U.S. Department of the Interior

U.S. Geological Survey

Open-File Report 2007–1170

External Quality Assurance Programs Managed by the U.S. 

Geological Survey in Support of the National Atmospheric 

Deposition Program/Mercury Deposition Network
External Quality Assurance Programs 

Managed by the U.S. Geological Survey 

in Support of the National Atmospheric 

Deposition Program/Mercury Deposition 

Network

By Natalie E. Latysh and Gregory A. Wetherbee

Open-File Report 2007–1170

U.S. Department of the Interior

U.S. Geological Survey
U.S. Department of the Interior

DIRK KEMPTHORNE, Secretary

U.S. Geological Survey

Mark D. Myers, Director

U.S. Geological Survey, Reston, Virginia: 2007

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Although this report is in the public domain, permission must be secured from the individual copyright owners to 

reproduce any copyrighted materials contained within this report.

Suggested citation: 

Latysh, N.E., and Wetherbee, G.A., 2007, External quality assurance programs managed by the U.S. Geological Survey 

in support of the National Atmospheric Deposition Program/Mercury Deposition Network: U.S. Geological Survey 

Open-file Report 2007-1170, 33 p.

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
iii

Contents

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

Introduction.....................................................................................................................................................1

Purpose and Scope of Report.............................................................................................................1

Mercury Analytical Laboratory Internal Quality Assurance Activities........................................3

Sample Collection in the NADP/MDN...............................................................................................3

System Blank Program..................................................................................................................................5

Sample Processing...............................................................................................................................5

System Blank Solution Preparation...................................................................................................5

Data Analysis.........................................................................................................................................7

Interlaboratory Comparison Program ........................................................................................................9

Preparation of Samples and Solutions..............................................................................................9

Natural Precipitation Solutions ........................................................................................................11

Spiked Mercury Solutions ................................................................................................................12

Ultrapure Deionized Water Solutions..............................................................................................14

Sample Mailing to Laboratory Participants....................................................................................14

Data Records.......................................................................................................................................14

Blind Audit Program ....................................................................................................................................16

Sample Processing.............................................................................................................................16

Data Records.......................................................................................................................................18

Summary........................................................................................................................................................18

References Cited..........................................................................................................................................18

Appendix 1. Cover letter mailed to National Atmospheric Deposition Program/Mercury 

 Deposition Network site operators participating in the U.S. Geological 

 Survey system blank program........................................................................................22

Appendix 2. Sample-processing instructions mailed to National Atmospheric Deposition 

 Program/Mercury Deposition Network site operators participating in 

 the U.S. Geological Survey system blank program ....................................................23

Appendix 3. Mercury Deposition Network observer form used by the National Atmospheric 

 Deposition Program/Mercury Deposition Network site operators for 

 recording sample information........................................................................................25

Appendix 4. Front and back sides of the postcard mailed to National Atmospheric 

 Deposition Program/Mercury Deposition Network site operators for 

 reporting system blank sample processing and shipping information to 

 the U.S. Geological Survey .............................................................................................26

Appendix 5. Cover letter mailed to laboratories participating in the U.S. Geological 

 Survey interlaboratory comparison program for the National 

 Atmospheric Deposition Program/Mercury Deposition Network.............................27
iv

Appendix 6. Example of control chart posted on the Internet displaying a 

 participating laboratory’s results for spiked mercury solutions 

 used in the U.S. Geological Survey interlaboratory comparison 

 program for the National Atmospheric Deposition 

 Program/Mercury Deposition Network........................................................................28

Appendix 7. Example of control chart posted on the Internet displaying a participating 

 laboratory’s results for deionized water solutions used in the U.S. 

 Geological Survey interlaboratory comparison program for the National 

 Atmospheric Deposition Program/Mercury Deposition Network ...........................29

Appendix 8. Cover letter mailed to National Atmospheric Deposition Program/Mercury 

 Deposition Network site operators participating in the U.S. Geological 

 Survey blind audit program ............................................................................................30

Appendix 9. Sample-processing instructions mailed to National Atmospheric Deposition 

 Program/Mercury Deposition Network site operators participating in the 

 U.S. Geological Survey blind audit program................................................................31

Appendix 10. Front and back sides of the postcard mailed to National Atmospheric 

 Deposition Program/Mercury Deposition Network site operators for 

 reporting blind audit sample processing and shipping information to the 

 U.S. Geological Survey...................................................................................................33

Figures

1. Map showing National Atmospheric Deposition Program/Mercury Deposition

 Nework site locations, fall 2006 ..........................................................................................2

2. Schematic showing the sampling train in the National Atmospheric Deposition

 Program/Mercury Deposition Network precipitation collector manufactured 

 by Loda Electronics...............................................................................................................3

3. Photograph showing the National Atmospheric Deposition Program/Mercury

 Deposition Network precipitation collector manufactured by N-CON 

 Systems...................................................................................................................................4

4. Schematic showing the sampling train in the National Atmospheric 

 Deposition Program/Mercury Deposition Network precipitation 

 collector manufactured by N-CON Systems.....................................................................4

 5. Flowchart for the National Atmospheric Deposition Program/Mercury 

 Deposition Network system blank program......................................................................6

6. Flowchart for the National Atmospheric Deposition Program/Mercury 

 Deposition Network interlaboratory comparison program..........................................10

7. Flowchart for the National Atmospheric Deposition Program/Mercury 

 Deposition Network blind audit program ........................................................................20
v

Tables

1. Mercury concentration values for solutions used in the U.S. Geological Survey 

 system blank program for the National Atmospheric Deposition 

 Program/Mercury Deposition Network .............................................................................8

2. Solution names, sample volumes, and approximate number of samples used 

 in U.S. Geological Survey system blank program for the National Atmospheric 

 Deposition Program/Mercury Deposition Network during 2006 ...................................9

3. Example of sample mailing schedule for U.S. Geological Survey interlaboratory 

 comparison program participants: Mercury Analytical Laboratory, North 

 Shore Analytical, Inc., and Northern Lake Service, Inc................................................11

4. Example of sample mailing schedule for U.S. Geological Survey interlaboratory 

 comparison program participants: ACZ Laboratories, U.S. Geological 

 Survey Wisconsin Mercury Research Laboratory, and IVL Swedish 

 Environmental Research Institute ....................................................................................12

5. Mercury concentration values for solutions used in the U.S. Geological 

 Survey interlaboratory comparison program for the National Atmospheric 

 Deposition Program/Mercury Deposition Network.......................................................13

6. Required volumes of diluted mercury standard for preparing 4-L batches 

 of spiked solutions used for U.S. Geological Survey external quality 

 assurance programs for the National Atmospheric Deposition 

 Program/Mercury Deposition Network ...........................................................................14

7. Mercury concentration values for solutions used in the U.S. Geological 

 Survey blind audit program for the National Atmospheric Deposition 

 Program/Mercury Deposition Network ...........................................................................15

8. Solution names, sample volumes, and approximate number of samples 

 used in the U.S. Geological Survey blind audit program for the National 

 Atmospheric Deposition Program/Mercury Deposition Network during 2006 .........18

Conversion Factors and Datums

Multiply By To obtain

centimeter (cm) 0.3937 inch

liter (L) 1.057 quart

milliliter (mL) 0.03381 ounce, fluid
vi

Abbreviations

Abbreviation Full name

ACM Aerochem Metrics Model 301

AIRMoN Atmospheric Integrated Research Monitoring Network 

DI Ultrapure (>16.7 megohm) deionized water

FORF Field observer report form

FGS Frontier GeoSciences, Inc.

HAL Mercury Analytical Laboratory

HCl Hydrochloric acid

HDPE High density polyethylene

Hg Mercury

HPS High-Purity Standards, Inc.

MDN Mercury Deposition Network

Me-Hg Methylmercury

MOF MDN observer form

MP Mercury precipitation

MPV Most probable value

MRL Minimum reporting limit

NADP National Atmospheric Deposition Program

NIST National Institute of Standards and Technology

NOS Network Operations Subcommittee

NTN National Trends Network

PDF Portable document format

QA Quality assurance

SAS SAS Institute Inc.

SP Synthetic precipitation

USGS U.S. Geological Survey

The following terms and abbreviations also are used in this report:

gram (g)

grams per mole (g/mol)

microgram (μg)

micrograms per liter (μg/L)

milligrams per gram (mg/g)

milligrams per liter (mg/L)

moles per liter (mol/L)

nanograms per liter (ng/L)
Abstract

The U.S. Geological Survey (USGS) Branch of Quality Systems operates external quality assurance programs 

for the National Atmospheric Deposition Program/Mercury 

Deposition Network (NADP/MDN). Beginning in 2004, three 

programs have been implemented: the system blank program, 

the interlaboratory comparison program, and the blind audit 

program. Each program was designed to measure error contributed by specific components in the data-collection process. 

The system blank program assesses contamination that may 

result from sampling equipment, field exposure, and routine 

handling and processing of the wet-deposition samples. The 

interlaboratory comparison program evaluates bias and precision of analytical results produced by the Mercury Analytical 

Laboratory (HAL) for the NADP/MDN, operated by Frontier 

GeoSciences, Inc. The HAL’s performance is compared with 

the performance of five other laboratories. The blind audit program assesses bias and variability of MDN data produced by 

the HAL using solutions disguised as environmental samples 

to ascertain true laboratory performance. This report documents the implementation of quality assurance procedures for 

the NADP/MDN and the operating procedures for each of the 

external quality assurance programs conducted by the USGS. 

The USGS quality assurance information provides a measure 

of confidence to NADP/MDN data users that measurement 

variability is distinguished from environmental signals. 

Introduction

In March 1995, a transition Mercury Deposition Network 

(MDN) was initiated, with seventeen collection sites (Clyde 

Sweet, Illinois State Water Survey, written commun., 2006), 

to monitor weekly concentration and temporal flux of total 

mercury (Hg) in precipitation (http://nadp.sws.uiuc.edu/mdn/; 

accessed April 26, 2007). The MDN became an official 

network in the National Atmospheric Deposition Program 

External Quality Assurance Programs Managed by 

the U.S. Geological Survey in Support of the National 

Atmospheric Deposition Program/Mercury Deposition 

Network

By Natalie E. Latysh and Gregory A. Wetherbee

(NADP) in 1996. As of fall 2006, precipitation for Hg analysis 

was being collected from 89 sites in the United States, Canada, 

and Mexico (fig. 1). The NADP comprises three networks: the 

MDN, the National Trends Network (NTN), and the Atmospheric Integrated Research Monitoring Network (AIRMoN). 

The U.S. Geological Survey (USGS) Branch of Quality 

Systems has been conducting external quality assurance (QA) 

programs for the NTN since the network’s inception in 1978 

and began QA monitoring for the MDN in 2004. The MDN 

QA programs are designed after the QA activities conducted 

for the NTN, incorporating methodologies acquired through 

many years of QA data acquisition. 

Since 2004, the USGS has operated three external QA 

programs for the NADP/MDN. The aim of these QA programs 

is to identify and quantify sources of variability in total Hg 

concentration and deposition data collected by the NADP/

MDN. Beginning in 2004, the USGS system blank program 

has assessed the effects of sample handling, shipping, and 

processing along with the effects of field exposure on MDN 

sample chemistry. The Mercury Analytical Laboratory (HAL) 

for the NADP/MDN is operated by Frontier GeoSciences, Inc., 

located in Seattle, Wash. The HAL analyzes all weekly precipitation samples collected by the NADP/MDN. The USGS 

interlaboratory comparison program, also initiated in 2004, 

evaluates the performance of the HAL. 

Beginning in 2006, the blind audit program has assessed 

the variability and bias of analytical results produced by 

the HAL by disguising QA samples as weekly precipitation 

samples from MDN sites. The USGS collaborates with the 

HAL and the NADP Program Office in designing, implementing, and monitoring QA programs for the MDN. 

Purpose and Scope of Report

This report documents methods used by the USGS 

for administering the external QA programs in support of 

the NADP/MDN. The external QA programs for the MDN 

evolved during the years 2004–2006. Procedural changes 

occurred as needed to improve data quality and to study 
0 100 200 300 400 500 MILES

N

Figure 1. Map showing National Atmospheric Deposition Program/Mercury Deposition Network site locations, fall 2006 (Mercury 

Deposition Network, 2006).

2 External Quality Assurance Programs Managed by the U.S. Geological Survey
different influences on Hg data 

quality. In each section of this 

report describing a QA program, a brief history is presented 

which describes the methods 

used to develop the program. 

Subsequently, methods used for 

solution preparation, sample 

processing, and data analysis are 

presented.

Mercury Analytical 

Laboratory Internal 

Quality Assurance 

Activities

The HAL conducts its own 

QA investigations focusing on 

field activities and laboratory 

practices. The HAL field blank 

program assesses the influence of 

field exposure on Hg concentration during weeks with no precipitation. All MDN sample collection 

bottles are precharged with 20 mL 

of hydrochloric acid (HCl) preservative prior to field installation in the 

precipitation collector. The HAL continuously measures Hg 

concentrations in randomly selected precharged MDN bottles. 

These samples are called “bottle blanks.” An MDN sample 

returned from the field to the HAL after a dry week is called a 

“field blank,” which contains only the HCl precharge. Differences in Hg concentrations between field blanks and bottle 

blanks can indicate influences of field exposure resulting in 

Hg contamination. The HAL monitors laboratory practices to 

ensure acceptable data variability resulting from laboratory 

analysis by routinely analyzing preparatory blanks, ongoing 

calibration blanks, ongoing calibration standards, matrix duplicates, matrix spikes, and reference materials (Gerard Van der 

Jagt, Frontier GeoSciences, Inc., written commun., 2006). The 

USGS does not evaluate HAL’s internal quality control data, 

but the internal quality control results may be used to help 

explain potential issues with the USGS external QA results.

Sample Collection in the NADP/MDN

For sample collection, the MDN uses (1) a modified 

Loda Electronics Model 2001 Precipitation Collector (www.

lodaelectronics.com; accessed April 26, 2007), modeled after 

the Aerochem Metrics Model 301 (ACM) precipitation collector (Aerochem Metrics, Inc., Bushnell, Fla.) used by the NTN, 

and (2) an N-CON Systems MDN Sampler (N-CON Systems 

Co., Inc., 2004), which the NADP approved for precipitation 

collection by the MDN in 2006. The MDN site sponsors can 

choose either the Loda Electronics collector or the N-CON 

Systems collector for use at their site. The availability of multiple precipitation collectors to MDN sites promotes competition among manufacturers, which minimizes cost and prevents 

monopolism. 

Figure 2 shows the sample collection train installed in 

the Loda Electronics MDN precipitation collector. The MDN 

collector is modified from the original NTN collector: in place 

of the wet-side bucket two smaller cylindrical inlets (known 

as chimneys), each with a 128-cm diameter, collect precipitation. The high density polyethylene (HDPE) collection bucket 

used by the NTN can be a source of contamination and is not 

suitable for the collection of Hg (Vermette and others, 1995). 

One of the chimneys, containing HDPE sampling train, collects precipitation for trace metal analysis, an option offered 

to MDN sites. However, these data are not reported by the 

NADP.

The primary chimney collects precipitation for total Hg 

analysis and holds a borosilicate glass funnel at the collection 

orifice. The funnel drains to an attached thin glass capillary 

tube, which empties into the 2-L glass sample collection bottle 

that has been precharged with 20 mL of 1 percent (volume/

volume) hydrochloric acid (HCl). The sample collection bottle 

is contained in an overflow bucket. The HCl keeps Hg ions in 

solution, preventing them from volatilizing and from adhering to the glass walls of the sample bottle (Bob Brunette, 

Frontier GeoSciences, Inc., personal commun., 2004). During 

precipitation, a precipitation sensor activates the opening of a 

Figure 2. Schematic showing the sampling train in the National Atmospheric Deposition 

Program/Mercury Deposition Network precipitation collector manufactured by Loda 

Electronics.

Introduction 3
Figure 3. Photograph showing the 

National Atmospheric Deposition 

Program/Mercury Deposition 

Network precipitation collector 

manufactured by N-CON Systems 

(N-CON Systems Co., Inc., 2004).

Figure 4. Schematic showing the 

sampling train in the National Atmospheric 

Deposition Program/Mercury Deposition 

Network precipitation collector 

manufactured by N-CON Systems (N-CON 

Systems Co., Inc., 2004).

Optical

Precipitation

Sensor

Drip Pan

Sample Train

Thermoelectric

Chiller

4 External Quality Assurance Programs Managed by the U.S. Geological Survey
lid, thus uncovering the chimneys and allowing for sample collection. When the precipitation ceases, the heated sensor dries 

and triggers the lid to close and cover the chimneys. 

Figure 3 shows a photograph of the N-CON collector 

and figure 4 shows a schematic of the sample collection train 

installed in the N-CON collector. Identical sample collection 

apparatus (glass funnel, capillary tube, 2-L sample bottle) are 

installed in the Loda Electronics collector and the N-CON collector for precipitation collection. The N-CON collector has 

one opening for collection of wet deposition for Hg analysis 

and relies on an infrared precipitation sensor to trigger the 

opening of the lid for sample collection. 

The collection apparatus is replaced with a clean glass 

sample train every Tuesday at each MDN site during the 

weekly site visit. The weekly precipitation sample and used 

glass sample train are mailed to the HAL. The glassware is 

cleaned and acid-leached in 30 percent HCl before being 

returned to a site for sample collection.

System Blank Program

The system blank program was initiated in 2004 to assess 

the effects of sample handling, shipping, processing, and field 

exposure on sample chemistry. The program evaluates the 

entire system of MDN sample collection. The system blank 

program mimics the field audit program operated for the NTN 

by the USGS (Wetherbee and others, 2006). Different solution 

volumes and mercury concentrations are used in the program 

to assess mercury contamination and loss resulting from 

sample collection.

Sample Processing

All MDN sites are supplied with a system blank sample 

to process annually. A dry week is required for processing the 

sample. If a dry week does not occur, the site operator is asked 

to return their unprocessed sample to the HAL for total Hg 

analysis and to participate again the following year. In December, prior to the beginning of the first quarter in January, all 

active MDN sites are scheduled to participate during a designated annual quarter of the following year. The number of 

active MDN sites is divided into four groups of 22 to 25 sites. 

Site operators are asked to process their system blank samples 

within six months of receiving the sample because experimental studies performed by the HAL confirm the stability of Hg 

for up to six months (Bob Brunette, Frontier GeoSciences, 

Inc., personal commun., 2004). Operators process the system blank sample on a Tuesday, to coincide with the weekly 

site visit when they collect the previous week’s precipitation 

sample and replace the glass sample collection train.

For processing a system blank sample, the site operator 

is shipped a 125-, 500-, or a 1,000-mL solution along with 

a cover letter (appendix 1), instructions for processing the 

system blank sample (appendix 2), two MDN observer forms 

(MOFs) (appendix 3), and a stamped postcard addressed to the 

USGS for reporting sample processing and shipping information (appendix 4).

During a weekly site visit, prior to processing the system 

blank sample, the site operator must verify that no precipitation occurred during the preceding week for which the weekly 

sample is being collected. The rain gage chart is checked to 

ensure the collector lid did not open and uncover the sample 

train. Exceptions are made for sites located in areas with high 

humidity. For these sites, samples may be processed if the 

collector lid opened, as long as no precipitation was recorded 

and the operator is sure the lid opened solely as a result of 

high humidity. This exception allows the participation of more 

sites in the system blank program. Otherwise, some MDN 

sites would never meet the dry-week criteria. The operator 

must also verify that the collector worked properly during the 

preceding week, with no clock stoppages or pen skips on the 

event recorder chart.

To begin sample processing, the site operator, wearing a clean pair of laboratory gloves to avoid contaminating the sample, triggers the collector lid to open and expose 

the sample collection orifice. The site operator removes the 

system blank sample bottle from the plastic bag in which it 

was shipped and pours 50 percent of the solution into the glass 

precipitation collection funnel. The bottom of a line drawn on 

the solution bottle marks the 50-percent portion of the solution 

that should remain in the bottle. The glass precipitation collection funnel is located in the chimney that collects precipitation 

for total Hg analysis. The solution passes through the funnel and glass capillary tube into the sample collection bottle, 

entraining potential contaminants adhering to the glass walls 

as well as potentially losing Hg ions as they adhere to the 

sampling equipment. The portion of the solution that passes 

through the sample collection train is known as the system 

sample, while the portion of the system blank solution remaining in the original bottle is known as the bottle sample (Wetherbee and others, 2006). The site operator closes the system 

blank sample bottle with the screw lid and places the bottle in 

the original plastic bag in which it was shipped. The operator screws the cap onto the sample collection bottle which 

contains the 50 percent of the transferred system sample and 

the HCl preservative. This bottle is removed from the overflow 

bucket, placed into a sample bottle bag, and transferred to a 

shipping cooler.

The site operator completes two MOFs: one for the bottle 

sample and one for the system sample, listing sample processing and shipping dates. The same information is included in 

the postcard mailed to the USGS. The MOFs, the system sample, the bottle sample, and the rain gage chart are packaged in 

a shipping cooler that is routinely used to ship precipitation 

samples and mailed to the HAL. The postcard is mailed to the 

USGS, indicating the sample has been processed and mailed. 

A flowchart of the system blank program is shown in figure 5.

System Blank Program 5
Figure 5. Flowchart for the National Atmospheric Deposition Program/Mercury Deposition Network system blank 

program.

Quarterly, U.S. Geological Survey (USGS) prepares and mails a 125-, 500-, or 1,000-

milliliter (mL) system blank solution to approximately 22 selected National Atmospheric

Deposition Program/Mercury Deposition Network (NADP/MDN) sites. 

Site operator waits for a dry week (no precipitation) to process the system blank sample.

Site operator processes the system blank sample by pouring 50% of the solution through

the glass precipitation collection funnel of the MDN collector into the sample collection

bottle that spent the previous week installed in the collector.

Site operator places the cap on the sample collection bottle (which contains 50% of the

transferred system blank sample and 20 mL of hydrochloric acid precharge solution),

removes it from the overflow bucket, and places it into a sample bottle bag. 

Both portions of the system blank sample (the 50% of the solution that is poured into the

sample collection bottle and the 50% of the solution that remains in the original bottle)

are sent to the Mercury Analytical Laboratory (HAL) at Frontier GeoSciences, Inc. in one

mailer, along with glassware through which the system blank sample was poured, a rain

gage chart, and MDN observer forms which document sample processing information. 

The HAL analyzes both portions of the system blank sample and provides the USGS 

with results annually. 

The USGS analyzes the system blank data and defines NADP/MDN variability and bias

attributed to field exposure, sample shipping and handling, and laboratory analysis.

The USGS presents the quality assurance results to the NADP/Network Operations

Subcommittee (NOS) annually. 

The USGS presents the quality assurance data in reports, publications, 

and on the Internet.

6 External Quality Assurance Programs Managed by the U.S. Geological Survey
System Blank Solution Preparation

The Hg concentrations in the solutions used in the 

system blank program are intended to represent the range of 

concentrations measured in precipitation samples collected 

by the NADP/MDN. At the start of the program, 2004–2006, 

solutions spiked with Hg were used in the system blank program. Three solutions, with different Hg concentrations, and 

three sample volumes are used in the system blank program 

to assess how sample-handling affects sample chemistry in 

solutions with different Hg concentrations and volumes. A 

description of the solutions and chemical compositions for the 

standard reference solutions used in the USGS system blank 

program is provided in table 1. 

Solutions used in the system blank program have 

included (1) low-concentration standard reference samples, 

prepared by High-Purity Standards, Inc. (HPS; Charleston, 

S.C.) and diluted by the USGS, (2) low-concentration standard 

reference samples, prepared by HPS, and diluted and spiked 

with Hg by the USGS, and (3) Ultrapure deionized water with 

a measured resistivity greater than 16.7 megohm (DI). System 

blank data are used to determine influences of the sampling 

process on Hg concentrations as precipitation passes through 

the glass sample train. Therefore, system blank solutions are 

not preserved with HCl, which could leach Hg ions from the 

sampling glassware and not replicate natural conditions.

In 2004, diluted low-concentration standard reference 

samples and DI were used to make system blank solutions. In 

2005, the USGS began spiking the diluted low-concentration 

standard reference samples with Hg and continued to spike 

solutions through the second quarter of 2006. The USGS-designated mercury precipitation (MP) spikes, MP3 and MP4 Hg 

spikes with target Hg concentrations of 14.3 and 20.0 ng/L 

respectively, were used for the system blank samples. 

Hg spiking of the system blank samples was discontinued during the third quarter of 2006. Large discrepancies 

in Hg concentrations between the system samples and the 

corresponding bottle samples raised concerns that samples 

were being mishandled or that the analytical data were being 

misinterpreted. Measured Hg concentrations in the system 

samples were lower than Hg concentrations in the bottle 

samples. Analysis of the data showed that Hg spike recovery 

may not be possible with unpreserved system blank sample 

aliquots. Without an HCl preservative, Hg ions in the original 

system blank container were assumed to have adsorbed to the 

container walls and Hg was not uniformly transferred to the 

sample collection bottle. As of the third quarter of 2006, only 

low-concentration standard reference solutions and DI solutions, with no additional Hg, are used in the system blank program to monitor contamination from field exposure, sample 

handing and shipping, and laboratory analysis. 

During 2004, the first year of the system blank program, 

ultraclean glass sample bottles, manufactured by I-CHEM 

(http://www.ichembrand.com/; accessed April 26, 2007) and 

guaranteed to meet the U.S. Environmental Protection Agency’s performance-based specifications for metals analysis, 

were used to store the system blank samples. Despite stringent 

shipping methods, the glass bottles were often damaged or 

leaked during transit from the USGS to the MDN sites. Teflon® bottles replaced the glass bottles in 2005, eliminating the 

disqualification of many damaged samples and resulting loss 

of data. Teflon bottles are used for collection of natural waters 

because they have been proven to contribute the least contaminants to the sample (Moody and Lindstrom, 1977). Teflon 

bottles are reused by the USGS for the MDN system blank 

and MDN interlaboratory comparison programs. The HAL 

receives Teflon bottles from site operators participating in the 

system blank program and from laboratories participating in 

the interlaboratory comparison program. The HAL cleans the 

Teflon bottles with HCl to leach Hg and other contaminants 

and returns the cleaned bottles to the USGS for reuse. 

A DI sample is collected prior to system blank sample 

preparation, and a second DI sample is collected after all 

system blank sample bottles are filled. The DI is the same 

water matrix used to dilute the standard reference solutions for 

the system blank samples. The DI samples are collected in the 

same Teflon bottles used to bottle the system blank samples. 

The HAL analyzes these two DI samples for total Hg a week 

prior to system blank sample mailing to ensure the system 

blank samples are not contaminated with Hg. This procedure 

was initiated in 2004 after system blank samples had higher 

than expected Hg concentrations resulting from the use of 

contaminated DI. 

Solutions used for the system blank program were 

changed in 2006. Solutions provided by HPS are used for 

both the MDN and NTN external QA programs. The solution 

chemistry used for the NTN QA programs was re-evaluated 

for 2006 and the solution matrices were changed to reflect the 

current precipitation chemistry collected by NADP/NTN sites. 

The change in solution types provided by HPS affected the 

solutions used for the MDN system blank program: solutions 

SP-97 and SP-98 were replaced by solutions SP-2 and SP-3. 

The MDN sites are randomly assigned a specific volume and 

solution type for the system blank program. Distribution of 

the solutions and their respective volumes used in the system 

blank program during 2006 are listed in table 2.

Data Analysis

Each quarter, the USGS records a description of the system blank solutions and corresponding volumes sent to each 

participating site in the MDN logbook and a Microsoft Excel 

workbook. Site operators mail the pre-addressed postcards 

to the USGS after processing the system blank samples. The 

USGS personnel transfer information from the postcards to the 

system blank program database, enter the date the card was 

returned to the USGS in the MDN logbook, and file the postcard for record. Card submission is evidence that the system 

blank sample has been submitted to the HAL. 

The HAL supplies the USGS with analytical results from 

the system blank program on a yearly basis. The USGS makes 

System Blank Program 7
Table 1. Mercury concentration values for solutions used in the U.S. Geological Survey system blank program for the National Atmospheric Depos

Program/Mercury Deposition Network.[all units are mercury concentration values in nanograms per liter; ng/L, nanograms per liter; Hg, mercury; SP, synthetic precipitation; Q1, 25th percentile; Q3, 75th percentile]

Solution

Agency preparing

the solution Description Q1 Median Q3

Ultrapure, 1% hydrochloric acid blanka

U.S. Geological Survey Deionized water with a measured resistivity greater than 

16.7 megohm, preserved with hydrochloric acid.

0.50 0.77 1.10 20

SP-2b

SP-3b

High-Purity Standards, IncU.S. Geological Survey

National Institute of Standards and Technology certified 

reference solutions prepared by High-Purity Standards, Inc. and diluted by the U.S. Geological Survey.

0.030.01

0.030.02

0.050.05

20Ju

SP-97c

SP-98d

High-Purity Standards, Inc.,U.S. Geological Survey

National Institute of Standards and Technology certified 

reference solutions prepared by High-Purity Standards, Inc. and diluted by the U.S. Geological Survey.

0.360.02

20

SP-97+14.3 ng/L Hg spikee

SP-98+20.0 ng/L Hg spikeeSP-2+20.0 ng/L Hg spikeeSP-3+20.0 ng/L Hg spikee

High-Purity Standards, Inc.,U.S. Geological Survey

National Institute of Standards and Technology certified 

reference solutions prepared by High-Purity Standards, Inc., diluted and spiked with Hg, preserved with 1% hydrochloric acid by the U.S. Geological Survey.

20Ja

a Concentrations calculated using Mercury Deposition Network interlaboratory comparison data submitted by six laboratory participants from 2004 through July 2006.b Concentrations calculated using Mercury Deposition Network system blank data for 2004.c Concentration is derived for one system blank sample analyzed during 2004.d Concentration is the average of the only two available system blank samples analyzed during 2004.e Concentrations for system blank solutions spiked with mercury were not considered accurate and were not calculated for these.
Table 2. Solution names, sample volumes, and approximate 

number of samples used in U.S. Geological Survey system 

blank program for the National Atmospheric Deposition 

Program/Mercury Deposition Network during 2006.

[mL, milliliters; HCl, hydrochloric acid; SP, synthetic precipitation]

Solution name

Sample 

volume 

(mL)

Number 

of 

samples

Ultrapure, 1% HCl blank 125 10

Ultrapure, 1% HCl blank 500 10

Ultrapure, 1% HCl blank 1,000 10

SP-2 125 10

SP-2 500 10

SP-2 1,000 10

SP-3 125 10

SP-3 500 10

SP-3 1,000 10

statistical interpretations of analytical data, which are published in annual reports (Wetherbee and others, 2004; Wetherbee and others, 2006), posted on the Internet (http://bqs.

usgs.gov/precip/new/frontpage_home.htm; accessed April 26, 

2007), and presented to the NADP/Network Operations Subcommittee (NOS). All system blank data are archived in SAS 

datasets and Microsoft Access databases, which are stored on 

a USGS file server.

The HAL analyzes both the system sample and the bottle 

sample for total Hg, and provides two sets of analyses for 

each system blank sample to the USGS for statistical data 

analysis and reporting. The USGS calculates system-minusbottle differences. Paired concentration differences between 

the system sample and the bottle sample may indicate network 

bias due to possible contamination or Hg loss during standard 

handling and processing of precipitation samples. Positive 

system-minus-bottle differences indicate Hg contamination of 

the system sample during its handling and exposure to sampling equipment. Quantifying differences in Hg concentration 

between the system sample and the minimally-handled bottle 

sample provides information for deducing Hg concentration 

in precipitation samples prior to their exposure to sources 

of sampling variability. Selected statistics (from SAS software) used for reporting the system-minus-bottle differences 

include median and other quartile values, the f-pseudosigma, 

and upper confidence limits. Examples of these statistics are 

presented in the annual reports for the USGS QA programs 

(Wetherbee and others, 2006).

Interlaboratory Comparison Program

The purposes of the MDN interlaboratory comparison 

program are to (1) quantify the uncertainty of chemical 

analyses determined by the HAL through comparison of the 

results with those produced by participating laboratories, 

(2) evaluate the analytical precision and accuracy of data 

produced by participating laboratories, and (3) enable comparison of laboratory data produced by other wet-deposition 

monitoring networks. The program requires participating laboratories to analyze DI solutions spiked with Hg or unspiked 

DI samples using U.S. Environmental Protection Agency 

Method 1631 (U.S. Environmental Protection Agency, 2002) 

or an equivalent atomic fluorescence spectrometry method. A 

flowchart of the MDN interlaboratory comparison program is 

provided in figure 6. 

The MDN interlaboratory comparison program was 

initiated in January 2004, with four participating laboratories: 

(1) HAL; (2) IVL Swedish Environmental Research Institute 

in Göteborg, Sweden (IVL); (3) North Shore Analytical, Inc.,

in Duluth, Minn. (NSA); and (4) USGS Wisconsin Mercury 

Research Laboratory in Middleton, Wis. (WML). Two additional laboratories joined the program in July 2004: (1) ACZ 

Laboratories, in Steamboat Springs, Colo. (ACZ), and (2) 

Northern Lake Service, Inc., in Crandon, Wis. (NLS).

Single-blind samples are those that are identified to the 

laboratories as QA samples but whose chemical characteristics are not known to the laboratories. From 2004 through 

June 2006, HAL, NSA, and NLS received four single-blind 

samples from the USGS every 2 weeks for chemical analysis; 

ACZ, IVL, and WML received two single-blind samples every 

month. As of July 2006, HAL, NSA, and NLS receive four 

single-blind samples every month; ACZ, IVL, and WML continue to receive two single-blind samples every month. Results 

submitted by the participating laboratories are compiled, 

analyzed, posted on the Internet (http://bqs.usgs.gov/precip_2/

mdn/index.html; accessed April 26, 2007), published in annual 

reports, and presented to the NADP/NOS at semiannual meetings. 

Preparation of Samples and Solutions

Samples for the interlaboratory comparison program are 

shipped by the USGS to the participating laboratories. Each 

laboratory receives samples derived from the same solutions 

for each mailing. Solutions that have been used in the program 

consist of (1) natural precipitation collected in Arvada, Colo., 

using an NTN Aerochem Metrics precipitation collector, (2) 

DI solutions spiked with Hg in a 1 percent HCl matrix, (3) DI 

solutions in 1 percent HCl, and (4) USGS standard reference 

water sample P41 (used for two sample mailings during 2004) 

(http://bqs.usgs.gov/srs/SRS_Fall03/P.xls; accessed April 26, 

2007). As of July 2006, only the synthetic DI solutions spiked 

with Hg in a 1 percent HCl matrix and DI solutions in 

1 percent HCl are used in the program. 

Solutions are made in 4-L flasks. These flasks are stored 

in a fume hood and filled with 10 percent HCl storage solution 

when not in use. Prior to solution preparation, the storage solution is emptied into an approved waste receptacle for proper 

neutralization and disposal and the flask is rinsed 3 times with 

DI. For each sample mailing, an attempt is made to minimize 

Interlaboratory Comparison Program 9
The U.S. Geological Survey prepares (1) Ultrapure deionized-water solutions in a 1%

hydrochloric acid matrix, or (2) Ultrapure deionized-water solutions spiked with

concentrated mercury in a 1% hydrochloric acid matrix. 

The U.S. Geological Survey bottles the solutions in 500-mL Teflon bottles, and packages

and labels all samples in preparation for mailing to laboratories. 

Four samples are mailed monthly to three

of the participating laboratories; each

laboratory receives 52 samples annually. 

Two samples are mailed monthly to three

of the participating laboratories; each

laboratory receives 26 samples annually. 

Mercury Analytical Laboratory (HAL)

managed by Frontier GeoSciences, Inc.

Seattle, Wash. 

North Shore Analytical, Inc. (NSA)

Duluth, Minn.

Northern Lake Service, Inc. (NLS)

Crandon, Wis. 

Swedish Environmental Research Institute

(IVL)

Göteborg, Sweden 

USGS Wisconsin Mercury Research

Laboratory (WML)

Middleton, Wis. 

ACZ Laboratories (ACZ)

Steamboat Springs, Colo.

Laboratories report analytical results to the U.S. Geological Survey.

Results summarized

in reports and

publications. 

Results reported to

participating laboratories

 on the Internet. 

Results presented to the

National Atmospheric

Deposition Program/Network

Operations Subcommittee

(NADP/NOS). 

Figure 6. Flowchart for the National Atmospheric Deposition Program/Mercury Deposition Network interlaboratory 

comparison program.

10 External Quality Assurance Programs Managed by the U.S. Geological Survey
solution volume to minimize storage solution waste. Three 

solutions are prepared for each mailing to the six laboratories. 

The same solution distributed to each laboratory in a monthly 

mailing must be derived from the same well-mixed solution 

in a flask, as the use of different batches of the same solution 

imparts variability on the results that cannot be differentiated 

from laboratory performance. 

All interlaboratory comparison samples are labeled with 

a unique 10-digit sample number assigned by the USGS. The 

10-digit sample identification is assigned as follows: the first 4 

digits of the 10-digit sample number represent the year during 

which the study is conducted, followed by the 3-digit Julian 

date of sample mailing, and the unique 3-digit identification of 

the sample, ranging from 001 to 018. Each laboratory receives 

samples with the same last 3-digit identifications for each 

sample mailing. For example, four interlaboratory comparison 

samples mailed to the HAL on June 26, 2006, would have the 

following identifications: 2006177001 through 2006177004. 

A random schedule of the monthly solution distribution is 

generated annually. An example solution distribution schedule 

is included in tables 3 and 4, for laboratories receiving four 

monthly samples and for laboratories receiving two monthly 

samples, respectively. Annually, 52 samples are distributed to 

each laboratory receiving four monthly samples. Of the 

52 samples, the USGS prepares 8 DI solutions and 44 DI 

solutions spiked with Hg. Laboratories receiving two monthly 

samples analyze 26 samples annually. Of the 26 samples, the 

USGS prepares 6 DI solutions and 20 DI solutions spiked with 

Hg. Table 5 lists descriptions, including Hg concentration values, of solutions that have been used in the USGS interlaboratory comparison program for the MDN.

Natural Precipitation Solutions

Natural precipitation solutions, known as HALNAT 

solutions, were used in the MDN interlaboratory comparison 

program from September 2004 through May 2006. The USGS 

selected weekly natural precipitation samples collected in 

Arvada, Colo., using an NADP/NTN Aerochem Metrics wetdeposition collector for use in the interlaboratory comparison 

program. Weekly precipitation samples were composited 

to obtain sufficient volume of a homogeneous solution for 

sample mailing to all laboratory participants. The precipitation solutions were combined in a volumetric flask and mixed 

on a magnetic stirrer with a Teflon-coated magnetic stir bar. 

Quantities of HCl and Hg were not added to these natural 

matrix solutions. The USGS bottled the precipitation aliquots 

in 500-mL Teflon containers. The USGS did not analyze the 

samples for target values prior to shipping the samples to the 

Table 3. Example of sample mailing schedule for U.S. Geological 

Survey interlaboratory comparison program participants: Mercury 

Analytical Laboratory, North Shore Analytical, Inc., and Northern Lake 

Service, Inc.—each receiving four monthly samples.

[MP, mercury precipitation]

Mailing

number

Sample mailing 

date MP1 MP2 MP3 MP4 Blank

1 January 9 1 1 2

2 February 6 2 1 1

3 March 6 2 1 1

4 April 3 1 2 1

5 May 1 1 2 1

6 May 29 2 1 1

7 June 26 1 2 1

8 July 24 1 2 1

9 August 21 1 1 2

10 September 18 2 1 1

11 October 16 1 1 2

12 November 13 2 1 1

13 December 11 2 1 1

Sample 

total 11 11 11 11 8

Interlaboratory Comparison Program 11
Table 4. Example of sample mailing schedule for U.S. Geological 

Survey interlaboratory comparison program participants: 

ACZ Laboratories, U.S. Geological Survey Wisconsin Mercury 

Research Laboratory, and IVL Swedish Environmental Research 

Institute—each receiving two monthly samples.

[MP, mercury precipitation]

Mailing

number

Sample 

mailing 

date MP1 MP2 MP3 MP4 Blank

1 January 9 1 1

2 February 6 1 1

3 March 6 1 1

4 April 3 1 1

5 May 1 1 1

6 May 29 1 1

7 June 26 1 1

8 July 24 1 1

9 August 21 1 1

10 September 18 1 1

11 October 16 1 1

12 November 13 1 1

13 December 11 1 1

Sample 

total 5 5 5 5 6

participating laboratories. Natural precipitation samples were 

kept refrigerated before being used in the program.

The use of HALNAT solutions was discontinued in June 

2006 because analytical results for these solutions were highly 

variable among the participating laboratories. Windborne contaminants and bugs can be entrained in NADP precipitation samples 

and the HALNAT solutions were not filtered prior to being sent 

to the laboratories. Even though particulates were not observed in 

the solutions, variable laboratory results may have been due to the 

presence of particulate-bound Hg, heterogeneously distributed in 

the solutions. Natural precipitation samples may be reintroduced 

to the interlaboratory comparison program, however the bulk 

solutions will have to be filtered and acidified prior to splitting 

the solutions to ensure homogenous composition.

Spiked Mercury Solutions 

Four solutions with specific Hg concentrations are used in 

the MDN interlaboratory comparison program. The USGS prepares solutions with approximate Hg concentrations that span the 

interquartile range (25th percentile to 75th percentile of concentrations) in precipitation collected by the MDN. For precipitation 

collected by the MDN from 1995 through 2004, the Hg concentration interquartile range is 5.8 to 15.0 ng/L. For QA sample 

preparation, USGS spikes DI with a diluted National Institute of 

Standards and Technology (NIST) Hg standard to produce 

target concentrations between 5.7 and 20.0 ng/L. The target 

Hg concentrations will change as needed to reflect Hg 

concentrations in precipitation collected by the MDN.

Hydrochloric acid is added to Hg-spiked solutions, 

keeping Hg in solution by preventing Hg from volatilizing 

and adhering to the glass flasks and Teflon bottles. The 

USGS prepares two HCl solutions diluted to 10 percent 

(volume/volume). The HCl storage solution, purchased 

from the USGS supply, is used to fill flasks and beakers 

during storage when they are not being used to prevent 

contaminants from adhering to the glass walls. The storage HCl has a certified Hg content less than 500 parts per 

trillion. The HCl, with a certified Hg content less than 

100 parts per trillion (certificate of analysis for Baseline® 

HCl obtained from Seastar Chemicals, Vancouver, British 

Columbia, Canada, http://www.seastarchemicals.com/cgibin/sci.exe?sproduct=BL04_ HydrochloricAcid; accessed 

April 26, 2007), is added to the diluted Hg standards, Hgspiked solutions, and blank solutions to yield a final HCl 

concentration of 1 percent. 

In 2003, the USGS purchased Standard Reference 

Material 3133, Lot Number 991304, with a gravimetric 

certified Hg value of 10.00 mg/g ± 0.02 mg/g from NIST 

(https://srmors.nist.gov/view_detail.cfm?srm=3133; 

accessed April 26, 2007) for preparing Hg-spiked solutions. The Hg standard was diluted to ease regularly 

scheduled preparation of low-concentration Hg-spiked 

synthetic wet-deposition samples. First, an intermediate 

standard with an Hg concentration of 125.25 mg/L in 1 

percent Baseline® HCl was prepared by pipetting 12.525 

g of the standard stock solution into a 1-L flask and bringing to volume with DI and 100-mL of 10 percent (volume/

volume) Baseline® HCl. Then, after thorough mixing, 1 

mL of this intermediate standard was further diluted in 

a 1-L flask with DI and 100-mL of 10 percent (volume/

volume) Baseline® HCl to yield the final standard with 

a calculated Hg concentration of 125.25 μg/L preserved 

in 1 percent Baseline® HCl. The additional dilution was 

required to enable the USGS to accurately formulate solutions with Hg concentrations in the parts per trillion range 

(ng/L). In October 2003, HAL analyzed the final standard 

and reported an Hg concentration of 121 μg/L. This value 

was used to determine target concentrations of prepared 

solutions spiked with the Hg standard.

Periodically, the HAL analyzes the Hg standard for 

the USGS because the Hg concentration decreases slightly 

with time, probably through volatilization. This ensures 

the target Hg concentrations in prepared QA samples are 

well known to the USGS. The measured Hg concentration 

of the standard was 114.5 μg/L in May 2005, suggesting 

an approximate negative monthly Hg difference of 

0.3 μg/L since the previous analysis in October 2003. 

The Hg standard has been used for preparing spiked Hg 

samples for the interlaboratory comparison program, the 

system blank program, and the blind audit program. 

For spiking solutions, a microliter-range pipette is 

used to measure the small volumes of Hg standard. Prior 

12 External Quality Assurance Programs Managed by the U.S. Geological Survey
Table 5. Mercury concentration values for solutions used in the U.S. Geological Survey interlaboratory comparison program for the National Atmo

Program/Mercury Deposition Network.

[all units are mercury concentration values in nanograms per liter; ng/L, nanograms per liter; Hg, mercury; MP, mercury precipitation; Q1, 25th percentile; Q3, 75th percentile; H

precipitation solution collected by the U.S. Geological Survey in Arvada, Colo.]

Solution

Agency preparing 

the solution Description Q1 Median Q3

HALNATa U.S. Geological Survey Natural wet-deposition collected using an Aerochem 

Metrics collector at a U.S. Geological Survey experimental site located in Arvada, Colo., and bottled by the U.S. Geological Survey. 

2.03 3.91 6.2

Ultrapure, 1% hydrochloric acid blanka

U.S. Geological Survey Deionized water with a measured resistivity greater 

than 16.7 megohm, preserved with hydrochloric acid.

0.50 0.77 1.1

MP1 (5.7 ng/L Hg spike)a

MP2 (8.6 ng/L Hg spike)aMP3 (14.3 ng/L Hg spike)aMP4 (20.0 ng/L Hg spike)a

U.S. Geological Survey Deionized water with a measured resistivity greater 

than 16.7 megohm, spiked with Hg, preserved with 1% hydrochloric acid.

6.108.90

14.9020.00

6.529.38

15.7221.32

7.0

10.017.023.0

P41 U.S. Geological Survey USGS standard reference water sample. 0.30 0.55 1.5

a Concentrations calculated using Mercury Deposition Network interlaboratory comparison data submitted by six laboratory participants from 2004 through July 2006.
Table 6. Required volumes of diluted mercury standard 

for preparing 4-L batches of spiked solutions used for U.S. 

Geological Survey external quality assurance programs 

for the National Atmospheric Deposition Program/

Mercury Deposition Network.

[mL, milliliters; ng/L, nanograms per liter; MP, mercury 

precipitation]

Solution 

name

Diluted mercury

 standard 

volume (mL)a

Target 

value

(ng/L)

MP1b,c 0.200 5.7

MP2b 0.300 8.6

MP3b,c,d 0.500 14.3

MP4b,d 0.700 20.0

a Volume is calculated for samples made with a standard having a 

 mercury concentration of 114.5 micrograms per liter.

b Solution used in the interlaboratory comparison program.

c Solution used in the blind audit program.

d Solution used in the system blank program.

to use, the pipette is checked for calibration by weighing 

known volumes of reagent water and recording the measurements in a logbook. Five gravimetric measurements are 

recorded for each volume between 100 μL and 1,000 μL, in 

increments of 100 μL. The average of the five measurements 

for each volume is plotted and a least-squares regression equation is generated which can be used to estimate the appropriate pipette settings to use for specific volumes. Table 6 lists 

the volumes of Hg standard required to prepare spiked Hg 

solutions with specific target values used in the interlaboratory comparison program, the system blank program, and the 

blind audit program. The solution Hg concentrations listed in 

tables 1, 5, and 7 are median values for numerous analyses of 

the solutions, and thus are considered the most probable values 

to make blank solutions for the MDN QA programs and Hg 

is never added to this flask. The DI blanks are prepared in the 

4-L volumetric flask by combining 3.6 L of DI with 400 mL 

of 10 percent HCl Baseline® solution to yield a DI solution 

preserved with 1 percent HCl. 

Sample Mailing to Laboratory Participants

Interlaboratory comparison samples are mailed biweekly 

to participating laboratories on a Tuesday morning in Teflon 

sample bottles which have been acid-washed and dried by the 

HAL and mailed to the USGS. These bottles are not conditioned with the solution prior to being filled. The QA solutions 

are poured directly into the bottles. 

A USGS label is affixed to each 500-mL sample bottle, 

identifying the sample as an interlaboratory comparison solution, listing the laboratory recipient name and the assigned 

10-digit USGS identification number. Each bottle is sealed in 

two Ziploc® bags. Because the cardboard boxes in which the 

samples are shipped to the laboratories can be a source of Hg 

(Chris Geske, Northern Lake Service, Inc., personal commun., 2005), the double bags help protect the samples from 

contamination. The USGS attempted to place identification 

labels on the Ziploc bags instead of the bottles to ease the 

effort of bottle cleaning, making label-removal unnecessary. 

A laboratory participant expressed disapproval citing the ease 

of separating unmarked bottles from their corresponding bags 

and sample identifications. Therefore, identification labels are 

placed directly on each bottle. 

For each monthly mailing, four samples are placed into 

a cardboard shipping container and mailed to the HAL, NSA, 

and NLS; while WML, ACZ, and IVL receive two samples in 

a padded envelope. The samples are not chilled with ice packs, 

like NTN QA solutions, because they are preserved with HCl, 

and MDN precipitation samples are not kept cold during 

shipment. A cover letter is included with each set of samples 

listing the sample identification numbers and the requested 

analyte determinations (appendix 5). Upon completion of analysis, laboratories are asked to mail the empty Teflon bottles to 

the HAL where they are acid-rinsed and returned to the USGS 

for reuse in the interlaboratory comparison program. Instructions for returning the Teflon bottles to the HAL and a preaddressed, postage-paid padded envelope are included with the 

interlaboratory comparison samples. 

Data Records

Participating laboratories provide analytical results to 

the USGS in electronic format including Microsoft Excel 

and portable document format (PDF). Data are compiled 

in a Microsoft Access database, processed, and graphically 

displayed using SAS Institute Inc. (SAS) software. The data 

are presented in control charts on the Internet (http://bqs.usgs.

gov/precip/new/mdninterlab_frontpage_data.htm; accessed 

April 26, 2007), an example of which is included in appendix 

(MPVs). The target Hg concentration values listed in table 

6 are considered approximate, as they are calculated by the 

USGS during sample preparation.

For preparing an Hg-spiked solution, a 4-L volumetric flask is filled approximately 75 percent full with DI and 

then 400 mL of 10 percent (volume/volume) Baseline® HCl 

is added. Finally, the appropriate volume of Hg standard is 

added, and the flask is diluted to volume with DI. The solution 

is mixed for approximately 15 minutes on a magnetic stirrer 

with a Teflon-coated magnetic stir bar. The USGS bottles the 

solution aliquots in 500-mL Teflon containers and mails them 

to laboratory participants.

Ultrapure Deionized Water Solutions

Ultrapure deionized water solutions are used for samples 

in the interlaboratory comparison program to detect low-level 

laboratory contamination. The 4-L volumetric flask used 

for preparing blank solutions is stored filled with 10 percent 

(volume/volume) HCl storage solution. This flask is only used 

14 External Quality Assurance Programs Managed by the U.S. Geological Survey
Table 7. Mercury concentration values for solutions used in the U.S. Geological Survey blind audit program for the National Atmospheric Depositi

Program/Mercury Deposition Network.

[all units are mercury concentration values in nanograms per liter; ng/L, nanograms per liter; Hg, mercury; SP, synthetic precipitation; Q1, 25th percentile; Q3, 75th percentile]

Solution Agency preparing the solution Description Q1 Median Q3 

Ultrapure, 1% hydrochloric acid blanka

U.S. Geological Survey Deionized water with a measured resistivity 

greater than 16.7 megohm, preserved with hydrochloric acid.

0.50 0.77 1.10 

SP-2+5.7 ng/L Hg spikeSP-2+14.3 ng/L Hg spikeb

High-Purity Standards, 

Inc.,

U.S. Geological Survey

National Institute of Standards and Technology 

certified reference solutions prepared by HighPurity Standards, Inc., diluted and spiked with Hg, preserved with 1% hydrochloric acid by the U.S. Geological Survey.

2.93 4.25

14.05

6.0 

a Concentrations calculated using Mercury Deposition Network interlaboratory comparison data submitted by six laboratory participants from 2004 through July 2006.b Concentration is the mean of the only two available blind audit samples spiked with 14.3 ng/L of Hg analyzed during 2006.
Sample Processing

Approximately 20 MDN sites are chosen to process a 

blind audit sample annually. During 2006, the MDN collectors 

had to be co-located with NTN sampling equipment to be considered for blind audit participation. The co-located NTN and 

MDN precipitation samplers share a precipitation rain gage 

whose chart shows the precipitation depth, along with two 

event records for the NTN and the MDN collector lid openings. The original weekly rain gage chart from a co-located 

site is mailed to the Central Analytical Laboratory along with 

the NTN precipitation sample, and a copy of the rain gage 

chart is mailed to the HAL in the same cooler containing the 

MOF and MDN precipitation sample. During 2006, USGS 

personnel thought that an authentic rain gage chart could not 

be created in the laboratory, consequently exposing the QA 

identity of the sample to the HAL personnel. The submittal 

of a photocopied and therefore, less legible weekly rain gage 

chart to the HAL with the QA sample helped disguise a USGS 

laboratory-created rain gage chart. Beginning in 2007, sites 

will not have to be co-located with NTN equipment to participate in the blind audit program because the USGS has refined 

the production of laboratory-created rain gage charts and now 

are less likely to be distinguishable from real rain gage charts.

In 2006, experimental fake rain gage charts were prepared using several methods. Laboratory-created rain gage 

charts were made using Adobe Photoshop. A blank rain 

gage chart was scanned and saved in a graphic format. Event 

recorder and precipitation depth lines, consistent with sample 

volume, were drawn on the graphic file and printed. The site 

operator was asked to fill in the pertinent weekly information 

on the laboratory-created rain gage chart, including site identification, time on, time off, and precipitation depth. The site 

operator copied this chart and shipped the copy along with the 

blind audit sample to the HAL. While this method produced 

believable laboratory-created rain gage charts, problems arose 

when the number of participants in the blind audit program 

had to be increased. The MDN sites without co-located NTN 

equipment now had to be included. These MDN sites send in 

original rain gage charts, so photocopying could not be relied 

upon to help disguise the laboratory-created charts; photocopies would no longer be believable. Believable event records 

and precipitation depths had to be transcribed to original 

Belfort charts to avoid raising suspicions at the HAL as to the 

identity of the QA samples. The USGS attempted to produce 

laboratory-created rain gage charts using a Belfort precipitation recording drum attached to the base mechanism, which 

had been removed from the rain gage’s housing and placed 

on a laboratory workbench. A specified volume of water was 

poured into the collection bucket and the depth was recorded 

on the rain gage chart. These charts were mailed to site operators, who were asked to fill in the weekly information and mail 

the chart with the blind audit sample to the HAL. Unfortunately, using the Belfort mechanism to make the laboratorycreated rain gage charts took a long time and it was difficult 

to regulate the water flow over a specified duration to produce 

6. Each laboratory’s data are only accessible by the individual 

laboratory and the USGS by way of password-protected web 

pages. 

Results for laboratories participating in the interlaboratory comparison program are posted on the Internet as analyses are received. The frequency of data reporting throughout 

the year varies for the individual laboratories. The data are 

posted on the Internet within one week of receipt of the data. 

Median concentrations for each solution are calculated as data 

are reported throughout the year. The USGS does not assign 

calculated target values based on the dilution of known Hg 

spike concentrations because the low-concentration solutions 

are difficult to prepare precisely. Median values calculated for 

each solution from submitted results are considered a more 

accurate measure of the MPVs of the solutions. 

Two kinds of bias are investigated in the interlaboratory 

comparison program. Interlaboratory bias (bias among participating laboratories) is determined by systematic differences 

of reported values from the median values determined from 

all participating laboratories. Intralaboratory bias (bias within 

a single laboratory) is identified by a systematic difference 

between the reported and expected values, at least as a preliminary indication. 

Interlaboratory results are graphically presented on 

control charts. The control charts are used to identify potential systematic error that might affect the quality of a laboratory’s data. Consistent, acceptable laboratory performance 

is indicated when the data remain within the control limits 

on the control charts. Positive and negative bias is indicated 

when data consistently exceed or are less than the control 

limits. Results for detected total Hg in the DI solutions are 

displayed in a plot showing the distribution of Hg concentrations reported by each laboratory; a line is graphed at the 

median Hg concentration for the DI solutions calculated from 

all reported results (appendix 7). For a complete explanation 

of statistical methods used to interpret USGS external QA 

program data, see Wetherbee and others (2004).

Blind Audit Program

The blind audit program for the MDN was initiated to 

assess the variability and bias of laboratory analysis of precipitation samples. The blind audit samples, whose Hg concentrations are known to the USGS, are disguised to the HAL as 

environmental precipitation samples. Differences between Hg 

concentrations reported for the blind audit samples and target 

values, determined by the USGS using interlaboratory comparison data, provide an insight into variability and bias resulting from laboratory analysis. A pilot MDN blind audit study 

was conducted in June 2005 to identify and remedy problems, 

and the program was implemented in January 2006.

16 External Quality Assurance Programs Managed by the U.S. Geological Survey
Blind Audit Program 17

believable precipitation depth. Consequently, the USGS 

attempted to draw the event record and precipitation depth, 

using the same color of ink as that used by Belfort rain gages. 

This procedure produced the most believable laboratory-created rain gage charts and was used to produce fake rain gage 

charts for blind audit samples mailed to site operators during 

2007. However, purposeful splotches of ink and ruffling of the 

chart were necessary to replicate the wear that paper charts 

experience under field conditions. The laboratory-created 

rain gage charts, recording depth corresponding to blind audit 

sample volumes, are mailed with the blind audit samples to 

MDN sites. 

In December each year, samples are prepared and mailed 

to sites chosen to participate in the blind audit program during the following year. Sites are asked to process the sample 

within a year of receiving the sample. Operators process the 

blind audit sample on a Tuesday, to coincide with the weekly 

site visit when they collect the previous week’s precipitation 

sample and replace the sample collection glassware. A dry 

week, when no precipitation occurs and the collector lid does 

not open to expose the sample train (figs. 2 and 4), is required 

for processing a blind audit sample. If a dry week does not 

occur during the year, the site is asked to return their unprocessed sample to the HAL and the site is asked to participate 

again the following year. 

The unprocessed sample is analyzed by the HAL to 

determine if Hg concentrations change during storage. It is 

suspected that Hg concentrations decrease through volatilization. The Hg concentrations are closest to the target value for 

blind audit samples that are processed soon after receipt by 

the site operator. The Hg concentrations generally decrease for 

samples that are stored for longer durations. If these preliminary interpretations are correct, the blind audit program will 

have to be modified to ensure that laboratory performance 

alone is being evaluated, without influences of Hg concentration decreases resulting from extended sample holding times. 

The Hg concentrations in the solutions used in the blind 

audit program are intended to represent the range of concentrations measured in precipitation samples collected by 

the NADP/MDN. Every two years, the USGS evaluates Hg 

concentrations measured by the MDN and confirms that the 

QA solution target values are representative of precipitation concentration ranges. Three solutions, with different 

Hg concentrations, and two sample volumes are used in the 

blind audit program to assess how laboratory analysis affects 

sample chemistry in solutions with different Hg concentrations 

and volumes. A description of the solutions and a summary 

of target values for solutions used in the blind audit program 

are provided in table 7. Distribution of the solutions and their 

respective volumes used in the 2006 blind audit program are 

listed in table 8.

For processing a blind audit sample, the site operator is 

shipped a 75- or 150-mL solution along with a cover letter 

(appendix 8), instructions for processing the sample (appendix 9), and a postcard addressed to the USGS for reporting 

sample processing and shipping information (appendix 10). 

The USGS personnel transfer information from the postcard 

to the blind audit program database, enter the date the card 

is returned to the USGS in the MDN logbook, and file the 

postcard for record. Card submission is evidence that the blind 

audit sample has been submitted to the HAL. 

In December, the HAL mails shipping coolers containing 

2-L sample collection bottles precharged with 20 mL of HCl 

to the USGS for use in the blind audit program. The USGS 

pours prepared blind audit solutions into the sample collection bottles, encloses the bottles in protective plastic bags, and 

ships the samples in HAL-supplied coolers to sites chosen to 

participate in the blind audit program. The HAL has the capability to track shipping coolers by using identification numbers 

inscribed on the container and lid of the cooler. Therefore, 

to avoid unveiling the identity of a blind audit solution to the 

HAL through recognition of a cooler tracking number, site 

operators are asked to transfer the blind audit sample to a different cooler out of their stock. 

Site operators are asked to fill in weekly information 

on the laboratory-created rain gage chart, recording dates for 

the dry week and time when the sample collection bottle was 

installed and removed from the field, along with precipitation 

depth indicated by the laboratory-created rain gage chart corresponding to solution volume of the QA sample. If the participating site is co-located with an NTN site, the site operator 

photocopies the rain gage chart to mail to the HAL with the 

blind audit sample and the original rain gage chart is mailed to 

the Central Analytical Laboratory with the NTN sample. If the 

site is not co-located with an NTN site, the original USGScreated rain gage chart is mailed to the HAL with the blind 

audit sample. The site operator fills out the MOF recording 

site information for the dry week but modifies the precipitation record by recording depth measurements drawn on the 

laboratory-created rain gage chart. The site operator mails the 

sampling train funnel and capillary tube that were installed 

in the collector during the previous dry week, the completed 

MOF and rain gage chart with the blind audit solution to the 

HAL, as if it were an actual precipitation sample. 

Site operators are instructed to place the sample collection bottle and rain gage chart that were installed in the field 

during the dry week, along with the MOF documenting the 

dry week’s sample collection, into the shipping cooler that 

was mailed to them by the USGS. They are asked to retain 

this cooler until the blind audit sample has been processed 

at the HAL and the data are reported to the USGS. Then, the 

USGS notifies the site operator to ship the actual dry-week 

sample to the HAL. The MDN precipitation samples are typically analyzed within three weeks of sample collection. The 

USGS compiles the returned blind audit postcards and notifies 

the HAL that blind audit samples have been processed in 

the previous month. The HAL grants the USGS access to its 

database to retrieve blind audit data and the USGS discloses 

the identity of sites that processed a blind audit sample to the 

HAL. The HAL identifies the QA sample data in its database 

and excludes them from monthly summary reports provided to 

MDN sites. Then, the USGS notifies participating sites to mail 
coolers containing the actual dry week sample bottle, MOF, 

and rain gage chart to the HAL.

For individual MDN sites requesting methylmercury 

(Me-Hg) analysis, the HAL extracts aliquots from weekly 

precipitation samples. These aliquots are analyzed individually for MDN sites requesting weekly Me-Hg analysis or, the 

weekly aliquots are composited and analyzed monthly for 

MDN sites requesting monthly Me-Hg analysis. In September 

2006, the USGS discovered that it had unknowingly asked 

several sites that determine Me-Hg to process blind audit 

samples. Since the identity of QA samples is not revealed to 

the HAL until after analysis, the processed blind audit samples 

were split for Me-Hg analysis. The splits were blended with 

precipitation samples and analyzed as part of monthly composite Me-Hg samples. This oversight corrupted several 

Me-Hg samples, the data for which had to be corrected. In 

the future, sites chosen for the blind audit program must not 

submit samples for Me-Hg analysis or for any other special 

analysis requiring the splitting and compositing of solutions. 

This mistake is documented herein to help avoid similar oversights in the future.

Data Records

The HAL supplies the USGS with analytical results for 

the processed blind audit samples on a yearly basis. Solutions used in the blind audit program are also analyzed by six 

laboratories participating in the interlaboratory comparison 

program. The MPVs are determined for each solution from 

the median Hg concentrations calculated using interlaboratory 

Table 8. Solution names, sample volumes, and approximate 

number of samples used in the U.S. Geological Survey blind 

audit program for the National Atmospheric Deposition 

Program/Mercury Deposition Network during 2006.

[mL, milliliters; ng/L, nanograms per liter; SP, synthetic precipitation]

Solution name

Sample 

volume

 (mL)

Number of 

samples

Ultrapure deionized water 75 3

Ultrapure deionized water 150 3

SP-2 + 5.7 ng/L mercury spike 75 4

SP-2 + 5.7 ng/L mercury spike 150 3

SP-2 + 14.3 ng/L mercury spike 75 3

SP-2 + 14.3 ng/L mercury spike 150 4

comparison program data. Statistical analysis of the differences between the reported Hg concentrations and the MPVs 

for the blind audit solutions quantify bias and variability in 

analytical data produced by the HAL. The USGS makes statistical interpretations of blind audit data, which are published in 

annual reports (Wetherbee and others, 2006) and presented to 

the NADP/NOS. All blind audit data are archived in SAS and 

Microsoft Access databases, which are stored on a USGS file 

server. A flowchart of the blind audit program is provided in 

figure 7.

Summary

Over the past 25 years, the USGS has been conducting external QA programs to quantify data variability for the 

NADP/NTN, and that expertise has been applied to implement 

programs for the MDN. The external QA programs conducted 

by the USGS in support of the NADP/MDN provide an ongoing assessment of data quality, quantify influences from most 

sources of contamination, and provide useful information for 

interpreting NADP/MDN data. Since 2004, three programs 

have been implemented: the system blank program, the interlaboratory comparison program, and the blind audit program.

The QA programs assess possible influences on data 

quality for precipitation samples collected by the NADP/

MDN. The system blank program quantifies Hg contamination of precipitation samples contributed by field exposure 

of sampling equipment and by routine sample handling and 

processing. The interlaboratory comparison program evaluates bias and variability of Hg data produced by the HAL by 

comparing HAL results with five other laboratories. The blind 

audit program assesses bias and variability of Hg data produced by the HAL using USGS-prepared solutions disguised 

as NADP/MDN precipitation samples which are submitted to 

the HAL by NADP/MDN site operators.

Preparing MDN QA solutions with specific Hg concentrations is more challenging than preparing solutions for the 

NTN because Hg concentrations in precipitation are very low 

and Hg can volatilize or adsorb to container surfaces. The use 

of HCl and Hg requires stringent safety and waste-disposal 

protocols. Preparation, processing, and storage methods for 

QA solutions and samples are continually being evaluated and 

modified when necessary. The QA evaluations of NADP/MDN 

sample collection help data users to differentiate between environmental indicators and sample collection influences.

18 External Quality Assurance Programs Managed by the U.S. Geological Survey
References Cited 19

The U.S. Geological Survey (USGS) prepares two

solutions in 1% hydrochloric acid (HCl): (1)

deionized water blanks with no added mercury

(Hg), and (2) synthetic, low-ionic strength

solutions spiked with different Hg concentrations. 

The Mercury Analytical Laboratory (HAL)

mails shipping coolers containing 2-L glass

sample-collection bottles each preserved with

20 mL HCl to the USGS. 

The USGS prepares the blind audit samples by adding 75- or 150-mL of solutions to the 2-L

glass bottles and mails them in Mercury Deposition Network (MDN) shipping coolers along

with a USGS laboratory-created rain gage chart depicting precipitation depth consistent with

sample volume, sample-processing instructions, and a cover letter to 20 site operators. 

After a dry week, the operator

processes the blind audit sample

by completing an MDN observer

form (MOF) and recording

precipitation information for the

laboratory-created rain gage chart.

Site operator ships the blind audit

sample, laboratory-created rain

gage chart, associated MOF, and

dirty glassware used in the field

during the dry week to the HAL in

a cooler from the site stock. 

Monthly, the HAL provides

USGS access to its preliminary

database, and USGS retrieves

data for blind audit samples

using information from

submitted postcards. Once 

USGS obtains the data, USGS

reveals the identity of a blind

audit sample to the HAL, and the

HAL flags the data in the

database as a QA sample. 

Site operator mails

postcard with

sample processing

information to

USGS. 

Site operator

notified by USGS

to ship real sample

and rain gage chart

to the HAL? 

Site operator stores the

real sample-collection

bottle from the dry week

and the real rain gage chart

in the cooler received from

the USGS until notified by

USGS to send sample and

chart to the HAL. 

NO 

The USGS compares the HAL-reported Hg concentrations for the blind

audit samples to median concentrations obtained for the same solutions

in the interlaboratory comparison program, which are considered the

most probable values. Results of the comparison are published in USGS

reports and presented to the National Atmospheric Deposition 

Program / Network Operations Subcommittee (NADP/NOS). 

YES

Site operator ships the real

sample and rain gage chart for

the dry week to the HAL. 

Figure 7. Flowchart for the National Atmospheric Deposition Program/Mercury Deposition Network blind audit 

program.
20 External Quality Assurance Programs Managed by the U.S. Geological Survey

References Cited

Mercury Deposition Network, 2006, MDN Sites: National 

Atmospheric Deposition Program, http://nadp.sws.uiuc.

edu/mdn/sites.asp, accessed September 12, 2006.

Moody, J.R., and Lindstrom, R.M., 1977, Selection and cleaning of plastic containers for storage of trace element samples: Analytical Chemistry, v. 49, no. 14, p. 2,264–2,267.

N-CON Systems Co., Inc., 2004, N-CON Systems Mercury 

Deposition Network Sampler (MDN): N-CON Systems Co., 

Inc., http://www.n-con.com/mdn.html, accessed April 26, 

2007.

U.S. Environmental Protection Agency, 2002, Method 1631, 

revision E: Mercury in water by oxidation, purge and trap, 

and cold vapor atomic fluorescence spectrometry: U.S. 

Environmental Protection Agency, Office of Water, EPA821-R-02-019, 38 p., http://www.epa.gov/ost/methods/

1631e.pdf, accessed April 26, 2007.

Vermette, S., Lindberg, S., and Bloom, N., 1995, Field tests 

for a regional mercury deposition network-sampling design 

and preliminary test results: Atmospheric Environment, 

v. 29, no. 11, p. 1,247–1,251.

Wetherbee, G.A., Latysh, N.E., and Gordon, J.D., 2004, External quality-assurance results for the National Atmospheric 

Deposition Program/National Trends Network, 2000–2001: 

U.S. Geological Survey Scientific Investigations Report 

2004–5034, 68 p.

Wetherbee, G.A., Latysh, N.E., and Greene, S.M., 2006, 

External quality-assurance results for the National Atmospheric Deposition Program/National Trends Network and 

Mercury Deposition Network, 2004: U.S. Geological 

Survey Scientific Investigations Report 2006–5067, 62 p.
Appendices

Appendices 21
Date 

Site Operator Address 

Dear Site Operator Name, 

Your site has been chosen to participate in the NADP/MDN System Blank Program during the 

specified quarter of year . This quality assurance program, conducted jointly by the U.S. 

Geological Survey and Frontier Geosciences, Inc., is intended to measure the effects of field 

exposure, handling, and processing on the chemistry of NADP/MDN precipitation samples. We 

appreciate your participation in this mandatory NADP/MDN quality assurance program. 

Please refer to the enclosed instructions for the exact steps that need to be followed to ensure that 

the System Blank sample is submitted correctly. Enclosed please find the System Blank sample, 

instructions and forms for your participation in the program. The samples used in the program 

consist of deionized water (blanks) and synthetic rainwater (reference materials). 

Fifty percent of the enclosed solution must be poured into the Aerochem Metrics collector, 

through the wet side sample collection train, and into the “sample collection bottle” that was 

installed in the field for an entire week with no precipitation. Please process the sample as soon 

as you have a dry week. If a dry week does not occur prior to deadline date please: 1) mail the 

sample to Frontier Geosciences, Inc. (it will be analyzed even though it was not processed) and 2) 

mail the enclosed postcard to the U.S. Geological Survey, stating that there was no dry week prior 

to deadline date. 

If you have any questions or concerns regarding the submission of the System Blank sample 

please call me at: (303) 236-1837, or e-mail: nlatysh@usgs.gov. Thank you in advance for your 

participation in the System Blank Program. 

Sincerely, 

Greg Wetherbee

United States Department of the Interior

U. S. GEOLOGICAL SURVEY

 Box 25046, M.S. 401

Denver Federal Center

Denver, Colorado 80225

Office of Water Quality

Branch of Quality Systems

 

IN REPLY REFER TO:

Appendix 1. Cover letter mailed to National Atmospheric Deposition Program/Mercury Deposition Network 

site operators participating in the U.S. Geological Survey system blank program. Highlighted text indicates fields 

customized for each site participating in the system blank study.

22 External Quality Assurance Programs Managed by the U.S. Geological Survey
NATIONAL ATMOSPHERIC DEPOSITION PROGRAM/MERCURY DEPOSITION NETWORK 

Instructions for submitting a system blank sample from your site to Frontier Geosciences, Inc.

ONLY SUBMIT A SYSTEM BLANK SAMPLE AFTER A WEEK 

 WITH NO PRECIPITATION AT YOUR SITE 

The sample you have been sent should be submitted prior to deadline date (6 months after sample 

mailing).

If this time period passes without a full dry week, please mail the sample to Frontier Geosciences, Inc. in the 

original box in which it was sent and mail the enclosed post card to the U.S. Geological Survey explaining 

that there were no dry weeks prior to deadline date (6 months after sample mailing).

Please verify that you have a 3-hole cooler for mailing all items to Frontier Geosciences Inc. If you do not 

have a 3-hole cooler, please contact Gerard Van der Jagt at Frontier Geosciences, Inc. (toll free: 877-622-

6960).

Step 1: Verify that no precipitation occurred

 Make sure that not even a trace of precipitation was measured by the rain gage. 

 Verify the collector worked properly and there were no clock stoppages or pen skips on the 

event recorder chart. If equipment malfunctioned during the preceding week, please 

do not process the system blank sample. 

 If a lid opening occurred, ensure that no precipitation was recorded. Explain any lid 

 openings due to humidity, dew or fog in the remarks section (Block 10) of the observer form. 

Step 2: Transfer half of the system blank sample into the collection bottle 

 Wet the sensor with MilliQ water to expose the sample train.

 Pour 50% of the system blank sample (down to the line marked on the bottle) into the collection

 funnel of the MDN collector. The sample will pass through the funnel and thistle tube, into the

 “sample collection bottle”. Wear clean gloves and be careful not to contaminate the sample.

 Replace the cap on the system blank bottle and seal it in its original ziploc bag. Screw the cap

 onto the “sample collection bottle” (which contains 50% of the transferred system blank sample

 and the pre-charge preservative) and remove it from the overflow bucket. Place this sample

 bottle into a sample bottle bag. 

 Please do not write anything on the bottles for identification purposes. The bag for the original

 system blank sample is labeled with sample information useful to Frontier Geosciences Inc. 

 Place both the bagged 2-L “sample collection bottle” and the bagged system blank bottle inside

 a standard cooler for shipping samples to Frontier Geosciences, Inc. Once both samples are

 capped and bagged you can begin the normal Tuesday field sampling procedures. 

Step 3: Fill out 2 MDN Observer Forms (MOFs) for the System Blank samples 

A) MOF for system blank sample bottle containing 50% of the original solution is labeled 

“System Blank Sample Bottle” in the Remarks section. Please fill in the following: 

1) Station. 2) Observer. 

Appendix 2. Sample-processing instructions mailed to National Atmospheric Deposition Program/Mercury Deposition 

Network site operators participating in the U.S. Geological Survey system blank program. Highlighted text indicates fields 

customized for a system blank study.

Appendices 23
Appendix 2. Sample-processing instructions mailed to National Atmospheric Deposition Program/Mercury Deposition 

Network site operators participating in the U.S. Geological Survey system blank program. Highlighted text indicates fields 

customized for a system blank study.—Continued

3) Bottle-Off Date and Time when you poured the system blank sample into the collection bottle (it is 

the off date because the process occurs the day you are taking the sample off the collector).

4) Site Operations–all equipment should have worked properly during the previous dry week in order 

to process the system blank sample. 

5) Enclosure Temperature.

B) MOF for the “sample collection bottle” that was installed in the collector for a full dry week is 

labeled “Sample Collection Bottle” in the Remarks section. Please fill in the following: 

1) Station. 2) Observer.

3) Bottle Id and the On and Off Dates and Times the collection bottle spent installed in the collector. 

(The On Date should be previous Tuesday’s date and Off Date should be Tuesday’s date when 

you process the system blank sample).

4) Analysis Type-please check the ‘System Blank’ box.

5) Observations-all should be checked ‘No’ since the bottle was covered by the lid all week and 

should not have been exposed to contamination.

6) Site operations–all equipment should have worked properly during the previous dry week in order to 

process the system blank sample.

7) Precipitation Record–please check the rain gage chart; precipitation values should all be zero.

8) Overflow–should be ‘No’.

9) Enclosure Temperature.

10) Please state the following in the Remarks section:

Poured system blank sample into the sample collection bottle on ‘date’.

Step 4: Fill out the enclosed pre-addressed post card and mail it to the U.S. Geological Survey.

Step 5: Ship the system blank samples to Frontier Geosciences, Inc.

Please send the following items to Frontier Geosciences, Inc. in the standard 3-hole shipping cooler: 

 The 2-L sample collection bottle containing 50% of the system blank sample plus pre-charge 

preservative

 The 1-L system blank bottle containing the remaining 50% of the solution placed into the 

cooler’s smaller opening normally used for trace metal samples

 Observer form for the sample collection bottle 

 Observer form for the system blank bottle 

 Rain gage chart 

 The dirty funnel and thistle tube 

Please place the paper work, including the rain gage chart, into a ziploc bag and lay it on top of the samples

inside the cooler.

*************************************************************************************** 

Thank you for participating in the System Blank Program. We appreciate your time.

If you have questions please contact Natalie Latysh at (303) 236-1874 or e-mail: nlatysh@usgs.gov 

*************************************************************************************** 

24 External Quality Assurance Programs Managed by the U.S. Geological Survey
Appendix 3. Mercury Deposition Network observer form used by the National Atmospheric Deposition Program/

Mercury Deposition Network site operators for recording sample information.

Appendices 25
Appendix 4. Front and back sides of the postcard mailed to National Atmospheric Deposition 

Program/Mercury Deposition Network site operators for reporting system blank sample 

processing and shipping information to the U.S. Geological Survey.

MDN –RECORD OF SYSTEM BLANK SAMPLE SUBMISSION 

PLEASE FILL IN THE BLANKS BELOW AND MAIL TO THE USGS.

THIS INFORMATION SHOULD MATCH WHAT IS LISTED ON

THE MDN OBSERVER FORM

SITE ID SITE NAME __________________ 

SITE OPERATOR __________________________ 

 Initials 

SAMPLE DATE SAMPLE SENT 

PROCESSING TO FRONTIER 

DATE ______________ GEOSCIENCES _______________ 

MS 401

U.S. Dept. of the Interior 

Geological Survey 

Box 25046 Federal Center 

Denver CO 80225-0046 

 U.S. Geological Survey 

 WRD / BQS 

 Bldg. 95 / MS 401 

 Box 25046 Denver Federal Center 

 Denver CO 80225 

26 External Quality Assurance Programs Managed by the U.S. Geological Survey
Appendix 5. Cover letter mailed to laboratories participating in the U.S. Geological Survey interlaboratory 

comparison program for the National Atmospheric Deposition Program/Mercury Deposition Network. 

Highlighted text indicates fields customized for each participating laboratory.

IN REPLY REFER TO:

United States Department of the Interior

U. S. GEOLOGICAL SURVEY

Box 25046 M.S. 401 

Denver Federal Center

Denver, Colorado 80225

Office of Water Quality

Branch of Quality Systems

Date 

Laboratory contact address 

Dear Contact: 

Enclosed are four samples for the Interlaboratory Comparison Program for the National 

Atmospheric Deposition Program / Mercury Deposition Network (NADP/MDN). This program 

is a component of the NADP/MDN External Quality Assurance (QA) Program, which is managed 

by the U.S. Geological Survey (USGS). 

The identification numbers for the enclosed samples are: 

Sample identification 1 

Sample identification 2 

Sample identification 3 

Sample identification 4 

Each sample should be analyzed for total mercury concentration in the same way that your 

laboratory would analyze an ordinary environmental sample. If possible, these samples should be 

processed immediately. Each sample is in a 1% hydrochloric acid matrix. Your laboratory 

identification code is “Laboratory”. Please email your data to us, preferably in an Excel 

spreadsheet or ASCII text file using the following example format: 

Sample Identification 

Number

(shown above)

Laboratory 

Identification 

Code

Date of

Analysis 

Total Mercury 

Concentration, in 

nanograms / Liter 

(ng/L) 

Minimum 

Reporting 

Limit 

(ng/L) 

Sample identification Laboratory Date ##.#### ##.#### 

Please direct any questions regarding the program to us at 303-236-1837 (Greg) or 303-236-1874 

(Natalie) or via e-mail at wetherbe@usgs.gov or nlatysh@usgs.gov. We appreciate your 

participation in this important quality assurance program for the NADP/MDN. 

Best Regards, 

 

Natalie Latysh 

Appendices 27
Appendix 6. Example of control chart posted on the Internet (http://bqs.usgs.gov/precip_2/mdn/index.html/) displaying 

a participating laboratory’s results for spiked mercury solutions used in the U.S. Geological Survey interlaboratory 

comparison program for the National Atmospheric Deposition Program/Mercury Deposition Network. 

28 External Quality Assurance Programs Managed by the U.S. Geological Survey
Appendix 7. Example of control chart posted on the Internet (http://bqs.usgs.gov/precip_2/mdn/index.html/) displaying 

a participating laboratory’s results for deionized water solutions used in the U.S. Geological Survey interlaboratory 

comparison program for the National Atmospheric Deposition Program/Mercury Deposition Network.

Appendices 29
Appendix 8. Cover letter mailed to National Atmospheric Deposition Program/Mercury Deposition Network 

site operators participating in the U.S. Geological Survey blind audit program. Highlighted text indicates fields 

customized for each site participating in the blind audit study.

Date

Site contact address

Re: MDN BLIND-AUDIT PROGRAM 

Dear Site Operator,

Please read this letter and the enclosed Mercury Deposition Network (MDN) Blind Audit 

Instructions carefully. Enclosed you will find the materials needed to process a MDN blind-audit 

sample. Please let us know if this package is missing any of the following items.

 MDN Blind-Audit sample 

 Illustrated instruction sheet 

 Self-addressed, stamped, MDN Blind-Audit postcard 

 FedEx shipping label and plastic sleeve 

You have been selected as a participant for blind-audit quality assurance program. The MDN 

blind audit is designed to measure the variability and bias in the HAL analytical data. It is very 

important that you take great care to disguise the blind-audit sample so that the HAL will not 

know that it is a quality assurance sample. Please submit your blind-audit sample(s) the next time 

your site has a dry week before study deadline date. If you are waiting for a dry week to submit a 

MDN system-blank sample, please submit the system-blank sample first, and then wait for the 

next dry week to submit the blind-audit sample.

MINIMIZE BLIND-AUDIT SAMPLE HANDLING!! DO NOT OPEN THE ZIPLOCK BAG 

CONTAINING THE BLIND-AUDIT SAMPLE BOTTLE NOR THE BOTTLE ITSELF.

If a dry week does not occur prior to study deadline date please ship the cooler containing the 

blind-audit materials to the HAL, using the enclosed FedEx shipping label and plastic sleeve. 

Since there is no specified submission date, it is important that you mail the self-addressed, 

stamped postcard to the USGS on the day you ship the blind-audit sample to the HAL.

If you have any questions or concerns regarding the submission of the blind-audit sample please 

call Greg Wetherbee at 303-236-1837 or Natalie Latysh at 303-236-1874, or e-mail:

nlatysh@usgs.gov. Thank you in advance for your participation in this program. 

Sincerely,

Greg Wetherbee, Chemist, Project Chief

United States Department of the Interior 

U. S. GEOLOGICAL SURVEY

 

IN REPLY REFER TO:

Box 25046 M.S. 401

Denver Federal Center

Denver, Colorado 80225

Office of Water Quality

Branch of Quality Systems

30 External Quality Assurance Programs Managed by the U.S. Geological Survey
Appendix 9. Sample-processing instructions mailed to National Atmospheric Deposition Program/Mercury Deposition 

Network site operators participating in the U.S. Geological Survey blind audit program.

MDN BLIND-AUDIT INSTRUCTIONS

1

STEP

2

STEP

3

STEP

Read and fill out the USGS-created rain gage chart just as you would for any raingage chart from your site. Fill

in: site id, date-on, date-off, time-on, time-off, and precipitation total. Make sure that the time-on and time-off

match the data from the real chart for the dry week. Make a small mark at the time-on and time-off with a pen.

If you typically photocopy the rain gage chart to mail to the HAL, photocopy the USGS-created chart and mail it

to the HAL with the blind audit sample. Then, you may discard the original USGS-created chart. If you send the

original rain gage chart to the HAL with your weekly samples, send the original USGS-created chart to the HAL

with the blind audit sample. 

GO TO REVERSE SIDE

Belfort chart shows no 

precipitation or collector 

openings.

The intent of the blind-audit program is to assess the variability and bias in the analytical results produced by

the Mercury Analytical Laboratory (HAL) by disguising a quality assurance sample as a weekly precipitation

sample from your site. 

Store in USGS cooler

Raingage chart

for dry week

MDN Observer

Form (MOF) for

dry week sample

Sample bottle

that spent a dry

week In the

collector 

Fill out USGS-created

raingage chart

Service MDN collector and rain gage. If no precipitation occurred and the collector did not open during

the preceding week, then you can process the blind-audit sample. Please DO NOT process the blindaudit sample if the collector lid opened even once. 

Transfer the blind-audit sample from the USGS cooler to an extra cooler in your stock. This is

IMPORTANT! It helps to disguise the blind-audit sample as a precipitation sample from your site since

the coolers can be tracked by the HAL. 

Complete a MOF for the field bottle that spent a dry week installed in the collector. Store the MOF, field

bottle and the rain gage chart that you just retrieved from the field in the cooler that you received from

the USGS until you are notified by the USGS to send it to the HAL. 

Appendices 31
Appendix 9. Sample-processing instructions mailed to National Atmospheric Deposition Program/Mercury 

Deposition Network site operators participating in the U.S. Geological Survey blind audit program.—Continued

5

STEP

4

STEP

Complete a second MOF for the blind-audit sample with information for a typical precipitation sample. Fill in

all fields on the MOF, except for field #7 Precipitation Record. For this field, use information from the

USGS-created rain gage chart (See step 3). Fill in the daily and weekly precipitation data on the blind-audit

MOF per the USGS-created rain gage chart. PLEASE DO NOT INDICATE ON THE BLIND-AUDIT MOF

THAT THIS IS A QUALITY ASSURANCE SAMPLE IN ANY WAY! 

Make sure that the cooler that you obtained from your stock (not the USGS cooler) sent to the HAL

contains: blind-audit sample, USGS-created rain gage chart, blind-audit MOF, and the used funnel and

thistle tube. Use standard shipping methods to mail the cooler to the HAL. DO NOT USE THE

ENCLOSED FEDEX LABEL. 

You Have Completed the MDN Blind Audit for Your Site! Thank You!

For more information, call Natalie Latysh at 303-236-1874 / e-mail: nlatysh@usgs.gov 

BLIND-AUDIT INSTRUCTIONS–Continued

Mail to the

HAL in a

cooler from

your stock 

USGS-created raingage

chart that you read

and filled out

Double blind sample

mailed to you by the USGS

Used funnel and thistle tube

installed in the collector

during the dry week

MOF with information for the

USGS created raingage chart

Fill out the stamped, pre-addressed postcard provided with the blind-audit sample and

mail to the USGS. 

Mail postcard

to USGS

 MDN – RECORD OF BLIND-AUDIT SAMPLE SUBMISSION PLEASE FILL IN THE BLANKS BELOW AND MAIL TO THE USGS. 

THIS INFORMATION SHOULD MATCH WHAT IS LISTED ON 

THE MDN OBSERVER FORM

SITE ID 

 SITE NAME __________________ 

SITE OPERATOR __________________________ 

 Initials 

DATE ON: ______________ 

TIME ON: ______________ 

 DATE SHIPPED 

DATE OFF: ______________ TO FRONTIER GEOSCIENCES 

TIME OFF: ______________ _______________ 

6

STEP

You will be notified in a few months to ship the actual field bottle, that spent a dry week installed

in the collector, to the HAL in the USGS cooler. Use the enclosed FedEx label to ship the USGS

cooler to the HAL. All fields have already been completed, please place the label in the plastic

sleeve and affix it to the USGS cooler. 

32 External Quality Assurance Programs Managed by the U.S. Geological Survey
Appendix 10. Front and back sides of the postcard mailed to National Atmospheric Deposition 

Program/Mercury Deposition Network site operators for reporting blind audit sample processing and 

shipping information to the U.S. Geological Survey.

MDN- RECORD OF BLIND-AUDIT SAMPLE SUBMISSION 

PLEASE FILL IN THE BLANKS BELOW AND MAIL TO THE USGS.

THIS INFORMATION SHOULD MATCH WHAT IS LISTED ON

THE MDN OBSERVER FORM

SITE ID SITE NAME __________________ 

SITE OPERATOR __________________________ 

 Initials 

DATE ON: ______________ 

TIME ON: ______________ 

DATE SHIPPED 

DATE OFF: ______________ TO FRONTIER GEOSCIENCES 

TIME OFF: ______________ _______________ 

MS 401

U.S. Dept. of the Interior

Geological Survey

Box 25046 Federal Center

Denver CO 80225-0046

U.S. Geological Survey

WRD / BQS

Bldg. 95 / MS 401

Box 25046 Denver Federal Center

 Denver CO 80225 

Appendices 33