Measurement of Opaque Liquids with UV-Vis and ATR

Importance of the Topic

Measuring optically opaque liquids is critical across industries such as paints, inks, food colorings, sunscreens and pharmaceuticals. Traditional UV-Vis analysis often demands extensive dilution of highly pigmented or emulsified samples, leading to increased solvent use, dilution errors, contamination risk and time-consuming procedures. ATR sampling combined with UV-Vis overcomes these limitations by enabling direct measurement of turbid or highly absorbing liquids without dilution.

Objectives and Study Overview

This study demonstrates the integration of an Agilent Cary 60 UV-Vis spectrophotometer with a Hellma Katana XP 6 three-bounce ATR fiber-optic probe. The main goals are to evaluate direct measurement of opaque paints, food colorings and sunscreens, and to monitor reaction kinetics in turbid media.

Instrumentation Used

• Agilent Cary 60 UV-Vis spectrophotometer with xenon flash lamp and fiber-optic interface
• Hellma Katana XP 6, three-bounce ATR probe (PK1KSSZUEASM210N) with sapphire crystal
• Agilent Cary WinUV software for data acquisition and processing

Methodology

Attenuated total reflection directs a focused light beam into a sapphire crystal at 60°, generating an evanescent wave that penetrates a few micrometers into the sample. Three internal reflections extend the effective path length (approx. 3 µm at 250 nm to 13 µm at 1,000 nm), allowing measurable absorbance without dilution. Measurement parameters:

• Mode: Absorbance
• Wavelength: 800–250 nm
• Spectral bandwidth: 1.5 nm
• Signal averaging time: 0.4 s
• Data interval: 2 nm
• Scan speed: 150 nm/min

Baselines were recorded with a clean crystal before each sample measurement.

Main Results and Discussion

• Paints: Yellow paint showed 0.15 Abs at 440 nm (≈6 µm path length), equivalent to 250 Abs in a 10 mm cuvette (~200× dilution).
• Food coloring: Royal blue dye exhibited 1.27 Abs at 650 nm (≈8.5 µm), corresponding to 1,500 Abs in a 10 mm cell (~1,000× dilution).
• Sunscreen: Absorbance spectra of SPF 15, 30 and 50 formulations revealed decreasing absorbance with increasing factor. Reflectance conversion confirmed higher reflectivity at higher SPF across UVA/UVB/UVC regions.
• Reaction kinetics: Real-time monitoring of Fmoc deprotection in a turbid peptide slurry tracked dibenzofulvene formation at 280 nm. The reaction plateaued at 6 min with a 0.05 Abs increase (~140 Abs in 10 mm cuvette).

Benefits and Practical Applications of the Method

• Direct measurement without dilution reduces solvent consumption and contamination risk.
• Minimized dilution errors and simplified sample handling.
• Real-time kinetic monitoring in turbid or emulsive systems.
• Applicable to quality control, R&D, industrial process monitoring and teaching laboratories.

Future Trends and Potential Applications

• Integration into automated in-line process analytics for manufacturing environments.
• Development of portable ATR-UV-Vis systems for field or on-site analysis.
• Coupling with chemometric models for advanced multivariate analysis.
• Expansion of ATR probes into near-IR and mid-IR spectral regions.
• Cloud connectivity for remote monitoring and data sharing.

Conclusion

The Agilent Cary 60 UV-Vis spectrophotometer combined with a Hellma ATR probe offers a robust, easy-to-use platform for direct analysis of opaque liquids and real-time reaction monitoring. This approach streamlines workflows, reduces solvent use and errors, and expands the scope of UV-Vis spectroscopy for diverse analytical applications.

Reference

• Clarke A., Wills R. Measurement of Opaque Liquids with UV-Vis and ATR: Combining the Agilent Cary 60 UV-Vis spectrophotometer with an ATR fiber-optic probe. Application Note, Agilent Technologies; October 2025.