The Balance That Drifted
A laboratory assistant, Marcus, is preparing a series of standard solutions for a calibration curve. He weighs 2.500 g of a primary standard on an analytical balance. The balance displays 2.500 g, but Marcus notices the balance was last calibrated eight months ago; the laboratory's procedure requires calibration every six months. The balance sits near an open window, and a fan is running. A colleague, Lena, weighed the same standard an hour earlier and recorded 2.497 g. Marcus needs to prepare five standards and has a 90-minute window before the solutions are needed for a class. He decides to proceed and record the masses as displayed.
1. Evaluate Marcus's decision to proceed. What quality-control issues are present?
Read the worked answer
Issues present:
- Overdue calibration - eight months exceeds the six-month requirement. The balance's accuracy is not verified; the displayed value may be biased.
- Environmental interference - an open window and a running fan create air currents and temperature fluctuation, both of which affect analytical balance stability and readings.
- Discrepancy with Lena's result - 2.500 g vs 2.497 g is a 3 mg difference, which for a 2.5 g sample is 0.12%. For a calibration curve this may matter, depending on the required tolerance. It is a signal worth investigating, not ignoring.
- Proceeding without investigation - Marcus is prioritising the 90-minute deadline over data integrity.
Judgement: Proceeding and recording the displayed masses without qualification is not defensible. The data may be used to calculate concentrations that feed into student results, and an unverified balance undermines the whole calibration curve.
2. Describe the quality-control actions Marcus should take, in order of priority.
Read the worked answer
Priority actions:
- Stop and control the environment - close the window, switch off the fan, and allow the balance to settle. Check the balance is level and on a stable bench.
- Verify the balance - perform a calibration check with certified check weights across the working range. If it passes, record the check. If it fails, the balance must be taken out of service, labelled, and serviced/calibrated before use.
- Investigate the discrepancy - reweigh the standard and compare with Lena's value. Consider causes: drafts, static, temperature, technique (e.g. not using weigh boats consistently), or genuine balance drift.
- Document - record the calibration check, the environmental conditions, and any deviation in the equipment log and the preparation record.
- Decide on the deadline - if the balance cannot be verified in time, Marcus should inform the teacher that the standards cannot be certified, and either use a verified alternative balance or reschedule. Releasing unverified standards is worse than a delay.
3. How should Marcus record this situation, and why does documentation matter here?
Read the worked answer
Recording:
- Equipment log: date, balance ID, calibration status, the fact calibration was overdue, the check-weight results, and any corrective action.
- Preparation record: the masses actually used, the balance ID, the date, the operator's name, and a note that the balance's calibration status was being verified at the time.
- Deviation/non-conformance note: what happened, what was done, who was informed, and the outcome.
Why it matters:
- Traceability - if student results later look anomalous, the record shows whether the balance was a contributing factor.
- Accountability - it demonstrates that the issue was identified and acted on, rather than hidden.
- Continuous improvement - repeated overdue calibrations point to a scheduling failure in the laboratory's maintenance system.
- Defensibility - in an audit or incident review, undocumented work cannot be shown to have been done correctly. An honest record of a deviation is far stronger than a clean-looking record that omits it.