Validation of Terahertz coating thickness measurements using X-ray microtomography

Area of Science:

• Pharmaceutical technology
• Non-destructive testing
• Metrology

Background:

• Terahertz pulsed imaging (TPI) is a non-destructive, non-contact method for measuring pharmaceutical tablet coating thickness.
• TPI offers high precision for relative coating thickness measurements without calibration.
• Accurate absolute coating thickness determination with TPI requires knowledge of the coating's refractive index (n).

Purpose of the Study:

• To develop and validate a method for independently quantifying pharmaceutical coating thickness.
• To assess the variability of the refractive index (n) in pharmaceutical tablet coatings.
• To establish the reliability of TPI for absolute coating thickness measurements.

Main Methods:

• Development of an X-ray microtomography technique for independent coating thickness quantification.
• Validation of TPI coating thickness measurements against X-ray microtomography data.
• Analysis of inter- and intra-tablet variation in refractive index (n) for pharmaceutical coatings.

Main Results:

• X-ray microtomography successfully validated TPI measurements for coating thickness.
• Inter-tablet variation in refractive index (n) was found to be less than 4% for coatings 25-270 μm thick.
• Intra-tablet variation in refractive index (n) was less than 3%.
• Estimated refractive index (n) for typical pharmaceutical coatings ranges from 1.45 to 2.01.

Conclusions:

• Terahertz pulsed imaging (TPI) is a robust and precise technique for measuring pharmaceutical tablet coating thickness.
• The low variation in refractive index (n) supports the accuracy of TPI for absolute thickness determination.
• TPI's simple principle makes it ideal for in-process control and quality monitoring in pharmaceutical manufacturing.