Quantification of color intensity of diluted textile dye by visible near-infrared spectroscopy

Significance of the Topic

The measurement of dye color intensity is critical for quality control in textile production. Combining visible and near-infrared spectroscopy into a single scan enables rapid, non-destructive analysis of color strength, dye type identification and impurity detection. This approach reduces analysis time and equipment costs compared to separate UV-Vis and NIR systems.

Objectives and Study Overview

This study aimed to demonstrate the capability of a Vis-NIR analyzer to:

• Quantify the color intensity of aqueous dye solutions as accurately as UV-Vis reference methods.
• Differentiating between distinct dye chemistries and suppliers.
• Detect impurities in undiluted dye batches during raw material control.

Methodology and Instrumentation

A two-step spectral approach was applied:

• Reflection scanning of undiluted dye samples in 4 mm glass vials using the NIRS DS2500 Analyzer (400–2500 nm) for type and impurity screening.
• Transmission scanning of diluted aqueous dye solutions with the NIRS XDS RapidLiquid Analyzer (400–2500 nm) for color intensity quantification.

Spectral data were preprocessed using a second derivative to correct baseline shifts. Partial Least Squares (PLS) regression was employed to correlate Vis absorbance changes with dye concentration.

Used Instrumentation

• Metrohm NIRS DS2500 Analyzer (reflection mode, 400–2500 nm)
• Metrohm NIRS XDS RapidLiquid Analyzer (transmission mode, 400–2500 nm)
• Disposable glass vials and sample cup holders
• Vision Air 2.0 software for data acquisition and chemometric model development

Main Results and Discussion

• Dye Type Differentiation: Distinct spectral features at 1570–1720 nm enabled robust classification of two dye chemistries.
• Supplier Discrimination: Subtle variations around 730–830 nm allowed differentiation of the same dye type from different suppliers.
• Impurity Detection: Contaminated dyes exhibited characteristic changes in the 1750–1800 nm region, facilitating quality screening.
• Quantification Performance: A six-factor PLS model (420–740 nm) achieved R2=0.9846, SEC=108, SECV=126 and SEP=133, matching the UV-Vis reference precision.
• External Validation: Seven independent dye samples were correctly classified by type and supplier and quantified within expected error margins.

Benefits and Practical Applications

• Single-scan, multi-parameter analysis reduces instrument footprint and operator time.
• Comparable accuracy to UV-Vis provides confidence for color strength measurement.
• Rapid impurity screening supports incoming raw material control.
• Flexibility to monitor multiple quality attributes in under 30 seconds.

Future Trends and Opportunities

Advances may include:

• Integration of inline Vis-NIR probes for real-time process monitoring in dyeing lines.
• Expansion of chemometric libraries to cover additional dye classes and pigments.
• Implementation of machine learning algorithms to enhance classification and quantification robustness.
• Miniaturized spectrometers for portable or handheld quality checks.

Conclusion

This application note demonstrates that combined Vis-NIR spectroscopy is a viable alternative to traditional UV-Vis methods for dye color intensity quantification, type identification and impurity detection. The approach offers significant time and cost savings while maintaining analytical performance.

References

1. http://www.essentialchemicalindustry.org/materials-and-applications/colorants.html
2. https://en.wikipedia.org/wiki/Dyeing