Raman Spectroscopy for Efficient Process Analytics

Raman spectroscopy is a non-contact method for the analysis of materials and ingredients.Raman spectroscopy characterizes the quality of a substance as well as the amount of a substance. Ingredients in biological, chemical and pharmaceutical applications are analyzed.Furthermore, it is used for material characterization of semiconductor materials, precious and semiprecious stones, catalysts, minerals, polymers and many other materials. Raman spectroscopy is based on the Raman effect, which was demonstrated experimentally in 1928 by the Indian physicist Sir Chandrasekhara Venkata Raman. For this discovery, Sir Raman in 1930 was awarded the Nobel Prize.

In Raman spectroscopy, the molecules of a substance to be examined are irradiated with monochromatic light [laser]. The majority [99.9%] penetrates the sample. Another part of light is scattered by the molecules in all directions of space [Rayleigh scattering]. Penetrating as well as scattering does not change the frequency of light. An even smaller part [approx. 10–6 %] shows the Raman effect. The Raman scattering is an interaction of the light with the electron shell of the molecules, which result in a change of the light frequency.

Each material shows a characteristic Raman signal that shows spectral lines whose frequency is shifted relative to the excitation frequency [Raman shift].The frequency shift corresponds to a change in light energy. The energy difference between excitation light and Raman scattering is typically used to describe the Raman effect. This energy difference is expressed in wavenumbers [cm-1]. This unit is proportional to the energy.

In Figure 1 Raman spectra are shown: The spectra show the Raman effect of acetone, ethanol and isopropyl alcohol.

Fig. 1 Raman Spectra of Acetone, Ethanol and Isopropylalcohol

Solids and liquids can be non-destructively analyzed without much preparation. The content of water does not interfere with the analysis of the spectra that are available within seconds. Similar to a fingerprint each spectrum is characteristic to a material, and can be very clearly analyzed. Samples can even be analyzed through glass or polymer packaging.

On the market a variety of spectrometer systems for laboratory use are available which can be used very efficiently for the analysis. For laboratories flexible measuring systems with variable ranges and alternative excitation sources offer interesting options. Usually for this flexibility recalibrations at regular intervals are taken into account. In addition, the laser safety briefing is mostly required for all staff in prosimity of the instrument.

For use in production environment, ease of the stability and maintenance must be the main focus. Furthermore, the systems should be able to measure autonomously 24/7 after commissioning and should be ideally maintenance free until the next revision of the system. In addition, a process spectrometer should have a different laser class as a research system to expose the personnel to any hazard.

These challenge tec5 AG faced by developing the Raman spectrometer system. Improving the detection sensitivity in order to detect low concentration in short measuring time. Special attention was paid to the laser safety as the system is installation in the plant. A high flexibility is achieved by allowing an installation over optical fibers to transport the laser and the Raman signal.

The tec5 engineers developed a compact and precise Raman system. Use was made of the distinctive in-house expertise in electronics, sensors, fiber optics and mechanics. A new member to our process-proven MultiSpec® series has been created: The MultiSpec® Raman system. A new process probe was developed, in cooperation with our partner, Hellma. A new version of the process software Multispec® Pro completes the measurement system

The sensitivity of a Raman spectrometer is mainly achieved by the high illumination intensity of a focused laser beam, a high light throughput of the fiber-optic probe, the quality of the spectrometer optics and high efficiency of the detector. The spectrometer system is controlled by a computer; spectral measurement data are recorded, and the results of spectral analysis calculated and displayed. In conjunction with process interfaces the system can be seamlessly integrated into a process environment.

Fig. 2: Schematic of a Raman spectrometer

The MultiSpec® Raman system tec5 uses a diode laser [Class 3B] with a wavelength of 785 nm and a power of 50–500 mW. The wavelength of this light source covers a very large part of possible applications. Lasers with other wavelengths could be envisaged in the future. The polychromator covers an area 300 to 3100 cm-1 with a resolution of about 7 cm-1. The fiber-optic probe has a focal length of typically 1.5 mm, optimized for measurement in liquids. Furthermore, it is offered in various embodiments, be able to operate with more measurement requirements and installation conditions. Chemical resistance and pressure resistance is ensured by the use of titanium and sapphire. The complete system is located in the classic MultiSpec® housing and provides different interfaces. Multi-channel versions with 2 or 4 channels are available. The special software MultiSpec® Pro II Raman detected spectral data and enables calculation of results. With optional modules such as

“Peak Finder”, ”chemometric Prediction” or interfaces for process communication, the software provides options to evaluate various application.

Raman Spectrometer System

Author Dr. Hanns Eckhardt, h.eckhardt@tec5.com

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