- Email: [email protected]
The development of a sensitive near-IR Raman system
sprreochimica Ana, Vol. 49A, No. 516, pp. 633-643, 1993 Printed in Great Britain
0584-8539/93 $6.00+ 0.00 @?J 1993 PergatnotlPress Ltd
The development of a sensitive near-IR Raman system S. M. MASON, N. CONROY,N. M. DIXON and K. P. J. WILLIAMS*? British Petroleum Research, Chertsey Road, Sunburyon Thames,MiddlesexTW167LN, U.K. (Received 6 October 1992; accepted 21 October 1992) Abstract-This paper describes the development of a highly sensitive near-IR laser Raman system which has been designed and built in our laboratory. The system comprises both a micro and macro sample handling facility. The approach that we have adopted uses a conventional spectrograph with a charge coupled device detector and optics optimized for the 700-1OOOnm spectral region. The laser source is a tunable titanium sapphire laser which provides intense radiation in the 700-900 nm spectral region. We show that the spatial resolution and spectral sensitivity that can be obtained with the microscope attachment are excellent. A comparison between data obtained from our system and that from an Ff’-Raman system is given.
INTIX~DUCTI~N
transform (PI) Raman spectroscopy has made a significant impact in recent years on the activities of Raman spectroscopists. The use of a near-IR (NIR) laser (1.064~m) with an FT-IR spectrometer has provided the Raman method with a much reduced incidence of laser induced sample fluorescence, together with a multiplexing advantage. The method was first demonstrated on a bench top instrument in our laboratories in 1987 [l]. Since that time the method has found a variety of applications [2-51. The NIR FT-Raman method has, in the main, been applied to sampling from large “macro” samples. These studies have encompassed a variety of applications including in situ methods such as reaction monitoring [6], curing [7] and electrochemistry [8]. To date, only a limited effort has been expended by academics and instrument manufacturers to develop a microscope attachment. In conventional Raman spectroscopy (visible lasers and dispersive spectrometers) the use of a microscope accessory is commonplace. The interfacing of a microscope to a dispersive spectrometer is optimal in respect of the image dimensions of the scattered Raman light (ca 2OOpm) and the slitwidths routinely used on Raman spectrometers (cu 400,um). The method offers very high spatial resolution, under optimum conditions, of cu lpm. This method has been used by a large number of groups in, for example, identifying inhomogeneities in polymers, monitoring in situ stress/strain structure relationships [9] and in the study of geochemical fluid inclusions [lo]. In the case of NIR PT-Raman spectroscopy, however, the interfacing of an optical microscope to an interferometer is difficult. The principal problem arises from the fact that an ET-IR spectrometer working in emission mode (as in the Raman experiment) has an entrance pupil (Jacquinot stop) of approximately 6 mm diameter. BERGIN [ll] demonstrated the feasibility of obtaining FT-Raman data from micrometre-sized samples using a conventional optical Raman microscope, and has shown it to be more inefficient when compared with the “macro” scale experiment. Later, ME~SERSCHMIDT and CHASE [12] coupled an IR microscope to an interferometer and improved upon data obtained from the previous approach. Most recently SOMMER and KATON [13] have detailed a similar approach to that of BERGIN (111 and characterized in some detail the spatial resolution that can be obtained. The indication is that the spatial resolution is significantly inferior to that routinely available in visible excited Raman microscopy. Indeed, the most detailed evaluation of this parameter has shown the limiting spatial resolution to be, at best 33pm. Some commercial systems are now coming onto the market. The indications are, from preliminary data released, that some technical modifications have FOURIER
l Author to whom correspondence should be addressed. t Present address: Department of Chemistry, University of Southampton, Southampton SO9 5NH, U.K.
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been made to improve this spatial resolution and performance. Further details on these advances will become available in the near future. Recently, BARIHLLAT et al. [14] have used a Nd-YAG laser with a conventional dispersive Raman spectrometer fitted with a microscope attachment. They demonstrated that with a charge coupled device (CCD) detector that anti-Stokes Raman data could be obtained down to 25 cm-’ of the laser line, with a spatial resolution of a few micrometres. It was also shown that considerable improvements in scattered light collection efficiency could be obtained, at l.O64pm, by using glass microscope objectives specially designed for the NIR wavelength range. The purpose of this paper is to discuss the development of our own NIR Raman microscope facility, together with a macro handling capability. This has been designed to provide fluorescence suppression whilst maintaining maximum sensitivity together with providing high spatial resolution when using the micro attachment. For this system we have opted to use a conventional dispersive spectrometer coupled to an optical microscope. The laser source we have used is a tunable (700-900 nm) titanium sapphire laser and the detector is a CCD camera. The rationale for our system will be discussed and the performance evaluated. A comparison between data obtained from a currently available FI-Raman microscope and conventional sampling arrangement will also be given.
EXPERIMENTAL Figure 1 is a schematic of our experimental arrangement. The optical microscope we have used is a Nikon system which has been modified by BGSC Ltd for Raman purposes. Problems were experienced initially with the beamsplitter incorporated within the microscope (optimized for visible radiation) but were easily overcome in the short term by purchasing a NIR (850nm) optimized optic. In the longer term the use of a Raman holographic edge (RHE) beamsplitter (Kaiser Optical) further improved the performance level. The spectrometer we have used is a Jobin Yvon THRlOOO single spectrograph incorporating a single holographic grating, 1200 grooves mm-‘, optimized at 750 nm. We have recently purchased a 300 grooves mm-’ ruled grating with 85% efficiency which has provided additional sensitivity together with increased spectral coverage. Traditionally single spectrographs show poor stray light rejection, which is manifested in an intense Rayleigh wing in the Raman spectrum and prevents any meaningful spectral analysis below ca 25OOcm-’ in the NIR. In order to circumvent this problem we have used novel edge filters specifically designed to provide efficient laser line rejection. RHE filters have been purchased from both Kaiser Optical and Physical Optics Corporation and are specified to have an optical density (OD) of >4.0 at the laser frequency. They are angle tunable to permit approach to within cu 200 cm-’ of the laser line, with a transmission of at least 80%. The filters we have obtained are for the rejection of 752 and 820 nm laser excitation wavelengths.
Schematic of near infrared Raman spectrometer Ti- sapphire laser
monochromator
Fig. 1. Schematic
diagram of the NIR Raman experiment using a spectrograph, and Ti-sapphire laser.
CCD detector
The development of a sensitive near-IR Raman system
635
It has been found, however, that for the microscope attachment no additonal filtering is required over and above that provided from the RHE beamsplitter. The characteristics of this optic are: (i) a reflectivity at 45’ to the incoming laser at a specified wavelength of w 90%; (ii) a rejection of the Rayleigh scatter of OD >4; and (iii) a transmission at 400 cm-’ from the laser line in excess of 85%. The stray light rejection was further improved by adding a mask with a 5 x 15 mm slot, just before the CCD which is the detector we have used in our system. The firstsignificantpaper on the
advantage of using an unintensified CCD detector for low light level Raman spectroscopy was published in 1986 [15]. Since then it has been widely recognized as the optimum detector for Raman spectroscopy with a high quantum efficiency and low noise characteristics, which is particularly advantageous when there are extremely weak Raman bands present [16]. The system we have employed is a Wright Instruments backthinned CCD which has a maximum quantum efficiency at 750 nm of 80%. This particular CCD is a Peltier cooled device with a twodimensional array of 600 X 400 discrete silicon elements (25 pm square) which provides detection in both spectral and spatial directions when coupled to the spectrometer. The arrangement we have adopted is to have the 600 elements on the spectral axis providing a detection range of cu 125 cm-’ (1200 grooves mm-’ grating) and cu 600 cm-’ (300 grooves mm-’ grating), while the height of the image on the detector is between 40 and 60 pixels. The background noise level under these conditions is only 1 photon count s-l. At this level the method becomes shot noise limited, in contrast to the FT-Raman method which is detector/preamplifier noise limited. The detector is controlled and driven through a Compaq 386 Deskpro computer operating with Lab-Calc software (Galactic Industries Corp). This software also supports the spectrometer drive routines and data processing. The standard operating range for this instrumentation is determined by the filters used and is between 100 and 3200 cm-’ Raman shift (A, 752.5 nm) and 100 and 2200 cm-’ Raman shift (1, 820 nm), the upper limit in each case being determined by the response of the CCD detector. The laser system we have used in these experiments is a tunable titanium sapphire laser pumped by a 6 W argon ion laser from Spectra Physics Ltd. It is capable of providing tunable far-red (700900 nm) laser wavelengths with a peak power of - 800 mW at 790 nm.
RESULTS AND DISCUSSION
The Raman spectrum of sulphur can be used to illustrate both the stray light levels and spectral resolution of the instrumentation. Figure 2 illustrates data obtained from
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Fig. 2. Raman spectrum recorded from sulphur using A, 820 mn 1OOpmslit and a POC filter.
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1 - Super notch filter 2 - Regular filter
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Fig. 3. Raman spectra obtained from sulphur using: (1) Kaiser optics super notch filter; and (2) Kaiser optic regular filter with 1,752 nm and a slitwidth of 1OO~m.
sulphur in the macro sampling configuration using an RHE filter from Physical Optical Corp. It can be seen from the spectrum that the filter permits approach to below 100 cm-’ from the laser line (820 nm in this instance). By comparison with literature spectra, recorded using conventional spectrometers, it would appear that this filter produces weak artifacts below 100 cm- ‘. The intense Raman band at 150 cm-’ is very well resolved and is indicative of a spectral resolution of better than 2 cm-‘, when using a slitwidth of 1OO~m to maximize the light throughput. However, for working closer to the laser line we have found the Kaiser super-notch filter to be optimum. Figure 3 shows a comparison between data recorded from sulphur
C3,,Hs2 super notch
80 Wavenumbers /cm”
Fig. 4. Raman spectrum of a longitudinal acoustic mode recorded from Cati super notch filter, 1,752 nm, slitwidth 6Opm.
using the Kaiser
The development of a sensitive near-IR Raman system
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Fig. 5. Raman spectrum recorded from cadmium sulphide filled polyethylene using, 100 mW, A, 752 nm, 1OO~mslit, accumulation time 10 s.
using a regular Raiser filter and a super notch tilter, respectively. The sharpness of the tilter edge and the relative band intensities using the super notch are much improved. In particular, it should be noted that the bands at 220 and 150cm-’ in the sulphur spectrum show different relative intensities depending on the filter used. The data obtained using the super notch filter, in this instance, compare favourably with those obtained using conventional instrumentation. However, we have found from our experimental work and from discussions with filter manufacturers that the angle tuning of the filter can produce band intensity distortions. Clearly, when working close to the laser line great care should be taken to set the filter angle correctly. An illustration of the sharpness of the edge of the super notch filter can be seen from analysis of the straight chain paraffin CJ& in the low frequency region. Figure 4 shows in this instance a strong Raman band at cu 60 cm-’ which is assigned to the longitudinal acoustic mode. We have found from some samples that the fluorescence rejection offered by operating at 752 and 820 nm laser wavelengths is not as effective as operating at 1.064~m. The
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Fig. 6. FM&man
spectrum from the same polyethylene as in Fig. 5, only 4 cm-’ resolution, 200 mW, d,l.M4~m, 400 scans, accumulation time 45 min.
S. M. MASONet al.
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Fig. 7. Raman spectrum recorded from 0.1 M aqueous solution of sodium sulphate, 100 mW, I., 752 nm, 120 s accumulation time, slits 1OOpm.
spectrum obtained from a plaque of high density polyethylene, filled with cadmium sulphide, is shown in Fig. 5. The Raman spectrum of the base polymer can be obtained easily with conventional lasers/spectrometers. However, when the cadmium sulphide additive is present its fluorescence dominates the spectrum and prevents any analysis. It is clear from Fig. 5 that no such problems exist when far-red laser wavelengths are used. Figure 6 illustrates the data obtained from the identical sample using an FT-Raman system. To obtain data of comparable quality, however, required twice the laser power and an accumulation time which was more than 250 times longer. Figures 7 and 8 show data obtained from a 0.1 M aqueous solution of sodium sulphate using the T&sapphire and FI-Raman systems, respectively. As with the polyethylene example the IT-Raman spectrum was recorded using a higher laser power and a longer accumulation time. The data obtained from the IT-Raman experiment are very inferior to those from the Ti-sapphire. Whilst this reflects in part the sensitivity levels of the two approaches it also highlights the problems of self-absorption of the Raman scatter when working from aqueous phase samples in the NIR (1.064 pm).
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Fig. 8. As Fig. 7 only FT-Raman spectrum, similar conditions but 45 min accumulation time.
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The development of a sensitive near-IR Raman system
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abo
6dO Wavenumbers/cm-’
-’
Fig. 9. Raman spectra recorded from anatase using the microscope attachment: (a) 20pm particle; (b) 2pm particle using 1,760 nm.
To test the spatial resolution of the microscope attachment we have obtained spectra from progressively smaller samples of anatase. Figure 9 illustrates data obtained from a 20pm particle together with data from a 2pm particle with a ~20 microscope objective. Clearly, the quality of the data deteriorates with sample size but little or no differences were seen when going from a bulk to a 20,~m sample. Changing to a higher magnification objective should provide a better spatial resolution and maintain good signal levels, assuming power densities are adjusted to avoid any sample damage. However, we have found that the transmission through the higher magnification objectives (> x50) was poorer than anticipated. This has been traced to the overfilling of the back of the
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/cm”
Fig. 10. Raman spectrum of a 15pm particle of a laser dye, Rhodamine 580, using I.,%!0 urn.
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objective and is to be the subject of further development. However, our current capability provides a limiting spatial resolution in the order of SlOpurn. The sensitivity of our microscope system has been increased by a factor of three by simply installing a RHE beam splitter (see experimental). Clearly the advantage of a 90% reflecting laser input and a 90% transmission of the Raman scatter is more efficient than the conventional 50/50 beamsplitter most commonly used. Figure 10 illustrates the spectrum recorded from a laser dye, Rhodamine 580, using the microscope attachment. The particle size of the sample was ca 15 pm in diameter and the spectum was obtained after only 5 s integration. No spectrum was obtained from this
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Fig. 11. Raman spectra obtained from a 20pm polyethylene fibre: (a) FT-Raman, 1 W, A, l.O64pm, 45 min acquisition time, 4 cm-’ resolution; (b) I., 752 nm, 300 mW, 10 s acquisition, 2 cm-’ resolution.
The development of a sensitive
near-IR Ramansystem
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sample using 752 nm laser excitation owing to sample fluorescence but by using 820 nm excitation no such problems were experienced. However, this example illustrates clearly that good fluorescence rejection can be achieved at these wavelengths. One of the clearest demonstrations of the enhanced sensitivity of this approach to NIR microscopy is the data we have been able to obtain from single polyethylene fibres (cu 2Opm diameter). Figure 11 illustrates data we have obtained using (a) a commercial FT-Raman microscope and (b) our system. Both spectra are of good quality and free from fluorescence. However, the FT-Raman data were acquired using 1 W of laser power and an accumulation time of 45 min, whereas the Ti-sapphire data were obtained with 300mW of laser power and an accumulation time of only 10 s. Clearly, this increased sensitivity is of great benefit when studying weaker scatterers, or carrying out dynamic experiments, such as on stress/strain effects in polymer fibres. In addition to increased sensitivity, we have also seen a much improved performance in terms of data quality from darkly coloured and black samples. The data recorded from a small carbon chip using the FT-Raman microscope could only be obtained by diluting the carbon in potassium nitrate which overcame a sample heating problem. The accumulation time was 50 min to achieve a signal-to-noise ratio of 4: 1. Figure 12 illustrates the spectrum obtained from the same sample using 820 nm laser excitation in an accumulation time of only 30 s. The superior response at 820 nm excitation is due to two factors: (a) the more sensitive CCD detector employed; and (b) the absence of any thermal emission which is frequently seen in NIR m-Raman spectroscopy of black samples. A third factor to be taken into account is the v4 advantage of working at 752 nm as opposed to 1.064flm which represents an additional factor of cu 3 improvement in scattering efficiency. Figure 13 illustrates another potential advantage for working with the T&sapphire laser wavelengths with a spectrum obtained from molten polyethylene at a temperature of 200°C. Some reference has been made in the literature to a fundamental limitation of the FYI’-Raman method for the analysis of samples at elevated temperatures. It is well known that the thermal black body emission for an object extends to the l-1.6pm spectral range. At temperatures above 160°C this intense emission makes analysis by FT’-Raman spectroscopy difficult. Methods, which require the use of pulsed lasers, to minimize this problem have been discussed elsewhere by CUTLER and PE-ITY [17]. Using laser excitation wavelengths to the blue of l.O64pm, in this instance 752 nm goes a long way to overcome this problem yet at the same time retain the fluorescence rejection advantage of the FT-Raman method.
Fig. 12. Raman spectrum of a carbon chip. I, 8u) mn, 10 mW, 30 s scan time.
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Fig. 13. Raman spectrum recorded from polyethylene at 200°C using 100 mW, .I, 752 nm.
CONCLUSIONS The system described in this paper represents a marked increase in sensitivity levels for fluorescence free Raman spectroscopy. Our results from a limited sample set indicate that the use of a Ti-sapphire- single spectrograph-CCD detector combination is effective for obtaining fluorescence-free Raman data with good sensitivity. The approach seems particularly well suited to micro samples where the coupling of the Raman light to the instrument is very efficient. The comparison between data obtained using our system with those using commercial FT-Raman systems shows that the sensitivity level is far higher. In addition, the advantages of using the T&sapphire laser wavelengths for the analysis of both aqueous phase and heated samples are clear. However, it is apparent that further advances in the FT-Raman method will be forthcoming. In particular, we have been aware during the course of our studies that instrument manufacturers are steadily improving the performance of their microscope attachments. The use of Ti-sapphire or diode laser wavelengths in the 750-850 nm range has to date not been used to any great degree with a highly sensitive detector on the FT-Raman experiment. However, it will only be a matter of time before such investigations are carried out and at that time many of the factors that make this approach so attractive (like detector performance) will be nullified. Acknowledgement-Permission
to publish has been granted by British Petroleum Research.
REFERENCES [l] [2] [3] [4] [S] [6] [7] [8] [9] [lo]
K. P. J. Williams, S. F. Parker, P. J. Hendra and A. J. Turner, Appl. Spectrosc. 42,762 (1988). C. H. Jones and I. J. Wesley, Spectrochim. Acta 47A, 1293 (1991). K. P. J. Wiiams, J. Raman Spectrosc. 21, 143 (1990). P. A. Evans, Spectrochim. Acra 47A, 1441 (1991). K. P. J. Williams and S. M. Mason, Trends Andyt. Chem. 9, 119 (1991). J. Clarkson, S. M. Mason and K. P. J. Williams, Spectrochim. Acta 47A, 1345 (1991). J. K. Agbenyega, M. Clayboum and G. Ellis, Spectrochim. Acta 47A, 1375 (1991). M. Hudson and D. N. Waters, Spectrochim. Acta 47A, 1467 (1991). B. J. Kip, M. C. P. Van Eijk and R. J. Meier, J. Polym. Sci. B Polym. Phys. 29,99 (1991). J. Konnerrup-Madsen, J. Dubessy and J. Rose-Hansen, Lithos 18,271 (1985).
The development [ll] [12] [13] [14] [15] [16] [17]
of a sensitive near-IR Raman system
F. J. Bergin, Spectrochim. Acta 46A, 153 (1990). R. G. Messerschmidt and D. B. Chase, Appl. Spectrosc. 43, 11 (1989). A. J. Sommer and J. E. Katon, Appl. Spectrosc. 45,527 (1991). J. Barbillat, E. Da Silva and B. Roussel, J. Raman Spectrosc. 22,383 (1991). C. A. Murray and S. B. Dierker, J. Opt. Sot. Am. A 3,215l (1986). J. E. Pemberton, R. L. Sobocinski, M. A. Bryant and D. A. Carter, Spectroscopy 5,26 (1990). D. J. Cutler and C. J. Petty, Spectrochim. Actu 47A, 1159 (1991).
U(A) 46:5/616-D
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0584-8539/93 $6.00+ 0.00 @?J 1993 PergatnotlPress Ltd
The development of a sensitive near-IR Raman system S. M. MASON, N. CONROY,N. M. DIXON and K. P. J. WILLIAMS*? British Petroleum Research, Chertsey Road, Sunburyon Thames,MiddlesexTW167LN, U.K. (Received 6 October 1992; accepted 21 October 1992) Abstract-This paper describes the development of a highly sensitive near-IR laser Raman system which has been designed and built in our laboratory. The system comprises both a micro and macro sample handling facility. The approach that we have adopted uses a conventional spectrograph with a charge coupled device detector and optics optimized for the 700-1OOOnm spectral region. The laser source is a tunable titanium sapphire laser which provides intense radiation in the 700-900 nm spectral region. We show that the spatial resolution and spectral sensitivity that can be obtained with the microscope attachment are excellent. A comparison between data obtained from our system and that from an Ff’-Raman system is given.
INTIX~DUCTI~N
transform (PI) Raman spectroscopy has made a significant impact in recent years on the activities of Raman spectroscopists. The use of a near-IR (NIR) laser (1.064~m) with an FT-IR spectrometer has provided the Raman method with a much reduced incidence of laser induced sample fluorescence, together with a multiplexing advantage. The method was first demonstrated on a bench top instrument in our laboratories in 1987 [l]. Since that time the method has found a variety of applications [2-51. The NIR FT-Raman method has, in the main, been applied to sampling from large “macro” samples. These studies have encompassed a variety of applications including in situ methods such as reaction monitoring [6], curing [7] and electrochemistry [8]. To date, only a limited effort has been expended by academics and instrument manufacturers to develop a microscope attachment. In conventional Raman spectroscopy (visible lasers and dispersive spectrometers) the use of a microscope accessory is commonplace. The interfacing of a microscope to a dispersive spectrometer is optimal in respect of the image dimensions of the scattered Raman light (ca 2OOpm) and the slitwidths routinely used on Raman spectrometers (cu 400,um). The method offers very high spatial resolution, under optimum conditions, of cu lpm. This method has been used by a large number of groups in, for example, identifying inhomogeneities in polymers, monitoring in situ stress/strain structure relationships [9] and in the study of geochemical fluid inclusions [lo]. In the case of NIR PT-Raman spectroscopy, however, the interfacing of an optical microscope to an interferometer is difficult. The principal problem arises from the fact that an ET-IR spectrometer working in emission mode (as in the Raman experiment) has an entrance pupil (Jacquinot stop) of approximately 6 mm diameter. BERGIN [ll] demonstrated the feasibility of obtaining FT-Raman data from micrometre-sized samples using a conventional optical Raman microscope, and has shown it to be more inefficient when compared with the “macro” scale experiment. Later, ME~SERSCHMIDT and CHASE [12] coupled an IR microscope to an interferometer and improved upon data obtained from the previous approach. Most recently SOMMER and KATON [13] have detailed a similar approach to that of BERGIN (111 and characterized in some detail the spatial resolution that can be obtained. The indication is that the spatial resolution is significantly inferior to that routinely available in visible excited Raman microscopy. Indeed, the most detailed evaluation of this parameter has shown the limiting spatial resolution to be, at best 33pm. Some commercial systems are now coming onto the market. The indications are, from preliminary data released, that some technical modifications have FOURIER
l Author to whom correspondence should be addressed. t Present address: Department of Chemistry, University of Southampton, Southampton SO9 5NH, U.K.
633
S. M.
634
MASON et al.
been made to improve this spatial resolution and performance. Further details on these advances will become available in the near future. Recently, BARIHLLAT et al. [14] have used a Nd-YAG laser with a conventional dispersive Raman spectrometer fitted with a microscope attachment. They demonstrated that with a charge coupled device (CCD) detector that anti-Stokes Raman data could be obtained down to 25 cm-’ of the laser line, with a spatial resolution of a few micrometres. It was also shown that considerable improvements in scattered light collection efficiency could be obtained, at l.O64pm, by using glass microscope objectives specially designed for the NIR wavelength range. The purpose of this paper is to discuss the development of our own NIR Raman microscope facility, together with a macro handling capability. This has been designed to provide fluorescence suppression whilst maintaining maximum sensitivity together with providing high spatial resolution when using the micro attachment. For this system we have opted to use a conventional dispersive spectrometer coupled to an optical microscope. The laser source we have used is a tunable (700-900 nm) titanium sapphire laser and the detector is a CCD camera. The rationale for our system will be discussed and the performance evaluated. A comparison between data obtained from a currently available FI-Raman microscope and conventional sampling arrangement will also be given.
EXPERIMENTAL Figure 1 is a schematic of our experimental arrangement. The optical microscope we have used is a Nikon system which has been modified by BGSC Ltd for Raman purposes. Problems were experienced initially with the beamsplitter incorporated within the microscope (optimized for visible radiation) but were easily overcome in the short term by purchasing a NIR (850nm) optimized optic. In the longer term the use of a Raman holographic edge (RHE) beamsplitter (Kaiser Optical) further improved the performance level. The spectrometer we have used is a Jobin Yvon THRlOOO single spectrograph incorporating a single holographic grating, 1200 grooves mm-‘, optimized at 750 nm. We have recently purchased a 300 grooves mm-’ ruled grating with 85% efficiency which has provided additional sensitivity together with increased spectral coverage. Traditionally single spectrographs show poor stray light rejection, which is manifested in an intense Rayleigh wing in the Raman spectrum and prevents any meaningful spectral analysis below ca 25OOcm-’ in the NIR. In order to circumvent this problem we have used novel edge filters specifically designed to provide efficient laser line rejection. RHE filters have been purchased from both Kaiser Optical and Physical Optics Corporation and are specified to have an optical density (OD) of >4.0 at the laser frequency. They are angle tunable to permit approach to within cu 200 cm-’ of the laser line, with a transmission of at least 80%. The filters we have obtained are for the rejection of 752 and 820 nm laser excitation wavelengths.
Schematic of near infrared Raman spectrometer Ti- sapphire laser
monochromator
Fig. 1. Schematic
diagram of the NIR Raman experiment using a spectrograph, and Ti-sapphire laser.
CCD detector
The development of a sensitive near-IR Raman system
635
It has been found, however, that for the microscope attachment no additonal filtering is required over and above that provided from the RHE beamsplitter. The characteristics of this optic are: (i) a reflectivity at 45’ to the incoming laser at a specified wavelength of w 90%; (ii) a rejection of the Rayleigh scatter of OD >4; and (iii) a transmission at 400 cm-’ from the laser line in excess of 85%. The stray light rejection was further improved by adding a mask with a 5 x 15 mm slot, just before the CCD which is the detector we have used in our system. The firstsignificantpaper on the
advantage of using an unintensified CCD detector for low light level Raman spectroscopy was published in 1986 [15]. Since then it has been widely recognized as the optimum detector for Raman spectroscopy with a high quantum efficiency and low noise characteristics, which is particularly advantageous when there are extremely weak Raman bands present [16]. The system we have employed is a Wright Instruments backthinned CCD which has a maximum quantum efficiency at 750 nm of 80%. This particular CCD is a Peltier cooled device with a twodimensional array of 600 X 400 discrete silicon elements (25 pm square) which provides detection in both spectral and spatial directions when coupled to the spectrometer. The arrangement we have adopted is to have the 600 elements on the spectral axis providing a detection range of cu 125 cm-’ (1200 grooves mm-’ grating) and cu 600 cm-’ (300 grooves mm-’ grating), while the height of the image on the detector is between 40 and 60 pixels. The background noise level under these conditions is only 1 photon count s-l. At this level the method becomes shot noise limited, in contrast to the FT-Raman method which is detector/preamplifier noise limited. The detector is controlled and driven through a Compaq 386 Deskpro computer operating with Lab-Calc software (Galactic Industries Corp). This software also supports the spectrometer drive routines and data processing. The standard operating range for this instrumentation is determined by the filters used and is between 100 and 3200 cm-’ Raman shift (A, 752.5 nm) and 100 and 2200 cm-’ Raman shift (1, 820 nm), the upper limit in each case being determined by the response of the CCD detector. The laser system we have used in these experiments is a tunable titanium sapphire laser pumped by a 6 W argon ion laser from Spectra Physics Ltd. It is capable of providing tunable far-red (700900 nm) laser wavelengths with a peak power of - 800 mW at 790 nm.
RESULTS AND DISCUSSION
The Raman spectrum of sulphur can be used to illustrate both the stray light levels and spectral resolution of the instrumentation. Figure 2 illustrates data obtained from
I 100
I
I
150
200
’
Wavenumbers/cm-’
Fig. 2. Raman spectrum recorded from sulphur using A, 820 mn 1OOpmslit and a POC filter.
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s. M. MASONet al.
1 - Super notch filter 2 - Regular filter
/
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L
2&I Wavenumbers /cm”
Fig. 3. Raman spectra obtained from sulphur using: (1) Kaiser optics super notch filter; and (2) Kaiser optic regular filter with 1,752 nm and a slitwidth of 1OO~m.
sulphur in the macro sampling configuration using an RHE filter from Physical Optical Corp. It can be seen from the spectrum that the filter permits approach to below 100 cm-’ from the laser line (820 nm in this instance). By comparison with literature spectra, recorded using conventional spectrometers, it would appear that this filter produces weak artifacts below 100 cm- ‘. The intense Raman band at 150 cm-’ is very well resolved and is indicative of a spectral resolution of better than 2 cm-‘, when using a slitwidth of 1OO~m to maximize the light throughput. However, for working closer to the laser line we have found the Kaiser super-notch filter to be optimum. Figure 3 shows a comparison between data recorded from sulphur
C3,,Hs2 super notch
80 Wavenumbers /cm”
Fig. 4. Raman spectrum of a longitudinal acoustic mode recorded from Cati super notch filter, 1,752 nm, slitwidth 6Opm.
using the Kaiser
The development of a sensitive near-IR Raman system
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L
1100
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Fig. 5. Raman spectrum recorded from cadmium sulphide filled polyethylene using, 100 mW, A, 752 nm, 1OO~mslit, accumulation time 10 s.
using a regular Raiser filter and a super notch tilter, respectively. The sharpness of the tilter edge and the relative band intensities using the super notch are much improved. In particular, it should be noted that the bands at 220 and 150cm-’ in the sulphur spectrum show different relative intensities depending on the filter used. The data obtained using the super notch filter, in this instance, compare favourably with those obtained using conventional instrumentation. However, we have found from our experimental work and from discussions with filter manufacturers that the angle tuning of the filter can produce band intensity distortions. Clearly, when working close to the laser line great care should be taken to set the filter angle correctly. An illustration of the sharpness of the edge of the super notch filter can be seen from analysis of the straight chain paraffin CJ& in the low frequency region. Figure 4 shows in this instance a strong Raman band at cu 60 cm-’ which is assigned to the longitudinal acoustic mode. We have found from some samples that the fluorescence rejection offered by operating at 752 and 820 nm laser wavelengths is not as effective as operating at 1.064~m. The
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Fig. 6. FM&man
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Fig. 7. Raman spectrum recorded from 0.1 M aqueous solution of sodium sulphate, 100 mW, I., 752 nm, 120 s accumulation time, slits 1OOpm.
spectrum obtained from a plaque of high density polyethylene, filled with cadmium sulphide, is shown in Fig. 5. The Raman spectrum of the base polymer can be obtained easily with conventional lasers/spectrometers. However, when the cadmium sulphide additive is present its fluorescence dominates the spectrum and prevents any analysis. It is clear from Fig. 5 that no such problems exist when far-red laser wavelengths are used. Figure 6 illustrates the data obtained from the identical sample using an FT-Raman system. To obtain data of comparable quality, however, required twice the laser power and an accumulation time which was more than 250 times longer. Figures 7 and 8 show data obtained from a 0.1 M aqueous solution of sodium sulphate using the T&sapphire and FI-Raman systems, respectively. As with the polyethylene example the IT-Raman spectrum was recorded using a higher laser power and a longer accumulation time. The data obtained from the IT-Raman experiment are very inferior to those from the Ti-sapphire. Whilst this reflects in part the sensitivity levels of the two approaches it also highlights the problems of self-absorption of the Raman scatter when working from aqueous phase samples in the NIR (1.064 pm).
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Fig. 8. As Fig. 7 only FT-Raman spectrum, similar conditions but 45 min accumulation time.
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Fig. 9. Raman spectra recorded from anatase using the microscope attachment: (a) 20pm particle; (b) 2pm particle using 1,760 nm.
To test the spatial resolution of the microscope attachment we have obtained spectra from progressively smaller samples of anatase. Figure 9 illustrates data obtained from a 20pm particle together with data from a 2pm particle with a ~20 microscope objective. Clearly, the quality of the data deteriorates with sample size but little or no differences were seen when going from a bulk to a 20,~m sample. Changing to a higher magnification objective should provide a better spatial resolution and maintain good signal levels, assuming power densities are adjusted to avoid any sample damage. However, we have found that the transmission through the higher magnification objectives (> x50) was poorer than anticipated. This has been traced to the overfilling of the back of the
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Fig. 10. Raman spectrum of a 15pm particle of a laser dye, Rhodamine 580, using I.,%!0 urn.
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objective and is to be the subject of further development. However, our current capability provides a limiting spatial resolution in the order of SlOpurn. The sensitivity of our microscope system has been increased by a factor of three by simply installing a RHE beam splitter (see experimental). Clearly the advantage of a 90% reflecting laser input and a 90% transmission of the Raman scatter is more efficient than the conventional 50/50 beamsplitter most commonly used. Figure 10 illustrates the spectrum recorded from a laser dye, Rhodamine 580, using the microscope attachment. The particle size of the sample was ca 15 pm in diameter and the spectum was obtained after only 5 s integration. No spectrum was obtained from this
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Fig. 11. Raman spectra obtained from a 20pm polyethylene fibre: (a) FT-Raman, 1 W, A, l.O64pm, 45 min acquisition time, 4 cm-’ resolution; (b) I., 752 nm, 300 mW, 10 s acquisition, 2 cm-’ resolution.
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sample using 752 nm laser excitation owing to sample fluorescence but by using 820 nm excitation no such problems were experienced. However, this example illustrates clearly that good fluorescence rejection can be achieved at these wavelengths. One of the clearest demonstrations of the enhanced sensitivity of this approach to NIR microscopy is the data we have been able to obtain from single polyethylene fibres (cu 2Opm diameter). Figure 11 illustrates data we have obtained using (a) a commercial FT-Raman microscope and (b) our system. Both spectra are of good quality and free from fluorescence. However, the FT-Raman data were acquired using 1 W of laser power and an accumulation time of 45 min, whereas the Ti-sapphire data were obtained with 300mW of laser power and an accumulation time of only 10 s. Clearly, this increased sensitivity is of great benefit when studying weaker scatterers, or carrying out dynamic experiments, such as on stress/strain effects in polymer fibres. In addition to increased sensitivity, we have also seen a much improved performance in terms of data quality from darkly coloured and black samples. The data recorded from a small carbon chip using the FT-Raman microscope could only be obtained by diluting the carbon in potassium nitrate which overcame a sample heating problem. The accumulation time was 50 min to achieve a signal-to-noise ratio of 4: 1. Figure 12 illustrates the spectrum obtained from the same sample using 820 nm laser excitation in an accumulation time of only 30 s. The superior response at 820 nm excitation is due to two factors: (a) the more sensitive CCD detector employed; and (b) the absence of any thermal emission which is frequently seen in NIR m-Raman spectroscopy of black samples. A third factor to be taken into account is the v4 advantage of working at 752 nm as opposed to 1.064flm which represents an additional factor of cu 3 improvement in scattering efficiency. Figure 13 illustrates another potential advantage for working with the T&sapphire laser wavelengths with a spectrum obtained from molten polyethylene at a temperature of 200°C. Some reference has been made in the literature to a fundamental limitation of the FYI’-Raman method for the analysis of samples at elevated temperatures. It is well known that the thermal black body emission for an object extends to the l-1.6pm spectral range. At temperatures above 160°C this intense emission makes analysis by FT’-Raman spectroscopy difficult. Methods, which require the use of pulsed lasers, to minimize this problem have been discussed elsewhere by CUTLER and PE-ITY [17]. Using laser excitation wavelengths to the blue of l.O64pm, in this instance 752 nm goes a long way to overcome this problem yet at the same time retain the fluorescence rejection advantage of the FT-Raman method.
Fig. 12. Raman spectrum of a carbon chip. I, 8u) mn, 10 mW, 30 s scan time.
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Fig. 13. Raman spectrum recorded from polyethylene at 200°C using 100 mW, .I, 752 nm.
CONCLUSIONS The system described in this paper represents a marked increase in sensitivity levels for fluorescence free Raman spectroscopy. Our results from a limited sample set indicate that the use of a Ti-sapphire- single spectrograph-CCD detector combination is effective for obtaining fluorescence-free Raman data with good sensitivity. The approach seems particularly well suited to micro samples where the coupling of the Raman light to the instrument is very efficient. The comparison between data obtained using our system with those using commercial FT-Raman systems shows that the sensitivity level is far higher. In addition, the advantages of using the T&sapphire laser wavelengths for the analysis of both aqueous phase and heated samples are clear. However, it is apparent that further advances in the FT-Raman method will be forthcoming. In particular, we have been aware during the course of our studies that instrument manufacturers are steadily improving the performance of their microscope attachments. The use of Ti-sapphire or diode laser wavelengths in the 750-850 nm range has to date not been used to any great degree with a highly sensitive detector on the FT-Raman experiment. However, it will only be a matter of time before such investigations are carried out and at that time many of the factors that make this approach so attractive (like detector performance) will be nullified. Acknowledgement-Permission
to publish has been granted by British Petroleum Research.
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