FTâRaman Spectroscopy: A Catalyst for the Raman Explosion

Waters Symposium: Raman Spectroscopy edited by

Waters Symposium

C. W. Gardner ChemImage Corporation Pittsburgh, PA 15208

FT–Raman Spectroscopy: A Catalyst for the Raman Explosion? Bruce Chase Corporate Center for Analytical Science, DuPont Central Research and Development, Experimental Station, Wilmington, DE 19880-0328; [email protected]

Fourier transform (FT) Raman spectroscopy involves the use of a multiplexing spectrometer, such as a Michelson interferometer, to detect and analyze the scattered radiation from a sample. In 1985, this represented a significant paradigm shift from the practice of Raman spectroscopy over the previous two decades. Dispersive instrumentation, using gratings and multistage monochromators, had been the method of choice and significant results had been obtained in a wide variety of fields using such instrumentation. Any shift away from existing spectrometers would obviously require a major effort. Whenever one contemplates such a significant shift in approach, there are two questions to be asked; WHY and HOW? The WHY is the more important question. Why bother to go to a new or different approach? What are the advantages and what do we stand to gain if this approach is successful? Are the perceived advantages significant enough to warrant the effort? HOW relates to technology. Is the technology available to successfully implement this new approach? In the case of FT–Raman spectroscopy, the answer to WHY was actually presented two decades earlier and then forgotten. In 1964, an article published in Nature by Chantry, Gebbie, and Hilsum (1) described an attempt to measure Raman scattering of iodine using an interferometer. The results were marginal and it was clear that there were severe technology limitations. The available lasers were pulsed and extremely noisy, the detectors were poor, and the interferometer performance was nowhere near that available today. It is probably significant that this article was never followed up with additional publications. Effectively, it disappeared from view. However, there is one paragraph that was probably the most important part of the manuscript. In it the authors said, Many pure substances and industrial intermediates are strongly colored and their Raman spectra can not be recorded using ultraviolet and visible lines, but nearly all compounds have a region of transparency in the near infrared. If, therefore, methods could be developed in which a selection of infrared exciting lines were used, Raman spectroscopy could be more widely applied and could become a valuable complement to standard infrared analysis.

This one statement pointed towards the WHY for FT–Raman spectroscopy. If absorption and the parallel process of fluorescence could be avoided, then Raman scattering could be significantly more applicable to a wide range of problems. Unfortunately, the HOW was missing. The technology at the time did not support the development of functional instrumentation for near infrared FT–Raman spectroscopy. www.JCE.DivCHED.org

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Now, let us move ahead two decades. It is 1984 and what is the state of Raman spectroscopy? In a series of biannual reviews (2), Don Gerrard provided such an assessment in Analytical Chemistry. In 1985, Gerrard found that the active areas of Raman spectroscopy included SERS (surface enhanced Raman spectroscopy), resonance Raman, and time-resolved measurements among others. Instrumentally, multichannel detectors such as photodiode arrays (PDA) and UV excitation were just beginning to be used. Micro Raman was clearly beginning to show its promise as shown by the work of Delhaye and co-workers. There were some 3000 articles published in the preceding two years. The limitations for Raman spectroscopy were the same as found in several of his previous reviews: fluorescence, cost, and complexity. At the same time, there was a virtual revolution proceeding in the sister field of vibrational spectroscopy involving infrared measurements. FT–IR was clearly establishing a dominant position and was well on the way to displacing dispersive instrumentation. The dramatic increase in sensitivity, coupled with the excellent frequency precision enabled major improvements in the practice of infrared spectroscopy. New (or improved) approaches to sampling included diffuse reflectance, infrared emission, and reflection absorption. There were many articles and presentations that clearly defined the advantages of Fourier transform measurements for infrared spectroscopy. The benefits of the three advantages, Fellget, Jacquinot, and Connes (3), were obvious to everyone. Early in the 1980s many researchers started to ask whether interferometric measurements could bring the same advantages to Raman scattering measurements. The problem at this time was that the major point of the Chantry article had been lost. Most people were focused on the current version of Raman instrumentation involving visible lasers and PMT (photomultiplier tube) detection. Hirschfeld addressed the issue in several articles (4, 5). He correctly pointed out that the major advantages of interferometric measurements were absent for visible lasers and shot-noise limited measurements. The multiplex advantage is only present when the experiment is detector-noise limited. Since a multiplex measurement places all of the integrated spectral intensity on the detector at one time, if the noise goes up as a result of this additional intensity, the multiplex gain is lost. This is exactly the situation for a shot-noise limited measurement, which Raman scattering in the visible with PMT detection encompasses. Furthermore, since the general practice called for tight focus of the laser on the sample, one could be throughput limited by the collection optics rather than the entrance slit of a dispersive spectrometer, minimizing the Jacquinot advantage. As Hirschfeld pointed out, there was really no effective argument for simply

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replacing a dispersive instrument with an interferometric instrument for visible Raman scattering. The most significant impediment to the widespread use of Raman scattering at this time was still fluorescence. Researchers had tried a variety of approaches to minimize the effects of sample and impurity fluorescence including photo bleaching, baseline filtering, time-based rejection, UV excitation, and red lasers (HeNe and Kr+). The end result was that there was still no universal approach to the problem. Fluorescent interference still was the rule rather than the exception, especially for industrially relevant samples. Combining the observations in the preceding paragraph, one can conclude that we were focused on the wrong question. Instead of asking, Will interferometric measurements of Raman scattering improve the sensitivity when compared to single channel dispersive measurements?

We should have been asking, How can we do Raman scattering with near infrared lasers to avoid the excitation of the interfering fluorescence?

Changes Needed

nificant. Since the measurement is detector-noise limited, combining all the scattered radiation onto the detector at once does not cause an increase in noise, so the interferometric measurement gains relative to the single-channel measurement by the square root of the number of resolution elements. FT–Raman measurements in the near infrared should show significant advantage when compared to dispersive singlechannel measurements in the near infrared. Fortunately, the technology had made significant strides since 1964, and it was now possible to use existing interferometers with nearinfrared beam splitters, existing near-infrared detectors with good performance, and existing CW (continuous wave) lasers. The WHY and the HOW could both be addressed and FT–Raman was both possible and worth the effort. The first measurements to demonstrate feasibility were of course made on strongly scattering samples (6). Results for more typical samples such as polyethylene were much less impressive as seen in Figure 1. It was quickly apparent that major optimization of the instrumentation was required. The components of an FT–Raman instrument can be separated into four logical groups: lasers, filters, interferometers, and detectors. The approach taken to improve the performance was to examine each of the components separately and to consider opportunities for improvement.

As Chantry pointed out twenty years earlier, we needed to move into the near infrared for our excitation source. The use of near infrared lasers would mean that PMT detection was no longer possible since these devices would not function at wavelengths longer than 1 µm. The detectors available for use at these longer wavelengths were much noisier than photomultipliers. In other words, the measurement would now likely be detector-noise limited. That, however, is exactly the situation where the multiplex advantage is sig-

Lasers The choice of laser was dictated by both performance needs and available lasers. In order to minimize fluorescence a move to the near infrared was dictated (7, 8). The lower the energy (longer the wavelength), the less the likelihood of excitation of the fluorescence. Unfortunately, the cross-section for Raman scattering varies as the fourth power of the frequency. There is a loss of approximately a factor of six-

Figure 1. First FT–Raman spectrum of polyethylene; 1064 nm excitation, 500 mW, 1-h measurement time.

Figure 2. FT–Raman spectrum and visible Raman spectra of a cyanine dye (reprinted with permission from ref 7).

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Filters The next challenge was the collection and optical filtering of the scattered radiation. Since an FT experiment looks at all the scattered light simultaneously, there was a major

problem associated with the Rayleigh line. The weak nature of Raman scattering meant that almost all of the scattered light arises from elastic scattering. This elastic component of the scattering carries no vibrational information, but if allowed to pass onto the detector, it will completely saturate the available dynamic range. Given an average Raman scattering efficiency of 10᎑8, it is clear that a filter with an optical density (OD) of close to 8 at the laser line wavelength is needed. In addition, since Raman scattering intensities are inherently low, the filter should only minimally attenuate wavelengths longer than the laser wavelength (the inelastically scattered Raman photons). The sharpness of this filter will determine how close to the Rayleigh line the FT–Raman spectrometer can go. Filtering out this elastic component represented one of the major challenges for FT–Raman spectroscopy. The effects of insufficient filtering are shown in Figures 3 and 4. Figure 3 shows FT–Raman spectra of anthracene, which has a large Raman scattering cross-section. The effective OD of the Rayleigh line filter at 1064 nm was 8 for trace A, 7 for trace B, and 6 for trace C. As the effective OD at 1064 nm is reduced, more Rayleigh light is allowed to strike the detector. It is clear that the noise throughout the entire spectrum is increased. This is an aspect of Fourier spectroscopy that is not always recognized. Introduction of excess signal with the accompanying noise at one frequency can perturb the spectrum at all frequencies. Thus, allowing extra Raleigh intensity through the filtering, as in Figure 3, causes the noise to increase across the entire spectrum. This is shown in more detail in Figure 4 where the traces A and B from Figure 3 are scale expanded. Even a small reduction in Rayleigh line filtering can affect the noise performance in the spectrum. For any sample, the

Figure 3. FT–Raman spectra of anthracene, showing the effects of reduced Rayleigh line filtering: (A) laser line OD 8, S/N 2450:1 (B) laser line OD 7, S/N 1300:1, and (C) laser line OD 6, S/N 410:1.

Figure 4. FT–Raman spectra (traces A and B) from Figure 3 scale expanded.

teen going from 541.5 nm to 1064 nm in laser wavelength. In addition, as the Raman spectrum is shifted deeper into the near infrared, the available detectors become significantly more noisy. FT–Raman spectroscopy benefited from the fortunate availability of a laser that had a near perfect compromise between fluorescence minimization and diminishing cross-section: the NdYAG laser that lases at 1064 nm. The ready availability of CW powers in excess of 1 W at 1064 nm allowed early experiments to proceed. The effectiveness of the fluorescence minimization is shown in Figure 2. The first versions of NdYAG lasers that were used for FT–Raman spectroscopy had significant noise issues. Since fluctuations in the laser output translated directly to noise at the detector, this meant that early FT–Raman experiments were actually source fluctuation-noise limited, the worst case scenario for a multiplex measurement. FT–Raman measurements improved as laser performance was improved with a decrease in fluctuation noise. Early systems used flash lamppumped versions of the NdYAG laser, where active stabilization resulted in improved noise performance. Most systems now are equipped with diode-pumped versions that are effectively noise free on the scale required by the FT–Raman experiment. The current versions of the NdYAG or NdYLF lasers offer the optimum wavelength for fluorescence minimization and the noise performance is well under the detector noise for the system. Therefore, further improvements in laser technology will not have a significant impact on FT– Raman performance.

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Rayleigh line needs to be reduced in intensity below that of the strongest line in the Raman spectrum. The search for suitable filters started with the same material used in protective laser goggles. These plastic sheets provided sufficient absorption strength to attenuate the Rayleigh scattering, but also showed significant absorption across the Raman spectrum. The second generation of Rayleigh line filters included multilayer dielectrics that had both sufficient absorption at the Rayleigh line and reasonable transmission over the Raman spectrum. However, multilayer dielectrics suffer from a structured transmission curve arising from multiple interferences within the layers. The filters currently in use in most spectrometers are volume phase holographics. These have excellent laser line attenuation with high transmission starting at about 100 cm᎑1 Stokes shift. Further improvements in filter technology would have a minimal effect on the FT–Raman spectrometer performance.

around the interferometer was the choice between a dedicated instrument and a multipurpose instrument, capable of both mid-infrared and FT–Raman operation. Since a multipurpose instrument required compromises in design, particularly around the number of reflecting surfaces, it was quickly apparent that for best sensitivity, a dedicated instrument was preferred.

Detectors

The third component of an FT–Raman spectrometer is the actual interferometer. Since few research groups were in a position to significantly alter the interferometer, it was clearly a case of “what you bought is what you have”. The only part of the interferometer subject to optimization was the beamsplitter. For near-infrared operation, two choices existed, quartz and CaF2. Either material could be produced at the requisite flatness, and modulation efficiencies approaching 75% were quickly achievable. Once again, further improvements in this component were unlikely and also of little potential impact on overall sensitivity. The other decision

Detectors are the final spectrometer element. The optimum detector would be responsive from 0.8 to 1.6 µm. This would allow coverage of both the Stokes and antiStokes regions, while minimizing the effect of background infrared radiation that would contribute to the detector noise. The two clear choices for detectors are germanium (Ge) and indium gallium arsenide (InGaAs). Both detectors have gone through improvement, primarily in the preamplifiers used. For either Ge or InGaAs the detectivity now approaches 1 × 10᎑15 W Hz᎑12. The spectral bandwidth is optimum for either of these detectors. Therefore, the only aspect that might be improved is the noise level or detectivity. Initially one might assume that further reduction in detector noise would result in higher sensitivity for the FT–Raman measurement. Unfortunately, this is not born out by experiment. Three separate singlebeam spectra recorded on an FT–Raman instrument are shown in Figure 5. Trace A shows the Raman spectrum of a sample of anthracene, trace B shows the spectrum of a white light with the source intensity adjusted to produce the same integrated intensity as observed in trace A, and trace C shows

Figure 5. Spectra of (A) anthracene, (B) white light intensity adjusted to produce equivalent integrated intensity as trace A, and (C) spectrum with no illumination (detector noise).

Figure 6. Difference noise spectra from the three spectra in Figure 5.

Interferometer

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the spectrum obtained with no illumination, that is, the spectrum of the detector noise. If further increases in detector noise performance are to be beneficial, the FT–Raman experiment needs to be detector-noise limited. In order to determine the noise level for the three traces shown in Figure 5, each spectrum was recorded twice and a difference spectrum generated. The difference spectrum should be the noise spectrum (times 1.414) under each of the three illumination conditions. These difference-noise spectra are shown in Figure 6. Clearly, as the intensity on the detector is increased, the noise increases. This can only occur when the measurement is either shot-noise or source fluctuation-noise limited. In either case, further reductions in detector noise will have no effect on the sensitivity of the measurement. So, as for each of the previous three components of the FT–Raman spectrometer, it appears that the detection systems are already operating near their limiting performance. Improved Sentitivity With all of these improvements, how has the sensitivity improved compared to the first FT–Raman spectrum of polyethylene? The FT–Raman spectrum of the same polyethylene sample taken with current instrumentation is shown in Figure 7: compare with the spectrum in Figure 1 The measurement time has been reduced to 30 s and the incident laser power reduced to 200 mW. There has been a significant improvement over those first few early spectra, but there is no reason to expect any further dramatic improvements. FT– Raman can clearly be considered to be a mature measurement technology.

Figure 7. FT–Raman spectrum of polyethylene, 30 s measurement time, 200 mW power.

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FT versus Dispersive Raman Now, it is important for the practitioner to ask how the FT–Raman experiment differs from conventional dispersive Raman measurements. There are six different aspects where there are significant differences: • Circular versus line imaging • Spot size requirements • Allowable power levels • Frequency precision • Thermal background • Nature of the noise

The imaging utilized in a Michelson interferometer is circular, as opposed to imaging in a slit-based instrument, which is a line. Optimum illumination of slit requires specialized optics such as cylindrical lenses or fiber optic bundles (circle to slit). In the FT–Raman measurement it is simpler to optically match the illumination to the instrument aperture. In terms of the instrument aperture, there is another crucial difference. For a dispersive instrument to get as much of the collected scattered radiation through a slit of several hundred microns (typical slit width), it is necessary to have an illuminated spot on the order of 50 µm or less for a f4 instrument if collection is done at f1. An interferometer does not have an entrance slit, but does have a resolution limiting aperture that is usually on the order of 4 mm. Under the same magnification conditions, the spot size at the sample for an FT–Raman measurement can be as big as 1 mm. This allows much lower power densities than can be achieved with conventional systems and can minimize thermal damage to the sample. Since FT–Raman measurements are carried out in the near infrared where sample absorption is minimized, high powers can be utilized with less chance of damage due to absorption. Michelson interferometers have inherently high frequency precision owing to the nature of laser referencing. This allows routine spectral subtractions to be done with minimal distortion owing to frequency registration from spectrum to spectrum. However, the near-infrared operation does come with a price. The presence of thermal backgrounds is much more severe for FT–Raman spectra in the near-infrared than for conventional Raman spectra in the visible. Sample temperatures of even 100 ⬚C can cause spectra to be distorted. A final difference involves the nature of the noise. An FT measurement by its very nature causes noise to be distributed across the entire spectrum. As discussed with respect to effective filtering of the Rayleigh line, a strong Rayleigh line with associated noise will cause the entire spectrum obtained with an FT instrument to be noisier. Of course, the final difference between the two approaches involves the relative sensitivities. As discussed in previous sections, with FT–Raman we have come about as far as we can go. Even with all the improvements, in terms of raw sensitivity, a CCDbased Raman spectrometer will still have between one and two orders of magnitude more sensitivity than an FT–Raman system IF there is no fluorescence!


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Figure 8. Number of Raman articles published per year.

Figure 9. Number of FT–Raman articles published per year.

Summary

we have reached the point where Raman measurements are just as routine as infrared measurements, and the presence of Raman instrumentation as a routine analytical tool has been established for the foreseeable future.

So, we come to the question raised in the title. Has FT– Raman been a catalyst for the Raman explosion? Since the rediscovery of FT–Raman in 1985, there has been a parallel development of dispersive Raman instrumentation utilizing CCD detectors. It is possible that the success of FT–Raman using near-infrared excitation for fluorescence rejection could have provided some direction. The first published CCD-based Raman data was by Murray et al. (9) in 1986. In 1989, both Pemberton and Sobocinski (10) and McCreery and co-workers (11) published on the utility of CCD detection with red excitation for fluorescence minimization. By 1990, both FT– Raman and CCD-based Raman were off and running. Figure 8 shows the number of articles published on Raman spectroscopy per year and Figure 9 shows similar data for FT– Raman. From these two tables, it appears that although FT– Raman grew significantly, the overall growth in the practice of Raman scattering was not directly attributable to FT–Raman. From the perspective of over a decade, it now seems clear to me that FT–Raman brought a level of enthusiasm and excitement to a field that had been languishing for the previous decade, but it was not the sole reason for the growth in the utility of Raman scattering. Along with developments in holographic filters, diode lasers, and CCD detectors, FT– Raman served to revitalize a field that was lagging. Today,

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Literature Cited 1. Chantry, G. W.; Gebbie, H. A.; Hilsum, C. Nature 1964, 203, 1052–1054. 2. Gerrard, D. L.; Bowley, H. J. Anal. Chem. 1986, 58, 6R–13R. 3. For further discussion of the Fellget, Jacquinot, and Connes advantages please see Griffiths, Peter; DeHaseth, James. Fourier Transform Infrared Spectrometry; Wiley & Sons: New York, 1986. 4. Hirschfeld, T. Appl. Spectrosc. 1976, 30, 68–70. 5. Hirschfeld, T. In Fourier Transform Infrared Spectroscopy; Ferraro, J., Ed.; Academic Press: New York, 1979; pp 13–50. 6. Hirschfeld, T.; Chase, B. Appl. Spectrosc. 1986, 40, 133–137. 7. Hallmar, V.; Zimba, C. G.; Swalen, J. D.; Rabolt, J. F. Spectroscopy 1987, 2, 40–45. 8. Chase, B. J. Am. Chem. Soc. 1986, 108, 7985–7988. 9. Murray, C. A.; Dierker, S. B. J. Opt. Soc. Am. 1986, 3, 2151– 2154. 10. Pemberton, J. E.; Sobocinski, R. L. J. Am. Chem. Soc. 1989, 111, 432–435. 11. Williamson, J.; Bowling, R.; McCreery, R. L. Appl. Spectrosc. 1989, 43, 372–375.


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FTâRaman Spectroscopy: A Catalyst for the Raman Explosion

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FTâRaman Spectroscopy: A Catalyst for the Raman Explosion