Differentiation of polymorphs using near infrared spectroscopy (NIRS)

Significance of the topic

Polymorph differentiation of pharmaceutical compounds is essential for ensuring drug efficacy and stability. Sulfathiazole polymorph I is the least stable form of an important sulfonamide antibiotic. Rapid and accurate monitoring of crystallization and final product form is vital to meet regulatory and quality control requirements. Near infrared spectroscopy provides a faster non destructive alternative to conventional techniques such as X ray diffraction.

Objectives and study overview

The study aimed to demonstrate that near infrared spectroscopy can reliably distinguish commercial sulfathiazole from its less stable polymorph form I by analyzing the first overtone region of N H stretching vibrations. The work included library development based on wavelength correlation and validation by internal cross validation.

Methodology

Samples of commercial sulfathiazole and synthesized polymorph form I were placed in glass vials and measured in stationary diffuse reflectance mode. Spectra were acquired with 32 co added scans. Data preprocessing using Standard Normal Variate was applied to reduce scatter effects. The spectral window 1400 1720 nm, corresponding to the N H first overtones, was selected to highlight polymorphic differences.

Instrumentation used

• NIRS DS2500 analyzer in diffuse reflectance mode
• Mini sample cup holder
• DS2500 Iris for sample positioning
• NIRS XDS SmartProbe with ReflectionProbe
• Vision Pharma 4.1 chemometric software

Main results and discussion

Distinct absorbance differences in the 1400 1720 nm region enabled clear differentiation of form I from the commercial form. A correlation in wavelength space algorithm with a 0.9 threshold yielded a robust calibration library. Internal cross validation confirmed method reliability. NIR allowed faster analysis and potential in situ monitoring compared to standard X ray diffraction.

Benefits and practical applications

• Rapid non destructive polymorph identification
• Real time process monitoring during crystallization
• High throughput quality control of raw and final materials
• Reduced analysis time versus conventional laboratory methods

Future trends and potential applications

Integration of near infrared probes into automated process analytical technology platforms can enable continuous monitoring of polymorphic transitions. Advances in fiber optic sampling and multivariate modeling may broaden applications to other antibiotics and high potency compounds, improving selectivity and sensitivity.

Conclusion

Near infrared spectroscopy provides a fast, reliable, and non destructive method to distinguish sulfathiazole polymorphs form I and the commercial form. The approach supports quality assurance and regulatory compliance in pharmaceutical manufacturing.

References

• Metrohm Application Note NIR 37 Differentiation of polymorphs using near infrared spectroscopy