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

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

Importance of the topic

Near-infrared (NIR) transmission spectroscopy offers practical advantages for rapid, non-destructive analysis of solids and liquids in pharmaceutical and polymer quality control. Because overtone and combination bands in the NIR are weaker than mid-infrared fundamentals, longer pathlengths and simpler sample containers (e.g., glass vials and tubes) can be used. The technique enables direct measurement of films and low-melting solids without dissolution or film casting, reduces sample preparation and cross-contamination, and supports temperature-controlled measurements for materials whose optical properties change with phase or particle morphology.

Objectives and study overview

• Demonstrate that Fourier transform NIR (FT-NIR) transmission measurements on the Thermo Scientific Antaris analyzer produce high-quality spectra from commercial polymer films, cast polymer alloys, and a waxy pharmaceutical formulation.
• Develop and validate a quantitative calibration for polymer blends (polystyrene/polyethylene) using Classical Least Squares (CLS) chemometrics.
• Investigate the effect of controlled temperature on the spectral quality of a waxy pharmaceutical sample and compare transmission to diffuse reflectance for this material.

Methodology

Spectra were recorded on a Thermo Scientific Antaris FT-NIR analyzer with the following routine acquisition parameters: 20-second measurement time and 8 cm-1 spectral resolution. Backgrounds were collected automatically using the instrument's internal background position prior to sample scans. Two sampling modes were evaluated: a non-heated three-position cardholder for polymer films and a heated cuvette/culture-tube holder for temperature studies on the waxy formulation. Sample types and handling:

• Commercial protective polymer sheet (~0.120 mm thickness) measured as received in transmission.
• Cast polymer films (0.190–0.225 mm) comprised of polystyrene/polyethylene blends used for quantitative calibration.
• Waxy pharmaceutical formulation broken into small pieces (<1 mm3) and loaded into 6 mm disposable glass vials for heated transmission measurements.

Chemometrics and software:

• Calibration standards created using RESULT software’s Collect Standards function.
• Quantitative CLS models built in TQ Analyst using three spectral regions.
• Temperature control and automated background collection managed through RESULT/RESULT Integration software.

Used instrumentation

• Thermo Scientific Antaris FT-NIR analyzer (transmission module).
• Three-position non-heated cardholder for film measurements.
• Heated cuvette/culture-tube holder capable of ramping temperature and holding disposable glass vials.
• Integrating sphere used for diffuse reflectance comparison.
• RESULT Operation and RESULT Integration software and TQ Analyst chemometric package (Classical Least Squares).

Main results and discussion

Transmission spectroscopy provided clear, linear absorbance bands from commercial polymer sheets measured as received; these NIR bands were within the instrument’s linear range and suitable for quantitation. By contrast, mid-infrared spectra of the same sheet exhibited strong absorption or non-linear behavior that would impair quantification. For quantitative polymer analysis, a CLS model built on 10 calibration standards (plus one validation) produced excellent fits: for polystyrene R = 0.99970 (RMSEC = 0.813%) and for polyethylene R = 0.99977 (RMSEC = 0.712%). These metrics indicate high accuracy and precision for composition prediction in the tested thickness range (≈0.19–0.225 mm).

For the waxy pharmaceutical formulation, diffuse reflectance using an integrating sphere yielded poor spectra because the sample surface did not behave as a diffusely scattering medium; signal-to-noise was low. Transmission at ambient temperature was also problematic due to strong light scattering from small particles, producing a high baseline (>2.5 absorbance units), broadened ill-defined peaks and poor S/N. Controlled heating (temperature ramp from 28 °C to 60 °C in 2 °C increments) softened the material, reduced scattering (sample became optically clearer), lowered the baseline, sharpened peaks and improved S/N. At ~60 °C the transmission spectra were of sufficient quality for analytical use. Figures in the original report illustrate spectral improvements with temperature and contrast NIR transmission vs mid-IR and diffuse reflectance.

Benefits and practical applications of the method

• Minimal sample preparation: thick polymer films and low-melting solids can be measured directly without pressing, dissolving, or casting.
• Use of inexpensive disposable glass tubes/vials reduces cross-contamination risk and simplifies workflow.
• Temperature-controlled sampling enables analysis of formulations whose optical properties depend on phase or particle morphology, extending NIR applicability to waxy or semi-solid samples.
• High-quality quantitative results are achievable using CLS chemometrics for composition assays in polymer blends, suitable for QC/production environments.
• FT-NIR advantages (fast acquisition, good S/N with appropriate sample handling, and automated background collection) make the approach practical for routine at-line and laboratory measurements.

Future trends and potential applications

• Broader adoption of temperature-controlled NIR sampling for semi-solid pharmaceuticals and formulations to manage scattering effects and phase transitions.
• Integration of advanced chemometric approaches (e.g., PLS, robust preprocessing, machine learning) to improve calibration transfer, outlier detection and multicomponent quantification across variable sample thicknesses and instruments.
• Development of inline or at-line transmission probes and automated sample handling to support process analytical technology (PAT) in polymer processing and pharmaceutical manufacturing.
• Combining transmission FT-NIR with complementary modalities (e.g., imaging or Raman) for richer material characterization and defect analysis in packaging and films.
• Increased use of disposable sampling consumables and standardized temperature-controlled holders to streamline regulatory-compliant workflows.

Conclusion

The Antaris FT-NIR transmission module yields reproducible, quantifiable spectra for thick plastics, polymer alloys and low-melting pharmaceutical solids with minimal preparation. Transmission measurements outperformed mid-IR for thick films and outperformed diffuse reflectance for waxy formulations when appropriate temperature control was applied. High correlation and low calibration error in CLS models demonstrate the method’s suitability for quantitative QC applications. The combination of FT-NIR sensitivity, disposable glass sampling and temperature control produces a practical high-precision analytical workflow for polymers and certain pharmaceutical formulations.

Reference

• McCarthy W. J. Measurement of Samples by Transmission Spectroscopy with the Thermo Scientific Antaris FT-NIR Analyzer. Application Note 51668. Thermo Fisher Scientific. ©2001, 2008.