4.2 Field Quality Control: Blank Preparation, Field Duplicates, and Acceptance Criteria

4.2 Field Quality Control: Blank Preparation, Field Duplicates, and Acceptance Criteria

Data quality objective (DQO) frameworks established by the EPA mandate that field risk assessors collect Quality Control (QC) samples alongside primary environmental samples. Field QC samples provide empirical evidence regarding the integrity of sampling tools, field handling practices, cross-contamination risks, and spatial sampling variability.

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Types of Field Quality Control Samples

1. Field Blanks

Field blanks evaluate potential contamination introduced during sample collection, handling, storage, or transit from the field to the analytical facility.

• Dust Wipe Field Blanks: Prepared by taking an unused, factory-sealed wipe from the same manufacturing lot used for sampling. The assessor opens the wipe container at the job site, unfolds and refolds the wipe using gloved hands exactly as done during actual sample collection, but without touching any environmental surface. The wipe is immediately placed into an identical rigid transport tube.
• Soil Equipment / Rinsate Blanks: Prepared when non-disposable sampling equipment (such as soil augers or stainless steel trowels) is decontaminated between sample locations. Deionized water is poured over the cleaned equipment and captured in a sample bottle to verify that decontamination procedures successfully eliminated carryover contamination.
• Air Monitoring Blanks: Unexposed filter cassettes opened and resealed immediately at the monitoring location without drawing air through the filter.

2. Field Duplicates

Field duplicates measure the combined precision of field sampling techniques and sample matrix spatial heterogeneity.

• Dust Wipe Side-by-Side Duplicates: Collected from an area immediately adjacent to the primary dust sample. For example, on a smooth uncarpeted floor, two identical 1 ft x 1 ft (144 sq in) templates are placed side-by-side. The primary sample is wiped in template A, and the duplicate is wiped in template B.
• Soil Split Duplicates: Collected by thoroughly mixing/homogenizing a soil sample in a clean stainless steel bowl before splitting the volume equally into two separate sample containers.

3. Blind Matrix Spikes / Performance Evaluation Samples

Spiked samples consist of clean media (blank wipes or filters) spiked with a precise, known quantity of lead (e.g., 50.0 µg Pb) prepared by an independent quality assurance source. These samples are submitted "blindly" to the laboratory (labeled with routine sample IDs) to audit laboratory analytical accuracy and matrix recovery.

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Required Sampling Frequencies

EPA guidelines and HUD Chapter 7 protocols establish explicit minimum frequencies for field QC sample submission:

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Quality Control Evaluation Metrics & Acceptance Criteria

1. Field Blank Acceptance Thresholds

Field blanks measure background contamination.

• Acceptance Criterion: The lead content of a dust wipe field blank must be below the Laboratory Reporting Limit (< RL) or < 5.0 µg/wipe (whichever is lower).
• Corrective Action if Blank Fails: If a field blank exhibits lead levels >= RL (e.g., 8.0 µg/wipe), all primary samples associated with that sampling batch or site session are suspect. The risk assessor must investigate the contamination source (e.g., contaminated wipe lot, dirty gloves, ambient air contamination) and typically must resample the affected area.

2. Relative Percent Difference (RPD) for Duplicates

Field duplicate precision is evaluated quantitatively using the Relative Percent Difference (RPD) equation:

RPD = (|S - D| / ((S + D) / 2)) * 100%

Where:

• S = Primary sample result (e.g., µg/ft² or ppm)
• D = Duplicate sample result (e.g., µg/ft² or ppm)

Worked Example:

Suppose a primary floor dust wipe yields S = 24.0 µg/ft² and its side-by-side duplicate yields D = 18.0 µg/ft²:

1. Difference |S - D| = |24.0 - 18.0| = 6.0 µg/ft²
2. Average (S + D) / 2 = (24.0 + 18.0) / 2 = 21.0 µg/ft²
3. RPD = (6.0 / 21.0) * 100% = 28.57%

• Acceptance Criteria:
  • Dust Wipe Side-by-Side Duplicates: RPD <= 25% (due to spatial micro-variance on building surfaces).
  • Homogenized Soil Split Duplicates: RPD <= 20% (homogenized material should show higher agreement).
• Action on RPD Exceedance: An RPD > 25% indicates high field variability or non-uniform lead distribution. Data must be qualified with an estimated ("J") flag, and risk assessment decisions near regulatory thresholds should be interpreted with caution.

3. Percent Recovery for Spiked Samples

Matrix spike accuracy is evaluated using the Percent Recovery (%R) equation:

Recovery % = (Measured Value / Known Spike Value) * 100%

• Acceptance Criteria: Spike recoveries must fall within 80% to 120%. Recoveries outside this window indicate analytical bias or matrix interference.