Anal. Chem. 1997, 69, 45-50
Noninvasive Confocal Raman Imaging of Immiscible Liquids in a Porous Medium Colin J. H. Brenan* and Ian W. Hunter
Department of Mechanical Engineering, Room 3-147, Massachusetts Institute of Technology, 77 Massachusetts Avenue, Cambridge, Massachusetts 02139 James M. Brenan†
University of California, Lawerence Livermore National Laboratory, L-202, P.O. Box 808, Livermore, California 94550
The physical properties of a porous medium impregnated by a liquid or vapor are dictated in part by the spatial distribution of these phases within the solid host matrix and by physicochemical interactions at the solid-liquidvapor boundaries. We report on a novel 3-D imaging technique based on the high spatial resolution and chemical specificity of the confocal Raman microscope, which allows for the first time the in situ study and visualization of these processes. Images showing the topology of immiscible liquids (trichloroethylene-water) in a natural porous sandstone demonstrate the potential of the technique and reveal a rich variety of interfacial phenomena. The physical properties of a porous medium impregnated by a liquid or vapor are dictated in part by the spatial distribution of these phases within the solid host matrix and by physicochemical interactions at the solid-liquid-vapor boundaries. These interactions often differ from those seen in macroscopic situations because of the unique thermodynamic and kinetic boundary conditions which apply inside pores. Pore geometry, size distribution, connectivity, pore pressure, surface-to-volume ratio, and pore stress states, in addition to such factors as solid-liquid-vapor wettability and interfacial chemistry, control local liquid or vapor penetration into the pore volume. The degree of penetration, in turn, can spatially modify such bulk material properties as fluid transport, electrical conductivity, acoustic wave propagation, magnetic permeability, elastic modulus, and tensile strength. Noninvasive in situ visualization of liquid and vapor topology within a given pore structure and corresponding measurement of liquidvapor-solid wetting properties is essential to understanding transport mechanisms of fluids in porous media of theoretical and economic importance. In particular, effective control of processes such as enhanced hydrocarbon recovery and quantitative evaluation of cleaning methodologies proposed for removal of volatile organic solvents from soil and groundwater aquifers is impossible without a detailed understanding of the microenvironments, phase behavior, mobility, topology, and reaction kinetics of fluids in complex geological materials.1,2 We report here a novel 3-D imaging technique based on the high spatial resolution and chemical specificity of the confocal * Address correspondence to this author. Phone: (617) 253-8116. FAX: (617) 258-7018. E-mail
[email protected]. † Present address: Department of Geology, University of Toronto, Earth Sciences Centre, 22 Russell St., Toronto, Ontario, M5S 3B1 Canada. (1) Thomas, M. M.; Clouse, J. A.; Longo, J. M. Chem. Geol. 1993, 109, 201. (2) Schwille, F. Dense Chlorinated Solvents in Porous and Fractured Media; Lewis Publishers: Chelsea, MI, 1988; 146 p; translated by J. F. Pankow. S0003-2700(96)00654-3 CCC: $14.00
© 1996 American Chemical Society
Raman microscope (CRM)3-5 which allows for the first time the in situ study and visualization of these processes. As a demonstration, a system important to groundwater contaminant studies involving a hydrophobic pollutant and on which little microscale experimental data are available was examined with the CRM; specifically, the wettability and penetration of two immiscible liquids, trichloroethylene and water, were studied in a porous sandstone matrix. Present experimental approaches suffer shortcomings in their ability to provide direct and noninvasive observation of the distribution of liquid or vapor within a porous medium. Serial sectioning and observation under an electron or optical microscope are destructive to the sample, cannot be performed in realtime, and may introduce distortion and artifacts into the image. Moreover, misregistration in serial section alignment prevents easy 3-D reconstruction of the imaged volume, and only fluids trapped within closed pore volumes can be observed. Optical microscopy is limited in the discrimination between fluids and fluid-solid interfaces when the differences in refractive indexes or scattering properties across the interfaces are small. Although X-ray tomography has proved to be useful for generating 3-D images of water distribution in such porous materials as natural soils6 and geothermal reservoir rocks,7 quantitative information regarding the exact chemical compounds present in the liquid phase cannot readily be obtained. Magnetic resonance imaging has imaged solvent penetration into ceramic voids8 but suffers from magnetic susceptibility artifacts. The initial impetus for applying the CRM to studying porous materials comes from the recent work of Fredrich et al.,9 who used confocal scanning fluorescence optical microscopy to image pore structures in various geological materials impregnated with a fluorescent dye-laden epoxy. Unlike this latter technique, the CRM does not require the presence of a fluorescing medium in order to replicate the sample porosity, thereby eliminating any concerns regarding sample expansion, pore distortion, and limited (3) Sharonov, S.; Nabiev, I.; Chourpa, I.; Feofanov, A.; Valisa, P.; Manfait, M. J. Raman Spectrosc. 1994, 25, 699. (4) Puppels, G. J.; de Mul, F. F. M.; Otto, C.; Greve, J.; Robert-Nicoud, M.; Arndt-Jovin, D. J.; Jovin, T. M. Nature, 1990, 347, 301. (5) Brenan, C. J.H.; Hunter, I. W. Applied Opt. 1994, 33, 7520. (6) Anderson, S. H.; Gantzer, C. J.; Boone, J. M.; Tully, R. J. Soil Sci. Soc. Am. J. 1988, 52, 35. (7) Bonner, B. P.; Roberts, J. J.; Schneberk, D. J. Trans.-Geotherm. Resour. 1994, 18, 305. (8) Wallner, A. S.; Ritchey, W. M. J. Mater. Res. 1993, 8, 655. (9) Friedrich, J. T.; Menendez, B.; Wong, T.-F. Science 1995, 268, 276.
Analytical Chemistry, Vol. 69, No. 1, January 1, 1997 45
penetration during the impregnation procedure. In general, the CRM has proved to be an invaluable tool for noninvasive 3-D analysis and visualization of chemically heterogeneous samples.3,10,11 The instrument combines focused laser illumination with spatially filtered optical detection to define within the sample a localized volume element (voxel) from which light is detected and spectrally analyzed. Attenuation of light from outside the voxel by a pinhole spatial filter increases image contrast and results in voxels typically less than several cubic micrometers in volume. Moving the specimen relative to the voxel and recording the scattered light Raman spectrum at each position generates a 3-D Raman spectral image of the specimen. It is well-established12 that fundamental molecular parameters, like bond stiffness, as well as other molecular attributes such as molecular symmetry, primary and secondary conformations, and side-chain reactions, can be deduced from a Raman spectral measurement, while characteristic Raman spectral patterns or “fingerprints” aid in molecular identification. Raman images can be interpreted either as a compositional map or as a means to view changes in local chemical functionality or reactivity of a particular chemical species. Raman images of optically similar yet chemically distinct materials can have high contrast, and acquisition of Raman images is minimally invasive because the detected signal originates from an intrinsic molecular property, i.e., its molecular polarizability. EXPERIMENTAL SECTION The CRM we employed for this study stems from an earlier correlation-based confocal optical microscope built for volumetric polarization, phase, and intensity imaging of sarcomeres in single isolated muscle cells undergoing mechanical testing with a highperformance, parallel drive micromotion robot.13 The present CRM integrates a visible light Fourier transform spectrometer14 with an object-scanning confocal optical microscope to create a Fourier transform confocal Raman microscope (FT-CRM)5,10,14,15 capable of volumetric Raman spectroscopic image acquisition. The microscope design rationale and operation are discussed in detail elsewhere5,14,15 and will be only briefly reviewed here. Plane polarized light at 0.488 µm from an Ar+ laser (Lexel Laser Model 75) illuminates through a high numerical aperture microscope objective, MO (NA ) 0.7, M ) ×20), a sample cell mounted on a three-axis microstepping motor translation stage (Figure 1a). Backscattered light from the sample is collected with the same objective, reflected from a wavelength-sensitive beamsplitter (DBS), and directed through a pinhole spatial filter, Pi, into the FT-Raman spectrometer. The optical autocorrelation (interferogram) output from the scanned Michelson interferometer is measured with a photomultiplier tube (PMT, Hammamatsu R464) after attenuation of the elastically scattered Rayleigh light at 0.488 µm with notch (Omega Optical) and long-pass (Physical Optics Corp.) optical filters. Numerical Fourier transformation of the interferogram with software in the computer (IBM RS6000) gives (10) Brenan, C. J. H.; Hunter, I. W. Appl. Spectrosc. 1995, 49, 997. (11) Feofanov, A.; Sharonov, S.; Valisa, P.; Da Silva, E.; Naviev, I.; Manfait, M. Rev. Sci. Instrum. 1995, 66, 3146. (12) Cf.: Long, D. A. Raman Spectroscopy; McGraw-Hill: New York, 1977. (13) Hunter, I. W.; Lafontaine, S.; Nielsen, P. M. F.; Hunter, P. J.; Hollerbach, J. M. IEEE Control Syst. Mag. 1990, 10, 4. (14) Brenan, C. J. H.; Hunter, I. W. Appl. Spectrosc. 1995, 49, 1086. (15) Brenan, C. J. J.; Hunter, I. W.; Korenberg, M. J. Proc. SPIE-Int. Soc. Opt. Eng. 1996, 2655, 130.
46 Analytical Chemistry, Vol. 69, No. 1, January 1, 1997
Figure 1. Schematics of (a, top) the Fourier transform confocal Raman microscope and (b, bottom) the sample cell.
the scattered light Stokes (long wavelength) portion of the Raman spectrum. A 3-D Raman spectroscopic image is generated on measurement of the Raman spectrum at each microstepper motor position, analyzing the recorded Raman spectra as a function of scan position. Insertion of a pinhole spatial filter into the image plane of the microscope objective increases the CRM spatial resolution and image contrast over that obtainable with a conventional Raman microscope but with a substantial decrease in spectrometer e´tendue by typically >104. Attenuation of the Raman signal by the pinhole has been addressed in the FT-CRM design in two ways: first, illumination of the sample with a relatively short wavelength (0.488 µm) to take advantage of the 1/λ4 increase in Raman scattered power, and second, application of a multiplexed (Fourier transform) spectrometer to maximize the photon flux incident on the photosensor. Detection by the photosensor of photons of all frequencies at each optical lag position in the interferometer, combined with high optical attenuation of the Rayleigh line, ensures that the spectral signal-to-noise ratio (SNR) will be limited by optical shot-noise, not detector thermal noise, even for the low Raman photon flux typical of the CRM. One drawback to this multiplex detection scheme is the potential degradation of spectral SNR in the advent of high photon fluxes incident on the photosensor compared with the SNR possible from a dispersive single (or multiple)-channel Raman spectrometer for the same optical signal. As pointed out by Kahn,16 Hirschfeld,17 (16) Kahn, F. D. Astrophys. J. 1959, 129, 518.
and others,18-20 the multiplex advantage depends on the square root of the ratio between the power in the spectral band under study and the mean power per spectral element averaged over the entire spectral range of photons reaching the photosensor. Given the wide variability in Raman spectra for any given imaging situation, it is difficult to predict in advance whether multiplexed detection will result in a comparative gain or loss in spectral SNR with respect to a dispersive Raman spectral measurement. There are other contributing factors that governed the selection of a FT spectrometer as the CRM spectral analysis subsystem. The large free spectral range of the spectrometer enables capture of an entire Raman spectrum with one interferogram measurement. When faced with complex systems comprised of polymineralic aggregates with potentially unknown and uncharacterized absorbates, measurement of the complete Raman spectrum at each image voxel provides the spectrochemical data needed to unravel the many and complex physicochemical interactions that may occur within the pore spaces. The spectral resolution of a FT spectrometer can be optimized to detect specific spectral features by simply changing the maximum optical path differences between the interferometer mirrors. Efficient coupling of the photon flux from microscope to FT spectrometer results from the minimal geometric aberrations inherent to the matched coaxial geometries of the FT spectrometer and microscope.21 A single optimized photosensor allows detection of the Raman interferogram over a large dynamic range (in the case of the FT-CRM, >20 bit) with good linearity. The independence of the spectral resolving power from pinhole radius, over the range of pinhole radii used in the FT-CRM, decouples the microscope spectral and spatial imaging properties14 and permits easy interchange of pinholes to optimize CRM performance for a given imaging situation. Finally, the measured Raman spectrum is of high accuracy because it is referenced to an internal and accurate spectral standard, the power-stabilized Ar+ laser wavelength at 0.488 µm. The sandstone sample used for this study is a well-sorted quartz arenite provided to the Experimental Geophysics Group at Lawrence Livermore National Laboratory by M. E. Hopkins of the Illinois Geological Survey. The sample was cored at 30 m depth in Section 4, Douglas County, IL. The sample has a bulk density of 2270 kg/m3, and assuming it contains 100% quartz, this yields a bulk porosity of ∼15%. Samples were prepared as cores, ∼10 mm in diameter, and then cut into ∼2 mm slabs using a diamond wafering saw. The samples were then cleaned ultrasonically in acetone, ethanol, and distilled water and then dried at 100 °C. After cleaning, the quartz sandstone samples were then saturated with trichlorethylene (TCE) and loaded into a sample cell (Figure 1b). The sample cell is constructed from a closedended hollow aluminum cylinder sealed at the opposite end with a 170 µm thick glass coverslip. Water is injected into the cell, the cell is sealed and placed on the microstepper motor stage, and the coverslip surface normal is oriented parallel to the microscope optical (Z) axis. The Raman spectra of HPLC-grade water and TCE (Figure 2) taken with the FT-CRM are in excellent agreement with previously (17) Hirschfeld, T. Appl. Spectrosc. 1976, 30, 68. (18) Luc, P.; Gerstenkorn, S. Appl. Opt. 1978, 17, 1327. (19) Nordstrom, R. J. In Fourier, Hadamard and Hilbert Transforms in Chemistry; Marshall, A. G., Ed.; Plenum Press: New York, 1982. (20) Thorne, A. P. Anal. Chem. 1991, 63, 57A. (21) Hirschfeld, T. Appl. Spectrosc. 1985, 39, 1086.
Figure 2. Raman spectra of (a) HPLC-grade water and (b) neat trichloroethylene. The 6287 point interferograms taken of water and TCE, respectively, were zero-padded to 8192 points, and a fast Fourier transform was applied to estimate the Raman spectrum. The conditions for each Raman spectral measurement are as follows: laser power on sample, 150 mW; integration time, 20 ms; spectral acquisition time, 126 s; spectral resolution, 20 cm-1; pinhole diameter, 50 µm.
published spectra22,23 and show Raman peaks indicative of their different molecular structure. The intense TCE 1601 cm-1 Raman line arises from CdC double bond stretching modes,24 while the dominant water Raman band centered at 3300 cm-1 is from the OsH stretch vibration.25,26 There were no observable differences between the neat TCE and water Raman spectra and those acquired of the same fluids in the saturated quartz sandstone. Moving the microscope focal volume relative to the sample and recording the Raman optical power in the 1601 and 3300 cm-1 spectral channels generates 3-D Raman spectral images of the TCE or water distributions within the sandstone. A separate Raman spectrum of the dry sandstone shows no Raman lines at either 1601 or 3300 cm-1 and a weak fluorescence background with a maximum at ∼500 cm-1. From an earlier study,10 a 50 µm (22) Schrader, B., Meier, W., Eds. Raman/IR atlas of organic compounds; Verlag Chemie: Weinheim GmbH, 1977; p C1-03. (23) Reference 22, p O-02. (24) Takagi, Y.; Udagawa, Y.; Mikami, N.; Kaya, K.; Ito, M. Chem. Phys. Lett. 1972, 17, 227. (25) Maeda, Y.; Kitano, H. Spectrochim. Acta Part A 1995, 51, 2433. (26) Scherer, J. R., In Advances in Infrared and Raman spectroscopy; Clark, R. J. H., Hester, R. E., Eds.; Heyden: London, 1978.
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Analytical Chemistry, Vol. 69, No. 1, January 1, 1997
Figure 3. Confocal Raman and Rayleigh image sequences of water and TCE in a quartz sandstone matrix. (a, Opposite page, top) Confocal water Raman image sequence at 3300 cm-1. (b, Opposite page, middle) TCE Raman image sequence at 1601 cm-1. (c, Opposite page, bottom) Corresponding set of Rayleigh images. (d, above) Composite water and TCE confocal Raman image sequence. Each image set consists of a series of ten 50 × 50 point 2-D confocal images (either Raman or Rayleigh) spaced axially by 5.2 µm. With a lateral distance of 5.2 µm between each image pixel, the total image volume scanned is 260 µm × 260 µm × 47 µm into the sample. The only signal processing applied to the images was fluorescence background subtraction and linear dynamic range expansion. For each image set, the laser power incident on the sample is 150 mW, the spectral integration time is 1 ms, and the spectral resolution is 112 cm-1. These parameters resulted in a per image acquisition time of ∼0.5 h and a total volumetric image acquisition time of 5 h.
diameter pinhole spatial filter was found to optimize confocal Raman image contrast, brightness, and axial resolution. In this study, however, a 100 µm diameter pinhole was used to increase confocal image brightness while sacrificing somewhat axial resolution and image contrast. The FT-CRM lateral resolution with the 100 µm diameter pinhole is 1.5 µm (fwhm), and its axial resolution is 10 µm (fwhm), resulting in ellipsoidal-shaped voxels of 12 µm3 in volume. Contact angle measurements were performed on both cut surfaces and natural faces of synthetic quartz crystals. Samples were cleaned in acetone and distilled water and fired at 400 °C to remove residual organic contaminants. We were able to observe TCE droplets on quartz surfaces immersed in H2O only when we suspected contamination of the surface by organic material. In those cases, contact angles are in the range of 100-170°, indicative of TCE “wetness” relative to water. In all other cases, the TCE wicked off of clean and smooth natural quartz faces, preventing contact angle measurements for this configuration. We successfully measured contact angles for water droplets on clean and smooth natural quartz faces immersed in TCE, and values are in the range 142-160°, indicating water “wetness” of these surfaces relative to TCE. RESULTS AND DISCUSSION Figure 3 shows a series of confocal Raman images at the 1601 (Figure 3a) and 3300 cm-1 (Figure 3b) Raman lines, indicating the spatial distribution of TCE and water, respectively, within the porous sandstone matrix. Measurements confirmed the long-term stability of the TCE and water in the sandstone under prolonged illumination with the focused laser beam. The corresponding confocal Rayleigh image sequence (Figure 3c) has poor contrast
because of the high attenuation by laser line filters placed in front of the photosensor and the small difference in reflectivity between TCE and water (
