Several statistical techniques are used for data analysis.
Results are used to compute long-term control limits, which,
in turn, are used to produce new acceptance criteria.
Frequently, a control limit is set at three times the standard
deviation of the expected value. If only normal random
errors are present, then 99.7 percent of the values will fall
within this limit (Friedman and Erdmann, 1982, p. 91).

Third Tier: Performance-Evaluation Studies

Performance-evaluation studies for organic compounds are
more difficult to administer than for inorganic constituents
because some of the compounds are unstable. Special care
is required to ensure that performance-evaluation studies
evaluate method performance and are not biased by natural
compound degradation. The special measures that must be
taken (for example, chilling samples and protecting them
from light), along with the high cost of organic sample
analysis, can make the studies expensive. Nevertheless, the
NWQL routinely participates in the following national
performance-evaluation studies:

* USEPA Water-Pollution (WP) and Water-Supply (WS)
studies,

+ NOAA Intercomparison Program for Marine Sediment
and Tissue, and

* National Water Research Institute of Canada (organic
carbon).

Field Supplies

The USGS warehouses stock the necessary supplies for
collecting environmental samples for organic analyses,
including sample containers, preservatives, filters, and
solid-phase extraction cartridges. In addition, organic blank
water suitable for pesticide and volatile analyses is
available. Blank water is rigorously tested and quality
assured at the laboratory and is distributed with a certificate
of analysis. Complete field spike kits, consisting of a
volumetric measuring device, necessary solvents, gloves,
and a waste storage container, can be purchased from the
NWQL. Verified spike solutions for several compound
classes are prepackaged and ready for use. Custom spike
solutions tailored to meet field QC activities also can be
prepared.

April 1998

7

SELECTED REFERENCES

Fishman, M.J., ed., 1993, Methods of analysis by the U.S.
Geological Survey National Water Quality Laboratory—
Determination of inorganic and organic constituents in
water and fluvial sediments: U.S. Geological Survey
Open-File Report 93-125, 217 p.

Fishman, M.J., and Friedman, L.C., 1989, Methods for
determination of inorganic substances in water and
fluvial sediments: Techniques of Water-Resources
Investigations of the U.S. Geological Survey, book 5,
chap. Al, 545 p.

Friedman, L.C., and Erdmann, D.E., 1982, Quality
assurance practices for the chemical and biological
analyses of water and fluvial sediments: U.S.
Geological Survey Techniques of Water-Resources
Investigations, book 5, chap. A6, 181 p.

Pritt, J.W., and Raese, J.W., 1995, Quality assurance/quality
control manual—National Water Quality Laboratory:
U.S. Geological Survey Open-File Report 95-443, 35 p.

Wershaw, R.L., Fishman, M.J., Grabbe, R.R., and Lowe,
L.E., 1987, Methods for the determination of organic
substances in water and fluvial sediments: Techniques
of Water-Resources Investigations of the U.S.
Geological Survey, book 5, chap. A3, 80 p.

— Kimberly D. Pirkey and Stephen R. Glodt

For more information, contact:

Chief, Branch of Analytical Services
USGS, National Water Quality Laboratory
Box 25046, Mail Stop 407

Denver, Colorado 80225

Fact Sheet FS—026—98



a USGS

science for a changing world

Quality Control at the

U.S. Geological Survey
National Water Quality Laboratory

U.S. Department of the Interior

U.S. Geological Survey

INTRODUCTION

The National Water Quality Laboratory (NWQL)-—the
primary analytical facility of the U.S. Geological Survey
(USGS)—analyzes environmental samples from across the
United States and around the world. Samples of water,
sediment, aquatic biological materials, and air are analyzed
at the laboratory using state-of-the-art instruments and
methods. These analyses require a comprehensive quality-
assessment process.

Quality assessment consists of two separate but related
components: quality-control data and quality-assurance
elements. Quality-control (QC) data are produced to
measure bias and variability of the analytical method.
Without QC data, the magnitude of errors, and thus the
quality of the environmental data, is not known. Some
examples of QC data sample types include standard
reference materials, laboratory replicates, laboratory reagent
blanks, spikes, and surrogates. Assessment of QC data is
essential to evaluating environmental analytical data
produced by a laboratory. Definitions of QC data sample

types used by the NWQL are emphasized in this Fact Sheet.

Quality-assurance (QA) elements are also essential to
producing high-quality analytical results. QA elements are
defined as the procedures used to control the unmeasurable
components of the analytical method. QA elements include
standard operating procedures, analysis-sequence protocols,
instrument-maintenance logs, and personnel training
records. Clearly, all of these elements could affect
laboratory data quality. However, it is often difficult to
ascertain the impact of QA elements on environmental
analytical results. Therefore, the assessment of QA
elements will not be discussed in this Fact Sheet.

Quality assessment is the process of reviewing the
application of QA elements and the analysis of QC data.
The objective of quality assessment is to provide evidence
that the measurement processes are under control.

QUALITY-CONTROL PROCESS

The NWQL uses a three-tiered approach to QC divided into
inorganic and organic analyses. For the first tier, “Method
Performance,” QC samples are analyzed by a chemist. At

this level, chemists use the QC sample data to control the
analytical process for each batch of environmental
samples. If prescribed acceptance criteria are not met, the
analysis is discontinued until corrected.

The second tier contains the “Data Review and Blind
Sample Programs.” Chemists use data from these programs
to monitor method performance throughout the laboratory
and over long periods.

The third and final tier consists of NWQL's participation in
“Performance-Evaluation Studies.” These studies are
managed by local, State, or Federal agencies external to the
NWQL. Data from these studies can be used to compare
laboratories or to select a laboratory for analytical work.
Summary reports from many of these studies are available
on the NWQL Home Page Website at

wwwnwal.cr.usgs.gov/USGS.
INORGANIC ANALYSIS

First Tier: Method Performance

Analysts trying to determine inorganic constituents in any
environmental sample evaluate two areas of performance:
instrument and method. Accurate instrument operation can
be verified by analyzing known standards at different
concentrations. Inorganic blank water, which has negligible
concentrations of the constituents of interest, is also
analyzed to evaluate and eliminate potential sources of bias.

The analyst uses QC samples to assess whether the method
is working properly. Generally, data produced from these
samples are the most useful for customers to evaluate the
entire analytical process. In addition to inorganic blank
water samples, standard reference materials are the primary
tools for evaluating method bias and variability. Laboratory
replicate data also might be evaluated to obtain method
variability information. Definitions of these sample types
follow.

Standard Reference Materials (SRMs). Generally, SRMs are
supplied in one of two ways: as an ampoule in concentrated
form that is added to the appropriate matrix, or as a prepared
reference ready for analysis. The prepared reference is the
predominant source when commercial materials are



available. In either form, the SRM is thought to be
sufficiently tested to be used to assess method performance.
Several SRMs of differing concentrations and constituents
are analyzed alongside environmental samples at the
NWQL. The SRM data are evaluated to assess bias for each
analysis. Additionally, as the data are compiled, long-term
estimates of method variability can be obtained.

Laboratory Replicate. A laboratory replicate is one type of
environmental sample replicate. The environmental sample
is split at the laboratory to provide information about
variability. If the sample is split into identical subsamples,
then the replicate will provide information about variability
inherent in the analytical process. However, true sample
splits are often difficult to obtain. More frequently, the split
samples will exhibit some differences because of sample
inhomogeneity. The sample variability inherent in more
challenging matrices, such as sediment, will often mask the
sample processing and analytical variability.

Second Tier: Data Review and Blind Sample Programs

Data Review Program. All inorganic analytical results
initially are stored in the Laboratory Information
Management System data base. A QC check program uses
chemical logic checks to review all analytical results in the
data base. All analytical results that fall outside the
acceptance limits are sent to chemists in the Quality
Assurance Unit for review. Examples of these logic checks
include the following:

+ Cation/Anion Balance

* Specific Conductance/Anion Ratio

* Specific Conductance/Cation Ratio

+ Dissolved Solids/Specific Conductance Ratio

* Field Value/Laboratory Value Agreement (pH,
alkalinity, specific conductance)

+ Filtered Component Compared with Unfiltered
Component

Blind Sample Programs. The inorganic blind sample
programs are administered by two different organizations,
one internal and the other external to the NWQL. The
internal program quantifies bias caused by random
laboratory contamination. It also supplements the external
blind sample program by submitting double-blind reference
samples for those analyses not evaluated externally
(cyanide, whole water recoverable constituents, sediment,
tissue). These reference samples are called “double blind”

because both the sample origin and constituent
concentrations are unknown to the analyst.

The external blind sample program is used to evaluate both
the NWQL and the USGS laboratory in Ocala, Fla. Bias
and variability information obtained from this program is
made available to laboratory customers on the Web at

wwwbtdgs.cr.usgs.gov.

Data analysis performed by either program typically
involves preparing control charts to monitor long-term
method performance. The number of standard deviations
from the most probable value (otherwise known as the
z-value) is calculated. Using this calculation, all data can be
easily displayed in a single chart. The z-value is calculated
as follows:

laboratory measured concentration — most probable value
standard deviation of most probable value

In figure 1, the z-values calculated for cadmium represent
concentrations ranging from 0 to 50 micrograms per liter.

eo

NUMBER OF STANDARD

DEVIATIONS FROM MOST
PROBABLE VALUE (Z-VALUE)

oo

J FMAM J J AS ON D
CALENDAR YEAR 1996
Figure 1. Control chart of cadmium determinations in

standard reference material samples by graphite furnace
atomic absorption spectrophotometry.

Third Tier: Performance-Evaluation Studies

The NWQL participates in the following national and
international interlaboratory performance-evaluation
studies:

+ US. Environmental Protection Agency (USEPA) Water-
Pollution (WP), Water-Supply (WS), and Safe Drinking
Water Act (radiochemistry) studies;

+ USGS Branch of Quality Systems Evaluation Program for
Standard Reference Samples;



+ National Water Research Institute of Canada; and

+ National Oceanic and Atmospheric Administration
(NOAA) Intercomparison Program for Marine Sediment
and Tissue.

Field Supplies

The USGS warehouses stock sample containers, filters, and
preservatives for collecting environmental samples.
Inorganic blank water is also available for customer use.
All supplies are rigorously tested to ensure compliance with
quality standards. Contamination is tested using the lowest
detection methodology for the constituents of interest.
Certificates of analysis are provided with each
manufacturer’s lot. Other field QC supplies, such as
inorganic spike materials, are available on request.

ORGANIC ANALYSIS

First Tier: Method Performance

Organic compounds may be degraded by heat, light,
temperature, microorganisms, and other factors. Because
the NWQL analyzes a wide range of samples and
specializes in trace and ultratrace compound concentrations,
there is an increased need for QC information compared to
inorganic analyses.

Samples received for organic determinations are grouped
together for similar analyses. Laboratory reagent blanks
and laboratory reagent spike samples are prepared and
analyzed with each group of environmental samples to
monitor method performance. Surrogate compounds are
also added to all environmental and QC samples. These QC
sample types are defined as follows.

Laboratory Reagent Blank. The laboratory reagent blank
(LRB) is prepared using an appropriate matrix. The
selected matrix should have negligible concentrations of
compounds for that particular analysis. The LRB is
processed in the same way as the accompanying
environmental sample and so is used to monitor for random
contamination introduced from laboratory processing. Ifa
compound is detected and identified in the LRB,
quantitation of that same compound in the environmental
sample would be qualified. Not only does the analyst track
LRB results with each sample set, but the laboratory can
track a history of LRB results to determine possible trends.

Laboratory Reagent Spike. A laboratory reagent spike (LRS)
is prepared and analyzed with each group of environmental
samples. The LRS is a sample where known concentrations
of the compounds of interest are added, usually into a

synthetic (reagent) matrix. An evaluation of reagent spike
data yields information about method performance. The
LRS analytical results are compared to acceptance criteria
for each method to assess potential bias of environmental
sample results. In addition, LRS data are charted
periodically to observe long-term method bias and
variability. A graph of the percent recovery of the herbicide
atrazine, an organic compound in the LRS, is shown in
figure 2.

PERCENT RECOVERY

J oF MAM J J

CALENDAR YEAR 1996

Figure 2. Percent recovery of the herbicide atrazine
analyzed by solid-phase extraction, gas chromatography/
mass spectrometry.

Surrogate. A surrogate is a compound that is expected to
perform similarly to the compounds being analyzed in a
laboratory method. The surrogate is not normally found in
the environment and therefore can be used to monitor the
recovery efficiency of analytical processes. The NWQL
adds surrogate compounds to all environmental and QC
samples to monitor compound recovery and potential
environmental effects. For example, high dissolved organic
carbon concentrations might interfere with analytical
techniques. Surrogate analytical results are released
electronically with environmental data for all organic
analyses.

Second Tier: Blind Sample Program and Data Review

Blind Sample Program. The first major component within
the second tier of quality control is the blind sample
program where double-blind samples are submitted for
analysis. Results of double-blind samples should accurately
reflect the state of laboratory processes since the samples do
not receive special treatment.

Data Review. Another major QC activity is long-term data-
quality assessment. This type of data review involves
evaluating control charts and analyzing statistics from the
LRBs, LRSs, and surrogates.