54 SPECTROSCOPY
RAMAN SPECTROSCOPY for PROCESS MONITORING
Karen Esmonde- White reveals fibre optic sampling probe considerations for in-process quantitative Raman spectroscopy
I
ntegrating analytical tools to understand, monitor and control product manufacturing has many benefits, including improved process efficiency and ensuring consistent quality. Raman spectroscopy is an important tool for laboratory, analytical and process applications. Successful Raman applications have been demonstrated at all scales, from at line in the laboratory to on-line in manufacturing in PAC and PAT environments. Kaiser has applied the measurement principles of Raman spectroscopy in a process or manufacturing environment for understanding, monitoring and controlling continuous processes or unit operations for 20 years. Process, logistical and technological aspects need to
be considered when integrating Raman spectroscopy in a process environment. One physical attribute that affects sampling of the process is optical scattering. Here we describe technologic and process considerations in sampling probe design in response to optical scattering and how these factors affect in situ implementation of Raman spectroscopy. Optical scattering arises from
differences in refractive index. Immiscible phases, particulates or bubbles are sources of optical scattering and can cause the reaction medium to appear turbid. Multiple scattering of photons results in diffusion of the excitation laser. Kaiser has a line of proven process- compatible fibre optic probes to ensure that the measurement is representative of the sample. Fig.1 shows a variant of Raman spectroscopy that can be used in process measurements. Backscattered Raman spectroscopy is achieved using a single excitation and single collection fibre in the probe and typically samples a small volume close to the probe window. Tis probe configuration is primarily used as an immersion probe for in situ liquids measurements
or to measure the surface of a solid. If a backscattered Raman probe is moved across the surface of a solid, then a wider area of the sample is measured. Wide area Raman can be used to measure compositional heterogeneity at the surface of a solid, and it can be employed to over-sample a surface to reduce the limits of detection or improve quantification.1, 2
volumetric Raman uses a large excitation spot and multiple collection fibres to achieve sampling in both the axial and lateral dimensions. Large volumetric Raman provides information on deeper layers in addition to the surface, which is useful for measuring a layered solid such as a tablet or capsule.
In situ Raman measurements Raman spectroscopy is a robust and proven analytical technology for processes involving liquids, solids and turbid media. Applications of Kaiser Raman for liquids include primary API reaction monitoring, sealed microwave systems, continuous flow reactors, NeSSI platform devices, and small volume thermal reactors. In applications involving liquids, optical
Large
Fig.1. The principles of Raman spectroscopy variants that are employed for in-process measurements, shown here for solids measurements. The probe optical configuration is shown on the first row and a relative sampled area is shown in the second row
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