Standard Operating Procedure for Calibration of pH and DO Sensors in Bioreactor Operations for Biosimilars

Department	Biosimilars
SOP No.	SOP/BS/095/2025
Supersedes	SOP/BS/095/2022
Page No.	Page 1 of 11
Issue Date	04/05/2025
Effective Date	06/05/2025
Review Date	04/05/2026

1. Purpose

To outline the standardized procedure for calibration of pH and dissolved oxygen (DO) sensors used in bioreactor operations for biosimilar production. This ensures sensors function accurately to support optimal cell culture conditions.

2. Scope

This SOP applies to all in-situ and ex-situ pH and DO sensors used in small-scale and large-scale bioreactors in the biosimilars department.

3. Responsibilities

• Engineering: Perform sensor calibration, maintain calibration records, and ensure periodic servicing.
• Production: Verify calibrated status of sensors before every batch run.
• QA: Review calibration records and ensure SOP compliance during audits.

4. Accountability

The Engineering Manager is accountable for ensuring the calibration of pH and DO sensors is performed on schedule and meets compliance with regulatory standards.

5. Procedure

5.1 General Guidelines

1. Calibrate all sensors before batch initiation and post-cleaning (CIP/SIP cycles).
2. Use only traceable, certified calibration solutions and gases.
3. Label sensors with calibration date, due date, and initials.

5.2 pH Sensor Calibration

1. Remove sensor from bioreactor if offline or perform in-situ if reactor allows.
2. Rinse sensor with distilled water and blot dry with lint-free tissue.
3. Immerse sensor in pH 7.00 buffer solution; allow to stabilize and set as first calibration point.
4. Rinse and then immerse in pH 4.01 or 10.01 buffer as the second calibration point (as per product requirement).
5. Verify slope (acceptable range: 95%–105%) and offset (±30 mV).
6. Document values in Annexure-1: pH Calibration Log.

5.3 DO Sensor Calibration

1. Perform a 2-point calibration:
2. Zero Calibration:
   • Immerse sensor in nitrogen-sparged water or sodium sulfite solution.
   • Set the output as 0% DO.
3. Span Calibration:
   • Expose sensor to air-saturated WFI or sparge with pure air.
   • Set output as 100% DO.
4. Stabilize readings before accepting each point. Acceptable sensor response time: ≤90 seconds.
5. Record calibration values in Annexure-2: DO Calibration Log.

5.4 Post-Calibration Checks

1. Verify calibration by checking pH and DO response in standard solutions or bioreactor media.
2. Recalibrate if deviation exceeds acceptable limits.

6. Abbreviations

• DO: Dissolved Oxygen
• CIP: Clean-In-Place
• SIP: Sterilize-In-Place
• QA: Quality Assurance

7. Documents

1. pH Sensor Calibration Log – Annexure-1
2. DO Sensor Calibration Log – Annexure-2

8. References

• USP <1058> – Analytical Instrument Qualification
• ICH Q10 – Pharmaceutical Quality System

9. SOP Version

Version: 2.0

10. Approval Section

	Prepared By	Checked By	Approved By
Signature
Date
Name
Designation
Department

11. Annexures

Annexure-1: pH Sensor Calibration Log

Date	Sensor ID	pH Buffers Used	Offset (mV)	Slope (%)	Calibrated By
04/05/2025	PH-BIO-21	4.01 & 7.00	5.2	98.6	Sunita Reddy

Annexure-2: DO Sensor Calibration Log

Date	Sensor ID	Zero Method	Span Method	Response Time (sec)	Calibrated By
04/05/2025	DO-BIO-09	Na₂SO₃	Air-Saturated WFI	82	Rajesh Kumar

Revision History:

Revision Date	Revision No.	Revision Details	Reason for Revision	Approved By
04/05/2025	2.0	Added post-calibration verification and response time criteria	GMP update