Raman spectroscopy, laboratory experiments

Raman spectroscopy s sensitivity to the local molecular enviromnent means that it can be correlated to other material properties besides concentration, such as polymorph form, particle size, or polymer crystallinity. This is a powerful advantage, but it can complicate the development and interpretation of calibration models. For example, if a model is built to predict composition, it can appear to fail if the sample particle size distribution does not match what was used in the calibration set. Some models that appear to fail in the field may actually reflect a change in some aspect of the sample that was not sufficiently varied or represented in the calibration set. It is important to identify any differences between laboratory and plant conditions and perform a series of experiments to test the impact of those factors on the spectra and thus the field robustness of any models. This applies not only to physical parameters like flow rate, turbulence, particulates, temperature, crystal size and shape, and pressure, but also to the presence and concentration of minor constituents and expected contaminants. The significance of some of these parameters may be related to the volume of material probed, so factors that are significant in a microspectroscopy mode may not be when using a WAl probe or transmission mode. Regardless, the large calibration data sets required to address these variables can be burdensome. 

Surface Enhanced Raman Spectroscopy A Novel Physical Chemistry Experiment for die Undergraduate Laboratory 35... 

Differences in perception of the technique s maturity may originate from a simple lack of widespread knowledge about the approach. There are only a few short courses offered. Many students learn of it only as a side note in a physical chemistry textbook and never have hands-on training to use it. Laboratory-based Raman instrumentation is not as ubiquitously available for familiarization and casual experimentation as FTIR. While most vendors will arrange to do feasibility studies and preliminary trials with potential customers, these kinds of short experiments make it difficult for new users to build a solid familiarity with the technique. If a group has years of experience with NIR, it can be difficult to remember to re-examine Raman spectroscopy for each new project to see if it would be a better technical choice and ultimately easier to do. 

Although the experiments referred to here demonstrate the wealth of kinetics and structural data that can be obtained from TR-XAFS data, application of XAFS spectroscopy combined with complementary techniques provides unique and even more detailed information. This statement refers to the most elegant way of using XAFS spectroscopy simultaneously with other methods (e.g., XRD Clausen, 1998 Clausen et al., 1993 Dent et al., 1995 Thomas et al., 1995), and it also refers to XAFS experiments complemented by experiments carried out under similar experimental conditions (e.g., laboratory techniques such as XRD, Raman spectroscopy, TG/DTA). More often than not, a detailed XAFS analysis is possible only when all additional data (characterizing phases, metal valences, and structure) representing the catalyst are available. Furthermore, the analysis of TR-XAFS data should aim at extracting as much information from the XANES part and the EXAFS part of a XAFS spectrum as possible. 

Raman Systems. - Raman instruments may be constructed by assembling components piece by piece, or one may rely on the experience and (business-driven) sense of Raman manufacturers/distributors and acquire a Raman "package." Figure 1 shows the piece by piece Raman spectroscopy apparatus used in the authors laboratory. The laser source is a Spectra-Physics argon-ion laser (Model 165), the monochromator is a SPEX Triplemate, and the detector is an IPDA/OMA (EG G, PAR) which is controlled with a dedicated... 

This was initiated by first choosing a simple test bed chemical reaction to evaluate and understand the functionality, flexibility and limitations of the microreactor platform. The reaction of acetic acid and methanol to form methyl ester was selected because the reaction was temperature sensitive and of minimal toxicity. This chemistry has been extensively studied in the author s laboratory previously by Raman spectroscopy in a typical batch reactor. The batch reactor results were a very useful foundation when trying to understand the reaction processes in the microreactor. The microreactor experiments were structured to study reaction response to reactor parameter changes (temperature and flow rate) using Raman spectroscopy. 

Introductory Raman spectroscopy fares better in the mainstream analytical chemistry and chemical education journals. Although they are not specifically aimed at students, the A-page articles in Analytical Chemistry and the Focal Point articles in Applied Spectroscopy are often written on the level of an advanced undergraduate text. The chemical education journals try to provide tutorial material and laboratory experiments in areas that are too new or too specialized for inclusion in standard texts. The Journal of Chemical Education publishes tutorials in specialized topics as well as broader reviews aimed at the college chemistry professor and the undergraduate chemistry student. 

Hendra has proposed similar IR-Raman experiments for a laboratory sequence based on FT-Raman spectroscopy [8]. Because the laboratory is based on a commercial instrument with l-cm resolution, it is possible to not only do a band assignment of chloroform but also to resolve isotopic splitting in the spectra. For the 368-cm umbrella mode, isotopic splitting is clearly visible. Once this effect is pointed out to students, most quickly realize that the band should contain four (incompletely resolved) components in the ratio 27 27 9 1. 

The development of Raman spectroscopy has generally lagged behind that of IR owing to greater technical difficulties. The advent of the laser was the most important event in its history and has enabled many special and esoteric Raman experiments to be conceived. It is no longer confined mainly to the laboratory and is now used extensively in industry. 

In 1970 two new experiments were added, one on Fourier transform infrared spectroscopy and the other on Raman spectroscopy with laser excitation. This was the last year of the laboratory offering. In 1971 there was a precipitous drop in attendance, perhaps related to the sharp economic downturn, and the first week (containing the laboratory) had to be canceled. Only 29 persons attended the second week. MIT informed Professor Lord that it no longer wanted to sponsor the course, so he asked Lippincott, Miller, and Mayo whether any of them wanted to offer the course at their institution. Lippincott and Miller could not do so, but Mayo, who by then was at Bowdoin College in Brunswick, Maine, was enthusiastic. Hence after 22 years at MIT, the course was moved to Bowdoin College, where the 1972... 

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