11.1 Laboratory SOPs, QA/QC & Defensible Records

11.1 Laboratory SOPs, QA/QC & Defensible Records

2025 WPI alignment: This section teaches following laboratory standard operating procedures in Laboratory Analysis, the 15-question area containing 3 recall, 12 application, and 4 calculation items.

Why this laboratory task matters

A laboratory SOP translates an approved method into controlled steps for a particular facility. It defines scope, responsibilities, safety, equipment, reagents, collection, preservation, calibration, quality control, calculations, acceptance criteria, records, and corrective action.

Analytical foundation

Laboratory workflow

1. Confirm the current SOP revision, method, analyte, matrix, range, reporting limit, and permit purpose.
2. Inspect instrument status, calibration, standards/reagents, glassware, temperatures, balances, and maintenance records.
3. Create the batch and QC plan before analysis so required controls are not added selectively after results appear.
4. Perform steps in sequence and record raw observations, times, volumes, weights, instrument readings, and deviations contemporaneously.
5. Evaluate every calibration and QC acceptance criterion before releasing sample results.
6. Complete calculation/review, preserve raw data, apply qualifiers, notify failures, and document corrective action.

Quality and diagnostic evidence

Calculation and interpretation

Transcribe raw data once and preserve sufficient significant figures for the method. Use dilution factors only when an aliquot was actually diluted, and apply blank, seed, tare, or moisture corrections exactly as the method directs. Independent review should recreate the result from raw values. A correct arithmetic total does not cure failed calibration or missing sample identity.

Worked laboratory scenario

An analyst notices after a nutrient batch that the continuing calibration check is outside acceptance. Reporting the samples because their values look typical would ignore evidence of drift. The analyst stops release, documents the failure, recalibrates, identifies samples since the last acceptable check, and reanalyzes or qualifies them according to the SOP.

Common exam traps

• An SOP cannot authorize a method the permit does not allow.
• Typical-looking data can still be invalid when QC fails.
• Never backdate calibration or sample information.
• A duplicate and a matrix spike evaluate different data-quality characteristics.

Field-to-exam checklist

• Use the current controlled SOP and underlying approved method.
• Plan and evaluate QC with every applicable batch.
• Preserve raw data, traceability, revisions, and corrective actions.
• Do not report results until calibration and QC decisions are complete.

Independent review questions

A reviewer should be able to trace a reported value backward from the report to calculation, instrument output, preparation, sample receipt, custody, and field collection. Then trace forward from each failed or qualified QC item to every affected sample and report. This bidirectional check catches copied dilution factors, transposed IDs, expired standards, and results released before review. Electronic systems help only when access, audit trails, calculations, and revision controls are themselves validated and monitored.

Define the affected-data window

When a blank, continuing check, duplicate, or spike fails, identify which samples share the affected preparation, calibration, reagent, instrument period, or analyst step. Do not invalidate an arbitrary whole month, but do not release only the favorable results either. Follow the method and SOP for troubleshooting, recalibration, reanalysis, qualification, or recollection. Preserve original observations and corrective-action records. The review should show why the final reported data remain valid and where the failure’s influence began and ended.

Method authority, holding times, and data-quality indicators

For NPDES compliance the approved method list is 40 CFR Part 136. A plant cannot substitute a faster or cheaper field kit for a reportable parameter without going through an approved alternate-test-procedure route. That is the boundary between process-control testing, where any useful method is legitimate, and compliance testing, where the method is prescribed.

Holding times and preservation an operator should recognize:

The pattern is worth internalizing: the more biologically active the measurement, the shorter the clock. Refrigerating a late bacteriological sample does not restore its validity.

Detection terminology is not interchangeable. The method detection limit is a statistical property of the method; the reporting limit is the lowest value the laboratory will report as a number. A result below the reporting limit is reported with the qualifier the permit requires, not as a bare zero, because zero asserts absence the method cannot demonstrate.

Precision and accuracy fail independently. Precision is evaluated from duplicates as relative percent difference; accuracy is evaluated from spike or laboratory-control-sample percent recovery. A batch can be beautifully precise and consistently biased.

Worked precision check. Duplicate results of 42 and 46 mg/L give RPD = |42 − 46| / ((42 + 46) / 2) x 100 = 4 / 44 x 100 = 9.1 percent. Whether that passes depends on the method's stated acceptance criterion, which is a documented limit rather than an analyst's judgement.