Calibration of FTIR Spectrophotometer

1. Introduction

The Fourier Transform Infrared (FTIR) spectrophotometer is used to identify and characterize materials by measuring their infrared absorption spectra. Calibration ensures the instrument provides accurate wavelength and absorbance readings in compliance with pharmacopoeial and regulatory requirements.

2. Objective

• To verify wavelength accuracy, resolution, and photometric accuracy of the FTIR instrument.

• To ensure consistent, reliable spectra for qualitative and quantitative analysis.

3. Principle

Calibration is performed by scanning standard reference materials with known, well-defined IR absorption peaks (such as polystyrene film) and comparing measured values to certified values.

• Wavelength accuracy: Checks if peak positions match reference values.

• Resolution: Checks instrument’s ability to resolve closely spaced peaks.

• Photometric accuracy: Checks absorbance response.

4. Equipment & Materials

• FTIR spectrophotometer (with calibrated temperature and humidity control if required)

• Certified polystyrene film (as per USP / NIST standards)

• Desiccant (to avoid moisture interference)

5. Procedure (Example per USP <857>)

1. Warm-up the FTIR as per manufacturer’s instructions.

2. Background Scan: Record baseline with no sample in the beam path.

3. Wavelength Accuracy:

   • Scan polystyrene film.

   • Identify characteristic peaks (e.g., 1601.4 cm⁻¹, 1583.0 cm⁻¹, 1154.5 cm⁻¹).

   • Compare to certified values; deviation should be within ±1 cm⁻¹.

4. Resolution:

   • Measure the doublet at ~1583 cm⁻¹ and 1601 cm⁻¹.

   • Ensure resolution ratio meets pharmacopoeial criteria.

5. Photometric Accuracy (optional for qualitative use, required for quantitative use):

   • Use certified absorbance standards and verify within acceptable % deviation.

6. Record Results: Document readings, deviations, and pass/fail status.

6. Acceptance Criteria (Example)

• Wavelength deviation: ±1 cm⁻¹ (per USP/Ph. Eur.)

• Resolution: ≥ specified value (e.g., ≥1.5)

• Photometric accuracy: Within ±5% of certified value

7. Frequency

• Before first use

• At least annually

• After any major maintenance or suspected performance issue

8. References

• USP <857>: Ultraviolet-Visible and Infrared Spectroscopy

• Ph. Eur. 2.2.24: Absorption Spectrophotometry, Infrared

• ICH Q2 (R1): Validation of Analytical Procedures