Standard Operating Procedure (SOP)

Here’s a sample Standard Operating Procedure (SOP) for the Calibration of a Refractometer used in pharmaceutical applications. It follows general cGMP guidelines and can be adapted to your organization’s specific requirements.

1. Purpose

To describe the procedure for the calibration of the refractometer used for determining the refractive index or Brix value of pharmaceutical solutions.

2. Scope

This SOP is applicable to all refractometers used in the Quality Control Department of [Company Name].

3. Responsibility

• QC Analyst: To perform the calibration as per SOP.

• QA Personnel: To verify and ensure compliance.

• QC Head: To review and approve the calibration records.

4.Definitions

• Refractometer: An instrument that measures the refractive index of substances.

• Brix: A unit representing sugar content in aqueous solutions.

5. Materials and Equipment

• Distilled water

• Certified reference standards (e.g., sucrose solution of known Brix)

• Lint-free tissue

• Calibration logbook or system

• Personal protective equipment (PPE)

6. Procedure

6.1 General Checks

1. Ensure the instrument is clean and free from previous samples.

2. Check the power supply and ensure the device is turned ON.

3. Allow the instrument to stabilize at room temperature (20–25°C).

6.2 Calibration Steps

1. Zero Calibration with Distilled Water:

   • Place a few drops of distilled water on the measuring prism.

   • Close the cover plate (if applicable).

   • Press the calibration or “Zero” button, or manually adjust to 1.3330 (Refractive Index at 20°C).

   • Wipe off the water with a lint-free tissue.

2. Calibration with Standard Solution:

   • Use a certified sucrose solution (e.g., 10% Brix, 20% Brix).

   • Place a few drops on the prism and record the reading.

   • Compare the observed value with the certified value.

   • Acceptable deviation: ±0.0003 RI units or ±0.1 Brix (as per instrument specification).

   • Repeat for additional standards if required.

3. Documentation:

   • Record all readings in the calibration logbook.

   • Note the date, time, analyst name, and standard used.

4. If Calibration Fails:

   • Do not use the instrument.

   • Label it as “Out of Calibration.”

   • Inform QA and initiate investigation.

7. Frequency of Calibration

• Daily (Zero Calibration)

• Weekly/Monthly (Standard Calibration) – as per equipment usage

• After major maintenance/repair

• After relocation or suspected malfunction

8. Acceptance Criteria

• Zero calibration: Must show 1.3330 for distilled water at 20°C.

• Standard solution: Must be within the specified deviation range.

9. Precautions

• Use only clean, dry tissues for cleaning the prism.

• Avoid touching the prism with fingers or metal objects.

• Ensure standard solutions are within expiry and properly stored.

10. Annexures / Attachments

• Annexure I: Calibration Log Sheet

• Annexure II: Instrument Label Template (Calibrated / Out of Calibration).

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