SOP: Pipette Calibration Procedure

Pipettes deliver the volumes that titrations and dilutions depend on, yet they drift with use, drops, and worn seals. Gravimetric calibration with water turns “it feels fine” into a verified number.

General laboratory procedures form the foundation every analytical result stands on. Balances, pipettes, glassware, and waste handling rarely make headlines, but errors here propagate silently into every test the lab performs. Treating these routines with the same rigor as the analyses themselves is what separates reliable laboratories from the rest.

Purpose

To verify micropipette accuracy and precision gravimetrically on a scheduled basis.

Scope

Applies to all adjustable air-displacement micropipettes used in the laboratory.

Equipment and Materials

• Analytical balance, 0.1 mg
• Deionized water equilibrated to room temperature
• Thermometer
• Weighing vessel with lid (to limit evaporation)
• Calibration log sheets
• Water density tables

Procedure

1. Equilibrate water, pipette, and room to the same temperature. Record the water temperature.
2. Select three test volumes: 10 percent, 50 percent, and 100 percent of nominal.
3. Fit a fresh tip, pre-wet it 3 times with water, then tare the weighing vessel.
4. Deliver the test volume into the vessel using consistent technique: smooth plunger motion, vertical pipette, touch-off on the vessel wall.
5. Weigh immediately and record. Repeat 10 times per volume.
6. Convert masses to volumes using the water density at the recorded temperature.
7. Calculate mean inaccuracy (percent of nominal) and imprecision (coefficient of variation).
8. Acceptance per manufacturer or ISO 8655 limits (for example plus or minus 1.0 percent inaccuracy and 0.5 percent CV at 100 percent volume for a 1000 uL pipette). Failures go out of service for adjustment or repair.
9. Calibrate every 6 months, or quarterly for high-use pipettes.

Recording Results

Log per pipette: ID, date, volumes tested, mean inaccuracy and CV, pass or fail, and next due date. Tag calibrated pipettes with the due date.

Quality Control

Document every activity in real time in bound logbooks or the validated electronic system; never backdate entries. Review logbooks weekly for completeness and sign off. Train all personnel on this procedure before independent work and reassess competence annually. Track calibration and maintenance due dates on a master schedule with alerts. Audit compliance with this SOP during internal audits and record findings with corrective actions. Retain all records per the document retention policy.

Troubleshooting

• High imprecision: inconsistent technique or worn tip seal. Retrain technique; replace seals.
• Systematic low volume: leaking seal or incorrect pipette for the viscosity. Service the pipette; use positive displacement for viscous liquids.
• Evaporation losses: work quickly with lidded vessels, especially at small volumes.
• Missed calibration due dates: implement automated reminders 2 weeks ahead; tag equipment with visible due-date labels.
• Incomplete logbook entries: review logs at shift handover; the next user should be able to reconstruct the work.

Interferences and Limitations

These procedures control the laboratory environment, not the analysis itself; their value depends on consistent execution by trained personnel. Human error remains the largest source of variation: rushing, skipping verification steps, or working while fatigued undermines the entire quality system. Equipment drifts between calibrations, so the scheduled verification frequencies are minimums, not targets. Review and update this procedure whenever equipment changes or audits reveal gaps.

Safety Notes

• No significant hazards. Never mouth-pipette.