Inlet Derivatisation for the GC Analysis of Organic Acid Mixtures

Importance of the Topic

Analysis of mixed organic acids such as succinic and levulinic acid is essential for real-time control and optimization of bulk chemical processes, particularly in acetic acid production.
The high temperature solubility and low temperature crystallization of these acids create sampling challenges, while trace-level components demand sensitive, reproducible GC methods.

Objectives and Article Overview

This work aimed to develop an in-liner derivatisation approach using a Programmable Temperature Vaporisation (PTV) injector to convert carboxylic acids into silyl esters for direct capillary GC analysis.
The goal was to minimize sample preparation, eliminate damaging reagents, and drastically reduce analysis time compared to conventional off-line methylation techniques.

Methodology and Instrumentation

Instrumentation

• Optic 200 PTV injector retrofitted to a Chrompack CP9001 GC with FID detection.
• Supelcoport liner and VG Multichrom data system.

Reagents and Conditions

• Derivatising agent: BSTFA in THF.
• Injection volume: 1 µL of sample/reagent mix.
• Inlet protocol: stopped-flow injection at 45 °C, ramp 16 °C/s to 250 °C, hold 2 min, followed by pressure-pulse transfer to column.

Main Results and Discussion

Key findings

• Succinic acid calibration over 50–100 % w/w showed linearity (R² = 0.9993).
• Repeat analysis of succinic acid sample yielded 78.8 % ± 1.0 % (95 % CI).
• Levulinic acid calibration performed over 1–10 % and 0–1 % w/w ranges, each with R² > 0.9998.
• Detection of levulinic acid down to 0.25 % ± 0.04 % in a succinic acid matrix.
• Chromatograms demonstrated sharp, well-resolved silyl ester peaks with minimal tailing.

Benefits and Practical Applications

This in-liner silylation method offers

• Five-fold reduction in total analysis time (35 min vs 3 h).
• Significant savings in reagents and solvent consumption.
• Minimal manual handling and no solvent extraction steps.
• Enhanced throughput and lower operational costs for process control laboratories.

Future Trends and Potential Applications

Further developments may include full automation of PTV derivatisation, extension to other carboxylic and polar analytes, coupling with mass spectrometry detectors, and integration into on-line process analytical technology for continuous monitoring.

Conclusion

PTV in-liner silyl ester derivatisation provides a robust, rapid, and cost-effective GC method for quantifying succinic and levulinic acids in industrial streams, supporting reliable process control and quality assurance.

Reference

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