Online LC Monitoring of a Hydrolysis of Organic Acid Esters by Means of a Flow Chemistry Reactor

Significance of the topic

Continuous flow reactors have emerged as a powerful alternative to traditional batch processes in chemical synthesis. They deliver enhanced heat and mass transfer, improved safety, and reduced reagent consumption. Real-time monitoring of reaction progress is essential for rapid optimization of flow conditions, quality control, and accelerated development of new processes in pharmaceuticals, fine chemicals, and academic research.

Objectives and overview of the study

This application note explores the integration of a Corning Advanced Flow Reactor with the Agilent InfinityLab Online LC Solution to monitor the acid-catalyzed hydrolysis of acetylsalicylic acid (aspirin). The primary goals are to:

• Automate sampling and analysis of a flowing reaction mixture.
• Evaluate the precision of peak area and retention time measurements.
• Assess the influence of residence time and acid concentration on conversion and impurity formation.

Methodology and instrumentation used

Reactants (0.016 M acetylsalicylic acid, 0.16–1.5 M H2SO4) were pumped via dual syringe pumps through a thermostatted Corning Low Flow microreactor at 86 °C. Residence times ranging from 1 to 60 min were achieved by varying flow rates. The reactor effluent was sampled every 3 min by an Agilent peristaltic pump, diluted, and injected directly into the online HPLC system. Key analytical parameters:

• Column: Agilent ZORBAX Eclipse Plus C18 (4.6 × 50 mm, 1.8 µm).
• Mobile phase: Water/0.1% formic acid – acetonitrile/0.1% formic acid (70:30, isocratic).
• Flow rate: 2 mL/min; column temperature: 50 °C.
• Detection: Diode Array Detector at 243 nm.
• Software: Agilent OpenLab CDS 2.6 and Online LC Monitoring Software 1.0.

Main results and discussion

Instrument performance tests using a 0.2 mg/mL aspirin/salicylic acid mixture yielded retention time RSDs ≤ 0.07% and area RSDs ≤ 1.3%, demonstrating high repeatability. In flow‐reactor experiments:

• Increasing residence time from 1 to 60 min led to gradual aspirin conversion: 4.98% at 1 min to 52.80% salicylic acid at 60 min, with an impurity level rising to 2.01%.
• Holding residence time at 60 min and varying acid concentration from 0.16 to 1.5 M increased aspirin conversion from 45.71% to 79.88%. However, the byproduct fraction grew from 1.86% to 15.87% at the highest acid strength.

Benefits and practical applications of the method

Combining flow reactors with online LC enables:

• Fully automated reaction monitoring and data collection.
• Rapid screening of reaction parameters for yield and impurity control.
• Resource-efficient process development by minimizing sample handling and downtime.

Future trends and potential applications

Future developments may include:

• Integration of machine learning algorithms for self-optimizing flow processes.
• Implementation of real-time process analytical technology (PAT) for feedback control.
• Expansion to multistep continuous syntheses and inline product isolation.

Conclusion

The study confirms that the Agilent InfinityLab Online LC Solution, when paired with a microflow reactor, delivers precise, repeatable analysis of flowing reaction streams. It facilitates rapid optimization of key parameters—residence time and catalyst concentration—to maximize product yield while monitoring impurity formation, supporting efficient development of continuous processes.

References

1. Corning Incorporated. Low Flow Reactor Web Information. www.corning.com.
2. Corning Advanced Flow Chemistry Reactor Datasheet. www.corning.com/media/worldwide/Innovation/documents/LF_WEB.pdf.
3. Naegele E.; Kutscher D. Online Reaction Monitoring by the Agilent InfinityLab Online LC Solutions. Agilent Technologies Application Note 5994-3528EN, 2021.