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Ia FT-Raman really worth it?

Is FT-Raman Really Worth It?

The past few years have seen a surge of interest in Raman spectroscopy stimulated by the development of new techniques such as Fourier-transform Raman Hadamar-transform Raman, deep-UV excitation, and the availability of sensitive multichannel detectors. This broadening capability has also brought Raman to the attention of many more potential users, especially in industry where product innovation often depends on comprehensive knowledge regarding the chemistry of a particular material or ingredient.

As one of various tools available to the analytical chemist, Raman has always had a number of attractive characteristics. Raman supplies detailed molecular and structural information in a non-contact, non-destructive manner. There is little or no sample preparation, and the technique is easily employed with aqueous solutions. The distinctive structural data accessible via Raman is critical to product development in many areas. That's because manufacturers tend to improve or differentiate their products by varying formulas or ingredients rather than concentrating on the introduction of totally new products.

Until recently, however, Raman had one major drawback -- fluorescence interference. For example, it has been estimated that more than 90% of all polymers could not be analyzed using conventional Raman spectroscopy because of unquenchable fluorescence. Fortunately, this problem has been eliminated by the commercialization of economical, easy-to-use instruments for Fourier-transform (FT) Raman spectroscopy in the near infrared. The answer to the question posed in our title is an unequivocal "yes". FT-Raman is definitely worth the investment required to place this technique at the disposal of a broad range of users, because Raman spectroscopy is now a practical analytical tool for work with virtually any sample.

Experimental The FT-Raman spectra presented in this paper were obtained on a SPEX Industries Inc., FT-R/IR system consisting of a 35W cw Nd: YAG laser, laser prefilter, sample compartment with 180| collection optics and built-in power meter, a Bomem 100 Michelson interferometer, cooled InGaAs detector, and a data system based on a PC/AT compatible computer. The resolution of the spectra is 4 cm raised to -1.

The conventional Raman spectra were acquired on a SPEX Ramalog-91 system consisting of a 1403 double spectrometer, 90| sample-collection optics, and multialkali photo-multiplier tube. The laser is a coherent Innova 70-4 4W argon ion unit. The spectra were obtained with either 514.5 nm or 488.0 nm radiation and 50 mW of power at the sample. Resolution was also 4 cm raised to -1. An optical diagram of the system is published elsewhere.

Results and Discussion The advantages of FT-Raman spectroscopy and the practicality of obtaining FT-Raman spectra with sophisticated interferometers have been amply demonstrated. In this paper, we show it is possible to produce high quality spectra of real-world samples with a relatively inexpensive interferometer.

Note the complete absence of fluorescence background. Such a spectrum is impossible to obtain under normal conditions with visible excitation. These spectra are ideal examples of the main benefit of FT-Raman spectroscopy with near-IR excitation, because the fluorescence in the conventional plot is due to the sample itself. No amount of purification or sample preparation could have eliminated this interference in experiments using visible excitation.

The rhodamine spectrum points out two corollary advantages of FT-Raman spectroscopy. This spectrum was obtained with 1.5 watts of power at the sample, which would have been impossible to do with visible excitation without rotating the sample. Because of strong absorption in the visible, that much power would have caused thermal degradation of the sample. If the spectrum had been obtained with deep UV excitation, it may have been possible to avoid the fluorescence. However, the result would have been a resonance Raman spectrum. This is not necessarily a detriment, although at times it is important to have information about the ground state of the molecule rather than the data about the excited state delineated in a resonance spectrum.

If the only problem materials for conventional Raman spectroscopy were compounds with strong natural fluorescence, FT-Raman would be interesting but it would not have broad practical appeal. Yet fluorescence creates interference in many samples where the material of interest has no intrinsic fluorescence. Such fluorescence may arise from additives like sizing or coloring agents, or from impurities. In any case, obtaining a Raman spectrum from these samples via conventional methodology can be tedious at best and, perhaps, even impossible.

The only Raman features observable above the fluorescent background are the C-H stretches. Nylon 6/6, being a condensation product of a straight chain amine and carboxylic acid, would not be expected to fluoresce. So this particular sample has some additional material which causes the interference. This is typical of the problems observed with polymer samples.

Antifreeze is another type of industrial sample which will not yield a Raman spectrum under conventional circumstances. The reason is that a fluorescent dye such as fluorescein is often used to give antifreeze a distinctive color. In addition to the elimination of fluorescence, these results confirm the throughput and multiplex advantages of interferometry, as well as the ability to illuminate the sample with higher laser power. And although ethylene glycol is not a strong Raman scatterer, it is still possible to maintain a signal-to-noise ratio sufficient for obtaining a survey scan in 10 minutes of data acquisition. Therefore, the sensitivity of the FT-R/IR system is clearly sufficient for routine analyses.

Conclusion The obvious conclusion to be drawn from the results presented above is that, yes, FT-Raman is really worth it. The elimination of fluorescence and the retention of reasonable data acquisition times clearly establishes the broad utility of this analytical tool. Similarly, the simplicity of the spectra and the narrowness of the bands relative to IR are additional advantages, particularly for analysis of mixtures.

Admittedly, as with any other technique, FT-Raman is not universally applicable. Consequently, it will not entirely supplant conventional Raman spectroscopy for a number of reasons. To begin with, low frequency modes below 300 cm raised to -1 are unavailable, at least for the present. Also, for samples like semiconductors, fluorescence is not a problem. There is even a distinct disadvantage with FT-Raman in that materials such as GaAs are transparent at the 1.064 um laser wavelength used for FT-Raman analysis. What's more, because the overwhelming majority of compounds have their electronic absorptions in the ultraviolet or visible, resonance Raman spectroscopy will continue to be done by conventional means.

On the other hand, FT-Raman spectroscopy does eliminate fluorecence interference--the main objection to Raman for numerous analytical applications. The improved frequency accuracy of the interferometer also makes spectral subtractions more reliable. And the routine characterization of many diverse samples with this technique should lead to the availability of spectral libraries in the near future, making the use of FT-Raman even more convenient.

Thus, there can be little doubt that FT-Raman spectroscopy is a significant addition to the battery of analytical techniques which are becoming increasingly important in many industries. The economy, versatility and convenience of FT-Raman constitutes an indispensable source of data with which to evaluate the potential profitability of a continually expanding range of products.
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Title Annotation:Fourier-transform Raman spectroscopy
Author:Purcell, F.J.; Heinz, R.E.
Publication:Canadian Chemical News
Date:May 1, 1989
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