Measurement Reproducibility – A Case Study of the Long-Term Performance of an FT Near-Infrared Analyzer

Importance of the Topic

The long-term stability of analytical instruments is a fundamental criterion for reliable routine measurement, method transfer and cost-effective quality control. In Near-Infrared (NIR) spectroscopy, where multivariate calibrations often rely on hundreds of standards and models deployed across multiple instruments, instrument drift or instability increases the burden of frequent revalidation and recalibration. Demonstrating sustained photometric and frequency stability over years supports robust process analytical technology (PAT), easier method transfer between systems, and lower operational overhead in regulated environments.

Objectives and Overview of the Study

This technical note reports a long-term case study of a single Thermo Scientific Antaris FT-NIR analyzer (serial AFA0000197). The primary objective was to assess photometric reproducibility and stability by repeatedly measuring NIST-traceable photometric linearity standards over a period of nearly three years (December 21, 2000 to November 6, 2003). The study evaluates absorbance repeatability at USP-recommended wavelengths and discusses consequences for method transfer and process measurements.

Methodology

Measurements were performed using the instrument's internal validation wheel, populated with NIST-traceable standards: a polystyrene frequency standard and five photometric transmission standards nominally at 2%, 10%, 20%, 40% and 80% transmission. Key experimental details:

• Measurement frequency: 60 measurement sessions distributed over ~3 years.
• Wavelengths evaluated (USP <1119>): 5000 cm-1 (2000 nm), 6250 cm-1 (1600 nm), and 8333 cm-1 (1200 nm).
• Data acquisition: duplicate measurements at each session (averaged), 16 co-added scans, 8 cm-1 spectral resolution, ~8 s collection time per measurement.
• Data output: absorbance values recorded and tracked over time to assess Y-axis (photometric) repeatability and drift.

Instrumentation Used

The Antaris FT-NIR Method Development System was the platform for this assessment. Relevant instrument design and qualification features described in the study:

• Fourier-transform NIR analyzer covering approximately 4000–12000 cm-1.
• Internal HeNe laser for X-axis (frequency) calibration, with reported frequency accuracy better than 0.001 cm-1 (measured on the 4332 cm-1 polystyrene peak).
• Internal validation wheel containing NIST-traceable polystyrene and five photometric transmission standards.
• Pinned-in-place optics and a dynamically aligned interferometer to minimize mechanical drift.
• Gasket-sealed validation compartment to protect standards from premature aging.
• Automatic, internal background collection to maintain consistent reference conditions.

Main Results and Discussion

Over 60 measurement events spanning nearly three years, absorbance values for the five photometric standards at three wavelengths produced 15 distinct time series that were essentially stable with no pronounced long-term trend. Statistical summarization across all runs yielded low standard deviations and low percent relative standard deviations (RSD):

• Percent standard deviations ranged from approximately 0.11% to 0.73% depending on the transmission level and wavelength.
• Lower-transmission (higher absorbance) standards showed the smallest percent RSD (e.g., ~0.11–0.16% for the 2% and 10% nominal standards at the evaluated wavelengths), while the highest transmission (80%) standards exhibited larger percent RSDs (up to ~0.73%).

These values indicate exceptional photometric repeatability for an FT-NIR bench over multi-year timescales. The presence of a stable internal frequency standard (HeNe laser) further reduces X-axis variability, which complements photometric stability and aids chemometric model robustness.

Benefits and Practical Applications

The observed long-term stability of the Antaris FT-NIR analyzer has several practical implications:

• Method transfer: stable instrument response across time supports transferring chemometric calibrations between instruments with reduced need for extensive local recalibration or model revalidation.
• Reduced maintenance burden: low drift limits the frequency of recalibration and revalidation activities, reducing downtime and labor costs for QA/QC laboratories.
• Process integration: reliable long-term performance is critical for in-line or at-line PAT applications and continuous process monitoring in pharmaceutical, chemical, food, pulp and paper, and polymer industries.
• Regulatory confidence: consistent instrument performance simplifies compliance with regulatory expectations for method validation and system suitability testing (e.g., as referenced in USP guidance).

Future Trends and Potential Applications

Building on demonstrated long-term stability, likely future developments and opportunities include:

• Networked instrument fleets with centralized calibration models and remote monitoring to further streamline method transfer and calibration maintenance.
• Adaptive calibration maintenance strategies that rely on sparse reference checks rather than full recalibrations, leveraging instrument stability to reduce workload.
• Increased deployment in continuous manufacturing and PAT architectures where long-term stability is essential for autonomous control loops.
• Integration with advanced chemometric transfer techniques (e.g., standardization, piecewise direct standardization) to exploit instrument stability and achieve near-seamless cross-instrument predictions.
• Regulatory harmonization and formalized protocols for long-term stability monitoring and validation tailored to FT-NIR platforms.

Conclusion

The Thermo Scientific Antaris FT-NIR analyzer demonstrated exceptional photometric and frequency stability over nearly three years of periodic measurements on NIST-traceable photometric linearity standards. Percent RSDs between 0.11% and 0.73% across standards and wavelengths indicate minimal drift, supporting robust method transfer, reduced calibration burden and reliable process deployment. Instrument design features—internal laser calibration, fixed optics, dynamic interferometer alignment and protected validation standards—were highlighted as contributors to this stability.

Reference

Hirsch J. Measurement Reproducibility – A Case Study of the Long-Term Performance of an FT Near-Infrared Analyzer. Technical Note 50814. Thermo Fisher Scientific; 2008.

United States Pharmacopeia. Chapter <1119> Near-Infrared Spectroscopy (referenced for photometric linearity test recommendations).