 37, 

the reference solution target pH was 4.47, and the 

target specific conductance was 26.3 μS/cm after 

the solution was spiked with KCl (table 2). The 

median pH of the intersite-comparison 37 reference solution was the same as the target pH—4.47. 

A total of 167 site operators that responded by the 

closing date of the study met the pH accuracy goals 

of 4.35 to 4.57 pH. The median specificconductance value was 26.3; 93.9 percent of site 

operators achieved the ±4 μS/cm of the median 

value accuracy goal.

The reference solution used in intersitecomparison study 38 had both a target and median 

pH of 4.22 and a target specific conductance of 

26.8 μS/cm. One hundred fifty-four site operators, 

submitting pH values on time, met the pH accuracy 

goals of 4.12 to 4.29. This means that 87.5 percent 

of the operators reporting by the closing date met 

the goals for pH. The median specific conductance 

was 26.8, with 94.9 percent of site operators 

achieving the ±4 μS/cm of the median value accuracy goal.

Scatterplots of the results of pH and specificconductance values for all participating site operators in intersite-comparison study numbers 35 and 

36 are shown in figure 2; the results for study numbers 37 and 38 are given in figure 3. The boundaries depicted on the scatterplots in figures 2 and 3 

are defined as the NADP/NTN accuracy goals set 

for each study. The boundaries delineate the pH 

and specific-conductance values for those site 

operators that meet the accuracy goals for pH or 

specific conductance, or both measurements as 

well as those not meeting the accuracy goals. The 

percentile distributions for the reported pH and 

specific-conductance values for study numbers 

35-38 are listed in figure 4.
External Quality-Assurance Results 9

Figure 2. Distribution of pH and specific-conductance values for intersite-comparison study numbers 

35 and 36, completed in 1995.
10 External Quality-Assurance Results for the National Atmospheric Deposition Program/National Trends Network, 1995-96

Figure 3. Distribution of pH and specific-conductance values for intersite-comparison study numbers 37 

and 38, completed in 1996.
External Quality-Assurance Results 11

Figure 20.--Bias for analyte concentration, deposition, and other physical 

parameters for weekly samples from collocated wet-dry precipitation 

collectors and precipitation depth from collocated rain gages. All data in 

percent.

PERCENTILES

INTERSITE-COMPARISON

STUDY NUMBER

FOR SPECIFIC-CONDUCTANCE MEASUREMENTS

37

38

 SPECIFIC CONDUCTANCE, IN 

MICROSIEMENS PER CENTIMETER

 AT 25 DEGREES CELSIUS

5 10 50 75 90

20.6 23.5 26.3 26.8 27.4

23.1 24.5 26.8 27.5 28.2

25

25.5

26.0

95

28.0

29.4

FOR pH MEASUREMENTS

37

38

pH, IN UNITS

5 10 50 90

4.35 4.40 4.47 4.53

4.13 4.16 4.22 4.29

25

4.44

4.20

75 95

4.49 4.57

4.25 4.37

FOR SPECIFIC-CONDUCTANCE MEASUREMENTS

35

36

 SPECIFIC CONDUCTANCE, IN 

MICROSIEMENS PER CENTIMETER

 AT 25 DEGREES CELSIUS

5 10 50 75 90

14.4 14.9 15.7 16.0 16.8

4.60 4.80 5.30 5.50 6.10

25

15.4

5.10

95

17.8

8.00

FOR pH MEASUREMENTS

35

36

pH, IN UNITS

5 10 50 90

4.80 4.84 4.95 5.04

4.70 4.81 4.92 5.03

25

4.90

4.87

75 95

5.00 5.08

4.98 5.11

(a)

(b) PERCENTILES

PERCENTILES

INTERSITE-COMPARISON

STUDY NUMBER

PERCENTILES

Figure 4. Percentiles for pH and specific conductance from intersite-comparison study numbers (a) 35 

and 36 and (b) 37 and 38.
12 External Quality-Assurance Results for the National Atmospheric Deposition Program/National Trends Network, 1995-96

Intersite-Comparison Study Follow-up Program

Results from the intersite-comparison studies 

are compiled and the operator’s performance is 

evaluated against the NADP/NTN accuracy goals 

applicable for each intersite study (tables 1 and 2). 

Accuracy is evaluated by converting reported values 

into standardized z-values. A z-value is analogous to 

a z-score, which is described by Iman and Conover 

(1983). To produce a z-value, nonparametric estimators replace the traditional parametric estimators 

used by Iman and Conover. The formulas for 

z-scores and z-values are as follows:

 and

where x = an individual observation;

= the mean of all observations;

= the median of all observations;

S = standard deviation of all observations; 

and 

fps = f-pseudosigma of all observations:

By using standardized z-values, each site 

operator’s performance relative to all other site 

operators can be evaluated objectively. The standardized z-values take into account the amount by 

which the pH measurement accuracy goals were 

missed, given the difficulty of measuring the pH of 

the solution on the basis of its hydrogen-ion concentration. By using a cumulative z-value total for the 

three most recent studies, each site operator that fails 

to meet the accuracy goals is placed into one of the 

following four categories:

Level 1. Operators receive a letter stating the 

problem with the recent results and 

suggestions for troubleshooting and 

improving field techniques.

Level 2. Operators are asked to remeasure and 

resubmit results for the current intersite 

study.

Level 3. Operators are given one additional 

check sample to measure.

Level 4. Operators are given two additional 

check samples to measure.

The categories are additive in that Level 3 site 

operators remeasure the original study solution and 

an additional check sample (solution A). Level 4 site 

operators remeasure the original study solution and 

two additional check samples (solutions A and B). 

The additional check samples used for follow up in 

Levels 3 and 4 are unopened aliquots of past 

intersite-comparison study samples that have been 

continuously chilled at 4°C since they were made. 

The pH and specific-conductance values of these 

additional check samples are within the ranges of 

natural precipitation. The stability of the hydrogenion concentration in synthetic precipitation samples 

stored as described is sufficient to allow use of the 

previous intersite samples in the follow-up analysis 

(Gordon and others, 1995). 

For intersite-comparison study number 35, 

14 sites were included in the follow-up study. Thirteen of these site operators were asked to perform 

additional pH measurements. Nine of these site 

operators met the accuracy goals for the pH measurements, and three did not respond. Intersitecomparison study number 36 included 21 follow-up 

study sites, with 16 included in Levels 3 and 4. 

Twelve out of the 18 site operators that responded 

met all the NADP/NTN goals for pH measurements; 

this indicates that the original measurement difficulties had been resolved. The follow-up results for 

study numbers 35 and 36 are summarized in 

figure 5, and the follow-up results for study numbers 

37 and 38 are summarized in figure 6.

z score 

x x –

S = ----------

z value x x –  ̃

fps = ---------- ,

x

x ̃

75th 25th ( ) – percentile

1.349 -------------------------------------------------------
External Quality-Assurance Results 13

 LEVEL

 ONE

 1 sites

LEVEL

 TWO

 1 sites

LEVEL

 THREE

 7 sites

LEVEL

 FOUR

 5 sites

1 met goals on

 remeasure of

 intersite 35

7 met goals on

 remeasure of

 intersite 35

6 met goals on

 for solution A

1 met goal on

remeasure of

 intersite 35

2 met goals

 for

 solution A

2 met goals

 for

 solution B

2 sites

did not

respond

1 site

did not

 respond

2 did not meet

goals on

remeasure

1 did not meet

goals for

solution A

1 did not meet

goals for

solution B

2 sites

did not

respond

 2 sites

 did not

 respond

 LEVEL

 ONE

 2 sites

LEVEL

 TWO

 3 sites

LEVEL

 THREE

 15 sites

LEVEL

 FOUR

 1 site

2 met goals on

 remeasure of

 intersite 36

 10 met goals

 on remeasure

of intersite 36

 9 met goals

 for

 solution A

1 met goals on

remeasure of

 intersite 36

 1 met

 goals for

solution A

 1 met

 goals for

solution B

 1 sites

 did not

 respond

2 did not meet

 goals on

 remeasure

3 did not meet

 goals for

 solution A

 3 sites did

 not

 respond

 3 sites did

 not

 respond

 FOLLOW-UP RESULTS FOR INTERSITE-COMPARISON STUDY NUMBER 36

 FOLLOW-UP RESULTS FOR INTERSITE-COMPARISON STUDY NUMBER 35

Explanation

Level 1 follow-up: Level 3 follow-up:

(1) Letter discussing common sources of (1) Letter discussing common sources of

 measurement errors measurement errors

(2) Request that site operator remeasure the

Level 2 follow-up: remaining portion of the intersite solution

(1) Letter discussing common sources of (3) Measure one additional aliquots – solutions A

measurement errors

(2) Request that site operator remeasure the Level 4 follow-up:

 remaining portion of the intersite solution (1) Letter discussing common sources of

 measurement errors

(2) Request that site operator remeasure the

 remaining portion of the intersite solution

 (3) Measure two additional aliquots – solutions A and B

Figure 5. Follow-up study results for intersite-comparison study numbers 35 and 36.
14 External Quality-Assurance Results for the National Atmospheric Deposition Program/National Trends Network, 1995-96

Figure 6. Follow-up study results for intersite-comparison study numbers 37 and 38.

 LEVEL

 ONE

 2 sites

LEVEL

 TWO

 3 sites

LEVEL

 THREE

 9 sites

LEVEL

 FOUR

 4 sites

2 met goals on

 remeasure of

 intersite 37

4 met goals on

 remeasure of

 intersite 37

4 met goals on

 remeasure of

 intersite 37

1 met goal on

remeasure of

 intersite 37

3 met goals

 for

 solution A

2 met goals

 for

 solution B

 1 site

 did not

 respond

1 did not meet

 goals on

 remeasure

2 did not meet

 goals on

 remeasure

3 sites did

 not

 respond

 1 site did

 not

 respond

2 sites did

 not

 respond

4 sites did

 not

 respond

3 sites did

 not

 respond

 LEVEL

 ONE

 7 sites

LEVEL

 TWO

 7 sites

LEVEL

 THREE

 6 sites

LEVEL

 FOUR

 9 sites

5 met goals on

 remeasure of

 intersite 38

4 met goals on

 remeasure of

 intersite 38

4 met goals

 for

 solution A

3 met goal on

remeasure of

 intersite 38

6 met goals

 for

 solution A

6 met goals

 for

 solution B

 2 sites

 did not

 respond

1 did not meet

 goals on

 remeasure

1 did not meet

 goals for

 solution A

6 did not meet

 goals on

 remeasure

3 did not meet

 goals for

 solution A

3 did not meet

 goals for

 solution B

 1 site did

 not

 respond

 1 site did

 not

 respond

 FOLLOW-UP RESULTS FOR INTERSITE-COMPARISON STUDY NUMBER 38

 FOLLOW-UP RESULTS FOR INTERSITE-COMPARISON STUDY NUMBER 37

Explanation

Level 1 follow-up: Level 3 follow-up:

(1) Letter discussing common sources of (1) Letter discussing common sources of

 measurement errors measurement errors

(2) Request that site operator remeasure the

Level 2 follow-up: remaining portion of the intersite solution

(1) Letter discussing common sources of (3) Measure one additional aliquot – solution A

measurement errors

(2) Request that site operator remeasure the Level 4 follow-up:

 remaining portion of the intersite solution (1) Letter discussing common sources of

 measurement errors

 (2) Request that site operator remeasure the

 remaining portion of the intersite solution

 (3) Measure two additional aliquots – solutions A and B

Figure 6. Follow-up study results for intersite-comparison study numbers 37 and 38.
External Quality-Assurance Results 15

Blind-Audit Program

The purpose of the blind-audit program is to 

assess the effects of routine sample handling, 

processing, and shipping of wet-deposition samples 

on analyte precision and bias. In this program, the 

site operator submits a portion of a synthetic precipitation sample disguised as a natural precipitation 

sample to the Central Analytical Laboratory (CAL) 

for analysis. The portion of the blind-audit sample 

disguised as a natural precipitation sample is subject 

to all of the normal processing and handling steps of 

a regular weekly sample. The remaining minimally 

handled portion of the blind-audit sample also is sent 

to the CAL for analysis, but under separate cover, 

and is analyzed independently of the portion subject 

to the same processing and handling as an actual 

weekly precipitation sample. Biased analytical data 

result if contamination is introduced during the shipping, handling, or processing of samples. Contact 

with the sample-collection container and the routine 

handling of wet-deposition samples have been identified as sources of wet-deposition sample contamination (Nilles and others, 1995; See and others, 

1989).

Throughout 1995 and 1996, on a quarterly 

basis, 32 blind-audit samples were sent to the operators of selected NADP/NTN sites. The sites selected 

on a quarterly basis were chosen to ensure a uniform 

geographic distribution throughout the United States. 

After a site has been selected for the blind-audit program (assuming successful participation), the site is 

not selected again for the blind-audit program until 

the operators of all other NADP/NTN sites have participated. All of the NADP/NTN sites participated at 

least once in the blind-audit program between 1995 

and 1996. Three different sample volumes were distributed for one of the sample matrices. Samples containing 250, 500, or 1,000 mL of the USGS solution 

were sent to the operators of selected sites to assess 

volume-related effects on biases. The volume of 

USGS solution that a site received was determined 

on a random basis. Additional information regarding 

the blind-audit program is available in previous 

reports (Gordon and others, 1997; See and others, 

1990). All components of the blind-audit program, 

from sample preparation to distribution of interpretive reports, are shown in figure 7.

The solutions used in the blind-audit program 

are intended to replicate the range of concentrations 

typical of what is found in natural precipitation 

samples collected at NADP/NTN sites. With the 

exception of the Ultrapure deionized-water samples, 

all of the median analyte-concentration values for 

the solutions used in the blind-audit program were 

between the 25th and 75th percentile of natural 

precipitation samples collected at NADP/NTN sites. 

The solutions used in the 1995-96 blind-audit program, the names of the agencies that prepared them, 

and any special remarks about each solution are 

listed in table 3. The target values for these solutions 

are presented in table 4.

Table 3. Solutions used in the 1995-96 blind-audit and 

interlaboratory-comparison programs

[MΩ, megohms]

Solution Preparing agency Remarks

CAL 4.3 Illinois State Water 

Survey, Central 

Analytical Laboratory

Dilute acid solution.

Ultrapure U.S. Geological 

Survey

Deionized water with a measured resistivity greater 

than 16.7 MΩ.

USGS

SP-1

SP-2

U.S. Geological 

Survey

Prepared from dissolved salts 

and deionized water.

P-17 U.S. Geological 

Survey

A low ionic strength reference 

sample prepared by the 

Standard Reference Sample Project. Dilution not 

required.

P-96 U.S. Geological 

Survey

Prepared from a low ionic 

strength reference sample 

collected by the Standard 

Reference Sample Project. 

Diluted with deionized 

water to achieve concentration levels similar to those 

found in precipitation.

2694A-I

2694A-II

National Institute of 

Standards and 

Technology

Supplied as certified reference 

solutions.
16 External Quality-Assurance Results for the National Atmospheric Deposition Program/National Trends Network, 1995-96

Figure 20.--Bias for analyte concentration, deposition, and other physical 

parameters for weekly samples from collocated wet-dry precipitation 

collectors and precipitation depth from collocated rain gages. All data in 

percent.

Analytical results of the

 blind-audit 

bottle sample compiled

Results presented to the

 National Atmospheric Deposition

 Program/National Trends Network

 Operations Subcommittee

Reports and publications

Analytical results of the

 blind-audit 

bucket sample compiled

Samples analyzed by the Illinois State

 Water Survey, Central Analytical Laboratory

Blind-audit samples prepared by the

 Illinois State Water Survey, 

Central Analytical Laboratory

25 percent of the blind-audit

 sample remains in original bottle

Operators of selected sites receive 250-, 500-, or 1,000-milliliter samples

Site operators process sample

Site operators collect regular weekly precipitation sample

Natural precipitation sample from wet side bucket Is labeled as a dummy sample

Blind-audit sample packaged for distribution to operators of selected sites by the

 U.S. Geological Survey

 75-percent of blind-audit sample is poured into a clean bucket

 Blind-audit sample is poured from bucket into a 1-liter shipping bottle

Blind-audit samples prepared

 by the U.S. Geological Survey

All blind-audit samples shipped to the

 Illinois State Water Survey,

 Central Analytical Laboratory

Figure 7. Blind-audit program of the U.S. Geological Survey.
External Quality-Assurance Results 17

 a Used in the 1995-96 blind-audit program. b Used in the 1995-96 interlaboratory-comparison program. c Concentration certified by the National Institute of Standards and Technology. d Concentration not certified by the National Institute of Standards and Technology.

 e At 25 degrees Celsius and 1 atmosphere pressure (Hem, 1985; Dean, 1979). f Used in the 1996 blind-audit program.

Table 4. Target values for solutions used in the 1995-96 U.S. Geological Survey blind-audit program and interlaboratorycomparison program

[pH, in units; specific conductance, in microsiemens per centimeter at 25 degrees Celsius; <dl, indicates value less than method detection limit; --,indicates 

no value available; significant figures vary because of differences in laboratory precision; Ca2+ , calcium; Mg2+, magnesium; Na+, sodium; K+, potassium; 

Cl-

, chloride; SO4

2-, sulfate; NH4+, ammonium; NO32-, nitrate]

Solution Ca2+ Mg2+ Na+ K+ NH4

+ Cl- NO32- SO42- pH

 Specific

conductance

a

CAL 4.3 <dl <dl <dl <dl <dl <dl 3.14 <dl 4.30 e21.8

a

P-17 0.30 0.045 0.283 0.057 0.10 0.416 1.12 0.50 5.55 e7.0

a,bUSGS 0.14 0.029 0.113 0.024 0.16 0.16 1.05 0.88 4.8 e

8.0

a,bUltrapure <dl <dl <dl <dl <dl <dl <dl <dl 5.65 e

0.064

a,bSP-1 0.46 0.092 0.420 0.076 0.680 0.590 2.100 3.850 4.36 e

30.27

a,bSP-2 0.46 0.070 0.360 0.060 0.560 0.450 3.000 2.334 4.51 e

23.47

b2694A-I c

0.0126 c0.0242 c0.208 c0.056 d0.120 d0.230 d0.530 c2.69 c4.3 e25.4

b2694A-II c

0.0364 c0.0484 c0.423 c0.108 d1.06 d0.940 c7.19 c10.6 c3.6 e129.3

f

P-96 0.28 0.06 0.21 0.09 -- 0.22 -- 0.39 5.72 e4.80

Site operators were provided detailed instructions on how to process the blind-audit samples. The 

instructions sent to site operators prescribed that 

75 percent of the blind-audit sample was to be poured 

into a standard, clean, NADP/NTN 13-L polyethylene 

collection bucket and processed as if it were the wetdeposition sample from the previous week. The blindaudit samples sent to the site operators are marked with 

a line circumscribing the bottle, which indicates how 

much sample the operator is to pour into the clean 

bucket. This poured-out portion of the blind-audit 

sample is referred to as the bucket sample. The operator 

determines the weight of the bucket containing 

75 percent of the blind-audit sample, then removes a 

20-mL aliquot in order to measure the pH and specific 

conductance. The instructions to the site operator specify that, after a minimum residence time of 24 hours, 

the sample is to be transferred from the bucket into a 

clean 1-L shipping bottle. During this entire process, all 

of the handling and processing steps to which a regular 

weekly precipitation sample is subject are duplicated as 

closely as possible. By using a fictitious NADP/NTN 

field-observer report form, the shipping bottle is disguised as a routine wet-deposition sample and submitted to the CAL for analysis. The site operator also 

collects a natural precipitation sample during the 

assigned week for submitting the blind-audit sample. 

The actual precipitation sample is submitted to the 

CAL using a "dummy" field-observer report form. Site 

operators then return the remaining 25 percent of the 

blind-audit sample, still in the original sample bottle, to 

the CAL for analysis. This portion of the blind-audit 

sample is referred to as the bottle sample. In order to 

keep their identities concealed, the actual precipitation 

sample and the two portions of the blind-audit sample 

are all shipped separately to the CAL. A report by 

Gordon and others (1997) contains additional details 

on the submission of blind-audit samples. 

Because of the order in which samples and fieldobserver report forms are processed, it is unlikely that 

the CAL staff could identify individual samples as 

being from an external QA program. Information 

concerning sample chemical composition was not 

provided to the CAL staff that did the analyses or to the 

site operators that did the processing. When the bottle 

portion of a blind-audit sample is submitted to the 

CAL, only the sample-processing group of the laboratory staff recognizes that it is not an actual NADP/NTN 

sample. By the time the analysis is performed, the 

samples appear to be regular network precipitation 

samples. The laboratory staff does not know the analyte 

concentrations in the bottle portions.
18 External Quality-Assurance Results for the National Atmospheric Deposition Program/National Trends Network, 1995-96

The CAL staff that receives and analyzes the 

actual precipitation sample cannot identify from which 

site the sample has been sent. After all the analyses for 

the bucket and bottle portions of the blind-audit sample 

and for the actual precipitation samples are completed, 

the identity of each of these samples is disclosed to the 

CAL Data Quality-Assurance Officer. The 

NADP/NTN data base then is corrected by matching 

the proper analytical data with each sample.

Assessing Analytical Bias

To assess analytical bias, differences between the 

results from the bucket and bottle portions are evaluated. The CAL analyzes all of the paired bucket and 

bottle samples within 21 days of each other. Analytical 

results of the bucket and bottle portions of the blindaudit sample provide paired analyses to determine if 

analyte concentrations have changed in the bucket 

samples as a result of sample handling, shipping, and 

processing protocols. Previous sample stability studies 

have indicated that the analytes in full, unopened 

quality-assurance samples similar in composition to 

those currently used in the blind-audit program are 

stable for at least 45 days (Peden and Skowron, 1978; 

Willoughby and others, 1991). In 1995 and 1996, complete bucket and bottle analyses were available for 124 

of the 128 blind-audit samples sent to participating site 

operators. The incidence of four operators failing to 

submit the blind-audit sample in each of these years 

was similar to the participation rate in previous years 

(Gordon and others, 1997).

If there is physical evidence of contamination 

and the chemistry is abnormal, the CAL assigns natural 

precipitation samples a C code to indicate the sample is 

contaminated (James, 1996). All quality-assurance 

samples, such as the bucket and bottle portions of the 

blind-audit samples that contain extrinsic material are 

assigned a C code, regardless of the sample chemistry. 

The C codes are assigned after the true identities of the 

bucket and bottle portions of the blind-audit samples 

have been disclosed to the CAL Data Quality-Assurance Officer. The Wilcoxon signed-rank test was used 

to compare the differences for the two groups of blindaudit samples (C coded samples and samples without C 

codes) for all major ions, and no statistically significant 

differences were found at the α =0.05 level in either 

1995 or 1996. These results are consistent with those 

from past years for C coded (contaminated) and uncontaminated blind-audit analyses. In 1995 and 1996, there 

were no significant differences in analytical results 

when differences between uncontaminated bottle samples and contaminated bucket samples were compared 

to the differences between uncontaminated bottle samples and uncontaminated bucket samples based on Wilcoxon signed-rank test results. Data from the five 

samples assigned a C code in 1995 and the 14 samples 

assigned a C code in 1996 were, therefore, included in 

the overall statistical analysis of blind-audit samples 

for each of these years.

Paired Blind-Audit Sample Differences

Paired bucket minus bottle differences were 

calculated for the purpose of determining analytical 

bias. Before determining paired bucket minus bottle 

differences (listed in tables 5 and 6), bucket and bottle 

values reported as less than the minimum detection 

limit were set equal to the minimum detection limit. 

The median paired bucket minus bottle differences for 

all analytes in 1995 and 1996 are presented in tables 5 

and 6, respectively, along with the minimum and maximum values, the upper and lower quartiles, and the 

interquartile range. In 1995, the median bucket-sample 

concentrations were larger than the median bottle-sample concentrations for calcium, magnesium, and specific conductance. In 1996, the median bucket-sample 

concentrations were larger than the median bottle-sample concentrations for calcium, magnesium, and potassium.

Between 1995 and 1996, the median bucket 

minus bottle differences for major ions ranged from 

-0.020 mg/L for ammonium and nitrate to 0.006 mg/L 

for calcium. Median paired differences for hydrogen 

ion and specific conductance were 0.000 μeq/L and 

0.200 μS/cm, respectively, in 1995. In 1996, the 

median paired differences for hydrogen ion and specific conductance were –0.357 μeq/L and 0.000 μS/cm, 

respectively. Boxplots in figures 8 and 9 graphically 

depict the paired bucket-sample minus bottle-sample 

concentration differences for all the major ions, as well 

as pH and specific conductance. The upper and lower 

lines defining the "box" portions, in yellow, of the 

graphs depict the interquartile range of the differences 

for each analyte. The "whisker" portions of the boxplots are defined by the largest value within +1.5 times 

the interquartile range extending from either the top or 

bottom of the box portion. A detailed explanation of the 

style of boxplot used throughout this report is shown in 

lower right portion of figures 8 and 9.
External Quality-Assurance Results 19

The interquartile ranges for all of the major ions 

in 1995 and 1996 were very similar to those in 1994 

and ranged from 0.003 mg/L for magnesium to 

0.075 mg/L for ammonium. The length and position of 

the boxplot whiskers were very similar in 1996 to the 

results for 1995, narrowing slightly in 1996 compared 

to 1995 for nitrate and ammonium. For hydrogen ion 

and specific conductance, both the box and whisker 

portions of the graphs were slightly wider in 1995 compared to 1996, reflecting slightly greater variance for 

these parameters in1995. In terms of chemical significance, the change in the NADP/NTN shipping protocol 

in 1994 continues to have a major effect. Since the 

1994 protocol change, there is no longer a significant 

loss of hydrogen ion, an observation first noted in the 

report on 1994 external quality-assurance results 

(Gordon and others, 1997) and again demonstrated by 

the 1995 and 1996 results. The quartiles of the hydrogen-ion differences indicated that 50 percent of the 

samples experienced a change in hydrogen-ion concentration of between –1.90 and +1.78 μeq/L, which is a 

very small percentage of the hydrogen-ion concentration present in the reference samples used in the 

program.

Table 5. Selected statistics for the paired bucket-sample concentration minus bottle-sample concentration differences 

in the blind-audit program during 1995 

 [All units in milligrams per liter except hydrogen ion, in microequivalents per liter, and specific conductance, in microsiemens per centimeter 

at 25 degrees Celsius; N, number of samples; Q1, the lower quartile in the data distribution; Q3, the upper quartile in the data distribution]

Analyte N Minimum Median

Quartiles

Maximum Interquartile 

Q1 Q3 range

Calcium 124 -0.365 0.002 -0.001 0.011 0.089 0.012

Magnesium 124 -0.091 0.001 -0.001 0.002 0.011 0.003

Sodium 124 -0.362 -0.017 -0.040 -0.003 0.063 0.037

Potassium 124 -0.045 0.000 -0.002 0.004 0.222 0.006

Ammonium 124 -0.390 -0.020 -0.060 0.000 0.140 0.060

Chloride 124 -0.410 -0.010 -0.030 0.010 0.330 0.040

Nitrate 124 -0.370 -0.020 -0.045 0.020 0.720 0.065

Sulfate 124 -0.850 0.000 0.000 0.020 0.080 0.020

Hydrogen ion 124 -10.310 0.000 -1.900 1.780 43.440 3.680

Specific conductance 124 -3.600 0.200 -0.400 0.800 10.900 1.200

Table 6. Selected statistics for the paired bucket-sample concentration minus bottle-sample concentration differences 

in the blind-audit program during 1996

[All units in milligrams per liter except hydrogen ion, in microequivalents per liter, and specific conductance, in microsiemens per centimeter 

at 25 degrees Celsius; N, number of samples; Q1, the lower quartile in the data distribution; Q3, the upper quartile in the data distribution]

Analyte N Minimum Median

Quartiles

Maximum Interquartile 

Q1 Q3 range

Calcium 124 -0.210 0.006 0.000 0.016 0.167 0.016

Magnesium 124 -0.007 0.001 0.000 0.003 0.055 0.003

Sodium 124 -0.087 -0.015 -0.033 0.002 0.052 0.035

Potassium 124 -0.026 0.001 -0.001 0.004 0.113 0.005

Ammonium 124 -0.290 -0.020 -0.070 0.005 0.100 0.075

Chloride 124 -4.900 -0.020 -0.040 0.000 0.090 0.040

Nitrate 124 -0.390 -0.010 -0.030 0.020 0.170 0.050

Sulfate 124 -0.090 0.000 0.000 0.020 0.230 0.020

Hydrogen ion 124 -12.100 -0.357 -1.890 0.755 9.180 2.650

Specific conductance 124 -6.600 0.000 -0.650 0.400 3.100 1.050
20 External Quality-Assurance Results for the National Atmospheric Deposition Program/National Trends Network, 1995-96

Figure 8. Paired bucket-sample concentrations minus bottle-sample concentrations in the blind-audit program for 1995.
External Quality-Assurance Results 21

Figure 9. Paired bucket-sample concentrations minus bottle-sample concentrations in the blind-audit program for 1996.
22 External Quality-Assurance Results for the National Atmospheric Deposition Program/National Trends Network, 1995-96

Statistical Significance

A Wilcoxon signed-ranks test (Conover, 1980) 

and paired t-test (Kanji, 1993) were used to determine if 

statistically significant differences existed between the 

analyte concentrations measured for the paired bucket 

and bottle portions of the blind-audit samples submitted 

in 1995 and 1996. Analytical results were evaluated for 

evidence of systematic bias by using both the paired ttest and the Wilcoxon signed-rank test because of the 

different advantages of each test. The paired t-test is a 

much more powerful test then the Wilcoxon signedranks test for paired observations (Kanji, 1993), and the 

blind-audit program uses a paired sample design. However, the paired t-test loses some of its power if the populations are not normal (Kanji, 1993). The Wilcoxon 

signed-ranks test is a distribution-free test with the 

assumption of random samples from independent populations (Conover, 1980). For the Wilcoxon signed-rank 

test, the differences between pairs of observations are 

calculated and then ranked. Where ties occur, the average of the corresponding ranks is used (Conover, 1980; 

Kanji, 1993). All blind-audit samples that had paired 

analyte determinations were included in both statistical 

analyses. Analyte concentrations reported as less than 

the minimum reporting limit were set equal to one-half 

the minimum reporting limit. (Alternatively, analyte 

concentrations reported as less than the minimum reporting limit were set equal to the minimum reporting limit 

and then to zero in order to determine if the results of the 

Wilcoxon signed-ranks test for bias would be different. 

Regardless of whether the values less than detection 

were set equal to zero, one-half the minimum reporting 

limit, or the minimum reporting limit, the results of the 

Wilcoxon signed-ranks test for bias were the same.) 

At a probability level of α=0.05, the Wilcoxon 

signed-ranks test indicated that a negative bias existed 

for ammonium in 1995, whereas the paired t-test indicated statistically significant negative bias for sodium 

and ammonium and positive bias for hydrogen ion and 

specific conductance (table 7). The negative bias for 

ammonium and sodium reflects the tendency for the 

portion of the blind-audit samples that was exposed to 

all of the handling and processing steps to actually have 

a lower concentration of these analytes than the minimally handled control portion of the blind-audit samples. This tendency has occurred since the NADP/NTN 

changed its sample shipping protocol in 1994.

The results for 1996 (table 8 and fig. 9) were generally similar to those for 1995. The probability level for 

ammonium differences in the Wilcoxon signed-rank test 

were fractionally higher than the α=0.05 level, and the 

paired t-test indicated statistically significant negative 

bias for sodium and ammonium and statistically significant positive bias for calcium, magnesium, potassium, 

and sulfate. The complete results of the paired t-test and 

Wilcoxon signed-ranks test for bias with values that 

were less than the minimum reporting limit set equal to 

one-half the minimum reporting limit are shown in 

tables 7 and 8 for 1995 and 1996, respectively.

Analyte Concentration Effects

Boxplots in figures 10 and 11 depict paired 

blind-audit differences by sample concentration. The 

paired blind-audit differences for hydrogen ion and 

specific conductance, arranged by sample concentration, are shown in figures 12 and 13. A relation 

between sample concentration and paired blind-audit 

differences is not readily apparent for most analytes 

upon visual inspection of these graphs. Figures 10 

through 13 do illustrate two important points: the data 

Table 7. Results of the tests for bias in the blind-audit program during 1995, using the paired t-test and a Wilcoxon signedrank test

[Bucket and bottle values less than the minimum reporting limit set equal to one-half the minimum reporting limit; Probability (Prob.), number of outcomes in the 

event divided by the total number of outcomes in the sample space; α , the maximum probability of making a Type I error; t, test statistic for the paired t-test; z, test 

statistic for the Wilcoxon signed-rank test; C coded, samples coded with a C to indicate possible contamination]

Analyte

Probability > |t|

C coded samples 

included

Probability > |z| 

C coded samples 

included

Probability > |z| 

C coded samples 

removed

Determined to be

 biased (α=0.05)?

(test 1) (test 2) (test 3) test 1 test 2 test 3 t Prob.>|t| z Prob.>|z| z Prob.>|z|

Calcium 0.815 0.4166 0.587 0.5571 0.571 0.5675 NO NO NO

Magnesium 0.233 .8161 0.611 .5410 0.641 0.5216 NO NO NO

Sodium -4.954 .0001 -1.822 .0685 -1.850 0.0643 YES NO NO

Potassium 1.466 .1452 0.179 .8582 0.096 0.9231 NO NO NO

Ammonium -6.220 .0001 -2.622 .0087 -2.698 0.0070 YES YES YES

Chloride -1.877 .0629 -0.921 .3571 -0.976 0.3292 NO NO NO

Nitrate -1.581 .1165 -0.843 .3991 -0.864 0.3877 NO NO NO

Sulfate -0.232 .8172 0.726 .4678 0.724 0.4692 NO NO NO

Hydrogen ion 3.370 .0010 0.785 .4322 0.881 0.3785 YES NO NO

Specific conductance 2.168 .0321 0.796 .4261 0.804 0.4213 YES NO NO
External Quality-Assurance Results 23

in these figures cluster around zero and display heteroscedasticity (Helsel and Hirsch, 1992). The 

Kruskal-Wallis test (Iman and Conover, 1983) was 

used to determine the relation between paired blindaudit sample differences and the target concentrations 

used in the blind-audit program in 1995 and 1996. 

Table 9 contains the p-values associated with the 

Kruskal-Wallis test statistic for each analyte, representing the level of significance of the data with respect to 

the test in question. The level of significance was evaluated to determine whether to accept or reject the null 

hypothesis. The value for α, the maximum probability 

of making a Type I error (Iman and Conover, 1983), 

was set to 0.05. Although results of a Kruskal-Wallis 

test indicate statistically significant relationships 

between the magnitude of paired blind-audit differences and sample concentration for potassium, ammonium, sulfate, hydrogen ion, and specific conductance 

in 1995 and 1996 (table 9), the heteroscedasticity of the 

data must be considered. Heteroscedasticity tempers 

the weight one can place on the results of the KruskalWallis test somewhat. The departure from equal variances was not large enough to nullify the results (Dr. 

Luther Smith, ManTech Environmental Technology, 

written commun., 1998). The implication from this 

analysis is that the larger the concentration in a blindaudit sample, the greater the magnitude of the paired 

blind-audit sample differences for these analytes.

Ultrapure Sample Analyses

Sixteen Ultrapure deionized-water samples were 

included in the blind-audit program in 1995 and again 

in 1996. Fifteen of the 16 Ultrapure samples were 

submitted correctly each year, resulting in complete 

data pairs. As with all blind-audit samples, deionized 

samples were processed in two parts; the first part was 

exposed to all normal sample-handling steps, which 

represented a field-exposed portion, and the second 

part was subject to minimal handling, which represented a control portion. In all of the deionized-water 

samples submitted in 1995 and 1996, sodium was 

detected in the field-exposed and the control portions. 

Ammonium, nitrate, and chloride also were commonly 

detected in both sample portions (table 10). As with the 

statistical evaluation of the entire blind-audit data sets 

discussed previously, the paired t-test and Wilcoxon 

signed-rank test were used in the statistical assessment 

of Ultrapure sample results. Statistical significance at a 

probability level of α =0.05 was reported if the probability of exceeding the test statistic for both the paired 

t- and Wilcoxon signed-rank tests were less than or 

equal to 0.05. The analysis of Ultrapure samples indicated positive bias for calcium, magnesium, and hydrogen ion in 1995. In 1995, the Ultrapure samples also 

indicated negative bias for ammonium. In 1996, potassium and specific conductance were the only two ions 

that indicated any bias (positive for both of these ions) 

in the Ultrapure samples. Below-detection-limit cases 

for the Ultrapure samples were handled in the same 

manner described for the analysis of all blind-audit 

samples. Unlike the analysis for the full data set where 

the treatment of values below detection limit had no 

effect on the results of the Wilcoxon signed-rank test, 

the treatment of values below detection limit in the 

Ultrapure samples did affect the results somewhat. The 

most conservative results of the below-detection-limit 

treatments were used, decreasing the number of ions 

with significant differences between paired bucket and 

bottle analytes in 1995. The treatment of values below 

detection limit did not affect the Wilcoxon signed-rank 

results for Ultrapure samples in 1996.

Table 8. Results of the tests for bias in the blind-audit program during 1996, using the paired t-test and a Wilcoxon signedrank test

[Bucket and bottle values less than the minimum reporting limit set equal to one-half the minimum reporting limit; Probability (Prob.) , number of outcomes in the 

event divided by the total number of outcomes in the sample space; α , the maximum probability of making a Type I error; t, test statistic for the paired t-test; z, test 

statistic for the Wilcoxon signed-rank test; C coded, samples coded with a C to indicate possible contamination]

Analyte

Probability > |t|

C coded samples 

included

Probability > |z| 

C-coded samples 

included

Probability > |z| 

C-coded samples 

removed

Determined to be

 biased (α=0.05)?

(test 1) (test 2) (test 3) test 1 test 2 test 3 t Prob.>|t| z Prob.>|z| z Prob.>|z|

Calcium 3.118 0.0023 1.466 0.1426 1.373 0.1699 YES NO NO

Magnesium 3.336 .0011 1.269 .2045 1.325 0.1852 YES NO NO

Sodium -6.255 .0001 -1.423 .1549 -1.339 0.1806 YES NO NO

Potassium 2.784 .0062 0.930 .3526 0.894 0.3716 YES NO NO

Ammonium -5.488 .0001 -1.876 .0607 -1.922 0.0546 YES NO NO

Chloride -1.447 .1503 -1.227 .2200 -1.171 0.2416 NO NO NO

Nitrate -1.756 .0815 -0.478 .6330 -0.558 0.5771 NO NO NO

Sulfate 3.682 .0003 1.017 .3091 0.955 0.3397 YES NO NO

Hydrogen ion 0.308 .7585 -0.577 .5643 -0.484 0.6282 NO NO NO

Specific conductance -0.930 .3540 -0.385 .7001 -0.342 0.7322 NO NO NO
24 External Quality-Assurance Results for the National Atmospheric Deposition Program/National Trends Network, 1995-96

Figure 10. Relation between paired blind-audit differences for major ions and the analyte concentrations 

of solutions used in the blind-audit program in 1995.
External Quality-Assurance Results 25

Figure 11. Relation between paired blind-audit differences for major ions and the analyte concentrations of 

solutions used in the blind-audit program in 1996.
26 External Quality-Assurance Results for the National Atmospheric Deposition Program/National Trends Network, 1995-96

Figure 12. Relation between paired blind-audit differences for hydrogen-ion and specific conductance and the 

analyte concentrations of solutions used in the blind-audit program in 1995.
External Quality-Assurance Results 27

Figure 13. Relation between paired blind-audit differences for hydrogen ion and specific conductance 

and the analyte concentrations of solutions used in the blind-audit program in 1996.
28 External Quality-Assurance Results for the National Atmospheric Deposition Program/National Trends Network, 1995-96

Table 9. Results of the Kruskal-Wallis analysis of variance test to determine the relation 

between paired blind-audit sample differences and the target concentrations used in the blind-audit 

program in 1995 and 1996

[All paired blind-audit differences expressed in milligrams per liter except specific conductance, in microsiemens per centimeter at 25 degrees 

celsius, and hydrogen ion, in microequivalents per liter]

Analyte

Bucket minus bottle differences in 

significance levels (p-values)

by target concentration

Statistically significant (α=0.05)?

1995 1996 1995 1996

Calcium 0.081 0.094 NO NO

Magnesium .085 .014 NO YES

Sodium .137 .787 NO NO

Potassium .002 .116 YES NO

Ammonium .048 .000 YES YES

Chloride .912 .628 NO NO

Nitrate .189 .625 NO NO

Sulfate .002 .017 YES YES

Hydrogen ion .0001 .003 YES YES

Specific conductance .0001 .0001 YES YES

Table 10. Number of determinations exceeding the method detection limit for

the Ultrapure deionized water samples submitted annually as part of the 

blind-audit program, 1995-1996

Analyte

Field-exposed bucket

portion

Minimally handled bottle 

portion

1995 1996 1995 1996

Calcium 5 0 3 1

Magnesium 6 1 3 3

Sodium 15 15 15 15

Potassium 5 0 11 3

Ammonium 12 12 9 8

Chloride 11 13 14 12

Nitrate 15 14 13 14

Sulfate 0 0 0 0
External Quality-Assurance Results 29

Percent Differences

The relative and absolute percent differences 

were calculated for all of the paired blind-audit samples. As long as the distribution of relative and absolute percent differences is considered, relative percent 

differences are useful for understanding bias in the data 

set, whereas absolute percent differences help to show 

the variability. The upper and lower quartiles as well as 

the median relative and absolute percent differences are 

listed in tables 11 and 12. The relative and absolute percent differences were determined by calculating each 

paired blind-audit difference as a percentage of the 

known sample concentration:

Relative percent difference = (C1- C2)/ C3 x 100 ,

and

Absolute percent difference = |(C1- C2)/ C3| x 100 ,

where

C1 = Sample concentration, in milligrams 

per liter, from the portion of the blindaudit sample that is exposed to all of 

the handling and processing steps of a 

normal weekly precipitation sample 

(bucket portion); 

C2 = Sample concentration, in milligrams 

per liter, from the control portion of the 

blind-audit sample subject to minimal 

handling and processing (bottle portion); and

C3 = Known concentration of the blind-audit 

sample, in milligrams per liter.

Bucket-bottle data pairs were excluded for a 

given analyte if the known concentration was less than 

or equal to the minimum reporting limit. The percent 

differences are inflated by the influence of large (on a 

percentage basis) bucket minus bottle differences when 

the known concentration was less than or equal to the 

minimum reporting limit (Nilles and others, 1995). 

The median (50th percentile) relative percent 

bias for most analytes was within a narrow range of 

–5 to +5 percent, indicating a low occurrence of either 

positive or negative bias. Exceptions were ammonium, 

which had a median relative bias of –9.72 percent in 

1995 and –12.50 percent in 1996. The median relative 

percent bias of –12.50 percent for ammonium in 1996 

was the largest for all of the analytes. The median relative percent bias also was fairly large for sodium at

-7.99 percent in 1995 and –7.00 percent in 1996. 

Chloride also had a median relative percent bias of 

–7.04 percent in 1996. 

One way to evaluate bias is to determine if the 

paired differences for a given analyte are evenly 

distributed about the zero difference line. If the paired 

differences for a given analyte were completely free of 

bias, the median difference would be zero and the 

upper quartile would be the same distance from the 

median difference of zero as the lower quartile. 

Sodium, potassium, chloride, and ammonium failed to 

display this type of pattern in 1995 or 1996. 

Table 11. Relative and absolute bucket minus bottle differences calculated as a percentage of the target bottle concentration 

for each analyte in 1995

Analyte

Relative bucket minus bottle differences, 

expressed as a percentage of corresponding 

target bottle concentration (selected data pairs 

only)

Absolute bucket minus bottle differences, 

expressed as a percentage of corresponding target 

bottle concentration (selected data pairs only)

Percentiles Percentiles

25th 50th 75th 25th 50th 75th

Calcium -1.35 1.43 4.62 1.43 2.86 6.02

Magnesium -2.25 0.36 2.70 0.96 2.70 5.41

Sodium -34.24 -7.99 -0.66 4.88 11.13 39.13

Potassium -9.48 0.00 5.26 3.45 8.00 24.00

Ammonium -37.50 -9.72 -1.67 6.25 12.50 37.50

Chloride -8.41 -2.29 0.80 2.17 7.04 14.08

Nitrate -3.22 -0.93 0.88 0.93 2.19 3.70

Sulfate -0.41 0.46 2.13 0.41 1.07 2.13

Hydrogen ion -6.64 2.19 14.48 4.61 8.95 16.71

Specific conductance -2.46 1.25 7.50 1.76 4.57 10.00
30 External Quality-Assurance Results for the National Atmospheric Deposition Program/National Trends Network, 1995-96

None of the analytes display a strong tendency 

for a positive bias in 1995. This is made clear by the 

fact that the interquartile range for relative percent difference spans zero for 8 of the 10 analytes. In contrast, 

virtually all of the paired differences for ammonium 

and sodium were negative values, reflecting the negative bias for these two analytes. In addition to depicting 

a negative bias, ammonium and sodium paired-sample 

differences also had the most variability of all of the 

analytes. This is illustrated by the fact that these two 

analytes have the highest median absolute percent 

differences of all 10 analytes.

The size of the interquartile range was a useful 

indicator of the amount of variability associated with a 

given analyte. Sodium and ammonium had interquartile ranges exceeding 15 percent on both a relative and 

an absolute basis in 1995 (table 11). This indicates that 

these analytes had the most variability associated with 

their paired blind-audit differences. In 1996, the interquartile range of relative and absolute percent 

differences exceeded 15 percent only for sodium, 

ammonium, and chloride (table 12). It is worth noting 

that ammonium and sodium paired sample differences 

in 1995 indicate that a quarter of the paired observations had over a third less ammonium and sodium in the 

bottle portion than in the bucket portion. This is evident 

in table 11, which depicts the lower quartile (25th percentile) of paired observations for sodium and ammonium as –34.24 and –37.50 percent, respectively. 

Table 12 depicts that, in 1996, only the lower quartile 

of paired bucket minus bottle relative percent differences for ammonium indicated over a third less ammonium (-34.92 percent) in the bottle portion in a quarter 

of all data pairs. The fact that the interquartile range of 

their paired differences for most analytes did not 

exceed 15 percent in 1995 or 1996 is an important indicator that users of NADP/NTN weekly precipitation 

data during this time period can be assured that the 

variability of the data for most analytes was not large.

Sample Volume Effects

A variety of sample volumes were used in the 

blind-audit program in order to investigate if a relationship existed between the volume collected in weekly 

NADP/NTN samples and the amount of contamination 

introduced through shipping and handling procedures. 

Differences between the bucket and bottle pairs were 

analyzed in their original concentration units and in 

units of mass. To convert differences from concentration units to mass units (micrograms), the concentration differences were multiplied by the volume, in 

liters, of sample poured into the bucket. Boxplots in 

figures 14 through 17 depict the differences for each 

analyte plotted by sample concentrations after converting the differences to units of mass.

Table 12. Relative and absolute bucket minus bottle differences calculated as a percentage of the target bottle concentration 

for each analyte in 1996

Analyte

Relative bucket minus bottle differences, 

expressed as a percentage of corresponding target 

bottle concentration (selected data pairs only)

Absolute bucket minus bottle differences, 

expressed as a percentage of corresponding target 

bottle concentration (selected data pairs only)

Percentiles Percentiles

25th 50th 75th 25th 50th 75th

Calcium 0.00 3.57 6.96 2.14 4.29 9.13

Magnesium 0.00 2.20 5.41 1.10 3.59 6.25

Sodium -21.74 -7.00 -0.46 5.37 12.88 22.83

Potassium -5.36 1.49 12.00 4.00 8.00 15.79

Ammonium -34.92 -12.50 -0.00 6.25 14.29 38.10

Chloride -21.13 -7.04 0.00 3.85 7.45 21.13

Nitrate -1.85 -0.34 1.29 0.93 1.85 2.78

Sulfate 0.00 1.07 2.13 0.42 1.27 2.67

Hydrogen ion -7.15 0.00 8.60 4.50 8.42 13.58

Specific conductance -3.75 0.00 5.00 2.15 4.54 8.75
External Quality-Assurance Results 31

Figure 14. Relation between paired blind-audit differences for the major ions and sample volume for 

the 250-, 500-, and 1,000-milliliter U.S. Geological Survey solution samples in 1995.
32 External Quality-Assurance Results for the National Atmospheric Deposition Program/National Trends Network, 1995-96

Figure 15. Relation between paired blind-audit differences for the major ions and sample volume for 

the 250-, 500-, and 1,000-milliliter USGS solution samples in 1996.
External Quality-Assurance Results 33

Figure 16. Relation between paired blind-audit differences and sample volume for pH and specific 

conductance for the 250-, 500-, and 1,000-milliliter USGS solution samples in 1995.
34 External Quality-Assurance Results for the National Atmospheric Deposition Program/National Trends Network, 1995-96

Figure 17. Relation between paired blind-audit differences for pH and specific conductance and 

sample volume for the 250-, 500-, and 1,000-milliliter USGS solution samples in 1996.
External Quality-Assurance Results 35

To determine if there was a statistically significant relation between paired blind-audit differences 

and the volume of the sample, a Kruskal-Wallis test 

was performed. As in past years, an equal number of 

samples containing 250, 500, or 1,000 mL of USGS 

solution were included in the 1995 and 1996 blind 

audits. Results of the Kruskal-Wallis test indicate no 

significant (α=0.05) relation between paired blindaudit sample differences in units of concentration 

and sample volume for any of the analytes in 1995, 

except for potassium (table 13). This is in marked 

contrast with the results from previous years when at 

least 5 of the 10 measured analytes displayed a statistically significant relation between paired blindaudit differences in units of concentration and sample volume. When paired differences were converted 

to units of mass, however, the outcome of the 

Kruskal-Wallis test was markedly different; all of 

the analytes, except sulfate and nitrate, had statistically significant (α=0.05) differences between the 

bucket and bottle pairs in 1995. Furthermore, the 

boxplots in figures 14 and 15 show that the bucket 

minus bottle differences in units of mass were 

usually negative.

Paired differences in units of mass tended to 

become more negative with increasing sample 

volume. This tendency has been observed since the 

protocol change in 1994 and is in sharp contrast to 

the results observed before 1994. Before 1994, 

paired differences in units of mass increased with 

increasing sample volume (Nilles and others, 1993). 

Because contamination from major sources, such as 

the o-ring bucket and bucket walls, have been 

removed from the system, contamination from other 

sources, such as the contaminated filters discussed 

previously, are exerting a more noticeable influence 

on the paired blind-audit results than previously 

observed from these sources. Upon arrival at the 

CAL, both the bucket and the bottle portions are 

subjected to a filtering process. 

Table 13. Results of the 1995 Kruskal-Wallis analysis of variance tests to determine if bucket minus bottle differences for the 

250-, 500-, and 1,000-mL samples of the USGS solution used in the blind-audit program have equivalent distributions

[mL, milliliter]

Analyte

Bucket minus bottle 

concentrations attained 

significance (p-value) 

levels on a concentration basis

Statistically significant

 (α=0.05) differences determined between 250-, 500-, 

and 1,000-mL USGS samples 

on a concentration basis

Bucket minus bottle 

concentrations attained 

significance (p-value) 

levels on a mass per 

bucket basis

Statistically significant

 (α=0.05) differences 

determined between 250-, 

500-, and 1,000-mL USGS 

samples on a mass per 

bucket basis

Calcium 0.503 NO 0.012 YES

Magnesium 0.088 NO 0.001 YES

Sodium 0.125 NO 0.000 YES

Potassium 0.014 YES 0.013 YES

Ammonium 0.082 NO 0.000 YES

Chloride 0.231 NO 0.025 YES

Nitrate 0.998 NO 0.204 NO

Sulfate 0.184 NO 0.141 NO

Hydrogen ion 0.067 NO 0.023 YES

Specific conductance 0.118 NO 0.006 YES
36 External Quality-Assurance Results for the National Atmospheric Deposition Program/National Trends Network, 1995-96

Before collecting the final aliquot for analysis 

from the bucket and bottle portions, the technician 

pours the excess sample volume through the filtering 

apparatus. If the filter is a source of contamination, 

then the amount of an analyte contributed to the 

aliquot collected for analyses will be inversely proportional to the amount of sample poured through the 

filter before collecting the aliquot. Bucket-sample 

portions have more water available to rinse the filter 

before collecting the final aliquot for analysis and 

would, therefore, be subject to less contamination 

than bottle samples, which have less volume available 

for rinsing. The maximum excess bucket-sample 

volume available for rinsing was 200 mL, the limit of 

volume the filtering apparatus can hold. The results 

for 1996 were similar to those observed in 1995, 

except that only 5 of the 10 analytes had statistically 

significant (α=0.05) differences between the bucket 

and bottle portions when the differences were converted to units of mass (table 14).

For 1995, sodium and ammonium differences 

become less negative with increasing volume while 

increasing in terms of magnitude, as indicated by the 

increasing spread in the interquartile range with 

increasing volume (fig. 15). The finding that the 

sodium bias becomes less negative with increasing 

volume supports the theory that the amount of 

sodium leached from the filters decreases with 

increasing rinse volume. In October 1998, the author 

discussed the results of the blind-audit program with 

Jane Rothert of the Illinois State Water Survey. 

Together, the author and Ms. Rothert traced the 

probable source of sodium contamination to the 

brand of filters used to process samples at the CAL 

prior to analysis. For 1996, the volume-based results 

of the blind-audit program indicate a situation similar to the situation indicated by the results in 1995, 

but sulfate differences became more positive as 

volume increased. 

Table 14. Results of the 1996 Kruskal-Wallis analysis of variance tests to determine if bucket minus bottle differences for the 

250-, 500-, and 1,000-mL samples of the USGS solution used in the blind-audit program have equivalent distributions

[mL, milliliter]

Analyte

Bucket minus bottle 

concentrations attained 

significance (p-value) 

levels on a concentration basis

Statistically significant 

(α=0.05) differences determined between 250-, 500-, 

and 1,000-mL USGS samples 

on a concentration basis

Bucket minus bottle 

concentrations attained 

significance (p-value) 

levels on a mass per 

bucket basis

Statistically significant 

(α=0.05) differences determined between 250-, 500-, 

and 1,000-mL USGS samples on a mass per bucket 

basis

Calcium 0.992 NO 0.116 NO

Magnesium 0.944 NO 0.020 YES

Sodium 0.393 NO 0.001 YES

Potassium 0.066 NO 0.076 NO

Ammonium 0.030 YES 0.000 YES

Chloride 0.205 NO 0.000 YES

Nitrate 0.914 NO 0.994 NO

Sulfate 0.326 NO 0.008 YES

Hydrogen ion 0.341 NO 0.410 NO

Specific conductance 0.972 NO 0.972 NO
External Quality-Assurance Results 37

Interlaboratory-Comparison 

Program

The two objectives of the interlaboratorycomparison program are: (1) to estimate the analytical precision of participating laboratories, and (2) to 

determine if statistically significant differences 

existed among the analytical results of participating 

laboratories. The following laboratories participated 

in the interlaboratory-comparison program in 1995 

and 1996: (1) Illinois State Water Survey, Central 

Analytical Laboratory (CAL) in Champaign, 

Illinois; (2) Atmospheric Environment Service, 

Environment Canada (AES) in Ontario, Canada; 

(3) QST Environmental (QST), formerly known as 

Environmental Science and Engineering, Inc., in 

Gainesville, Florida; (4) Ontario Ministry of the 

Environment, Water Quality Section (MOE), in 

Ontario, Canada; (5) Global Geochemistry 

Corporation (GGC) in Canoga Park, California; and 

(6) Shepard Analytical Services (SA) in Simi Valley, 

California. GGC dropped out of the program and 

was replaced by Shepard Analytical Services in the 

third quarter of 1996. The same analyst that analyzed 

the samples at GGC analyzed the samples at SA 

using the same analytical methods. Therefore, 

GGC/SA is used in some of the figures to signify 

data reported collectively by the two laboratories in 

1996, with annotation to indicate when the change in 

laboratories occurred.

Samples from three sources were used in the 

interlaboratory-comparison program in 1995 and 

1996: (1) synthetic wet-deposition samples and 

Ultrapure deionized-water samples prepared by the 

USGS, (2) standard reference samples (2694A-I, and 

2694A-II) prepared and certified by the National 

Institute of Standards and Technology (NIST) 

(National Institute of Standards and Technology, 

1991), and (3) natural wet-deposition samples collected at NADP/NTN sites and bottled by the CAL. 

Table 3 contains information on the preparation of 

the synthetic solutions made by the USGS or the 

CAL, as well as the solution names of the NIST-certified samples. Target values for all of the synthetic 

wet-deposition solutions used in the interlaboratorycomparison program are listed in table 4. Each of the 

five participating laboratories received 104 samples 

in 1995 and 1996. In each year, 52 were aliquots of 

natural precipitation bottled by the CAL, 12 were 

synthetic samples made by the USGS and referred to 

as USGS solution, 18 were NIST-certified samples, 

8 were synthetic samples made by the USGS and 

referred to as SP-1 solution, 8 were synthetic samples made by the USGS and referred to as SP-2 solution, and 6 were Ultrapure deionized-water samples 

bottled by the USGS.

Natural wet-deposition samples collected at 

NADP/NTN sites that had volumes greater than 

750 mL were selected randomly by the CAL for use 

in the interlaboratory-comparison program. These 

natural wet-deposition samples were divided into 

10 aliquots by using a deca-splitter (a device used to 

split samples into ten parts). The aliquots were 

bottled in 125-mL polyethylene bottles and shipped 

in chilled, insulated containers to the USGS in 

Denver, Colorado. Natural samples were kept refrigerated and were reshipped to participating laboratories within 10 days of receipt by the USGS. For the 

analysis of all samples, except the natural samples, 

the concentration of each analyte was known either 

because the sample was a certified reference sample 

or because it was prepared by the USGS and tested 

to verify the target values. The analysis of the natural 

samples was limited to comparing differences 

between laboratories on identical sets of replicate 

samples (26 replicate pairs per laboratory) in a given 

year.

Samples used in the 1995-96 interlaboratorycomparison program were relabeled and shipped by 

the USGS to the five participating laboratories 

approximately every 2 weeks. All samples were 

relabeled with only a sample number to ensure that 

laboratory personnel could not determine the type of 

sample (natural or synthetic) or the actual analyte 

concentrations in the samples until the chemical 

analyses were performed. Each laboratory received 

four samples per shipment, and each laboratory 

received the same type of samples for a given 

mailing. Specifically, the first shipment for all laboratories in a 4-week period consisted of triplicate 

synthetic wet-deposition samples prepared by NIST 

and a single aliquot of Ultrapure deionized water or 

four replicates of synthetic wet-deposition samples. 

The second shipment consisted of two natural wetdeposition samples, in duplicate. A flowchart of the 

interlaboratory-comparison program is shown in 

figure 18.
38 External Quality-Assurance Results for the National Atmospheric Deposition Program/National Trends Network, 1995-96

125-milliliter interlaboratorycomparison program samples 

prepared by the

 Illinois State Water Survey,

 Central Analytical Laboratory

Natural 

wet-deposition

samples

60-milliliter 

interIaboratory-comparison 

program samples prepared

 by the National Institute of 

Standards and Technology

125-milliliter interlaboratorycomparison program

 samples prepared 

by the 

U.S. Geological Survey

Mailed to participating 

laboratories for analysis

Synthetic standard 

reference samples

Results presented to

 the National Atmospheric 

Deposition Program/National 

Trends Network Operations 

Subcommittee

Quarterly report sent to 

participating laboratories

Reports 

and publications

Analytical results reported to the U.S. Geological Survey

QST

 Environmental

Central

Analytical 

Laboratory

Atmospheric 

Environment 

Service

Ontario Ministry

 of the

 Environment

Ultra pure 

deionized-water 

samples

Synthetic 

wet-deposition 

samples

Global Geochemistry 

Corporation

Shepard Analytical 

Services

Figure 18. Interlaboratory-comparison program of the U.S. Geological Survey.
External Quality-Assurance Results 39

Laboratory Precision

Precision is defined by the extent to which a 

given set of measurements of the same sample agrees 

with its mean. For this analysis, laboratory precision 

was estimated for each analyte by calculating the 50th

and 90th percentile of the absolute differences for the 

results reported for the replicate natural and synthetic 

wet-deposition samples (tables 15 and 16). Differences 

were calculated from 26 sample pairs of replicate natural samples for each laboratory in 1995 and in 1996. 

Analyte concentrations reported as less than the minimum reporting limit were set equal to the minimum 

reporting limit. 

In 1995 and 1996, cation precision estimates at 

the 50th percentile were quite similar among laboratories. All participating laboratories had median absolute 

differences that were less than or equal to 0.004 mg/L 

for the cations. Larger differences between laboratories 

were apparent for precision estimates at the 90th percentile for cations. In 1995, cation precision estimates 

at the 90th percentile ranged from 0.001 to 0.038 mg/L. 

In 1996, the 50th percentile for cation precision estimates were less than or equal to 0.004 mg/L, whereas 

the 90th percentile for cation precision estimates ranged 

from 0.000 to 0.019 mg/L in 1996.

Anion precision estimates at the 50th percentile 

exhibited greater variability among laboratories than 

was observed for the cations. Median absolute differences ranged from 0.000 to 0.015 mg/L in 1995-96. 

Differences among laboratories in 1995 for precision 

estimates at the 90th percentile for anions ranged from 

0.010 to 0.050 mg/L. Differences among laboratories 

were more apparent in 1996 for precision estimates at 

the 90th percentile for the anions, ranging from 0.01 to 

0.085 mg/L. 

The pH (expressed as hydrogen ion) precision 

estimates at both the 50th and 90th percentile exhibited 

scant variability among laboratories in 1995-96. The 

50th percentile precision estimates for hydrogen ion 

ranged from 0.075 to 0.687 μeq/L in 1995; in 1996 the 

range was 0.000 to 0.448 μeq/L. Precision estimates at 

the 90th percentile ranged from 0.746 to 1.649 μeq/L in 

1995 and from 1.040 to 1.838 μeq/L in 1996. 

Four laboratories routinely reported specificconductance results. The AES does not routinely report 

specific-conductance measurements. There was very 

little variability among laboratories in their reported 

specific-conductance measurements in 1995: The 50th

percentile precision estimate ranged from 0 to 

0.200 μS/cm in 1995 for all four participating laboratories, whereas the 90th percentile ranged from .80 to 

1.65 μS/cm. In 1996, the 50th percentile precision estimate for specific conductance ranged from 0.050 to 

0.100 μS/cm and the 90th percentile ranged from 0.200 

to 1.500 μS/cm.

Table 15. Fiftieth and ninetieth percentile absolute differences for analyses of replicate samples determined by five 

laboratories participating in the 1995 interlaboratory-comparison program

 [All units in milligrams per liter except hydrogen ion, in microequivalents per liter, and specific conductance, in microsiemens per centimeter at 25 degrees 

Celsius; CAL: Illinois State Water Survey, Central Analytical Laboratory; AES: Atmospheric Environment Service; QST: QST Environmental; MOE: 

Ontario Ministry of the Environment, Water Quality Section; GGC: Global Geochemistry Corporation; --: not calculated]

Analyte

CAL AES QST GGC MOE

50th 90th 50th 90th 50th 90th 50th 90th 50th 90th

Calcium 0.000 0.010 0.003 0.010 0.001 0.003 0.001 0.004 0.002 0.009

Magnesium .000 .001 .000 .001 .000 .001 .000 .002 .001 .002

Sodium .002 .006 .001 .005 .001 .003 .001 .005 .003 .023

Potassium .001 .019 .002 .016 .000 .001 .001 .005 .001 .038

Ammonium .000 .010 .001 .005 .003 .010 .002 .005 .001 .006

Chloride .000 .020 .002 .017 .001 .010 .002 .005 .000 .000

Nitrate .000 .010 .010 .024 .002 .009 .006 .026 .000 .018

Sulfate .000 .010 .005 .026 .004 .020 .005 .010 .005 .050

Hydrogen ion .094 .800 .163 .746 .687 1.649 .320 1.391 .075 1.040

Specific conductance .000 .300 -- -- .200 .400 .050 .200 .100 1.000
40 External Quality-Assurance Results for the National Atmospheric Deposition Program/National Trends Network, 1995-96

Interlaboratory Bias

Interlaboratory bias (bias among multiple 

laboratories) is defined as a systematic difference in 

reported values for a given laboratory observed 

when the results from several laboratories are 

compared. To examine potential bias in the analytical results among the laboratories, a Friedman test 

was performed. A Friedman test is used to investigate the significance of the differences in response 

to multiple treatments for more than two subjects 

without assuming the data are from normal distributions (Kanji, 1993). In this application, the multiple 

treatments are the different sample matrices, and the 

subjects are the various laboratories. This allows the 

comparison of paired data from each of the 

participating laboratories, while controlling for the 

different sample matrices sent in different mailings. 

In 1995, results of the Friedman test indicated 

significant (α=0.05) differences in analyte measurements for calcium, potassium, sulfate, nitrate, and 

hydrogen ion among the five laboratories. In 1996, 

the Friedman test indicated significant 

(α=0.05) differences in analyte measurements for 

potassium, ammonium, chloride, sulfate, nitrate, and 

hydrogen ion.

In order to facilitate a visual comparison of 

interlaboratory differences, graphs of each laboratory’s analyte concentrations minus the analyte 

medians calculated for all laboratories are presented 

in the control charts shown in figures 19 through 28. 

The warning limits are positioned at +2 F-pseudosigmas from the zero difference line, whereas the 

control limits were placed at +3 F-pseudosigmas 

from the zero difference line. Control limits 

(3-sigma) define the bounds of virtually all values 

produced by a system in statistical control. Modern 

control charts commonly have additional limits 

called warning limits (2-sigma) within which most 

(95 percent) of the values should lie (Taylor, 1987). 

There are some important features in figures 

19-28 of which the reader should be aware. First, the 

graph for each analyte is scaled to reflect the maximum scatter for a given analyte in the data for one of 

the five participating laboratories. That means that 

the graph for each analyte is scaled to reflect the 

maximum range of difference between a participating laboratory’s measured values and the median 

value of all participating laboratories. When viewing figures 19-28, the reader should first note the 

different scale for each analyte depicted in figure 19. 

Scaling differences are visual clues of differences in 

performance in the analysis of a given analyte for the 

laboratories as a group. For example, the graphs for 

sulfate and nitrate are scaled the widest of any of the 

graphs for the major ions, indicating that, as a group, 

the laboratories had the most variability in measured 

minus median values for these two analytes. It also 

is important to note that the scales for a given analyte are consistent in figures 19-28. This facilitates 

comparison of performance in the analysis of a 

given analyte among laboratories. This approach has 

its drawbacks in that comparison of a given laboratory’s performance on the measurement of various 

analytes puts an extra burden on the reader to adjust 

Table 16. Fiftieth and ninetieth percentile absolute differences for analyses of replicate samples determined by five 

laboratories participating in the 1996 interlaboratory-comparison program

[All units in milligrams per liter except hydrogen ion, in microequivalents per liter, and specific conductance, in microsiemens per centimeter at 25 degrees 

Celsius; CAL: Illinois State Water Survey, Central Analytical Laboratory; AES: Atmospheric Environment Service; QST: QST Environmental; MOE: 

Ontario Ministry of the Environment, Water Quality Section; GGC: Global Geochemistry Corporation; --: not calculated]

Analyte CAL AES QST GGC MOE

50th 90th 50th 90th 50th 90th 50th 90th 50th 90th

Calcium 0.000 0.000 .002 .006 0.001 .008 .001 .009 0.001 .007

Magnesium .000 .001 .000 .004 .000 .003 .001 .002 .000 .002

Sodium .001 .004 .001 .007 .001 .011 .002 .009 .002 .007

Potassium .001 .002 .000 .008 .000 .019 .001 .002 .001 .006

Ammonium .000 .010 .001 .005 .004 .018 .002 .014 .001 .006

Chloride .000 .010 .006 .023 .002 .085 .003 .012 .000 .010

Nitrate .000 .020 .011 .034 .004 .013 .012 .029 .000 .027

Sulfate .010 .020 .005 .025 .002 .010 .015 .055 .010 .050

Hydrogen ion .000 1.040 .448 1.756 .000 1.838 .265 1.456 .180 1.372

Specific conductance .050 .200 -- -- .100 1.500 .030 .200 .000 .500
External Quality-Assurance Results 41

to changing analyte scales. However, if the graphs 

for all of the analytes had been scaled to facilitate 

comparison of analytes, then comparison of laboratory performance would be very difficult because 

the scaling of the graphs would obscure these differences. Differences between laboratories would be 

difficult to detect, and the main purpose is to compare laboratory performance.

Intralaboratory Bias

Intralaboratory bias (bias within a single laboratory) is defined as a systematic difference between 

the measured and expected values due to laboratory 

sample handling and analysis procedures and is 

detected when each laboratory is reviewed independently. Potential bias for laboratories participating in 

the interlaboratory-comparison program was evaluated by two methods: (1) comparison of laboratory 

results to the certified values and the estimated 

uncertainties reported by NIST for standard reference material 2694A, Level I and Level II; and (2) 

comparison of laboratory results to those expected 

for Ultrapure deionized-water samples (table 4). 

National Institute of Standards and 

Technology Standard Reference Samples

Potential bias was examined by comparing the 

median laboratory values to the certified values 

reported by the NIST. The NIST certifies values for 

seven measurements from standard reference sample 2694A-I samples and eight measurements from 

standard reference sample 2694A-II samples. Bias 

could be indicated if laboratory median values were 

outside the NIST-certified values by a chemically 

significant amount, plus or minus the estimated 

uncertainty reported by NIST (table 17). In 1995, 

interlaboratory-comparison study results indicated 

that 5 of 7 median values at CAL and at QST for 

Table 17. Summary of laboratory median values per analyte in 1995 and the analyte range reported by the National Institute 

of Standards and Technology (NIST) for standard reference materials 2694A-I and 2694A-II

[All units in milligrams per liter except: hydrogen ion, in microequivalents per liter, and specific conductance, in microsiemens per centitmeter at 

25 degrees Celsius. AES, Atmospheric Environment Service; CAL, Illinois State Water Survey, Central Analytical Laboratory; QST, QST Environmental; 

GGC, Global Geochemistry Corporation; MOE, Ontario Ministry of the Environment, Water Quality Section; shading indicates that the median value 

reported by a given laboratory for a given analyte is outside NIST range; NIST ranges are from NIST Certificate for Standard Reference Material 2694A, 

dated June 10, 1991; boxed areas indicate a median value was calculated from less than nine (maximum possible) reported values]

ANALYTE

Hydrogen 

ion

Specific conductance Calcium Magnesium Sodium Potassium Ammonium Chloride Nitrate Sulfate

NIST RANGE, 2694A-I

Lower 46.77 24.2 0.011 0.024 0.206 0.054 1 0.12 1 0.23 1 0.53 2.66

Upper 53.70 26.6 0.014 0.024 0.210 0.058 1 0.12 1 0.23 1 0.53 2.72

LABORATORY MEDIAN, 2694A-I

AES 53.70 (2) (2) 0.023 0.205 0.054 0.12 0.23 0.52 2.64

CAL 57.54 28.1 0.010 0.025 0.207 0.056 0.12 0.24 0.54 2.75

QST 57.54 27.5 0.013 0.024 0.193 0.053 0.11 0.22 0.53 2.63

GGC 56.23 26.5 0.013 0.025 0.204 0.053 0.12 0.22 0.55 2.68

MOE 53.70 25.7 0.020 0.024 0.208 0.054 0.12 0.22 0.54 2.70

NIST RANGE, 2694A-II

Lower 234.4 128.0 0.036 0.047 0.411 0.105 1 1.06 1 0.94 7.03 10.5

Upper 269.2 130.6 0.037 0.049 0.435 0.111 1 1.06 1 0.94 7.35 10.7

LABORATORY MEDIAN, 2694A-II

AES 257.0 (2) 0.038 0.049 0.419 0.103 1.06 0.94 7.13 10.8

CAL 257.0 132.7 0.040 0.049 0.429 0.105 1.10 1.00 7.16 10.8

QST 281.8 133.0 0.038 0.050 0.398 0.108 1.09 0.96 7.13 10.7

GGC 269.2 130.6 0.036 0.050 0.412 0.102 1.07 0.96 7.15 10.4

MOE 263.0 129.0 0.036 0.052 0.412 0.107 1.06 0.99 7.08 9.8

1

Value not certified by NIST.

2Data not available.
42 External Quality-Assurance Results for the National Atmospheric Deposition Program/National Trends Network, 1995-96

sample 2694A-I were outside the NIST-certified 

ranges for a given analyte. The GGC had four 

median values that were outside the NIST-certified 

range for a given analyte, whereas AES had three 

and the MOE had one median value that was outside 

the NIST-certified range for a given analyte. 

In 1996, MOE had 6 of 7 median values outside the NIST- certified range for a given analyte for 

certified sample 2694A-I (table 18). The GGC had 

five median values that were outside the NIST-certified range for a given analyte. CAL and QST each 

had four median values outside the NIST-certified 

range in sample 2694A-I, while AES had three 

median values outside the NIST-certified range. The 

similarity, in general, among laboratories in terms of 

their ability to achieve the NIST-certified range was 

noteworthy. The concentrations measured in sample 

2694A-II are commonly much higher than the 

values measured in precipitation and usually require 

dilution before analysis. It is, therefore, important 

not to read too much into an assessment of laboratory results analyzing sample 2694A-II. 

Furthermore, the certified ranges supplied by NIST 

for both sample types are generally extremely narrow or a single point value; it is not of great concern 

that many of the laboratories were only able to 

achieve these narrow ranges for a few analytes. 

Median values outside the range usually were different from the NIST range by only 0.01 to 0.05 mg/L 

for the major ions. A summary of the estimated 

uncertainty ranges for the NIST standard-reference 

materials 2694A-I and 2694A-II and of the median 

values for each laboratory is presented in tables 17 

and 18.

Table 18. Summary of laboratory median values per analyte in 1996 and the analyte range reported by the National Institute 

of Standards and Technology (NIST) for standard reference materials 2694A-I and 2694A-II 

[All units in milligrams per liter except: hydrogen ion, in microequivalents per liter, and specific conductance, in microsiemens per centitmeter at 

25 degrees Celsius. AES, Atmospheric Environment Service; CAL, Illinois State Water Survey, Central Analytical Laboratory; QST, QST Environmental; 

GGC, Global Geochemistry Corporation; MOE, Ontario Ministry of the Environment, Water Quality Section; shading indicates that the median value 

reported by a given laboratory for a given analyte is outside NIST range; NIST ranges are from NIST Certificate for Standard Reference Material 2694A, 

dated June 10, 1991; boxed areas indicate a median value was calculated from less than nine (maximum possible) reported values]

ANALYTE

Hydrogen 

ion

Specific Conductance Calcium Magnesium Sodium Potassium Ammonium Chloride Nitrate Sulfate 

NIST RANGE, 2694A-I

Lower 46.77 24.2 0.011 0.024 0.206 0.054 1 0.12 1 0.23 1 0.53 2.66

Upper 53.70 26.6 0.014 0.024 0.210 0.058 1 0.12 1 0.23 1 0.53 2.72 

LABORATORY MEDIAN, 2694A-I

AES 53.70 (2) (2) 0.025 0.201 0.052 0.11 0.22 0.53 2.68

CAL 54.95 28.7 0.010 0.024 0.207 0.055 0.11 0.22 0.53 2.78

QST 60.25 26.7 0.013 0.024 0.195 0.055 0.11 0.23 0.53 2.78

GGC 57.54 27.6 0.014 0.027 0.203 0.051 0.12 0.22 0.56 2.68

MOE 56.23 27.1 0.019 0.025 0.210 0.050 0.12 0.24 0.53 2.49

NIST RANGE, 2694A-II

Lower 234.4 128.0 0.036 0.047 0.411 0.105 1 1.06 1 0.94 7.03 10.5

Upper 269.2 130.6 0.037 0.049 0.435 0.111 1 1.06 1 0.94 7.35 10.7 

LABORATORY MEDIAN, 2694A-II

AES 265.5 (2) 0.033 0.052 0.416 0.105 1.05 0.89 7.03 10.5

CAL 263.0 133.8 0.040 0.048 0.416 0.106 1.04 0.99 7.15 10.7

QST 275.4 121.5 0.038 0.051 0.408 0.107 1.06 0.98 7.15 10.8

GGC 281.8 130.7 0.037 0.051 0.409 0.101 1.06 0.96 7.24 10.4

MOE 263.0 130.0 0.044 0.050 0.417 0.102 1.08 1.00 7.22 10.2

1

Value not certified by NIST.

2Data not available.
External Quality-Assurance Results 43

Figure 19. Difference between the measured value reported by the Atmospheric Environment Service and the 

median value calculated for all participating laboratories in the interlaboratory-comparison program during 

1995. Samples reported as less than the detection limit (about 3 percent of the samples) were removed.
44 External Quality-Assurance Results for the National Atmospheric Deposition Program/National Trends Network, 1995-96

Figure 20. Difference between the measured value reported by the Atmospheric Environment Service and 

the median value calculated for all participating laboratories in the interlaboratory-comparison program during 

1996. Samples reported as less than the detection limit (about 3 percent of the samples) were removed.
External Quality-Assurance Results 45

Figure 21. Difference between the measured value reported by the Illinois State Water Survey, Central Analytical 

Laboratory and the median value calculated for all participating laboratories in the interlaboratory-comparison program 

during 1995. Samples reported as less than the detection limit (about 3 percent of the samples) were removed.
46 External Quality-Assurance Results for the National Atmospheric Deposition Program/National Trends Network, 1995-96

Figure 22. Difference between the measured value reported by the Illinois State Water Survey, Central Analytical 

Laboratory and the median value calculated for all participating laboratories in the interlaboratory-comparison program 

during 1996. Samples reported as less than the detection limit (about 3 percent of the samples) were removed.
External Quality-Assurance Results 47

Figure 23. Difference between the measured value reported by QST Environmental and the median value 

calculated for all participating laboratories in the interlaboratory-comparison program during 1995. Samples 

reported as less than the detection limit (about 3 percent of the samples) were removed.
48 External Quality-Assurance Results for the National Atmospheric Deposition Program/National Trends Network, 1995-96

Figure 24. Difference between the measured value reported by QST Environmental and the median value 

calculated for all participating laboratories in the interlaboratory-comparison program during 1996. Samples 

reported as less than the detection limit (about 3 percent of the samples) were removed.
External Quality-Assurance Results 49

Figure 25. Difference between the measured value reported by the Global Geochemistry Corportation and 

the median value calculated for all participating laboratories in the interlaboratory-comparison program during 

1995. Samples reported as less than the detection limit (about 3 percent of the samples) were removed.
50 External Quality-Assurance Results for the National Atmospheric Deposition Program/National Trends Network, 1995-96

GGC<- ->SA GGC<- ->SA

GGC<- ->SA GGC<- ->SA

Potassium

Specific Conductance

Chloride

GGC<- ->SA

GGC<- ->SA

GGC<- ->SA GGC<- ->SA

GGC<- ->SA

GGC<- ->SA

Warning limits (+ 2 and -2 F - pseudosigmas from zero difference line)

Control limits (+ 3 and - 3 F - pseudosigmas from zero difference line)

EXPLANATION

GGC Global Geochemistry Corporation, January 1, 1996 through September 30, 1996

SA Shepard Analytical Services, November 18, 1996 through December 31, 1996

Magnesium

Sodium Calcium

Sulfate

Nitrate

pH

Ammonium

Jan Apr Jul Oct Jan

0.10

0.05

0.00

- 0.05

- 0.10

DIFFERENCES, IN UNITS CONCENTRATION DIFFERENCE, IN MILLIGRAMS PER LITER

Jan Apr Jul Oct Jan

5.0

2.5

0.0

- 2.5

- 5.0

0.10

0.05

0.00

- 0.05

- 0.10

Jan Apr Jul Oct Jan

Apr Jul Oct Jan

Jan Apr Jul Oct Jan

Jan Apr Jul Oct Jan

Jan Apr Jul Oct Jan Jan Apr Jul Oct Jan

Jan Apr Jul Oct Jan

Jan Apr Jul Oct Jan

3.0

1.5

0.0

- 1.5

- 3.0

0.30

0.15

0.00

- 0.15

- 0.30

0.30

0.15

0.00

- 0.15

- 0.30

0.10

0.05

0.00

- 0.05

- 0.10

20

10

0

- 10

- 20

1.0

0.5

0.0

- 0.5

- 1.0

0.50

0.25

0.00

- 0.25

- 0.50

DIFFERENCES, IN

 MICROSIEMENS

 PER CENTIMETER AT

25 DEGREES CELSIUS

Jan

Figure 26. Difference between the measured value reported by the Global Geochemistry Corporation/Shepard 

Analytical Services and the median value calculated for all participating laboratories in the interlaboratory-comparison 

program during 1996. Samples reported as less than the detection limit (about 3 percent of the samples) were removed.
External Quality-Assurance Results 51

Figure 27. Difference between the measured value reported by the Ontario Ministry of the Environment, Water Quality 

Section, and the median value calculated for all participating laboratories in the interlaboratory-comparison program 

during 1995. Samples reported as less than the detection limit (about 3 percent of the samples) were removed.
52 External Quality-Assurance Results for the National Atmospheric Deposition Program/National Trends Network, 1995-96

Figure 28. Difference between the measured value reported by the Ontario Ministry of the Environment, Water Quality 

Section, and the median value calculated for all participating laboratories in the interlaboratory-comparison program 

during 1996. Samples reported as less than the detection limit (about 3 percent of the samples) were removed.
External Quality-Assurance Results 53

Ultrapure Deionized-Water Samples

In order to detect possible low-level sample 

contamination resulting from laboratory analyses, six 

Ultrapure deionized-water samples were included 

among the samples submitted to the participating 

laboratories throughout 1995. Table 19 shows the number of times each laboratory reported a concentration 

greater than the “standardized” minimum reporting 

limit in a solution not expected to contain detectable 

analyte concentrations. In order to facilitate the intercomparison among laboratories using different reporting limits, all data for a given ion less than the largest 

minimum reporting limit used by any of the five participating laboratories were set equal to the largest 

minimum reporting limit. If the reporting limits were 

not standardized in this manner, a comparison among 

laboratories would be heavily influenced by differences in reporting limits. Laboratories with lower 

reporting limits would have a much higher incidence of 

“hits” for the deionized-water samples than laboratories with higher reporting limits. In 1995 and again in 

1996, QST reported two values greater than the 

“standardized” reporting limit for ammonium. None of 

the other laboratories reported any ion values greater 

than the “standardized” reporting limit in either year 

(tables 19 and 20). Measured concentrations greater 

than the minimum reporting limit for the Ultrapure 

deionized-water samples is an indication of possible 

contamination.

Collocated-Sampler Program

The collocated-sampler program has been operational since October 1988 and provides a method of 

estimating the overall precision of the precipitationmonitoring system used by the NADP/NTN. Variability from the point of sample collection through 

storage of the data in the data base is included in this 

estimate of NADP/NTN precision (Gordon and others, 

1997). Nilles and others (1991) provide a detailed 

description of the collocated-sampler program. Unlike 

1994, when the collocated-sampler program was operated on a calendar-year basis because of special circumstances (Gordon and others, 1997), the collocated 

sites were operated on a water-year basis (October 

through September) in 1995 and 1996. Since the study 

began in 1988, collocated sites have been operated on 

a water-year basis every year except 1994. 

New sites are selected for each year of the study 

and provide estimates of precision on a networkwide 

basis. Four sites participated in the collocated study 

between October 1, 1994, and September 30, 1995–

water year 1995. In water year 1996, four new sites 

were selected for participation in the collocated study. 

The locations of sites participating in the collocatedsampler study in water years 1995 and 1996 are shown 

in figure 29.

Table 19. Number of analyte determinations greater than 

the largest minimum reporting limit used by any participating 

laboratory for each ion for the Ultrapure deionized-water 

samples during 1995

[CAL, Illinois State Water Survey, Central Analytical Laboratory; AES, 

Atmospheric Environment Service; QST, QST Environmental; MOE, 

Ontario Ministry of the Environment, Water Quality Section; GGC/SA, 

Global Geochemistry Corporation/Shepard Analytical]

Analyte CAL AES QST MOE GGC

Calcium 0 0 0 0 0 

Magnesium 0 0 0 0 0 

Sodium 0 0 0 0 0 

Potassium 0 0 0 0 0 

Ammonium 0 0 2 0 0 

Chloride 0 0 0 0 0 

Nitrate 0 0 0 0 0 

Sulfate 0 0 0 0 0 

Table 20. Number of analyte determinations greater than 

the largest minimum reporting limit used by any participating 

laboratory for each ion for the Ultrapure deionized-water 

samples during 1996

[CAL, Illinois State Water Survey, Central Analytical Laboratory; AES, 

Atmospheric Environment Service; QST, QST Environmental; MOE, 

Ontario Ministry of the Environment, Water Quality Section; GGC/SA, 

Global Geochemistry Corporation/Shepard Analytical]

Analyte CAL AES QST MOE GGC/SA

Calcium 0 0 0 0 0 

Magnesium 0 0 0 0 0 

Sodium 0 0 0 0 0 

Potassium 0 0 0 0 0 

Ammonium 0 0 2 0 0 

Chloride 0 0 0 0 0

Nitrate 0 0 0 0 0

Sulfate 0 0 0 0 0 
54 External Quality-Assurance Results for the National Atmospheric Deposition Program/National Trends Network, 1995-96

Figure 20.--Bias for analyte concentration, deposition, and other physical 

parameters for weekly samples from collocated wet-dry precipitation 

collectors and precipitation depth from collocated rain gages. All data in 

percent.

Collocated-sampler sites for water year 1996

13MD

03MO

99VT

EXPLANATION

ASHLAND WILDLIFE AREA, MISSOURI

65NY

63IL

00MT

99NE

06SC

Collocated-sampler sites for water year 1995

63IL

NORTH PLATTE AGRICULTURAL EXPERIMENT STATION, NEBRASKA

SANTEE NATIONAL WILDLIFE REFUGE, SOUTH CAROLINA

LITTLE BIGHORN BATTLEFIELD NATIONAL MONUMENT, MONTANA

DIXON SPRINGS AGRICULTURAL CENTER, ILLINOIS

JASPER, NEW YORK

UNDERHILL, VERMONT

Collocated Original

Site Code Site Code Site Name

WYE, MARYLAND

IL63

00MT MT00

65NY NY65

99VT VT99

13MD MD13

03MO MO03

99NE NE99

06SC SC06

Collocated Original

Site Code Site Code Site Name

Figure 29. Location of National Atmospheric Deposition Program/National Trends Network sites with collocated 

samplers in water years 1995-96.
External Quality-Assurance Results 55

Sites must meet preestablished criteria to be 

considered for the collocated study. NADP/NTN 

guidelines for site selection and installation 

(Bigelow, 1984) are used in the establishment of 

each collocated site. Site selection is made with the 

goal of distributing sites among diverse regional 

locations and precipitation regimes. To minimize 

data loss due to changes in personnel, only those 

sites with stable operational histories are considered. 

The lack of room at the site for collocated equipment 

is, unfortunately, another reason sites are sometimes 

dropped from consideration. 

After the sites for the collocated-sampler 

program were selected, equipment was shipped by 

the USGS to each site, installed by USGS personnel 

with assistance from site personnel, and field tested 

by USGS personnel to ensure that the equipment was 

in good working order. The principal investigator for 

the collocated-sampler study inspects new collocated sites in either August or September before 

sampling begins at the site in October. Site inspection takes place immediately after equipment 

installation and before collection of the first sample. 

The site operator processes samples from each pair 

of collectors, according to standard NADP/NTN 

procedures (Bigelow and Dossett, 1988). Onsite pH 

and specific-conductance measurements on the 

samples from the newly installed collocated samplers were not required; however, a 20-mL aliquot 

was removed from samples of 70 mL or larger to 

provide equivalent processing to both samples from 

the collocated-sampler site. All samples were analyzed as routine weekly NADP/NTN samples by the 

CAL.

Data from the original and collocated equipment were analyzed in two ways. For the purpose of 

site characterization (that is, determining the median 

sample chemistry or median precipitation), the data 

from the original and collocated sites were pooled. 

For the purpose of comparing an original site and a 

collocated site, the data from the original and collocated sites were analyzed for differences in 1995 and 

1996. Data from an original site and a collocated site 

are formally referred to by the four character site 

code of the original site, followed by the four character site code of the collocated site. For instance, 

the Wye, Maryland, site is formally referred to as 

MD13/13MD. For this analysis, the data used were 

from wet-deposition samples with volume greater 

than 35 mL (laboratory type “W”) that did not 

require dilution. Samples requiring dilution are 

inherently prone to a greater error component. 

Median concentrations for selected analytes in 

weekly samples that were collected at the 1995 and 

1996 collocated sites are depicted in figure 30 and 

figure 31, respectively. Figures 32 and 33 depict the 

median hydrogen-ion concentration, median specific 

conductance, median sample volume, and median 

precipitation depth for the collocated sites in 1995 

and 1996.

Median sample chemistry varied considerably 

among sites, reflecting their diverse climate, human 

influence, and wet-deposition regimes. For example, 

the median annual chloride concentration is about 

4 times higher at the two marine-affected sampling 

locations, Santee National Wildlife Refuge, South 

Carolina (SC06/06SC), and Wye, Maryland 

(MD13/13MD), than at the two inland sampling 

locations, Ashland Wildlife Refuge, Missouri 

(MO03/03MO), and North Platte Agricultural 

Experiment Station (NE99/99NE). The median 

annual ammonium concentration at NE99/99NE is 

more than double the median ammonium concentration at any of the other sites, reflecting the intensive 

livestock activity in the area of Nebraska where 

NE99/99NE is located; such activity is not found in 

the vicinity of MD13/13MD, MO03/03MO, or 

SC06/06SC. Sulfate, nitrate, and hydrogen-ion concentrations are generally highest in the northeastern 

United States (National Atmospheric Deposition 

Program, 1996). Reflecting this, the collocated site 

nearest the northeastern United States in 1995 

(MD13/13MD) had the highest median sulfate and 

nitrate concentrations and the most acidic precipitation (highest median hydrogen-ion concentration) of 

any of the four 1995 collocated sites. The median 

hydrogen-ion concentration for MD13/13MD was 

nearly 10 times higher than the median hydrogen-ion 

concentration from NE99/99NE. 

Annual summaries of NADP/NTN data 

describe precipitation chemistry in units of concentration and deposition for ionic constituents (National 

Atmospheric Deposition Program, 1996); statistical 

summaries for both concentration and deposition of 

ionic constituents are provided by the NADP Program 

Office (http://nadp.sws.uiuc.edu/). The weekly precipitation depth associated with each Belfort recording 

rain gage was used to calculate deposition values at the 

collocated sites. Concentration, in milligrams per liter, 

is multiplied by 10-1 times the rainfall depth in centimeters to yield deposition in kilograms per hectare. 

The variability in deposition amounts due to differences in rain-gage collection efficiency at collocated 

sites provides an estimate of the variability in deposition amounts at other NADP/NTN sites. 
56 External Quality-Assurance Results for the National Atmospheric Deposition Program/National Trends Network, 1995-96

Figure 30. Median sample chemistry for selected analytes at four National Atmospheric Deposition 

Program/National Trends Network sites with collocated samplers in 1995.
External Quality-Assurance Results 57

Figure 31. Median sample chemistry for selected analytes at four National Atmospheric Deposition 

Program/National Trends Network sites with collocated samplers in 1996.
58 External Quality-Assurance Results for the National Atmospheric Deposition Program/National Trends Network, 1995-96

Figure 32. Median hydrogen-ion concentration, specific conductance, sample volume, and precipitation depth for 

four National Atmospheric Deposition Program/National Trends Network sites with collocated samplers in 1995.
External Quality-Assurance Results 59

Figure 33. Median hydrogen-ion concentration, specific conductance, sample volume, and precipitation depth for 

four National Atmospheric Deposition Program/National Trends Network sites with collocated samplers in 1996.
60 External Quality-Assurance Results for the National Atmospheric Deposition Program/National Trends Network, 1995-96

In the analysis of collocated data, care was taken 

to select statistics that were meaningful in describing 

overall sampling precision and that were not overly 

sensitive to a few extreme values. Precision estimates 

for each site are calculated from the relative and 

absolute differences between the pairs of collocated 

samplers and are expressed as median relative and 

median absolute error for a given site and analyte. The 

equations used to estimate median relative and absolute 

error from collocated data are:

Median relative error = M| C1- C2/(C1+ C2)/2|x100,

and

Median absolute error = M| C1- C2|,

where

M = Median of all paired differences; 

C1 = Sample concentration, in milligrams per 

liter, from the original precipitation sampler, 

or deposition, in kilograms per hectare, 

from the original precipitation sampler and 

rain gage; and

C2 = Sample concentration, in milligrams per 

liter, from the collocated precipitation 

sampler, or deposition, in kilograms per 

hectare, from the collocated precipitation 

sampler and rain gage.

Precision estimates defined by the median of the 

unsigned absolute or relative percent difference are 

fairly insensitive to a few extreme values. For sample 

pairs with low concentrations of ionic constituents, the 

relative percent error can be very large, although the 

absolute difference between the samples is small. In 

1995, sites MD13/13MD, MO03/03MO, and 

SC06/06SC each had 44 valid sample pairs that were 

used to calculate precision estimates. (A sample pair 

refers to the collection of a valid sample from both the 

original and collocated sampler.) NE99/99NE had 

42 valid sample pairs. In 1996, there were generally 

fewer sample pairs: only 37, 38, and 39 valid sample 

pairs were available from NY65/65NY, MT00/00MT 

and IL63/63IL, respectively. The other site that 

operated in 1996, VT99/99VT, had the highest number 

of valid samples pairs (47) of any collocated site operated in 1995-96. Typically, sites located in more arid 

climates have fewer valid sample pairs than sites 

located in humid climates (Gordon and others, 1997). 

Surprisingly, the number of valid sample pairs was not 

strongly correlated with the annual amount of precipitation at 1995-96 collocated sites. For example, the 

sampling locations with the least amount of precipitation, MT00/00MT and NE99/99NE, had a number of 

valid samples comparable to SC06/06SC, a sampling 

location with much greater precipitation.

In 1995, the median relative error (MRE) for 

chemical determinations was the smallest for nitrate 

concentration and was less than 5 percent at each of the 

four sites. The MRE’s for sulfate in 1995, at 6 percent 

or less, were almost as small as those measured for 

nitrate. Upon converting concentration amounts to 

deposition totals, the MRE’s for most constituents 

increase (figs. 34 and 35). For clarity of information 

shown in the graphs, only the four digit codes of the 

original sites are displayed in figures 34 and 35 in lieu 

of the formal convention of referring to data from an 

original site and a collocated site by the four character 

site code of the original site followed by the four character site code of the collocated site. For some analytes, 

the magnitude of the MRE for deposition totals is more 

than twice the magnitude of the MRE for concentration 

(for example, sulfate at NE99/99NE). 

General agreement of precision was observed for 

specific conductance, sample volume (measured from 

the Aerochem Metrics wet-deposition collectors), and 

precipitation depth (measured from the Belfort rain 

gages). The MRE’s for these properties were uniformly 

small (less than 10 percent) and fairly consistent at all 

four 1995 sites despite differences in typical sample 

chemistry and precipitation amounts among the sites. 

More variability in the MRE’s for specific conductance, sample volume, and precipitation depth was 

measured at the collocated sites in 1996 than in 1995. 

For example, the precipitation depth MRE ranged from 

5 to 15 percent at the four collocated sites in 1996. 

Graphical depictions of all MRE’s for collocated sites 

are shown for concentration, for deposition, and for the 

physical measurements of sample volume and precipitation depth in figures 34 and 35.

As in past years, the MRE's were larger and more 

variable from site to site for cations and particularly for 

cations whose concentrations typically were near laboratory detection limits. Estimates of network precision 

covering several previous years of collocated sampling 

are given in Nilles and others (1993). The MRE’s for 

cations exceeded 7 percent at most sites in 1995 and 

1996 and frequently exceeded 20 percent. Assuming 

that random contamination is independent of sample 

concentration and that laboratory error increases as 

analyte concentration decreases, an increase in relative 

error at sites with lower concentrations would be 

expected. For example, the MRE for potassium concentration, an analyte detected at very low levels in 

precipitation, ranged from 18.0 percent to 40.1 percent 

at the 1995-96 collocated sites.
External Quality-Assurance Results 61

Figure 34. Median relative error for analyte concentration, deposition, and other physical parameters for 

weekly samples from collocated wet-dry precipitation collectors and precipitation depth from collocated 

rain gages in 1995. All data are in percent.
62 External Quality-Assurance Results for the National Atmospheric Deposition Program/National Trends Network, 1995-96

Figure 35. Median relative error for analyte concentration, deposition, and other physical 

parameters for weekly samples from collocated wet-dry precipitation collectors and precipitation 

depth from collocated rain gages in 1996. All data are in percent.
External Quality-Assurance Results 63

Consistent with the results from previous 

years of this study, the precision for hydrogen-ion 

concentration and deposition varied in absolute and 

in relative terms among the sites, depending upon 

the acidity of the precipitation at a given collocated 

site. The sites with the lowest median hydrogen-ion 

concentrations had the highest median relative 

errors associated with their paired-sample analyses. 

The MRE’s for hydrogen-ion concentration were 

less than 10 percent at sites with median hydrogenion concentrations of 20 μeq/L or greater. For sites 

with median hydrogen-ion concentrations less than 

20 μeq/L, the MRE’s exceeded 15 percent (figs. 32 

through 35).

Collocated-analyte precision estimates in 

tables 21 and 22 can be compared to analytical 

precision estimates calculated in the same manner 

from 90 sample pairs submitted in 1995 and 1996 to 

the CAL as part of the interlaboratory-comparison 

program. The interlaboratory-comparison program 

is described in the preceding section of this report. 

Aliquots of natural, weekly, wet-deposition samples 

with volumes greater than 750 mL are used in the 

USGS interlaboratory-comparison programs as well 

as synthetic precipitation samples. The natural interlaboratory samples had slightly lower specific conductance and median concentrations of analytes 

when compared to the median values for all 

NADP/NTN samples analyzed at the CAL. 

Table 21. Median absolute error for analyte concentration and deposition values determined from weekly collocated 

precipitation samples and replicate samples measured by the Illinois State Water Survey, Central Analytical Laboratory 

in 1995

[All units in milligrams per liter except: hydrogen ion, in microequivalents per liter; specific conductance, in microsiemens per centimeter at 

25 degrees Celsius; and precipitation depth, in millimeters; CAL, Illinois State Water Survey, Central Analytical Laboratory; --, no data]

Analyte

Sampling site

Wye, Maryland Ashland Wildlife 

Area, Missouri

North Platte Agricultural Experiment

Station, Nebraska

Santee National Wildlife 

Refuge, South Carolina CAL

Concentration Deposition Concentration Deposition Concentration Deposition Concentration Deposition Concentration

Calcium 0.009 0.003 0.011 0.009 0.022 0.009 0.004 0.004 0.000

Magnesium 0.002 0.001 0.002 0.002 0.003 0.002 0.002 0.002 0.000

Sodium 0.009 0.009 0.009 0.007 0.014 0.005 0.012 0.017 0.002

Potassium 0.004 0.002 0.003 0.002 0.009 0.004 0.006 0.005 0.001

Ammonium 0.020 0.010 0.020 0.017 0.090 0.038 0.020 0.019 0.000

Chloride 0.010 0.015 0.010 0.007 0.010 0.005 0.020 0.016 0.000

Nitrate 0.046 0.043 0.055 0.065 0.055 0.042 0.020 0.025 0.010

Sulfate 0.050 0.045 0.045 0.058 0.055 0.046 0.030 0.032 0.000

Hydrogen ion 1.638 1.504 1.302 1.145 0.335 0.120 1.272 1.394 0.361

Specific conductance 1.000 -- 0.800 -- 0.800 -- 0.600 -- 0.100

Precipitation depth 0.020 -- 0.020 -- 0.030 -- 0.020 -- --
64 External Quality-Assurance Results for the National Atmospheric Deposition Program/National Trends Network, 1995-96

COMPARISON OF LABORATORY AND 

NETWORK ERROR

A comparison of the laboratory random error to 

the overall network error estimated from the collocated-sampler program indirectly provides a method to 

apportion the relative amount of error attributable to 

laboratory operations. Laboratory random error, as 

calculated from replicate samples submitted to the 

CAL for analysis in the interlaboratory comparison 

program, typically accounted for between one-sixth to 

one-fourth of the overall collocated-sampling error, 

although the fraction of sampling error attributable to 

laboratory random error varies with site and with 

analyte. Comparisons of laboratory random error to 

sampling error from specific NADP/NTN sites has limitations because sampling error is site specific for some 

analytes, such as hydrogen ion. Specific partitioning of 

error at a given site would only be meaningful if the 

laboratory error term was calculated from a number of 

replicate samples collected at sites with similar hydrogen-ion concentration values.

Bias was evaluated for each site and analyte by 

using the median signed difference between collocated-sample concentrations (figs. 36 and 37). Bias 

estimates for sample volume from the precipitation 

collectors and precipitation depth from the recording 

rain gages also were calculated. Because the collocated 

paired samples were shipped from the sites weekly to 

the same laboratory at the same time, the author 

attributes bias in the data-set pairs to systematic differences in: (1) sampler response, (2) sample collection, 

and (3) sample handling before shipment. Bias for 

analytes accounted for less than 20 percent of the overall relative error in collocated measurements. The 

absence of bias as a significant contributor to overall 

variability in NADP/NTN wet-chemistry measurements from the collocated sites was attributed to good 

precision and low bias associated with the collectors 

used in 1995 and 1996. The small amount of bias that 

was associated with one of the collectors was usually 

due to small differences in lid opening and closing rates 

caused by sensor differences. Bias in sample volume 

between collectors varied from -1.5 to +3.5 percent at 

collocated sites operated during 1995 or 1996. These 

results are not as good as the results from the 1994 collocated program but are more in agreement with the 

results from previous years in the collocated program 

when bias in sample volume was as large as -7.0 percent and frequently exceeded +1.5 percent.

Table 22. Median absolute error for analyte concentration and deposition values determined from weekly collocated 

precipitation samples and replicate samples measured by the Illinois State Water Survey, Central Analytical Laboratory in 1996

[All units in milligrams per liter except: hydrogen ion, in microequivalents per liter; specific conductance, in microsiemens per centimeter at 25 degrees 

Celsius; and precipitation depth, in millimeters; CAL, Illinois State Water Survey, Central Analytical Laboratory] 

Analyte

 Sampling site

Dixon Springs

Agricultural Center, Illinois

Little Bighorn

Battlefield National 

Monument, Montana

Jasper, New York Underhill, Vermont CAL

Concentration Deposition Concentration Deposition Concentration Deposition Concentration Deposition Concentration

Calcium 0.017 0.023 0.013 0.005 0.016 0.011 0.007 0.007 0.000

Magnesium 0.002 0.003 0.002 0.001 0.003 0.001 0.002 0.002 0.000

Sodium 0.012 0.018 0.020 0.003 0.013 0.006 0.012 0.009 0.001

Potassium 0.004 0.003 0.006 0.001 0.003 0.002 0.003 0.002 0.001

Ammonium 0.025 0.047 0.055 0.012 0.040 0.023 0.020 0.012 0.000

Chloride 0.010 0.021 0.020 0.004 0.020 0.011 0.010 0.009 0.000

Nitrate 0.035 0.174 0.060 0.021 0.140 0.104 0.045 0.057 0.005

Sulfate 0.040 0.231 0.035 0.010 0.100 0.132 0.050 0.052 0.010

Hydrogen ion 1.340 3.244 0.840 0.152 2.688 2.634 2.373 2.695 0.289

Specific conductance0.500 -- 0.500 -- 1.450 -- 0.900 -- 0.100

Precipitation 

depth

0.140 -- 0.010 -- 0.020 -- 0.030 -- --
Comparison of Laboratory and Network Error 65

MD13 MO03 NE99 SC06

-10

-5

0

5

10

MD13 MO03 NE99 SC06

-10

-5

0

5

10

MD13 MO03 NE99 SC06

-10

-5

0

5

10

MD13 MO03 NE99 SC06

-10

-5

0

5

10

MD13 MO03 NE99 SC06

-10

-5

0

5

10

MD13 MO03 NE99 SC06

-10

-5

0

5

10

MD13 MO03 NE99 SC06

-10

-5

0

5

10

MD13 MO03 NE99 SC06

-10

-5

0

5

10

MD13 MO03 NE99 SC06

-20

-10

0

10

20

MD13 MO03 NE99 SC06

-10

-5

0

5

10

MD13 MO03 NE99 SC06

-10

-5

0

5

10

Calcium Sodium

Potassium Ammonium

Nitrate Sulfate Hydrogen Ion

Specific Conductance Volume Precipitation Depth

Figure 20.--Bias for analyte concentration, deposition, and other physical 

parameters for weekly samples from collocated wet-dry precipitation 

collectors and precipitation depth from collocated rain gages. All data in 

perBIAS IN PERCENT

MD13 MO03 NE99 SC06

-10

-5

0

5

10

Magnesium

BIAS, IN PERCENT

ORIGINAL SAMPLING SITES*

EXPLANATION

* Data is from the original (C1) and collocated (C2) sites. For clarity, only the four digit code of the

 original sampling sites is displayed. Bias in Percent = (C1-C2)/(C1+C2/2)X100.

ORIGINAL SAMPLING SITES*

ORIGINAL SAMPLING SITES*

ORIGINAL SAMPLING SITES*

Chloride

Figure 36. Bias for analyte concentration, deposition, and other physical parameters for weekly samples from collocated 

wet-dry precipitation collectors and precipitation depth from collocated rain gages in 1995. All data are in percent.
66 External Quality-Assurance Results for the National Atmospheric Deposition Program/National Trends Network, 1995-96

MD13 MO03 NE99 SC06

-10

-5

0

5

10

MD13 MO03 NE99 SC06

-10

-5

0

5

10

MD13 MO03 NE99 SC06

-10

-5

0

5

10

MD13 MO03 NE99 SC06

-10

-5

0

5

10

MD13 MO03 NE99 SC06

-10

-5

0

5

10

MD13 MO03 NE99 SC06

-10

-5

0

5

10

MD13 MO03 NE99 SC06

-10

-5

0

5

10

MD13 MO03 NE99 SC06

-10

-5

0

5

10

MD13 MO03 NE99 SC06

-20

-10

0

10

20

MD13 MO03 NE99 SC06

-10

-5

0

5

10

MD13 MO03 NE99 SC06

-10

-5

0

5

10

Calcium Sodium

Potassium Ammonium

Nitrate Sulfate Hydrogen Ion

Specific Conductance Volume Precipitation Depth

Figure 20.--Bias for analyte concentration, deposition, and other physical 

parameters for weekly samples from collocated wet-dry precipitation 

collectors and precipitation depth from collocated rain gages. All data in 

perBIAS IN PERCENT

MD13 MO03 NE99 SC06

-10

-5

0

5

10

Magnesium

BIAS, IN PERCENT

ORIGINAL SAMPLING SITES*

EXPLANATION

* Data is from the original (C1) and collocated (C2) sites. For clarity, only the four digit code of the

 original sampling sites is displayed. Bias in Percent = (C1-C2)/(C1+C2/2)X100.

ORIGINAL SAMPLING SITES*

ORIGINAL SAMPLING SITES*

ORIGINAL SAMPLING SITES*

Chloride

Figure 37. Bias for analyte concentration, deposition, and other physical parameters for weekly samples from collocated 

wet-dry precipitation collectors and precipitation depth from collocated rain gages in 1996. All data are in percent.
Summary 67

SUMMARY

During 1995 and 1996, the U.S. Geological 

Survey used four programs to provide external qualityassurance monitoring for the National Atmospheric 

Deposition Program/National Trends Network 

(NADP/NTN). An intersite-comparison program was 

used to estimate the accuracy and precision of onsite 

pH and specific-conductance determinations. A blindaudit program was used to assess the effects of routine 

sample handling, processing, and shipping of wetdeposition samples on the precision and bias of 

NADP/NTN wet-deposition data. As part of the 

interlaboratory-comparison program, analytical results 

from five laboratories that routinely analyze wet-deposition samples were examined to determine estimates 

of analytical bias and precision for major constituents 

in wet deposition from each laboratory. A collocatedsampler program was used to determine the overall 

precision of NADP/NTN wet-deposition data at 

selected sites in the network. 

Intersite-comparison study numbers 35 through 

38 were conducted between May of 1995 and November of 1996. As in previous years, these studies were 

made on a semiannual basis. In the four studies, the 

percentage of site operators that responded on time and 

that met the accuracy goals ranged from 87.5 to 

94.6 percent; about 94 to 98 percent of the site operators met the accuracy goals for specific conductance in 

these same studies. The solutions used in the intersite 

comparison program are designed to simulate natural 

rainwater and are prepared to replicate the wide range 

of hydrogen-ion concentration found in natural precipitation samples throughout the United States. In 1995-

96, intersite samples were prepared with median pH 

values that ranged from 4.95 in study number 35 to 

4.22 in study number 38.

For the blind-audit program, the Wilcoxon 

signed-rank test and the paired t-test were used to determine if significant statistical differences existed 

between the analyte concentrations measured in paired 

blind-audit samples. The paired t-test is based on blindaudit samples that are paired analyses, whereas the Wilcoxon signed-rank test is based on ranks instead of the 

actual data, but assumes the data consist of random, 

independent samples. One part of the blind-audit sample was subject to all of the normal onsite handling and 

processing steps to which a regular weekly precipitation 

sample is subject, whereas the other part received only 

minimal handling. The change in network sample-shipping protocol from 13-L buckets to 1-L bottles in 1994 

(and the corresponding change in the blind-audit procedures) continued to affect the results of the blind-audit 

program in 1995 and 1996. Before the protocol change, 

statistically significant positive bias between the intensively handled bucket portion and the minimally handled bottle portion of the blind-audit sample was 

routinely observed. During 1995-96, the paired 

t-test indicated significant (α=0.05) positive bias for 

four or fewer analytes each year: hydrogen ion and 

specific conductance in 1995 and calcium, magnesium, 

potassium, and sulfate in 1996. Significant (α=0.05) 

negative bias was determined using the paired t-test for 

ammonium and sodium in 1995 and 1996.

The Wilcoxon signed-rank test for bias indicated positive bias for ammonium in 1995 (α=0.05), 

but this same test indicated that none of the analytes 

were biased significantly (α=0.05) in 1996. During 

1995-96, the median paired differences between the 

bucket and bottle portions ranged from -0.020 mg/L 

for ammonium to +0.006 mg/L for calcium. The 

median relative and absolute percent bias values were 

determined for all of the paired blind-audit sample differences by calculating each signed and unsigned 

paired difference, respectively, as a percentage of the 

concentration measured in the bottle portion of the 

bucket minus bottle data pair. The median relative percent bias for hydrogen ion ranged from 0.00 percent in 

1995 to 2.19 percent in 1996, indicating virtually no 

change in the amount of hydrogen ion lost as a result 

of routine shipping and handling procedures. The 

median relative percent bias for specific conductance 

also was miniscule, ranging from 1.25 percent in 1995 

to 0.00 percent in 1996. In 1995 and 1996, the median 

relative percent difference was within a range of –5 to 

+5 percent for all major ions, except ammonium and 

sodium. The magnitude of the median relative percent 

bias for sodium continued to decrease slightly in 1995 

and 1996 (–7.99 percent in 1995, and –7.00 percent in 

1996), compared to –10.96 percent in 1994.

During 1995 and 1996, the median relative 

percent bias values for ammonium were –9.72 and 

–12.50 percent, respectively. The absolute percent bias 

values were similar in magnitude to the relative percent 

bias values for calcium, magnesium, nitrate, sulfate, 

hydrogen ion, and specific conductance. The absolute 

percent bias was much larger than the relative percent 

bias for ammonium, chloride, potassium, and sodium.

Sixteen 250-mL, sixteen 500-mL, and sixteen 

1,000-mL bottles of the same solution (USGS) were 

sent to the operators of selected sites in 1995 and again 
68 External Quality-Assurance Results for the National Atmospheric Deposition Program/National Trends Network, 1995-96

in 1996 to test volume effects on paired sample results. 

The use of different sample volumes allowed the determination of whether a relation existed between the 

volume collected in the bucket and the differences 

between the analyte amounts in the bucket and bottle 

portions of the blind-audit sample. Results of a 

Kruskal-Wallis test indicated no significant (α=0.05) 

difference in bucket minus bottle differences on a mass 

per bucket basis for nitrate in 1995 and 1996, whereas 

statistically significant (α=0.05) differences were 

determined in 1995 and 1996 for magnesium, sodium, 

ammonium, and chloride. The magnitude of the negative bias of sodium, ammonium, and chloride in units 

of mass increased with increasing sample volume.

In 1995 and 1996, sodium, ammonium, nitrate, 

and chloride were reported at concentration levels that 

exceeded the method detection limit in more than onehalf of the Ultrapure deionized-water samples submitted each year as part of the blind-audit program. 

Sodium was detected at levels greater than the method 

detection limit in all 15 of the Ultrapure samples used 

in 1995 and 1996. 

In 1995, results of the Friedman test indicated 

significant (α=0.05) differences in analyte measurements for calcium, potassium, sulfate, nitrate, and 

hydrogen ion among the five laboratories that participated in the interlaboratory comparison program. In 

1996, the Friedman test indicated significant (α=0.05) 

differences in analyte measurements for potassium, 

ammonium, chloride, sulfate, nitrate, and hydrogen 

ion. Cation precision estimates at the 50th percentile 

exhibited less variability than anions, pH (expressed as 

hydrogen-ion concentration), and specific-conductance precision estimates at the 50th percentile. 

Precision estimates at the 90th percentile exhibited 

some variability among the laboratories for the cations, 

anions, pH, and specific conductance. All participating 

laboratories had median absolute differences that were 

less than or equal to 0.004 mg/L for cations during 1995-

96. Anion precision estimates at the 50th percentile 

exhibited greater variability among laboratories than 

observed for the cations. For the anions, all of the participating laboratories in 1995-96 had median absolute 

differences that were less than or equal to 0.015 mg/L.

In order to detect possible low-level sample 

contamination resulting from laboratory analyses, six 

Ultrapure deionized-water samples were included 

among the samples submitted to the participating 

laboratories in 1995 and 1996. For the purpose of 

comparing deionized analyses uniformly, reporting 

limits for all five laboratories were standardized: if a 

minimum reporting limit was lower than the reporting 

limit used by the CAL for a certain analysis, the higher 

reporting limit used by the CAL was substituted. The 

CAL, AES, GGC, and MOE did not detect concentrations greater than the “standardized” reporting limits for 

the six Ultrapure samples for any analytes. The QST 

reported two concentrations greater than the “standardized” reporting limit for ammonium in 1995 and again 

in 1996. 

A collocated-sampler program was used to 

estimate the overall variability of chemical measurements of wet-deposition data collected for the 

NADP/NTN. The estimates of precision include all 

variability in the data-collection system, from the point 

of sample collection through storage in the 

NADP/NTN data base. Weekly wet-deposition 

samples and precipitation measurements from collocated NADP/NTN sites were compared. Estimates of 

precision were calculated in terms of median relative 

and median absolute difference for concentration and 

deposition of the ionic constituents. The median relative error for sulfate and nitrate concentrations and for 

collected sample volumes was typically less than the 

median relative error calculated for the other analytes 

examined. Relative error was greater for cations, with 

median relative error exceeding 7 percent at most sites. 

As in previous years, the precision for hydrogen-ion 

concentration and deposition varied greatly in absolute 

and relative terms among the sites, depending upon the 

acidity of the precipitation at a given collocated site. 

Bias was not a major component of collocated results; 

bias in collocated measurements accounted for less 

than 20 percent of the overall error in collocated measurements. By comparing results from the interlaboratory program (replicate natural samples analyzed at the 

CAL) with results from the blind-audit and collocated 

programs, laboratory error was estimated in 1995-96 to 

account typically for one-sixth to one-fourth of the 

overall sampling error. 

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