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13 October 2026 · Dusko Kadijevic, Publisher Analytic Journal
Category: Spectroscopy | Chemical Industry | Reaction Monitoring
Author: Dusko Kadijevic, Publisher Analytic Journal
Last updated: October 2026
Reading time: approx. 9 minutes
Target audience: Process engineers, PAT specialists, plant safety managers in the chemical industry
Inline Raman spectroscopy for reaction monitoring is the use of Raman probes placed directly in the reaction medium of a synthesis reactor to track molecular conversion, intermediates and reaction endpoints in real time without sampling. The method delivers chemically specific information even in aqueous matrices, which makes it particularly suitable for batch and semi-batch syntheses with safety-critical reaction endpoints.
Chemical synthesis reactors are often run on fixed time or temperature profiles — not on actual reaction progress. The result: over-reaction, lost yield and, in exothermic systems, a potential safety problem in the worst case. Temperature and pressure are the established safety-relevant variables in reaction engineering, but they only show how the process responds — not directly what is happening chemically. Raman adds molecular spectroscopic information. Quantitative concentration values require a validated calibration. Qualitative endpoint or intermediate monitoring often works without elaborate chemometrics.
In process analytics, NIR spectroscopy is by far the more established technique — in use for decades, with mature calibrations, application templates and operator know-how in the plants. Inline Raman for reaction monitoring is far less widespread in pharma, chemicals and petrochemicals than its technical advantages would suggest. For a long time this was also a question of price: Raman systems used to be expensive and offered by only a few suppliers. That is changing — lasers and detectors have become noticeably cheaper, and the number of manufacturers is growing. Two technical reservations weigh just as heavily in practice: fluorescence interference and the question of whether a probe survives the process at all.
Raman spectroscopy is based on inelastic light scattering: photons from a laser excite molecular vibrations, and the shifted frequency of the scattered light is a substance-specific fingerprint. Unlike MIR/FTIR spectroscopy, which usually needs direct ATR contact because of strong self-absorption, Raman — like NIR — can be used through glass or quartz windows and in aqueous media, provided windows, fouling and optical geometry are under control. Water itself is a relatively weak Raman scatterer, but can become relevant at low wavenumber shifts or with weak analyte bands. Fibre-optic probes can often be installed well over 100 metres from the spectrometer — more than is typical for NIR, where systems usually hit spectral limits at around 100 metres. This also applies in hazardous areas (where laser power and further requirements for intrinsic explosion protection must also be considered — see the NAMUR document further below).
No sampling is needed: the probe is immersed directly in the reactor contents, the measurement is non-destructive and runs continuously. Raman and IR spectroscopy are complementary, not competing. Raman detects vibrations through a change in polarisability, IR through a change in dipole moment. Some bands are therefore IR-active but Raman-inactive, and vice versa. In practice: for symmetric molecules, non-polar bonds and certain crystal or polymer structures, Raman provides information that IR cannot access or only weakly.
| Technique | Matrix suitability | Sample preparation | Typical interference |
|---|---|---|---|
| Raman | aqueous and organic | none | fluorescence |
| NIR | aqueous and organic | none, but matrix-sensitive | water bands, scattering by particles |
| MIR/FTIR | mainly organic, low water content | ATR contact often required | strong water absorption |
Commercially available Raman systems differ mainly in detection technology and in how they deal with fluorescence.
| System type | Operating principle | Handling of fluorescence |
|---|---|---|
| Dispersive Raman (CCD-based) | grating dispersion of the scattered light, CCD detector; excitation at 532, 633 or most commonly 785 nm — 785 nm is regarded as the most popular compromise between signal strength, low fluorescence and CCD sensitivity | most susceptible to fluorescence at short excitation wavelengths; technically the dominant approach for compact, process-capable Raman systems |
| FT-Raman | interferometer-based, typically 1064 nm excitation, Ge/InGaAs detector | low fluorescence thanks to the long 1064 nm excitation, but also the strongest form of the general Raman signal loss at long wavelengths (1/λ⁴ dependence); interferometers also place higher demands on mechanical stability — modern designs mitigate this but add complexity and cost. Mainly relevant in laboratory and research settings and for strongly fluorescent samples; for compact process probes the dispersive concept is technically more common |
| Shifted excitation Raman difference spectroscopy (SERDS) | two slightly shifted excitation wavelengths; computed difference removes the broadband fluorescence background | computational fluorescence suppression; primarily a measurement/evaluation method, usually running on dispersive systems, but requiring special fast-switching or dual laser sources |
| Time-gated Raman | pulsed laser combined with time-gated detection separates the fast Raman signal from the more slowly decaying fluorescence | active fluorescence suppression independent of excitation wavelength; a younger technology with commercial systems, e.g. in bioprocessing — broad industrial adoption is not yet documented |
The overview covers the currently commercially relevant technology categories but makes no claim to completeness — further approaches may emerge as the market develops.
Independently of the system type, the coupling to the process also differs:
| Coupling type | Description |
|---|---|
| Fibre-optic immersion probe | probe immersed directly in the reaction medium, light guided via optical fibre to the spectrometer |
| Window coupling at the reactor wall | probe measures from outside through a sapphire or quartz window, no direct media contact |
| Bypass flow cell | measurement in an external side stream, decoupled from the main reactor, easier maintenance and cleaning |
In theory, Raman works in almost any reaction medium. In practice, three factors decide between success and silent measurement error.
| Challenge | Cause | Countermeasure |
|---|---|---|
| Fluorescence interference | impurities, catalyst residues or chromophores fluoresce under laser excitation and swamp the weak Raman signal | adapt the excitation wavelength to the matrix (e.g. 785 nm or 1064 nm instead of 532 nm) — always a compromise, because Raman scattering intensity scales approximately with 1/λ⁴: every longer excitation wavelength that reduces fluorescence also costs signal. Plus SERDS, baseline correction |
| Probe fouling and wear | precipitates, polymer films, solid deposits — with aggressive media also direct material attack on windows and sealing faces | rinse cycles, window geometry and material selection tailored to the medium, periodic reference measurement |
| Field installation | mechanical stress on the fibre coupling, Ex certification, laser safety labelling, limited fibre range | certified Ex probe hardware, clear labelling and on-site briefing of personnel |
Probe window versus aggressive medium. From practice: when monitoring a highly corrosive process, it took a good year until a probe was developed that could withstand the medium. Two probes were literally burnt through in the meantime — the sealing faces at window and optics were simply too sensitive for the process. In an aggressive medium, the probe itself is the critical component, not the spectroscopy behind it.
Spectra that suddenly drifted away. From practice: in a process where fluorescence had never been an issue, it suddenly appeared. Cause: thermal oil had entered the process through a leak in the heat exchanger. Even small amounts were enough for a clear fluorescence signal — the Raman measurement detected the leak before the plant itself noticed anything. A lucky find, not a planned feature, but a good example that fluorescence interference is not only a disturbance but can sometimes be an early warning signal.
Laser safety on site. The mandatory labelling at the reactor initially caused confusion among staff and contractors unfamiliar with the symbols. In the analyser house I was once urgently advised by shift personnel not to go near the probe — there was "radiation" there. What they meant was the laser warning sign, not radioactivity — but when the same yellow warning signs hang everywhere, the mix-up runs deep. A short on-site briefing solved the problem — still, this point should not be underestimated in installation planning.
The common lesson from all three cases: a system that is supposed to run at three in the morning without laboratory backup needs a signal quality threshold in the control system — not just a raw spectrum plot. Otherwise the software cannot distinguish fouling-related signal loss from the actual reaction endpoint, and in an exothermic process the reaction continues beyond the intended conversion.
| Reactor type | Typical use | Benefit |
|---|---|---|
| Batch synthesis reactor (specialty chemicals, API precursors) | endpoint detection of the main conversion | shorter hold times, less over-reaction |
| Polymerisation reactor | monomer conversion tracking | trend-based control instead of time profile |
| Continuous flow reactor | real-time coupling to the control loop | automated process control without manual sampling |
| Exothermic reactions with safety relevance* | detection of the reaction endpoint | reduced risk of uncontrolled post-reaction |
* "Safety relevance" describes the context of the reaction here, not the suitability of the Raman measurement itself as a safety function. As described in the section on standards and regulations, inline Raman with chemometric evaluation and typically Windows-based software is currently not a SIL-capable measurement — it supports process monitoring and provides an early warning signal, but does not replace a SIL-qualified protective device.
| Standard/Directive | Short description | Relevance |
|---|---|---|
| ATEX 2014/34/EU | EU directive for equipment in potentially explosive atmospheres | Ex certification of probe and enclosure hardware |
| UKEX (UK SI 2016/1107) | UK regulations for equipment in potentially explosive atmospheres, post-Brexit counterpart to ATEX | Ex conformity for installations in Great Britain |
| IEC 60079 series | international series of standards for equipment classification in hazardous areas | basis for zoning and equipment selection |
| IEC 60079-28 | specifies optical protection concepts (Ex op is, Ex op pr, Ex op sh) for radiation in hazardous areas | governs how laser power in the Ex area is safely limited or interlocked |
| NAMUR AK-PRAXIS: RAMAN in Ex-Zonen (WG 3.6.3, PAT Innovations, 23 July 2025) | official WG-PRACTICE document of NAMUR working group 3.6.3 (PAT Innovations) on implementing Raman solutions in Ex zones; an English translation is available on the NAMUR WG 3.6 pages | supplements ATEX/IEC 60079 with equipment-specific implementation practice specifically for Raman systems |
| NAMUR NE 107 | recommendation on self-monitoring and status signals of field devices | diagnostic concept for signal quality/fouling detection |
| IEC 60825-1 | international standard for laser classification and safety, basis for the EU and worldwide outside the USA | defines laser class, limits and labelling of the probe hardware |
| ANSI Z136.1 / FDA 21 CFR 1040.10 | US laser safety standard (ANSI) and US federal regulation for laser products (FDA) | applies in addition to or in parallel with IEC 60825-1 for installations in the USA |
| ASTM E1840 | Standard Guide for Raman Shift Standards for wavenumber calibration | reference values for wavenumber calibration — not a complete calibration procedure, the standard itself defines no procedure |
Laser safety is regulated differently by region: IEC 60825-1 applies in the EU and most regions of the world. In the USA, ANSI Z136.1 (occupational safety practice) and the FDA regulation 21 CFR 1040.10 (product law) apply in parallel — a laser classification according to IEC alone is not sufficient for a US installation. For plants with sites in both regions, check both sets of rules separately.
No SIL function without dedicated assessment: Inline Raman in the form described here — with chemometric evaluation and typically Windows-based spectrometer software — cannot simply be used as a safety-related measurement within a SIL loop (Safety Instrumented Function). The method is suitable for process monitoring and early warning, but does not replace SIL-qualified instrumentation as the sole protective layer. Dedicated Raman photometers with reduced, hard-wired evaluation logic are a different device class and deserve their own assessment — they are not covered in this article.
Note: check current revisions with the relevant standards organisations before starting a project — this article cites the standards as entry references, not as a binding interpretation.
What is inline Raman spectroscopy for reaction monitoring? Inline Raman spectroscopy is the use of a Raman probe directly in the reaction medium to track molecular composition and reaction progress continuously and without sampling. It provides a chemically specific fingerprint instead of an indirect indicator such as temperature or pressure.
What is the difference between Raman and NIR spectroscopy for reaction monitoring? Raman delivers sharp, substance-specific bands and works well in aqueous media because water scatters only weakly. NIR spectra are broader and more overlapping, but NIR hardware is often more robust against scattered light from particles. The choice depends on the matrix and on the functional groups to be tracked.
When is Raman better suited than NIR? Raman is better suited when small functional groups with weak NIR overtones need to be distinguished or when the matrix is highly aqueous. NIR is often preferable when the sample tends to fluoresce — NIR is typically much less affected by fluorescence than short-wavelength Raman, but not fundamentally immune, and is in turn sensitive to scattering, particles, temperature and water bands.
Which standards apply to Raman probes in hazardous areas? In the EU, ATEX 2014/34/EU and the IEC 60079 series govern the certification of probes and enclosures for potentially explosive atmospheres; in Great Britain, the UKEX regulations apply. IEC 60079-28 adds the optical protection concepts for laser radiation. The specific zoning of the plant determines which equipment category is required.
What are typical mistakes when operating inline Raman probes? Common mistakes are an unsuitable laser wavelength for a fluorescent matrix, missing periodic reference measurements for drift detection, and ignoring fouling until the signal drops noticeably. All three can be avoided with a fixed maintenance and diagnostic concept.
Why is fluorescence a critical problem in Raman measurements? Fluorescence is not a fundamental problem of Raman spectroscopy; it occurs mainly with aromatic compounds, dyes, biological materials and certain impurities — many colourless, simple chemicals or aqueous solutions can be measured by Raman without difficulty. When it does occur, however, it quickly becomes the dominant factor: impurities or chromophores in the matrix emit light themselves under laser excitation, and this fluorescence signal can be orders of magnitude stronger than the actual Raman signal and completely mask it.
How is probe fouling detected and avoided in practice? Fouling usually first shows up as a slow loss of signal intensity over several batches. Periodic reference measurements, defined rinse cycles and a signal quality threshold stored in the control system detect the trend before it leads to wrong decisions.
What distance can the fibre-optic connection between probe and spectrometer cover? Depending on laser power, wavelength and fibre type, distances from tens of metres to a few hundred metres are possible — there is no universal figure because too many individual factors play a role. The actual limit should always be agreed with the spectrometer manufacturer before installation, based on the signal-to-noise ratio required for the specific measurement task.
Why is Raman less widespread than NIR despite its technical advantages? Lasers and detectors have become significantly cheaper in recent years. The remaining cost driver lies mainly in the high-quality suppression of the much more intense Rayleigh scattering — notch or edge filters — and in the calibrated overall system architecture; complete process Raman systems therefore often cost several tens of thousands of euros. On top of that, NIR has an infrastructure head start: it has been established in pharma and petrochemicals for decades, with mature calibrations, application templates and operator know-how that Raman still has to build in many plants.
Quantitative chemometric modelling Multivariate calibration (PLS, PCA) for converting raw Raman spectra into quantitative concentration values is a methodological topic in its own right with its own practical part.
Chemometric calibration for inline spectroscopy is planned as a separate topic for a future Analytic Journal article, currently without a fixed date.
Raman in crystallisation (polymorph control) Polymorph discrimination in crystallisation processes uses Raman differently from the reaction endpoint detection covered in this article and deserves its own treatment.
Raman spectroscopy in crystallisation monitoring is also planned as a separate topic for a future article, currently without a fixed date.
Comparison of specific instrument manufacturers This article deliberately remains manufacturer-neutral. A market overview of specific Raman systems for the process industry is a separate, editorially labelled format.
Inline Raman delivers molecular information directly from the process, not just indirect indicators such as temperature or pressure. The spectroscopy behind it is rarely the problem. The difficulties lie in choosing the laser wavelength against fluorescence, in fouling management and in a field installation that withstands harsh media and Ex requirements. Solve these properly and you get a control loop that bases night-shift decisions on real measurement data — not on a rigid time profile.
1. Standards and directives ATEX 2014/34/EU; IEC 60079 series (equipment for explosive atmospheres); UKEX (UK SI 2016/1107); IEC 60079-28 (optical radiation in explosive atmospheres); IEC 60825-1 (safety of laser products); ANSI Z136.1 / FDA 21 CFR 1040.10 (US laser safety); ASTM E1840 (Raman Shift Standards)
2. Industry standards/recommendations NAMUR NE 107 (self-monitoring and diagnosis of field devices); NAMUR WG-PRACTICE "RAMAN in Ex-Zonen" (WG 3.6.3, PAT Innovations, 23 July 2025)
3. Technical literature: Esmonde-White, K.A.; Cuellar, M.; Uerpmann, C.; Lenain, B.; Lewis, I.R.: "Raman spectroscopy as a process analytical technology for pharmaceutical manufacturing and bioprocessing." Anal. Bioanal. Chem. 409 (2017), 637–649. — Lenain, B.; Lucas, H.; Uerpmann, C.; Davis, K.L.; Ehly, M.A.; Kemper, M.S.; Lewis, I.R.: "Raman Spectroscopy for Process Control in Chemical and Pharmaceutical Manufacturing." Proceedings ICheaP-8, 2007. — Hamlin, T.A.; Leadbeater, N.E.: "Raman spectroscopy as a tool for monitoring mesoscale continuous-flow organic synthesis." Beilstein J. Org. Chem. 9 (2013), 1843–1852.
4. Organisations and data sources NAMUR e.V.; DECHEMA e.V.
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Dusko Kadijevic | Publisher Analytic Journal | analyticjournal.com
Written October 2026. Created with AI assistance.
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This article has been written to the best of our knowledge based on publicly available technical information, standards, and manufacturer data. Specifications may change — for binding information, consult current manufacturer datasheets and relevant standards bodies. Analytic Journal produces editorial content independently of manufacturers and advertising partners.
Author: Dusko Kadijevic — 20 years of experience as a PAT practitioner in the process industry