Protocol for the Collection, Transfer and ANSI/AARST MW-RN 2020 Measurement of Radon in Water 27 10.1.5 External testing programs—Intercomparisons and performance testing To be considered qualified, a laboratory shall participate in at least one external proficiency testing program or large-scale intercomparison exercise annually and report a result within 10% of the known for a concentration above 300 pCi/L (11 Bq/L) for each method utilized in analyzing a water sample for radon. Laboratories shall continue to maintain satisfactory participation in annual intercomparison studies and/or proficiency testing. A failure shall result in the provision of: a) a corrective action report; b) analytical proof of an effective remedy; and c) the successful participation in a follow-up intercomparison study and/or proficiency testing study. Where a laboratory or method has failed acceptable criteria for intercomparisons or performance testing, the Qualified Laboratory status or designation for use of the method shall be terminated after 90 days: a) unless identifying corrections and successfully completing another intercomparison or performance test; or b) if failing two consecutive intercomparison or performance tests, successfully complete three intercomparisons or performance tests with Qualified Laboratories. 10.2 General quality assurance and quality control (QA/QC) The frequency of conducting samples for quality assurance is dependent on the measurement instrument being used and the sample measurement load. Qualified Laboratories, Qualified Measurement Professionals, and all users of this document must comply with ANSI/AARST MS-QA “Radon Measurement Systems Quality Assurance” for details that relate to proper quality assurance during collection and measurement of the radon content of water. In general, a laboratory shall have demonstrated minimum detectable limit ( MDL ) and precision studies. 10.3 Quality control (QC) Analytical laboratories that measure and report concentrations of radon in water shall maintain a demonstrated and documented QA/QC program. The quality assurance plan shall, at aminimum, contain the following requirements. 10.3.1 Routine validation Performance of the measurement instrument shall be confirmed each day before use, and if a large sample load, at least one for every 10 samples on average. Blank samples, consisting of laboratory grade pure water, shall be constitute at least 5% of measured samples. Duplicate samples from the same bottle, of a concentration greater than 300 pCi/L (11 Bq/L), shall constitute at least 10% of the measured samples. 10.3.2 Reference standards and the quality assurance plan A QA Plan shall be written, followed, and verifiable (documented). Quality assurance is typically performed using reference standards, either purchased or created in-house. The reference standards do not necessarily have to be traceable to an accrediting organization but are instead used to determine that the instrument-measurement parameters have not changed since the previous check(s) and the last external standardization, proficiency test, or intercomparison . The value of the reference standard is determined following each instrument standardization, and then shall be tabulated (or charted) with each use to aid in the identification of a trend (e.g., loss of efficiency). Warning and control limits shall be established. Documentation regarding the quality assurance that is conducted shall be kept current.