Protocol for the Collection, Transfer and ANSI/AARST MW-RN 2020 Measurement of Radon in Water 29 10.5.2 Intercomparisons Prior to a laboratory offering commercial services involving the analysis of water samples for radon, a qualified laboratory shall have demonstrated proficiency by successfully participating in an intercomparison study with at least three approved laboratories. The new laboratory shall prove the capability to achieve, report (to certifying bodies and/or the public), and maintain test results that are within 25% of the established mean (unless noted) for radon at three distinct concentrations: a) one to evaluate levels below 1000 pCi/L (37 Bq/L) (within 50% of the established mean); b) one about five times a); and c) one at significantly greater levels. The National Primary Drinking Water Regulations (NPDWR) defines the minimum detection limit ( MDL ) as a concentration that can be measured with a precision of ± 100% at the 95% confidence level (2 s ). Detection limits are dependent on the following: a) Background or blank count rate; b) Instrument sensitivity (i.e., efficiency); c) Volume of sample that is analyzed; d) Sample decay time; and e) Counting time. As the 1991 proposed a maximum contaminant level for waterborne radon was 300 pCi/L (11 Bq/L), the MDL for an approved method shall be less than 200 pCi/L (7.4 Bq/L). In lieu of a nationally recognized or federal program to qualify laboratory reference facilities, external laboratories that successfully meet requirements for state programs or intercomparisons in accordance with Section 10.5.2 shall be acceptable for use as a reference facility when conducting performance tests. SECTION 11.0 LIQUID SCINTILLATION—LABORATORY PROCESSING Note—A variety of analytical methods are permitted following successful participation in interlaboratory intercomparison or proficiency testing studies and approval of the methodology. The text below provides details pertaining to the U.S. EPA-accepted methods which are most prevalent. The sample transfer protocols may be applicable to other methods. 11.1 Analytical measurement of the water sample 11.1.1 Sample preservation The person responsible for handling the samples in the field and in the laboratory should take measures, as appropriate, to avoid compromising the integrity of the sample due to temperature in the environment where the sample is stored or transported. Water samples shall not be preserved with acid as this is unnecessary, potentially dangerous for the collector, and would introduce air contact with the water sample (and potential loss of radon). 11.1.2 Time limits The cocktail vial shall dark-adapt a minimum of 1 hour after sample preparation before beginning measurement. Note—As radon decays with a half-life of 3.82 days, measurement of the radon activity commonly commences within 4 days of sample collection and the measurement is often completed within 8 days of collection.