Measurement of Samples by Transmission Spectroscopy with the Thermo Scientific Antaris FT-NIR Analyzer

Significance of the Topic

The application note evaluates transmission-mode Fourier transform near-infrared (FT-NIR) spectroscopy for direct analysis of solid polymer films and low-melting pharmaceutical formulations. Transmission FT-NIR offers practical advantages for quality control and materials characterization because NIR absorptions are weaker than mid-IR fundamentals, allowing longer pathlengths, simple glass sampling, and minimal sample preparation. These properties make transmission FT-NIR attractive for rapid compositional analysis of packaging materials, polymer alloys and heatable pharmaceutical samples.

Objectives and Study Overview

The work aims to demonstrate the capabilities of the Thermo Scientific Antaris FT-NIR analyzer in transmission mode for:

• Acquiring high-quality spectra of commercially produced polymer films without modification.
• Building quantitative models for polymer blends (polystyrene/polyethylene) using classical least squares (CLS).
• Assessing temperature effects on spectral quality for a waxy pharmaceutical formulation and comparing transmission versus diffuse reflectance for such samples.

Methodology and Instrumentation

Key experimental conditions and instruments are summarized below:

• Instrument: Thermo Scientific Antaris FT-NIR analyzer (transmission module).
• Acquisition: 20 s measurement time, spectral resolution 8 cm-1; automatic internal background collected prior to samples.
• Sample holders: three-position non-heated cardholder for polymer sheets; heated cuvette/culture-tube holder with automated sample shuttle for temperature studies.
• Polymer samples: commercial protective sheet (~0.120 mm thick) and cast films (0.190–0.225 mm) composed of polystyrene/polyethylene blends.
• Quantitation software: TQ Analyst (CLS) and RESULT Operation/Integration software (Collect Standards and Build TQ Analyst features).
• Pharmaceutical sample handling: waxy formulation broken into <1 mm3 pieces, placed in 6 mm disposable glass vials and measured from 28 °C to 60 °C in 2 °C increments; diffuse reflectance data obtained with an integrating sphere for comparison.

Main Results and Discussion

Polymer transmission data:

• FT-NIR spectra of the commercial plastic sheet produced well-defined absorbance bands within the linear range; peaks were suitable for quantitative work.
• In contrast, mid-IR spectra of the same sheet showed saturation or non-linear absorbances that hinder quantitation.

Quantitative modelling for cast polymer films:

• Calibration used ten standards, with one validation standard; three spectral regions were employed in the CLS model.
• Polystyrene: correlation coefficient R = 0.99970 and RMSEC = 0.813%.
• Polyethylene: R = 0.99977 and RMSEC = 0.712%.

Temperature effects on a waxy pharmaceutical formulation:

• Diffuse reflectance was ineffective: the formulation surface lacked a diffuse appearance and reflectance spectra showed low signal-to-noise.
• At ambient temperature transmission spectra were poor due to strong scattering from solid particles (baseline >2.5 absorbance units and low S/N).
• Progressive heating softened the material, reduced scattering, lowered the baseline, sharpened peaks and improved S/N; at 60 °C the transmission spectra became suitable for analysis.
• Spectra were displayed on a common scale to demonstrate these trends; heating enabled successful transmission measurements where reflectance failed.

Benefits and Practical Applications of the Method

Principal advantages and uses identified:

• Minimal sample preparation: commercial polymer films can be analyzed as-received without pressing, dissolution or film casting.
• Disposable glass vials and culture tubes reduce cross-contamination risks and permit straightforward handling of low-melting samples.
• Temperature control allows analysis of materials that require softening or melting to obtain representative spectra (e.g., waxy pharmaceuticals).
• High precision and rapid measurement (20 s acquisitions) support routine QC tasks for packaging materials, polymer composition assays, and formulation screening.
• Strong chemometric performance (CLS models with R ~0.9997) demonstrates quantitative suitability for compositional analysis.

Future Trends and Applications

Likely developments and broader opportunities include:

• Integration of transmission FT-NIR modules into at-line and in-line process analytical technology (PAT) for polymer and pharmaceutical manufacturing.
• More advanced chemometric approaches (e.g., multivariate regression variants, machine learning) to improve robustness across production variability.
• Enhanced temperature-controlled sample accessories for automated melt-state and semi-solid analyses.
• Detector and optics improvements to further raise signal-to-noise for highly scattering samples, expanding applicability to particulate or composite materials.
• Standardized workflows for using disposable glass sampling in regulated environments to simplify compliance and reduce contamination risk.

Conclusion

The Antaris FT-NIR transmission module delivers reproducible, quantifiable spectra for thick plastics, polymer blends and low-melting solids and liquids. Transmission FT-NIR outperforms mid-IR for thick polymer films in terms of linear response and avoids saturation. For waxy pharmaceutical formulations, controlled heating enables transmission measurements where diffuse reflectance fails due to surface scattering. The combination of short acquisition times, automatic background collection, disposable glass sampling and effective chemometric modelling makes transmission FT-NIR a practical, high-precision option for routine analytical and QC applications.

References

1. McCarthy WJ. Measurement of Samples by Transmission Spectroscopy with the Thermo Scientific Antaris FT-NIR Analyzer. Application Note 51668, Thermo Fisher Scientific; 2008.