Robust on-li ne Raman spectrometer for process control applications ABOUT VTT VTT is Finland’s leading R&D service provider. About 3000 employees currently organized into six units, one of which is VTT Electronics in Oulu (Northern Finland). The CEO is located in Espoo, near Helsinki. Accounting is by US GAAP, operations are ISO9000 certified. Proven project-based methodology for working with industrial customers in R&D and Product Engineering. Successful track records with several industries. State-o f-theart lab facilities, instrumentation, workshops and machiner. More than seventy (70) scientists and engineers make up the optoelectronic group at VTT Electronics, forming a one-stop shop for the development of optical instrumentation; capabilities include optics, mechanics, electronics, software, and many niche hardware skills. Contact Information Research Manager Jouni Tornberg Tel. +358 8 551 2288 GSM +358 40 555 0756 E-mail: [email protected] Group Manager Janne Suhonen Tel. +358 8 551 2298 GSM +358 40 702 3285 E-mail: [email protected] VTT TECHNICAL RESEARCH CENTRE OF FINLAND VTT ELECTRONICS Kaitoväylä 1, Oulu P.O. Box 1100 FIN-90571 Oulu, Finland www.vtt.fi/ele Science-based innovations. Robust on-line Raman spectrometer for process control applications BACKGROUND Recent developments in CCD detector and diode laser technologies have brought out the possibility of constructing a small, rugged Raman spectrometer, which could be applied to industrial process measurements. Raman spectroscopy provides some major benefits compared to other spectroscopic techniques such as IR and NIR. Raman works well with aqueous samples and a simple calibration method can be utilized in most cases, because of the well-r esolved Raman spectra. What’s more, Raman needs minimal sample preparation, which enables on-line measurements directly from the process line. The contemporary device constructions are, however, versatile measurement systems designed mainly for laboratory analysis. VTT has developed a dedicated device construction for process measurement applications, and the performance of VTT’s Raman spectrometer is equal to the well-kn own laboratory instruments. TECHNOLOGY The principal components of the device construction are a diode laser, a spectrograph and a multi-element CCD detector, placed in a protective housing, and a fiber-optic probe, serving as an interface between the sample and the device. The excitation source is a self-contained laser module with a fiber-optic output of 300 mW at 830 nm. The high-power external-cavity diode laser provides the single longitudinal output mode needed in Raman spectroscopy. The frequencyst abilized diode laser is also temperature controlled via a two-stage TE-cooler and thermistor. The laser diode and optics are sealed and desiccated to serve as protection against dirt and dust. cooler and selecting a detector with an MPP (Multi-Pin-Phasing) option. More than 20 % quantum efficiency up to 1000 nm (=2000 cm-1 ) is ensured by using a back-il luminated version of the CCD detector. A high dynamic range of the acquired data is achieved with a 16-bit A/D converter. The spectrograph of the Raman spectrometer has been designed and constructed through cooperation between VTT Electronics and Specim Ltd. An L-shaped axial transmissive configuration, comprising a holographic transmission grating and two Cooke triplets, was chosen for the spectrograph. Its main components are two multielement lenses, optical axes perpendicular to each other, and a holographic transmission grating, situated at the crossing point at an angle of 45o. High resolution (<8 cm-1 ) combined with high throughput can be obtained with this simple configuration. VTT has also developed a new fiber-optic probe for process measurements. The probe can be inserted into a process pipe for process monitoring, or it is also possible to use it as a hand-held probe head. The probe is suitable for noncontact measurements from samples in bottles and vials through a glass window. The sampling optics and the working distance of the probe are easily changeable for different applications by changing the focusing lens of the probe. The filtering needed for Raman spectra measurements is placed in the head of the fiber-optic probe. The collection side of the probe utilizes a fiber-optic bundle, adapting the head optics to the entrance slit of the spectrograph and ensuring higher sample volumes when inhomogeneous process samples are measured. The compact fiber-optic probe of the VTT Raman spectrometer. The range of our pharmaceutical applications is wide, including determination of component concentrations, homogeneity of pharmaceutical mixtures, monitoring of manufacturing processes of pharmaceutical substances and identification of their polymorphic forms. An example from the food industry is the quantitative analysis of yoghurt content. Raman spectroscopy has been utilized to measure fat, protein and sucrose content in yoghurt production plant. The accuracy obtained both in milk and yoghurt form was ca. 0.5 % for fat and protein and 1 % for sucrose. On-Line Raman measurements from yoghurt factory. APPLICATIONS Thanks to developments in Raman spectrometer component technologies, the Raman techniques have aroused wide interest in various industrial areas. The extensive application area for Raman Spectroscopy covers applications from structure determination to multi-component qualitative/ quantitative analysis. Our Raman studies have been focused on applications from chemical, pharmaceutical, food, and the pulp and paper industry. A self-c ontained CCD camera is chosen as the Raman detector. The high-s ensitive CCD array is placed in a maintenance-free hermetic package on a two-stage Peltier cooler, which can cool the temperature of the CCD detector to 55 °C below the ambient temperature. The dark current has been reduced to an adequate low level by using a two-stage Peltier The robust Raman spectrometer for process applications. ADVANTAGES • No need for sample preparation => enables on-li ne measurements • Fast measurement • Selectivity of Raman spectra => less chemometrics works and simple calibrations • Works well with aqueous samples • Multi-point measurement (i.e.10 points) => enables measurement of many processes simultaneously with one instrument • No moving components => robust technology for process measurements As an example from the chemical industry, we have studied the curing process of polyester resins. The resins were cured, and the Raman spectra were recorded during the curing reaction. The spectral changes were identified (C=C, C=O), the cure process could be monitored from the intensities. The gel time given by the resin suppliers correlated well with the Raman results. It could also be seen that the curing process continues for a long time, up to several weeks. Post-curing will finally complete the curing process. Curing of laminates containing 50-70 % of glass fiber mat could also be followed by this method. Raman spectra of polyester resins during the curing reaction. Monitoring of the curing process.
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