6.2 Quality Control Measurements: Duplicates, Blanks & Spikes

6.2 Quality Control Measurements: Duplicates, Blanks & Spikes

At the core of ANSI/AARST MS-QA-2023 is a mandatory trio of Quality Control (QC) measurements: Duplicates, Blanks, and Spikes. These tactical measurements provide empirical evidence of data reliability, enabling Radon Measurement Professionals (RMPs) to detect random variability, background contamination, and systematic instrument bias before client reports are finalized.

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1. Duplicate Measurements (Side-by-Side Co-located Detectors)

Duplicate measurements consist of two identical detectors deployed side-by-side in the same test location for the exact same exposure duration.

Mandatory Frequency

Under MS-QA-2023, duplicates must be deployed at a minimum rate of 10% of total measurement locations (or 1 duplicate pair for every 10 test sites). High-volume measurement operations must distribute duplicate deployments evenly across all active field technicians and device batches.

Primary Purpose

Duplicates evaluate Precision—the degree of agreement among independent, repeated measurements under identical environmental conditions. Precision checks detect random errors, such as manufacturing variability in passive detectors, minor micro-climate fluctuations, or optical reader noise.

Placement Configuration

Duplicate detectors must be placed 4 inches (10 cm) apart from each other, maintaining identical distances from walls, windows, floors, and HVAC vents. They must be exposed and retrieved simultaneously.

Evaluation Metric: Relative Percent Difference (RPD)

Duplicate performance is evaluated using Relative Percent Difference (RPD): $\text{RPD} = \frac{|R_1 - R_2|}{\frac{R_1 + R_2}{2}} \times 100\%$ Where $R_1$ and $R_2$ represent the radon concentrations recorded by the primary and duplicate detectors.

For radon concentrations $\ge 4.0 \text{ pCi/L}$, MS-QA-2023 sets the Warning Limit at an RPD of 28% and the Action/Control Limit at an RPD of 36% (the assumed in-control average RPD target is 14%). For concentrations between 2.0 and 4.0 pCi/L, the warning limit rises to 50% and the control limit to 67% (in-control target 25%). At low concentrations (< 2.0 pCi/L), the warning limit is reached when the absolute difference between the two results exceeds 1.0 pCi/L, reflecting the inherent statistical variability of radioactive decay counting at low activity levels.

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2. Blank Measurements (Unexposed Detectors)

Blank measurements consist of detectors that are handled and analyzed without receiving exposure to environmental radon at the testing site.

Mandatory Frequency

Blanks must be conducted at a minimum rate of 5% of total measurement locations (or 1 blank for every 20 test sites).

Primary Purpose

Blanks evaluate Contamination and Background Signals. They determine whether radon gas is penetrating detector housings during storage or transport, or whether analytical instruments (such as alpha track readers or electret voltage readers) exhibit baseline noise or dark current drift.

Types of Blanks

1. Field Blanks: Transported to the testing site in their sealed protective packaging, briefly unsealed in the field to simulate handling, immediately resealed, stored alongside active detectors during transport, and submitted to the laboratory for analysis.
2. Office / Storage Blanks: Retained in a designated low-radon storage environment (e.g., an office storage cabinet with verified radon levels < 0.4 pCi/L) to verify storage integrity over time.

Evaluation Criteria

Blank measurement values must not exceed the instrument's Lower Limit of Detection (LLD), typically < 0.4 pCi/L to 1.0 pCi/L. A blank reading exceeding 1.0 pCi/L indicates background contamination, compromised packaging, or analytical reader defect, requiring immediate investigation.

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3. Spike Measurements (Chamber-Exposed Detectors)

Spike measurements (also called matrix spikes or blind spikes) consist of detectors exposed to a precisely known, certified radon concentration within an accredited calibration facility.

Mandatory Frequency

Spikes must be conducted at a minimum rate of 3% of total measurements, with a strict absolute minimum of 3 spike measurements per calendar year (and a maximum cap of 6 spikes per month for large-scale operations).

Primary Purpose

Spikes evaluate Accuracy and Systematic Bias. While duplicates assess random variability, spikes measure how close the measurement system's recorded values are to the true, known radon concentration. Spikes detect systematic calibration drift, detector sensitivity loss, or systematic lab calculation errors.

Execution Process

The RMP submits detectors to an NRPP-approved radon calibration chamber. Chamber operators expose the detectors to a known radon concentration (typically between 5.0 and 25.0 pCi/L) under controlled temperature and relative humidity. The exposed detectors are returned to the tester labeled in a manner indistinguishable from routine field samples (blind spikes) and submitted for standard laboratory analysis.

Evaluation Metric: Relative Percentage Error (RPE)

Spike accuracy is evaluated using Relative Percentage Error (RPE): $\text{RPE} = \frac{\text{Measured Concentration} - \text{Chamber Target Value}}{\text{Chamber Target Value}} \times 100\%$ Alternatively, Percent Recovery may be calculated: $\text{Percent Recovery} = \frac{\text{Measured Concentration}}{\text{Chamber Target Value}} \times 100\%$ Under MS-QA-2023, an individual spike result with an RPE outside 0 ± 20% has exceeded the warning limit, and an RPE outside 0 ± 30% has exceeded the control (action) limit. The cumulative annual mean bias should remain near zero (the default limits assume an in-control standard deviation of 10%).

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The Quality Control Trio Comparison Table