Peptide therapeutics are becoming increasingly important across modern pharmaceutical pipelines. At the same time, peptide manufacturing remains under pressure to improve process efficiency, reduce solvent and reagent consumption and strengthen real-time process understanding.
Process engineering, manufacturing and manufacturing science and technology (MSAT) teams face the challenge of making peptide synthesis more measurable and controllable without adding unnecessary analytical complexity. Inline refractive index (RI) monitoring offers a range of advantages as a process analytical technology (PAT). In this article, Tamara Murusidze from Vaisala explores its practical applications.
Why refractive index?
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, by-product formation, solvent exchange and washing. As these liquid-phase concentration profiles change, the RI signal also changes, providing a continuous process fingerprint rather than molecular identification.
Practical applications for RI monitoring include reagent and solvent transitions, concentration changes during coupling and deprotection, wash-out profiles after reaction steps, end-point behaviour, deviations from expected process trends and batch-to-batch or scale-to-scale consistency.
B.G. de la Torre et al. (1) evaluated RI as a process analytical tool for the real-time monitoring of solid-phase peptide synthesis (SPPS). The aim was to determine reaction end points and optimise reagent and solvent use to deliver a more sustainable process. RI technology adds direct, continuous visibility to peptide synthesis.
Where does RI add value?
Real-time visibility of liquid-phase composition can provide valuable insights at several stages of peptide synthesis.
Optimising washing
Washing steps are often intentionally conservative because of limited real-time information on when residual reagents, by-products 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 a more informed approach to developing washing steps and may reduce unnecessary solvent use without exposing proprietary process details or relying solely on fixed time or volume assumptions.
Monitoring deprotection
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 a better understanding of deprotection kinetics and step completion.
This is especially valuable during development, troubleshooting and scale transfer, where comparing real-time profiles may reveal whether a process is behaving consistently across equipment, resin batches, solvent systems or operating conditions.
Tracking coupling and reaction trends
Coupling reactions involve the uptake and consumption of dissolved reagents by the solid phase. Although RI is not selective for a single molecule, it can provide useful insight into process trends reflected in changes in the surrounding liquid phase.
This may help identify abnormal reaction behaviour, delayed mass transfer, incomplete displacement or unexpected process drift. Used alongside established offline analysis, RI can be a practical tool for building process understanding without requiring complex chemometric models.
Monitoring solvent transitions and consistency
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 the monitoring of solvent transitions, wash consistency and process reproducibility across development, pilot and manufacturing scales.
The need for real-time insight
Recent work in peptide manufacturing shows a clear direction: the industry is moving towards greener, faster processes informed by more data. Studies on wash reduction, greener solvents, Raman-based monitoring, computer vision and intensified synthesis platforms all point to the same need for better real-time process information.
However, not every manufacturing environment needs a complex spectroscopic model. In many cases, the immediate requirement is a robust, scalable signal that is easy to integrate and helps operators and process teams understand whether the process is progressing as expected. Inline RI offers a continuous, stable, real-time measurement that is comparatively simple to implement.
Putting RI into practice
Successful RI implementation requires application-specific evaluation. Important factors include the solvent system, temperature behaviour, installation point, process pressure and flow conditions, baseline definition and the intended use of the signal.
For implementation under good manufacturing practice (GMP), RI should be positioned as a complementary PAT signal supporting process understanding, end-point detection and process consistency. It does not need to disclose or depend on proprietary reaction recipes to deliver value.
A practical starting point is to profile processes at development scale, compare RI trends with existing analytical data and then identify where the signal can support decision-making.
Designed for demanding environments
Vaisala Polaris process refractometers are designed for continuous industrial liquid measurement in demanding process environments. Their robust optical design, with no moving parts, compact sensor body and stable digital measurement principle make them well suited to inline monitoring in peptide manufacturing.
Integrated temperature measurement and compensation support reliable trend monitoring during changing process conditions. Multiple mounting options allow installation in development, pilot and manufacturing systems, while ATEX-certified configurations are available for solvent-based and hazardous-area applications.
The instrument can be integrated directly into automation systems as a standalone sensor or used with a Vaisala Indigo transmitter for local visualisation, diagnostics, configuration and ease of operation.
This flexibility allows process teams to apply the same measurement principle from early process development to commercial manufacturing, supporting scale-up, process understanding and digitalisation without adding unnecessary analytical complexity.
Supporting greener production
As peptide manufacturing moves towards greener, more efficient production informed by process data, the need for practical, scalable PAT tools continues to grow. Inline refractive index monitoring offers a simple, robust way to improve real-time visibility during peptide synthesis, supporting process understanding, solvent optimisation and manufacturing consistency.
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