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The U.S. Geological Survey (USGS) conducts an inter-laboratory comparison study semiannually. This project provides a variety of Standard Reference Samples (SRSs) for laboratory quality assurance testing and are available for internal quality control. The majority of samples are prepared with water from Colorado streams and spiked with reagent grade chemicals to measurable concentrations.

Fact Sheet
Schedule of Events
Benefits of Participation
Frequently Asked Questions
SRS's Available from our Inventory
Answer Sheets and Method Codes
Standard Reference Sample Listing
Round Robin Sample Concentration Ranges
Active Labs (participating within last 2 years)

Quick Links:
Quality Systems Branch
U.S. Geological Survey

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Standard Reference Sample Project
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The project began in 1962 with a single sample containing major constituents that was prepared from distilled water and reagent grade chemicals. Twenty-three USGS laboratories participated in the determinations of six analytes. Since that time, objectives of the project have been to accomplish the following:   

Approximately one hundred and twenty-five USGS and non-USGS laboratories are actively participating  (from 40 states) in the project and six standard reference sample types are available (Trace Element, Major Ion, Precipitation, Mercury, Low-level Nutrient, and Regular-level Nutrient). If you would like to enroll, please contact the project chief at

This project is not open to international laboratories. U.S. Federal, State, municipal, and university laboratories can participate even though they do not provide data to the USGS. Laboratories can compare their results to other labs based on the deviation (z-value) from the median and percent difference. Laboratories may order previously made SRSs by calling the number below.

bullet Benefits of Participation

Note: All samples are in the dissolved phase (filtered to 0.2 um) so bottle aliquots are homogenous. Creating an unfiltered SRS cannot be prepared without very high variability.

bullet Contacts

SRS Project Chief 
U.S. Geological Survey
Quality Systems Branch
Denver Federal Center, Bldg. 95
P.O. Box 25046, MS 401
Denver, CO  80225-0046

Other numbers:
Jim Simmons, Branch Chief, Quality Systems Branch (303) 236-1835

If you can not fully access the information on this website, please contact SRS Project Chief .

bullet Sample List

To order samples for the current inter-laboratory comparison, please visit our order page. To order previously prepared SRSs, please email View the list of available SRSs from our inventory.

All samples are in the dissolved phase (filtered to 0.2 um).

Trace element sample (500 mL): chlorinated with sodium hypochlorite and acidified with nitric acid   
Major ion sample (500 mL): chlorinated with sodium hypochlorite
Precipitation (low ionic-strength) sample (500 mL): typically non-preserved snowmelt
Nutrient (low-level) sample (60 mL; dilute 1:10): must remain chilled
Nutrient (regular-level) sample (250 mL): must remain chilled
Mercury sample (250 mL): concentration <0.1 ug/L; acidified with hydrochloric acid

bullet Event Schedule

December 14, 2020
invitation to participate in the inter-laboratory comparison June 22, 2020
January 22, 2021
sample order deadline for participating laboratories July 24, 2020
Jan 26 - Feb 3, 2021
mail out samples to laboratories Jul 28 - Aug 6, 2020
March 26, 2021
data entry deadline September 25, 2020
March __, 2021
results made available on the web September 28, 2020

bullet Answer Sheets and Method Codes

The answer sheets below were created to help laboratories collect analytical results for electronic data entry. These forms are not to be used for data submittal. A listing of the SRS method codes has also been provided for your assistance.
Answer sheets for | Trace Sample | Major Sample | Precipitation Sample | Mercury Sample | Nutrient Samples |
SRS Method Codes are used when entering data and are included in the results reports.

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