Fourier Transform Infrared Spectroscopy for Rapid Cleaning Verification of Mixing Vessels and Reaction Chambers

Significance of the topic

The validation of cleaning procedures in pharmaceutical and biopharmaceutical manufacturing is critical to ensure product purity, prevent cross‐contamination, and comply with regulatory standards. Rapid, nondestructive techniques that reduce downtime, solvent consumption, and analytical costs are highly desirable. Handheld FTIR provides a promising solution to verify residual active pharmaceutical ingredients (APIs) and detergents directly on reaction chamber surfaces.

Objectives and overview of the study

This application note investigates the use of the Agilent 4300 handheld FTIR spectrometer for fast, in situ cleaning verification of stainless steel surfaces. The main goals were:

• To build calibration models correlating FTIR absorbance to surface concentration of common contaminants (acetaminophen, caffeine, clarithromycin, CIP-92 detergent).
• To demonstrate detection limits down to sub-microgram per square centimeter levels.
• To evaluate the feasibility of replacing traditional swab-and-HPLC methods with a rapid, field-deployable FTIR approach.

Methodology and used instrumentation

Sample preparation:

• 5 × 5 cm stainless steel coupons polished to a mirror finish (no. 7).
• Automated deposition of analyte solutions (1–4 g/L) onto coupons using a solid‐printer to create arrays of 0.021 µL dots spaced 1 mm apart, yielding surface concentrations of 1–5 µg/cm².
• Drying overnight to remove all solvent.

Used Instrumentation:

• Agilent 4300 handheld FTIR spectrometer (2 kg, IP54, battery or AC powered).
• Sampling modules: diffuse reflectance for rough/porous surfaces; external reflectance for smooth films; grazing‐angle specular reflectance for submicron layers.
• MicroLab mobile software for data collection and automated module recognition.

Spectral acquisition and modeling:

• Spectra collected at 8 cm⁻¹ resolution over 50 scans (~15 s per spectrum), at 0.5 mm probe distance.
• Background correction using unprinted steel coupon.
• First‐derivative preprocessing to remove baseline shifts.
• Partial least squares (PLS) regression using a single latent variable in the 650–1 500 cm⁻¹ fingerprint region.
• Model acceptance criteria: R² ≥ 0.95, low RMSEC and RMSECV values.

Results and discussion

All four calibration models exhibited excellent linearity and sensitivity: acetaminophen, caffeine, clarithromycin, and CIP-92 detergent models achieved R² ≈ 0.97–0.98, with root‐mean‐square errors of calibration and cross‐validation under 7 % of the stock surface concentration. The lowest detectable surface coverage reached 0.62 µg/cm² for APIs. Randomizing spot selection across coupons minimized local heterogeneity effects. The results suggest that similarly contaminated vessel surfaces can be reliably quantified without extensive swabbing or solvent extraction.

Benefits and practical applications of the method

Key advantages include:

• Rapid turnaround: results in minutes rather than hours or days.
• Nondestructive, solvent‐free analysis directly on equipment surfaces.
• Low technical barrier: minimal FTIR expertise required.
• Portability: handheld operation enables field or plant‐floor measurements.
• Potential to reduce water and detergent use, vessel downtime, and analytical costs.

Future trends and potential applications

Future work should focus on:

• Validating models on actual vessel interiors with independent swab/HPLC verification.
• Investigating the influence of surface roughness and shininess on sensitivity to extend application to worn or textured equipment.
• Optimizing deposition mimicking real‐world thin films rather than discrete dots to improve calibration fidelity.
• Expanding the method to detect a wider range of APIs, cleaning agents, and excipients in multistep manufacturing.

Conclusion

This study demonstrates that the Agilent 4300 handheld FTIR spectrometer can rapidly and accurately quantify residual contaminants on stainless steel surfaces down to sub‐microgram levels. The approach offers a compelling alternative to traditional swab‐and‐HPLC cleaning verification, delivering speed, cost savings, and ease of use for pharmaceutical manufacturing quality assurance.

Reference

1. Schmidt A H. Validated HPLC Method for the Determination of Residues of Acetaminophen, Caffeine, and Codeine Phosphate on Swabs Collected from Pharmaceutical Manufacturing Equipment. J. Liq. Chromatogr. Relat. Technol. 2006, 29, 1663–1673.
2. Boca M A.; Apostolides Z.; Pretorius E A. Validated HPLC Method for Determining Residues of a Dual Active Ingredient Anti-Malarial Drug on Manufacturing Equipment Surfaces. J. Pharm. Biomed. Anal. 2005, 37, 461–468.
3. Forsyth R J.; Haynes D. Cleaning Validation in a Pharmaceutical Research Facility. Pharm. Technol. 1998, 22(9), 104–112.