Raman optical cells

Two types of Raman optical cells have been used so far. Fused silica in the form of cylindrical tubes of inner diameter 2-10 mm are the proper and simplest material for non-corrosive melts. Windowless cells made of graphite or noble metals have proved adequate for studying corrosive fluoride and/or oxide melts. 

In general, a Raman adsorption cell consists of a length of pyrex or silica tubing, one end of which is sealed with an optical flat, and the other either connected to a gas line for admitting the adsorbate or to a vacuum line for evacuating the cell. Activation of the samples may then be carried out in situ 27). 

A trivial yet important application is following ethanol production via a bioprocess. Sivakesava et al.1 simultaneously measured glucose, ethanol, and the optical cell density of Saccharomyces cerevisiae during ethanol fermentation, using an off-line approach. Samples were brought to an instrument located near the fermentation tanks and the measurements made in short order. While they eventually used MIR due to the interfering scatter of the media, they proved that Raman could be used for this application. 

Sivakesava et al. also used Raman (as well as FT-IR and NIR) to perform a simultaneous on-line determination of biomass, glucose, and lactic acid in lactic acid fermentation by Lactobacillus casei.2 Partial least squares (PLS) and principal components regression (PCR) equations were generated after suitable wavelength regions were determined. The best standard errors were found to be glucose, 2.5 g/1 lactic acid, 0.7 g/1 and optical cell density, 0.23. Best numbers were found for FT-IR with NIR and Raman being somewhat less accurate (in this experiment). 

Problems can arise in the investigation of rapid reactions if the reactants are not heated sufficiently fast to the desired temperature, and if the samples from the reactor are not cooled rapidly to stop the reaction. A more sophisticated approach consists of monitoring the changes in concentration in an optical cell, in situ, by means of spectroscopy. Both infra-red and Raman spectroscopy can be used, depending on the sensitivity of characteristic bonds and the wave-number range of interest. 

An optical cell for pressures of up to 200 MPa and temperatures to 200°C is presented in Chapter 4.3.4. The cell can be coupled with a commercial Raman spectrometer to measure the course of the intensity of a bond s signal with time. By calibration, the intensity versus time curve can be converted into a concentration versus time curve, from which the rate of reaction and kinetic parameters can be evaluated. The method is explained in Chapter 3.3.2, considering the decomposition of an organic peroxide. 

An optical cell developed for Raman spectroscopy is presented in Fig. 4.3-29 [42],... 

Fig. 3.12. Non-invasive Raman spectra of pharmaceutical capsules. The spectra were obtained using a laboratory instrument configured in the transmission Raman geometry and a standard commercial Raman microscope (Renishaw) in conventional backscattering geometry. The Raman spectra of an empty capsule shell (lowest trace) and the capsule content itself (top trace, the capsule content was transferred into an optical cell) are shown for comparison. The dashed lines indicate the principal Raman bands of the capsule and of the API (this figure was published in [65], Copyright Elsevier (2008))...

Even if the focus is exclusively on optical cells, it is impossible to review the large number of published experimental arrangements in a chapter of limited size. Since the field has already been treated by Sherman and Stadtmuller (1987), this text is only concerned with optical cells for vibrational spectroscopic studies (IR, NIR, Raman) of fluid phases under conditions involving high pressure and high temperature up to a maximum of about 650 °C and 7 kbar. This is the area of the authors greatest personal experience (Buback, 1981 Buback et al., 1987). 

Harvey TJ et al (2008) Spectral discrimiuation of live prostate and bladder cancer cell lines using Raman optical tweezers. J Biomed Opt 13(6) 064004... 

The inner volume and maximum working pressure of the high-pressure optical cell for the Raman spectroscopic analysis were 0.2 cm and 400 MPa, respectively. The cell had a pair of sapphire (or quartz) windows on both the upper and lower sides. The thermostated water was circulated constantly in the exterior jacket of the high-pressure optical cell. A ruby ball was enclosed to agitate the contents by the vibration from outside. 

Figure 4 Raman spectra of rotation for H2 (a), and intramolecular vibration for H2 (b) and CO2 (c) molecules in the gas and hydrate phases at 280.1 K and 4.3 MPa. The mole fraction of THF is 0.056. The high base line less than 520 cm and the broad peaks at 600, 810 and 1060 cm are due to the quartz window of high-pressure optical cell.

Figure 2 Raman spectra of intramolecular stretching vibration of 1,1-DMCH under four-phase equilibrium condition at 298.28 K and 25.5 MPa for the CH4+1,1-DMCH hydrate system, (a) liquid LGSphase, (b) hydrate phase. The single peak of 751 cm is derived from the sapphire M indow of optical cell.

Frantz JD, Dubessy J, Mysen B (1993) An optical cell for Raman spectroscopic studies of superoritieal fluids and its application to the study of water to 500°C and 2000 bar. Chem Geol 106 9-26 Friedman I, O Neil JR (1977) Compilation of Stable Isotope Fraetionation Factors of Geochemical Interest. U SGeol Surv Prof Paper 440-KK... 

The Raman cell for the study of electrodes in an aqueous electrolyte is relatively simple and more flexible in design compared with the nonaqueous system. Figure 10 shows the two most commonly used cells. Cell (a) is suitable for the front collection mode with different incident angle and the back-scattering mode in the macro-Raman system. Cell (b) is for the micro-Raman system. An optically transparent quartz or glass window may be used, especially to avoid contamination from the ambient atmosphere. Usually, it is not necessary to... 

Lu, W., Guo, H., Chou, I.M., Burruss, R.C., Li, L., Determination of diffusion coefficients of carbon dioxide in water between 268 and 473K in a high-pressure capillary optical cell with in situ raman spectroscopic measurements, Geochim. Cosmochim. Acta. 115 (2013) 183-204. 

An excellent review on vibrational spectroscopy in supercritical fluids was published in 1995 by Poliakoff et al. [6]. In the late 1990s, Kessler et al. [7] developed IR and Raman spectroscopy for the investigation of rapid high-pressure reactions in optical cells. Raman was preferred to IR for the determination of the decomposition rate of peroxides under high pressure. They studied the decomposition of tert-butyl peroxypivalate at pressure up to 180 MPa and temperatures of 90-160 °C. A typical Raman spectrum is presented in Fig. 5.3. 

T. J. Harvey, E. Correia Faria, E. Gazi, A. D. Ward, N. W. Clarke, M. D. Brown, R. D. Snook and P. Gardner, The Spectral Discrimination of Live Prostate and Bladder Cancer Cell Lines Using Raman Optical Tweezers, J. Biomed. Opt., 2008, 13, 064004. 

Very recently, Addleman et al. described a high-pressure cell for the study of TRLIF of uranyl complexes in supercritical CO2 (21). A schematic of the optical cell is shown in Figure 3. The cell has two perpendicular optical paths that are both orthogonal to the SCF flow, allowing absorption, fluorescence, and Raman measurements. The cell body was machined from stainless steel with an internal volume of 0.3 ml. The cell windows were made of 2-mm-thick synthetic... 

Optical measurements (/) such as Raman Scattering, Fluorescence techniques. Vibrational Circular Dichroism, (VCD), Optical Rotational Dispersion (ORD), Raman Optical Activity (ROA) and infrared absorption spectroscopy can overcome many of the obstacles mentioned above due to the fact that optical techniques are non-invasive and can monitor proteins in their native environment and with accurate time resolution. One disadvantage is the low sensitivity. However, the use of Surface Enhanced Raman Scattering (SERS), techniques (2-4) means that proteins can be observed down to the single molecule level. Thus, optical teclmiques hold great promise for the future investigation of protein dynamics processes provided that proteins can be maintained in a suitable and controllable sample cell. 

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