Simultaneous Analysis of Carboxylic Anhydrides and Hydrolysates Using Supercritical Fluid Chromatography

Significance of the Topic

Carboxylic anhydrides are widely used intermediates in polymer synthesis, but their propensity to hydrolyze in the presence of water or alcohol complicates conventional reversed-phase liquid chromatography analysis. Normal-phase LC alleviates this but relies on large volumes of hazardous organic solvents. Supercritical fluid chromatography (SFC) leverages CO₂ as a nonpolar mobile phase to reduce solvent consumption, minimize environmental impact, and limit analyte decomposition.

Goals and Overview of the Study

This work aimed to develop a rapid, robust SFC method for simultaneous analysis of four biphthalic and oxydiphthalic anhydride isomers and their hydrolysis products. Key objectives included identifying an appropriate stationary phase, optimizing separation conditions to achieve baseline resolution in under 7 minutes, and demonstrating simultaneous detection of intact anhydrides and their hydrolysates by online modifier switching.

Methodology and Instrumentation

A systematic column scouting experiment evaluated six Shim-pack UC stationary phases (Diol II, Sil II, PolyVP, PolyBT, PBr, ODS) using CO₂/acetonitrile gradients. Model compounds (s-, a-BPDA, ODPA-C, a-ODPA) were dissolved in superdehydrated acetonitrile to suppress pre-injection hydrolysis. Separation parameters such as mobile-phase composition, backpressure (15 MPa), temperature (column 40 °C, BPR 50 °C) and flow rate (3 mL/min) were optimized using Method Scouting Solution software for automated method generation.

Instrumentation

• Nexera UC Chiral Screening System with Method Scouting Solution software
• Shim-pack UC Series analytical columns (250 mm × 4.6 mm, 5 µm)
• CO₂/Acetonitrile mobile phase with backpressure regulator set to 15 MPa
• PDA detector at 300 nm; injection volume 2 µL of superdehydrated acetonitrile solutions

Key Results and Discussion

The UC-PolyBT column provided the best resolution among the tested phases, achieving baseline separation of all four anhydrides within approximately 7 minutes. Calibration curves over 5–125 mg/L exhibited excellent linearity (R² > 0.999) and precision (%RSD < 1.0). By switching the modifier post-elution to 0.1 % phosphoric acid in methanol, hydrolysis products were eluted and detected in the same run, demonstrating SFC’s capability for simultaneous multi-fraction analysis.

Benefits and Practical Applications of the Method

The developed SFC approach offers:

• Reduced solvent usage and environmental footprint compared to normal-phase LC
• Minimized analyte decomposition by avoiding aqueous mobile phases
• High throughput with sub-10 min total run time
• Automated method development to reduce human error and accelerate validation

Future Trends and Potential Applications

Advancements in SFC stationary phases and automated scouting tools will further streamline method development for labile analytes. Integration with mass spectrometry detection could extend applicability to trace analysis of anhydrides and related impurities in pharmaceutical, polymer, and materials research. Continuous improvements in green chromatography will drive broader adoption of CO₂-based separations.

Conclusion

This study demonstrated a fast, reproducible SFC method for concurrent analysis of carboxylic anhydrides and their hydrolysates. By leveraging automated column scouting and modifier switching, the approach addresses challenges of analyte stability, environmental impact, and analytical throughput in quality control and research settings.