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newSchoolWorks.com › VoTechWorks › Agriculture, Environment & Public Service › Water / Wastewater Operator › Sampling & Laboratory Procedures for Water and Wastewater Operations › Lesson 9
AGR-WATE71-05 · Lesson 9 · 13 clock hours · theory

Quality Assurance and Quality Control in the Environmental Lab

Objective. Students will define QA/QC terms (blank, duplicate, spike, matrix spike, method detection limit), calculate percent recovery and relative percent difference, and determine whether a QC result is within acceptable control limits using a control chart.

⚠ Pending professional review

This lesson, its worked examples and its practice questions has not yet been verified by a licensed professional in the field or by a certification or approval body. It is a study draft and may contain errors. Do not use it as instructions for work on a job site or on live equipment. Always follow your instructor, your supervising professional, and current official references and regulations.

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Why QA/QC Matters in Environmental Labs

Every water and wastewater sample you collect tells a story about public health and environmental compliance. Quality Assurance (QA) is the overall management system that ensures your laboratory produces defensible, accurate data. Quality Control (QC) is the set of specific technical practices—the checks and measurements—that prove each analytical run met its accuracy and precision targets. Without both, a reported result is just a number with no proof behind it. Regulators, engineers, and plant operators make real decisions from your data, so errors have consequences: permit violations, public health risks, or costly re-testing.

Core QA/QC Terms

Method Blank (Reagent Blank): A sample of analyte-free water carried through the entire analytical procedure alongside real samples. It checks whether your reagents, glassware, or environment are contributing contamination. A blank result that exceeds the method detection limit signals a problem that must be investigated before sample results can be reported.

Field Blank: Analyte-free water opened and exposed at the sampling site, then processed like a real sample. It checks for contamination introduced during collection and transport.

Duplicate (Field or Lab Duplicate): A second portion of the same sample analyzed independently. Duplicates measure precision—how repeatable the measurement is. The statistic used is Relative Percent Difference (RPD).

Matrix Spike (MS): A sample to which a known, measured amount of target analyte is added before analysis. The spike tests whether the sample's physical or chemical characteristics are interfering with the method. The statistic used is Percent Recovery (%R).

Matrix Spike Duplicate (MSD): A second aliquot spiked with the same amount as the MS, analyzed independently. The RPD between MS and MSD measures precision in the spiked matrix.

Method Detection Limit (MDL): The lowest concentration of an analyte that can be measured and reported with a stated level of confidence that the concentration is greater than zero. The MDL is determined experimentally using replicate low-level standard analyses. Results reported below the MDL are flagged as estimated or non-detect.

Control Chart: A time-series graph that displays successive QC results (such as a control standard) against a center line (the mean) and upper and lower control limits (UCL/LCL, typically set at ±3 standard deviations) and warning limits (±2 standard deviations). The chart gives a visual signal when a process drifts out of statistical control.

Calculating Percent Recovery

Percent recovery tells you how much of the spiked analyte the method actually found:

%R = [(Spiked Sample Result − Unspiked Sample Result) ÷ Known Spike Amount] × 100

Most methods require %R to fall between 80% and 120% (some methods allow 70–130%). Results outside the accepted window indicate matrix interference, a preparation error, or instrument problems.

Calculating Relative Percent Difference

RPD measures the spread between two duplicate measurements:

RPD = [|Result 1 − Result 2| ÷ ((Result 1 + Result 2) ÷ 2)] × 100

A lower RPD means better precision. Many methods require RPD ≤ 20% for environmental samples; check your specific method for the required limit.

Reading and Using a Control Chart

Plot each QC result on the chart as soon as it is available. Apply these standard rules to identify out-of-control conditions:

When a run is out of control, do not report sample results until the problem is resolved.

Corrective Action and Documentation

When a QC failure occurs, the lab must follow a documented corrective action process:

1. Identify which check failed and which samples are affected.

2. Investigate the likely cause (contaminated reagent, instrument drift, calculation error, cross-contamination).

3. Act: re-analyze affected samples, recalibrate or service the instrument, replace reagents, or review the method.

4. Document: record the problem, investigation findings, corrective actions taken, and the outcome in a corrective action log. All documentation must be legible, dated, and signed.

Safety in the QA/QC Lab

Many QC standards and reagents contain hazardous chemicals. Always consult the Safety Data Sheet (SDS) before using any chemical. Wear appropriate PPE—at minimum, safety glasses, chemical-resistant gloves, and a lab coat. Work in a fume hood when handling volatile or corrosive reagents. Never pipette by mouth. Dispose of chemical waste according to your facility's hazardous waste plan and applicable regulations. When handling wastewater samples, treat all material as potentially infectious: wear gloves and wash hands thoroughly after any contact.

Key terms10

Quality Assurance (QA)The overall management system of policies and procedures designed to ensure that laboratory data are accurate, defensible, and fit for their intended use.
Quality Control (QC)The specific technical measurements and checks—blanks, duplicates, spikes, control standards—performed during each analytical run to verify accuracy and precision.
Method BlankAnalyte-free water carried through the full analytical procedure to check for laboratory contamination from reagents, glassware, or the environment.
Matrix Spike (MS)A sample aliquot to which a known amount of target analyte is added before analysis, used to calculate percent recovery and detect matrix interference.
Percent Recovery (%R)The ratio of the measured spike concentration to the known spike concentration, expressed as a percentage; indicates method accuracy in a given sample matrix.
Relative Percent Difference (RPD)A measure of precision calculated as the absolute difference between two duplicate results divided by their average, multiplied by 100.
Method Detection Limit (MDL)The lowest analyte concentration that can be measured and reported with statistical confidence that it is above zero, determined experimentally by the laboratory.
Control ChartA time-series graph of successive QC results plotted against a center line (mean) and control/warning limits to detect trends, bias, or out-of-control conditions.
Corrective ActionThe documented process of identifying a QC failure, investigating its cause, taking corrective steps (re-analysis, reagent replacement, instrument service), and recording the outcome.
Upper/Lower Control Limits (UCL/LCL)The boundaries on a control chart, typically set at ±3 standard deviations from the mean, beyond which a QC result is considered out of statistical control.

Worked examples4

1. A matrix spike is prepared by adding a known spike of 2.00 mg/L of ammonia to a wastewater sample. The unspiked sample result is 3.40 mg/L and the spiked sample result is 5.28 mg/L. Calculate the percent recovery.
  1. Step 1 – Identify the values: Spiked result = 5.28 mg/L; Unspiked result = 3.40 mg/L; Known spike added = 2.00 mg/L.
  2. Step 2 – Apply the formula: %R = [(Spiked − Unspiked) ÷ Known Spike] × 100.
  3. Step 3 – Subtract: 5.28 − 3.40 = 1.88 mg/L (amount of spike measured).
  4. Step 4 – Divide: 1.88 ÷ 2.00 = 0.94.
  5. Step 5 – Multiply by 100: 0.94 × 100 = 94%.
  6. Step 6 – Evaluate: 94% falls within the typical 80–120% acceptance window, so the result is acceptable.
Answer: Percent recovery = 94%; the QC check passes.
2. Two turbidity measurements are taken from duplicate field samples. Result 1 = 18.4 NTU; Result 2 = 21.2 NTU. Calculate the RPD and determine whether precision is acceptable if the control limit is RPD ≤ 20%.
  1. Step 1 – Identify the values: Result 1 = 18.4 NTU; Result 2 = 21.2 NTU.
  2. Step 2 – Calculate the absolute difference: |18.4 − 21.2| = 2.8 NTU.
  3. Step 3 – Calculate the average: (18.4 + 21.2) ÷ 2 = 39.6 ÷ 2 = 19.8 NTU.
  4. Step 4 – Apply the RPD formula: RPD = (2.8 ÷ 19.8) × 100.
  5. Step 5 – Divide: 2.8 ÷ 19.8 = 0.1414.
  6. Step 6 – Multiply by 100: 0.1414 × 100 = 14.1%.
  7. Step 7 – Evaluate: 14.1% is less than the 20% limit, so precision is acceptable.
Answer: RPD = 14.1%; the duplicate result meets the ≤ 20% precision requirement.
3. A method blank analyzed for chlorine shows a result of 0.08 mg/L. The MDL for the method is 0.05 mg/L. Determine whether the blank indicates contamination and what action is required.
  1. Step 1 – State the rule: A method blank result that exceeds the MDL indicates the analytical system may be contaminated and the blank has 'failed.'
  2. Step 2 – Compare: Blank result (0.08 mg/L) > MDL (0.05 mg/L).
  3. Step 3 – Conclusion: Because the blank result exceeds the MDL, contamination is indicated.
  4. Step 4 – Required action: Do not report sample results from this run. Investigate the source of contamination (reagents, glassware, environment), correct the problem, and re-analyze the blank and associated samples.
Answer: The blank result (0.08 mg/L) exceeds the MDL (0.05 mg/L); contamination is indicated. Sample results cannot be reported until the problem is corrected and a clean blank is confirmed.
4. A laboratory control standard (LCS) is analyzed three days in a row. Results are 98%, 104%, and 87% recovery. The control chart has a center line at 100%, UCL at 120%, LCL at 80%, and warning limits at 115% and 85%. Plot each result on the chart and determine the status of the analytical process.
  1. Step 1 – Plot Day 1 result: 98% — between warning limits (85%–115%) and between control limits (80%–120%). In control.
  2. Step 2 – Plot Day 2 result: 104% — between warning limits and control limits. In control.
  3. Step 3 – Plot Day 3 result: 87% — this falls between the LCL (80%) and the lower warning limit (85%). It is inside the control limits but outside the lower warning limit.
  4. Step 4 – Apply control chart rules: One point outside a warning limit does not alone trigger an out-of-control action, but it is flagged as a warning. Examine whether this is part of a downward trend.
  5. Step 5 – Look for trend: Day 1 = 98%, Day 2 = 104%, Day 3 = 87%. The drop to 87% is a single warning-limit exceedance; no seven-in-a-row or two-of-three rule violations exist yet.
  6. Step 6 – Conclusion: The process is currently at a warning level on Day 3. Continue monitoring; investigate possible causes (reagent change, instrument drift). If the next result also falls below the warning limit on the same side, corrective action is required.
Answer: Days 1 and 2 are in control. Day 3 (87%) triggers a warning limit flag but does not cross the LCL. The run is at a warning status; investigation and heightened monitoring are required.

The math in this lesson1

Learners compute spike percent recovery (5.28 − 3.40) ÷ 2.00 = 94%.
7.RP.3
Brush up in the math halls at newSchoolWorks.com:
In class: Lecture and guided notes on QA/QC terminology; students build a personal glossary of at least 12 QA/QC terms with definitions and examples drawn from tests performed in previous lessons. · Work through four QC calculation problems: (1) calculate percent recovery of a matrix spike; (2) calculate RPD between duplicate turbidity measurements; (3) determine whether a blank result indicates contamination; (4) plot three successive QC results on a pre-drawn control chart and identify whether the run is in or out of statistical control. · Case study: students review a printed lab QC summary sheet with two out-of-control results and write a corrective action memo identifying the problem, likely cause, required actions (re-analysis, method review, instrument service), and documentation steps.
Materials: QA/QC terminology handout, calculator, pre-drawn control chart worksheet, QC calculation problem set, case study QC summary sheet (instructor-prepared), Standard Methods QA/QC chapter excerpt, EPA QA/QC guidance for environmental labs reference
Check: All four QC calculations correct (or within rounding); control chart plotted accurately; corrective action memo addresses all required elements; pass = 80% correct on calculation set and complete memo with no omitted required elements.

Practice check8

Answer each question, then check it. Nothing is recorded; this is your own practice.

Sampling and Laboratory Procedures
1. A method blank is used in environmental lab analysis primarily to:
Sampling and Laboratory Procedures
2. A matrix spike of 5.00 mg/L is added to a sample. The unspiked sample reads 2.10 mg/L and the spiked sample reads 6.65 mg/L. What is the percent recovery?
Sampling and Laboratory Procedures
3. Duplicate turbidity readings are 12.6 NTU and 15.4 NTU. What is the RPD?
Sampling and Laboratory Procedures
4. On a control chart with a center line at 100% recovery, warning limits at ±2 SD, and control limits at ±3 SD, which result requires immediate out-of-control action?
Sampling and Laboratory Procedures
5. The Method Detection Limit (MDL) of an analytical method represents:
Sampling and Laboratory Procedures
6. A lab analyst notices a blank result above the MDL for nitrate. The correct first response is to:
Sampling and Laboratory Procedures
7. Which of the following QC samples is specifically designed to detect matrix interference in a wastewater sample?
Sampling and Laboratory Procedures
8. A corrective action memo written after a QC failure MUST include all of the following EXCEPT:
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