Applications of PTV Injectors for Problem Solving in the Petrohemical Industry Part 2:- In-Liner Derivatisation for the Analysis of Organic Acid Mixtures

Importance of the Topic

The control of acetic acid production streams is critical for maintaining product quality and process efficiency. Dicarboxylic and keto acids such as succinic and levulinic acids pose analytical challenges due to crystallization and low volatility. Reliable, rapid methods improve process monitoring and reduce downtime.

Study Objectives and Overview

This work aimed to develop a fast in-liner derivatization technique using a Programmable Temperature Vaporization injector to convert carboxylic acids to silyl ethers for capillary gas chromatography. The goal was to quantify succinic and levulinic acids in industrial process streams with higher throughput, lower reagent use, and improved chromatographic performance compared to conventional off-line methylation.

Methodology

Samples and standards were prepared by dissolving acids in tetrahydrofuran. Aliquots of sample solution were combined with BSTFA derivatizing reagent, then injected into the PTV injector under stopped flow at 45°C. The injector temperature was ramped at 16°C/s to 250°C and held for 2 minutes to complete silylation. Derivatized compounds were transferred to the GC column via a pressure pulse.

Instrumentation Used

• Gas chromatograph: Chrompack CP9001 with FID detection
• PTV injector: Optic 200 model equipped with Supelcoport packed liner
• Data system: VG Multichrom
• Column: CPSIL8, 25 m length, 0.53 mm internal diameter, 0.5 μm film
• GC temperature program: 45°C initial, ramp 10°C/min to 250°C, detector at 270°C
• PTV program: 45°C hold, ramp 16°C/s to 250°C, 2 min hold; carrier gas pressure varied from 0.55 to 0.65 bar

Main Findings and Discussion

• Trimethylsilyl derivatives obtained with good conversion, reproducibility and minimal tailing
• External calibration provided superior precision compared to internal standard approach
• Succinic acid quantification over 50–100% w/w was linear (R2 = 0.9993); levulinic acid calibration in 0–1% and 1–10% ranges achieved R2 > 0.9998
• Analysis of process samples yielded succinic acid at 78.8% ± 1.0% (95% confidence) and levulinic acid detection down to 0.25% ± 0.02%
• Run time reduced to ~35 minutes per sample versus ~180 minutes for off-line methylation

Benefits and Practical Applications

• Fivefold decrease in analysis time enhances laboratory throughput
• Minimal sample preparation reduces labor and potential errors
• Lower reagent consumption and no solvent extraction step lower cost and environmental impact
• Improved chromatographic resolution supports accurate process control

Future Trends and Potential Applications

The in-liner derivatization approach could be extended to other organic acid mixtures and complex matrices. Integration with mass spectrometry may enhance specificity. Automation and real-time inline monitoring represent promising directions for process analytics. Further development of reagent chemistries and PTV programming could broaden applicability across petrochemical and biochemical industries.

Conclusion

The PTV in-liner silylation method offers a robust, rapid and cost-effective solution for the quantification of succinic and levulinic acids in acetic acid production streams. The approach delivers high precision, reduced analysis time and streamlined workflow, supporting efficient process control.

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