Silicone Quantitation from Siliconized Rubber Stoppers by Using FTIR

Significance of the Topic

Silicone coatings on rubber stoppers play a crucial role in pharmaceutical packaging by ensuring smooth plunger movement in prefilled syringes, maintaining container closure integrity, and minimizing extractables. Accurate quantification of silicone levels is essential for quality control, regulatory compliance, and the assessment of extractable and leachable substances in drug delivery systems.

Study Objectives and Overview

This work aims to develop a rapid, reliable Fourier Transform Infrared (FTIR) method for quantifying polydimethylsiloxane (PDMS) on siliconized rubber stoppers. The study includes calibration with USP reference standards, validation of extraction efficiency, and assessment of method accuracy through recovery experiments.

Methodology and Instrumentation

Sample Preparation:

• Siliconized and non-siliconized rubber stoppers (50 units each) were placed in a beaker with hexane, agitated, and evaporated at 90 °C.
• The residue was redissolved in hexane, centrifuged, and the supernatant collected for FTIR analysis.

Standard Preparation:

• PDMS reference standard at 5000 ppm was diluted to obtain calibration points (250–2500 ppm).

Instrumentation:

• FTIR Spectrophotometer: IRSpirit-ZX
• Measurement Cell: 0.1 mm NaCl fixed-thickness cell
• Resolution: 4 cm⁻¹, Scans: 45, Wavelength Range: 1250–1270 cm⁻¹
• Detector: DLATGS, Apodization: Happ-Genzel

Main Results and Discussion

A linear calibration curve (r = 0.999) was obtained using peak height at 1259 cm⁻¹. Quantitation results showed an average silicone content of 284.2 µg per siliconized stopper, while non-siliconized stoppers fell below the lowest calibration level. Recovery tests at 1000 ppm PDMS yielded 82.1 % for siliconized and 90.7 % for non-siliconized samples, demonstrating acceptable accuracy and precision.

Benefits and Practical Applications

• The method offers a straightforward extraction protocol compatible with routine laboratory workflows.
• High repeatability and robustness make it suitable for quality control of pharmaceutical packaging components.
• Adaptable to studies on extractables and leachables in medical devices such as prefilled syringes and tubing.

Future Trends and Potential Applications

Advancements may include automation of sample preparation, expansion to other silicone types or more complex matrices, and integration with chemometric tools for multicomponent analysis. The approach can be extended to real-time monitoring of siliconization processes and regulatory compliance studies.

Conclusion

This FTIR-based method using hexane extraction and IRSpirit-ZX instrumentation provides a fast, accurate, and reproducible tool for quantifying silicone on rubber stoppers. The demonstrated calibration linearity and recovery performance support its routine application in pharmaceutical quality assurance.

References

1. USP Monograph of Dimethicone