Feature article - Toward greener real-time peptide manufacturing with inline RI monitoring

Tamara Murusidze of Vaisala looks at the advantages of inline refractive index monitoring in process analytical technology 

Process engineering and manufacturing teams are facing the challenge of making peptide synthesis more measurable and controllable without adding unnecessary analytical complexity. Inline refractive index (RI) monitoring offers a wide range of advantages in process analytical technology (PAT).

RI is a direct physical measurement related to the concentration and composition of dissolved material in the liquid phase. In peptide synthesis, many process steps involve changes in the liquid surrounding the resin: reagent introduction, coupling, deprotection, byproduct formation, solvent exchange, and washing. 

As these liquid-phase concentration profiles change, the RI signal also changes. This makes RI useful not as a molecular identification tool, but as a continuous process fingerprint.

Practical applications for RI monitoring include reagent and solvent transitions, concentration changes during coupling and deprotection, wash-out profiles after reaction steps, endpoint behavior, deviations from expected process trends, and batch-to-batch or scale-to-scale consistency. 

De la Torre et al. evaluated RI as a PAT tool for the real-time monitoring of SPPS to determine the endpoint of reactions and to optimize the use of reagents and solvents to deliver a more sustainable SPPS process.1 The benefit of RI technology is that it adds direct continuous visibility to the peptide synthesis process.

Where does RI add value?

Real-time visibility of liquid-phase composition can provide valuable insights into a variety of stages in peptide synthesis. 

Washing steps are often intentionally conservative because of limited real-time information on when residual reagents, byproducts, or solvents have been sufficiently displaced. RI can provide a continuous wash profile, helping teams understand when the liquid phase has returned to the expected baseline.

This supports more rational wash development and may reduce unnecessary solvent use without exposing proprietary process details or relying only on fixed time/volume assumptions.

During Fmoc deprotection, the liquid phase composition changes as deprotection progresses and reaction products are removed. RI can follow these bulk concentration changes in real-time, supporting better understanding of deprotection kinetics and step completion. 

This is especially valuable during development, troubleshooting, and scale transfer, where the comparison of real-time profiles may reveal whether a process is behaving consistently across equipment, resin lots, solvent systems, or operating conditions.

Coupling reactions involve the uptake and consumption of dissolved reagents by the solid phase. While RI is not selective to a single molecule, it can still provide useful insight into process trends that are reflected by changes in the surrounding liquid phase. 

This may help identify abnormal reaction behavior, delayed mass transfer, incomplete displacement, or unexpected process drift. Used together with established offline analytics, RI can become a practical tool for building process understanding without requiring complex chemometric models.

Modern peptide manufacturing increasingly explores greener solvent systems, solvent reduction, recycling, flow-based synthesis, and alternative process intensification strategies. In these contexts, real-time visibility of liquid-phase composition becomes even more important. RI can support monitoring of solvent transitions, wash consistency, and process reproducibility across development, pilot, and manufacturing scales.

Recent work in peptide manufacturing shows a clear direction: the industry is moving toward greener, faster, more data-rich processes. Studies on wash reduction, greener solvents, Raman-based monitoring, computer vision and intensified synthesis platforms all point to the same need: better real-time process information. 

However, not every manufacturing environment needs a complex spectroscopic model. In many cases, the immediate need is a robust, scalable, easy-to-integrate signal that helps operators and process teams understand whether the process is progressing as expected. This is where inline RI can be attractive: it is universal, real-time, continuous, stable and comparatively simple to implement.

Practical implementation

Successful RI implementation requires application-specific evaluation. Important factors include solvent system, temperature behavior, installation point, process pressure and flow conditions, baseline definition, and the intended use of the signal.

For GMP implementation, RI should be positioned as a complementary PAT signal supporting process understanding, endpoint detection and process consistency. It does not need to disclose or depend on proprietary reaction recipes to deliver value. A good start is to begin with development-scale process profiling, comparing of RI trends with existing analytical data, then defining where the signal provides practical decision-making value.

Among the products on the market are Vaisala’s Polaris* process refractometers, which are designed for continuous industrial liquid measurement in demanding process environments. Their robust optical design, with no moving parts, a compact sensor body and a stable digital measurement principle make them well suited for inline monitoring in peptide manufacturing environments.

Summary

As peptide manufacturing moves toward greener, more efficient, and more data-driven production, the need for practical and scalable PAT tools continues to grow. Inline refractive index monitoring can offer a simple and robust way to improve real-time process visibility during peptide synthesis, supporting better process understanding, solvent optimisation and manufacturing consistency.

*  - Polaris is a registered trademark of Vaisala

Reference

1. B.G. de la Torre, S. Ramkisson, F. Albericio & J. Lopez, OPRD 2021, 25(4), 1047-1053: DOI: 10.1021/acs.oprd.1c00051