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Anal. Chem. 1989, 61, 650-656 BetIOn. J.-M.; Desmadrll, M.; Mitraki, A.; Yon, J. M. Biochemkby 1984, 23 6654-6681.
Craig, S.;Hollecker, M.; Creighton, T. E.; Pain, R. H. J. Mol. Biol. 1985. 185. 681-687. Brems, D. N.; Plaisted, S.M.; Havel, H. A.; Kauffman, E. W.; Stodola, J. D.; Eaton, L. C.; White, R. D. Blochemisby 1985, 24, 7662-7668. Holzman. T. F.; Brems, D. N.; Douaherty. . J. J.. Jr. Biochemlst~y1986, 25, 6907-6917. Kuwaiima, K.; Hiraoka, Y.; Ikegushi, M.; Sugai, S.Biochemistry 1985, 24 074-88 1. Frontlcelli, C.; Bucci, E. Biophys. Chem. 1985, 23, 125-128. Brems, D. N.; Plaisted, S. M.; Dougherty. J. J., Jr.; Holzman. T. F. J. Bioi. Chem. 1987. 262, 2590-2596. Kuwajlma, K.; Yamaya, H.; Mlwa, S.;Sugal. S.;Nagamura, T. F€BS Lett. 1987, 221, 115-118. Rosenheck, K.; Doty, P. Proc. Natl. Acad. Sci. U . S . A . 1981, 4 7 , 1775- 1785. Lakowicz, J. R. Rlnclples of Fiuorescence Spectroscopy; Plenum: New York, 1983; Chapter 11. Teale, F. W. J.; Weber, G. Biochem. J. 1957, 65, 476-482. Havel, H. A.; Elzinga, P. A.; Kauffrnan, E. W. Biochim. Biophys. Acta 1988, 955, 154-163. Beechem, J. M.; Brand, L. Ann. Rev. Biochem. 1985, 54, 43-71. Burger, H. G.; Edelhoch, H.; Condliffe, P. G. J. Biol. Chem. 1988, 24 1 , 449-457. Butler, W. L. Methods Enzyrnol. 1979, 56, 510-515. Terada, H.; Inoue, Y.; Ichikawa, T. Chem. Pharm. Bull. 1984, 32, 585-590. Horwltz, J.; Strickland, E. H.; Billups, C. J. Am. Chem. SOC. 1969, 91, 184-190. Strickland, E. H.; Horwitz, H.; Billups, C. Siochemlsfry 1989, 8 , 3205-32 13. Horwltz, J.; Strkkland, E. H.; Billups, C. J. Am. Chem. SOC. 1970, 92, 2119-2129. Havei. H. A.; Kauffman, E. W.; Plaisted, S.M.; Brems, D. N. Biochemistry 1986, 25, 6533-6538. ~
(44) Brems, D. N.; Plaisted, S. M.; Havei. H. A,; Tomich, C.4. C. Proc. Natl. Acad. Sci. U . S . A . 1988, 85, 3367-3371. (45) Kauffman, E. W.; Thamann, T. J. I n Pittsburgh Conference & Exposition on Analytical Chemistry and Applied Spectroscopy; New Orleans, LA, 1968; Abstract 734. (46) Harada, I.; Muira, T.; Takeuchi, H. Spectrochim. Acta, Part A 1988, 42A, 307-312. (47) Siamwiza, M. N.; Lord, R. C.; Chen, M. C.; Takamatsu, T.; Harada, I.; Matsuura, H.; Shimanouchi, T. Biochemistry 1975, 74, 4870-4876. (48) Van Wart, H. E.; Scheraga, H. A. J . Phys. Chem. 1978, 8 0 , 1812-1822. (49) Van Wart, H. E.; Scheraga, H. A. J. Phys. Chem. 1978, 8 0 , 1823-1732. (50) Van Wart, H. E.; Scheraga, H. A. R o c . NaN. Acad. Sci. U . S . A . 1977, 7 4 , 13-17. (51) Van Wart, H. E.; Cardinaux, F.; Scheraga, H. A. J. fhys. Chem. 1978, 80,625-630. (52) Van Wart, H. E.; Scheraga, H. A.; Martin, R. 8. J. fhys. Chem. 1976, 80, 1832. (53) McConnell, M. L. Anal. Chem. 1981, 53, 1007A-1018A. (54) Nystrom, 8.; Roots, J. Chem. Phys. Lett. 1982, 97, 236-240. (55) Ostrowsky, N. D.; Sornett, P. P.; Pike, E. R. Opt. Acta 1981, 28, 1059-1070. (56) Provencher, S. W. Comput. fhys. Commun. 1982, 27, 213-227. (57) Koppei. D. E. J. Chem. fhys. 1972, 57, 4814-4820. (58) Provencher, S. W. Comput. fhys. Commun. 1982, 27, 229-242. (59) Dorshow, R.; Nicoli, D. F. J. Chem. fhys. 1981, 75, 5853-5856. (60) Yarmush, D. M.; Morel, G.; Yarmush, M. L. J. Biochem. Biophys. Methods 1987, 14, 279-289. (61) Wlllis, P. R.; Georgalis, Y. J. fhys. Chem. 1981, 85, 3978-3984.
RECEIVED November 9, 1988.
ARTICLES
Transient Infrared Emission Spectroscopy Roger W.Jones' and J o h n F. McClelland*JJ Center for New Industrial Materials and Ames Laboratory-USDOE, Iowa State University, Ames, Iowa 50011 Translent infrared emisslon spectroscopy (TIRES) is a new method that produces analytically useful emlsslon spectra from optlcally Ihlck, solid samples by greatly reducing selfabsorptlon of emitted radlatlon. The method reduces selfabsorption by creating a thln, short-llved, heated layer at the sample surface and coHectlng the transient emission from thls layer. The techdque requires no sample preparation and may be appiled to both moving and statlonary samples. The slngle-ended, noncontact TIRES measurement geometry is Ideal for on-llne and other remote-senslng applications. TIRES spectra acquired via a Fourler transform infrared spectrometer on movlng samples of coal, plastic, and palnt are presented and compared to photoacoustlc absorptlon spectra of these materlals. The TIRES and photoacoustk results are In close agreement as predlcted by Klrchhoff's law. Center for N e w I n d u s t r i a l Materials. Ames Laboratory-USDOE.
INTRODUCTION Conventional infrared emission spectroscopy, with the sample held at an elevated uniform temperature, has not been a practical method for infrared analysis of most bulk materials due to the phenomenon of self-absorption. Self-absorption in optically thick samples causes severe truncation of strong bands and leads to emission spectra that closely resemble black-body emission spectra and contain little spectral structure characteristic of the material being analyzed (1,Z). The applications of emission spectroscopy could be greatly increased if self-absorption could be controlled. For example, remote, on-line infrared analysis of bulk materials requires a single-ended measurement geometry that could be uniquely satisfied by emission spectroscopy if self-absorption were sufficiently reduced. Conventionally,self-absorption is reduced by thinning the sample. High-quality spectra of free-standing films and thin layers on low-emittance substrates are routinely measured
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(1-5). Rather than thinning the sample itself, it is only necessary that the thickness of the portion of the material actually emitting radiation be reduced. This can be accomplished by generating a thin, transiently heated surface layer in the material and collecting radiation emitted by this layer for analysis. This method, termed transient infrared emission spectroscopy (TIRES), is shown in this paper to reduce self-absorption in thick samples to a level where analytically useful infrared emission spectra can be obtained. A TIRES measurement is made by using a laser beam or other energy source that will be absorbed very near the sample surface so that only a thin surface layer is directly heated. The layer is, of course, transient since thermal diffusion will cause it to thicken and cool rapidly. If a pulsed laser is used on a stationary sample, the layer is present in the sample a t the position of the laser beam for a short time immediately following the laser pulse. If a continuous laser is used, it must be scanned across the sample surface or the sample must be translated through the beam path. The transient layer then exists in the beam track across the sample at and immediately behind the beam position (6). The experiments described in this paper involve a fixed, continuous laser beam focused on a rotating sample which is used to simulate an on-line measurement arrangement. Studies where a pulsed laser is used will be reported at a later time. Once the layer is created, the emission from it is analyzed by a Fourier transform infrared (FT-IR) or other infrared spectrometer to obtain an emission curve that can be converted, according to Kirchhoff s law, to a spectrum analogous to an absorbance spectrum. Spectral information obtained from TIRES, therefore, is similar to that yielded by other types of infrared measurements (transmission, photoacoustic, or diffuse reflectance) which, however, are generally incompatible with on-line analysis. Like infrared absorption, TIRES should be able to determine nondestructivelynot only molecular properties but also many other material properties that are related to molecular structure. Such determinations can be made with the aid of existing FT-IR software which correlates properties with infrared spectral structure. TIRES spectra of heterogeneous samples can be expected to have some band intensity differences relative to spectra of the other measurement techniques due to differences in the heating efficiency of the excitation beam for different components. This effect can be compensated for in the data treatment. The peak temperature increase, AT-, reflects how strong the sample emission will be, and the emitting layer thickness, 1, controls how much self-absorption will occur. These two parameters are the most important in the TIRES process. A detailed theory for the TIRES method is currently being developed; however, ATrnm and 1 may be roughly estimated by using a very simple model. For simplicity, assume that the absorptivity of the sample at the laser wavelength is so high that all of the laser energy is absorbed in an infinitesimally thin layer at the sample surface. Let us also assume that thermal diffusion occurs only one-dimensionally, inward, and ignore other means of heat transport. Given these assumptions, the temperature increase within the sample for the energy absorbed at the surface at time t = 0 is given by the following equation (7):
AT=
E pc(rDt)'/'
where x is the depth into the sample, D is the thermal diffusivity, E is the energy deposited per unit surface area at t = 0, and c and p are the specific heat and density of the sample. If the t 1 / 2term in the preexponential factor of eq 1 is neglected for purposes of estimation, the equation shows
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that a specific value of AT moves inward such that x 2 is proportional to 4Dt. Accordingly, a reasonable estimate of the emitting-layer thickness is 1 = (4Dt)'i2. In the case of a pulsed laser on a stationary sample, the maximum value for t is the period between pulses and so I , = (4D/R)1/2where R is the repetition rate of the laser. For a continuous beam moving across a sample at a velocity u, the maximum value of t is r / u where r is the width of the region behind the laser observed by the spectrometer. That means 1, = (4Dr/u)'/2. If the laser power is P, and the area illuminatedby the beam is A, then the energy absorbed per unit surface area, or energy density, E, is PT,/A where 7, is the time a point on the sample surface is irradiated. This again assumes that all of the incident energy is absorbed. For a pulsed laser on a stationary sample, 7 , is the pulse length and A = rd2/4, where d is the diameter of the laser spot. For a continuous beam moving over the sample, T , = d / u , so the beam moves a distance d along a path d wide in time T , and thus A = d2. During the time T the heat will diffuse inward a distance 1,equal to (4DTP)'/'. If the absorbed energy is assumed in this simple model to be spread uniformly in a layer of this thickness at time T,, then AT,, = E/(cpl,). Substituting in for the various parameters gives D- 112
17,
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To illustrate the range of values involved, we will consider two examples presented in the Results and Discussion, one at low velocity, the other high. Spectrum 3A is of coal ( c = 0.25 cal/(gK), p = 1.3 g/cm3, and D = 0.0015 cm2/s) at a low velocity of 30.6 cm/s with P = 2.2 W, d = 0.08 cm, and r = 0.4 cm. These conditions give AT,, = 167 "C and 1, = 89 Mm. Spectrum 5A, on the other hand, is of a phenolic plastic (c = 0.298 cal/(gK) and p = 1.25 g/cm3 (8), and D = 0.0020 cm2/s (9))at a high velocity of 245 cm/s with P = 3.4 W and d and r as before. These conditions give AT- = 69 O C and 1,- = 36 Mm. These values suggest that the temperature exposure will not be excessive for coal and most other organic materials and that the transient layer thicknesses are much thinner than the effective emission thickness of a stream of even finely ground heated coal or other material. Layer thicknesses of a few tens of micrometers will still lead to substantial self-absorption, but, as the experimental results presented here show, not to the extent that the infrared spectrum of such materials cannot be readily observed. Other considerations are expected to be important in TIRES measurements. Heat build-up has to be controlled whenever the same area is repeatedly sampled, as in pulsed measurements on stationary samples and in continuous measurements with a rotated (as opposed to linearly translated) sample or laser beam. Successful experiments using a cooling jet of gas on a rotating sample are described in the Results and Discussion. In pulsed measurements, the combination of a cooling jet and a pulse length that is short compared to the period between pulses should prove successful, but this has yet to be confirmed. In the case of a continuous flow of new material, heat build-up is avoided. TIRES has a number of advantages over other methods. Especially important is the fact that TIRES does not require any sample preparation, unless the composition of the surface layer probed by TIRES is not related in a known way to the bulk constituents of interest. TIRES is applicable to a very wide range of situations. Depending on the analysis geometry, TIRES can be applied to either a moving stream of material or a stationary object of any size. With a pulsed laser on a stationary target, it may be possible to perform high-resolution infrared microscopy of microsamples by tightly focusing the
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Experimental arrangement used for
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spectra. excitation beam. At the other extreme, it is possible using a moving stream of material to average over much more material with TIRES than would be practical by conventional infrared methods. For instance, with a 1m/s material velocity and a 1-mm-diameter laser spot, a TIRES measurement can potentially examine 3.6 m2 of surface per hour (assuming a 100% duty cycle). The type of instrumentation used can be tailored to meet the specific measurement requirements. A spectrometer based on a number of filters and detectors that operate simultaneously can collect TIRES spectral data 100% of the time from a flowing stream of material. An FT-IR spectrometer, on the other hand, can provide much more detailed spectral information but does not collect data at every instant. Details of the excitation method can also be selected to match the particular situation. Laser type (pulsed vs continuous), wavelength, power, beam size, and optical geometry can all be adjusted to produce the optimum result, and yet a single set of choices can have broad applicability. The studies reported here successfully examined several disparate materials, yet were all done with a single laser, a single beam size, and a single optical geometry.
EXPERIMENTAL SECTION Figure 1 shows the arrangement for recording TIRES spectra with a fixed continuous wave (CW) laser beam and a moving sample. A disk either made of or covered with the sample material was mounted on the shaft of a variable-speed motor and placed at the normal source position of a Perkin-Elmer 1800 Fourier transform spectrophotometer. The beam from an argon-ionlaser operating in the multiline mode at up to 3.5 W was focused on the disk at a 45' angle to a spot approximately0.8 mm in diameter, positioned 3.9 cm from the center of the disk. The spectrometer observed the sample normal to the sample surface with the laser focus centered in the spectrometer's 8-mm-diameterfield of view. The entry port of the spectrometer was 5 cm from the disk and was covered with a salt window. No special additional optics were used to better match the small TIRES source size to the 8-mmwide field of view. The spectrometer was fitted with a wide-band liquid-nitrogen-cooled HgCdTe detector (D*= 1 X 1Olo c m Hz112/W)and accumulated 256 scans in single-beam mode with a 1.50 cm/s optical-path-difference velocity and 4-cm-' nominal resolution. In some cases a sample-coolingjet of chilled helium gas was used. A coil of 1.6-mm-diameter stainless-steel tubing carrying helium was immersed in liquid nitrogen. The open end of the tubing directed the jet onto the disk 0.5 cm from the laser focus so that the rotation of the disk carried the area irradiated by the
laser into the jet immediately after it left the spectrometer field of view. The intensities of our observed emission spectra fall off with increasing wavenumber in the same manner as black-body emission cwves. Additionally, sources other than the sample (such as the spectrometer itself')may contribute a background emission. These may be corrected for by converting the emission curves to emittance curves. According to Kirchhoff's law, emittance is proportional to the fraction of light absorbed when it strikes the surface of a body and so an emittance spectrum is analogous to an absorbancespectrum. Although Kirchhoffs law strictly applies only at thermal equilibrium, emittance curves based on our TIRES emission spectra closely resemble absorbance spectra. An emittance spectrum t may be calculated from an emission spectrum by using the equation t = (S, - S 2 ) / ( B 1- B2),where S and B are the observed sample and black-body emission spectra and the subscripts refer to two temperatures, Tland T2( 3 , 4 ) . Typically, T1> T2 and T2is ambient. S2and B2 correct for background emission while the division by B compensates for the Planck black-body modulation and for the response curve of the spectrometer and detector. For our TIRES spectra, S2 and B2 were at room temperature (i.e., no laser heating or jet cooling), while SI was the raw emission spectrum being converted to emittance. The black-body curves were generated by placing a heated plastic plate covered with carbon black at the source position of the FT-IR spectrometer and recording spectra at a series of closely spaced temperatures. The specific black-body curve selected as Bl in a particular case could not be chosen on the basis of temperature since the TIRES emission is not defined by a single, well-defined temperature. Instead, the fact that the emittance of an object cannot exceed one was used as the criterion. The black-body curve chosen in each case was the lowest temperature curve that equaled or exceeded the sample emission spectrum at all wavelengths. Noise and small zeroing errors produced zero and negative values in the weak, high-wavenumbertails of the emission curves. To correct for this and avoid division by zero, a small constant (always less than 1% and normally less than 0.1% of the spectrum maximum) was added to each emission curve prior to ratioing. These constants had some effect on the relative size of features at high wavenumbers in the resulting emittance spectra, and so they were chosen by using as guides the reference spectra described below. For comparison with the TIRES results, infrared absorption spectra were recorded using photoacousticdetection. An MTEC Model 200 photoacoustic cell was mounted in the FT-IR spectrophotometer (with the spectrophotometer's normal light source) and 32 scans were accumulated at 0.05 cm/s optical-path-difference velocity and 8cm-l nominal resolution. The times required to record a reference photoacoustic spectrum and a TIRES spectrum were both about 3 min.
RESULTS AND DISCUSSION Emission spectra recorded for a smooth-surfaced, 3.0-mmthick, red, filled-phenolic-plastic (Synthane brand) disk are compared to a black-body curve in the top half of Figure 2. Conditions were adjusted so that the observed total emission intensities were identical in the three cases and the curves are shown on the Same vertical scale. Spectrum 2B is the emission from the stationary disk when heated by the laser operating at 0.18 W, and so it is a conventional emission spectrum. Such laser heating has been successfully applied to the emission spectroscopy of thin samples by Lin et al. (2). For this thick phenolic-plastic sample, however, self-absorption has so severely truncated the spectral features that the differences between spectrum 2B and the black-body curve, spectrum 2C, are very small and subtle. Contrasting with this is spectrum 2A, the emission from the disk rotating at 75 rpm with the laser a t 3.4 W. In spectrum 2A the self-absorption is sufficiently reduced that spectral features are readily apparent atop a black-body envelope. In the lower panel of Figure 2 the emittance spectra, spectra 2D and 2E, which were calculated from the emission curves, spectra 2A and 2B, respectively, are compared to a reference
ANALYTICAL CHEMISTRY, VOL. 61, NO. 7, APRIL 1, 1989
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Figure 2. Observed emission spectra from laserheating a phenolicplastic disk (A) while rotating at 75 rpm and (6)while stationary and (C) from carbon black heated to 60 OC. Emittance spectra calculated (D) from spectrum A and (E) from spectrum B. (F) Reference photoacoustic absorption spectrum of the phenolic plastic.
absorption spectrum (2F) of the plastic. The three spectra have been scaled vertically to make them appear the same size. The TIRES spectrum is actually about 2.7 times larger than the stationary spectrum. The differences between TIRES and stationary emission are even more striking for these emittance spectra. Note that in spectrum 2E the band at 1740 cm-' is very roughly the same height as all features to the right of it, while in the absorption spectrum, spectrum 2F, the 1740cm-' band is a weak shoulder. In spectrum 2E the features to the right of 1740 cm-I have been truncated by self-absorption down to the level of this weak shoulder. Such severe truncation makes the spectrum useless for analytical purposes. On the other hand, the TIRES emittance, spectrum 2D, closely resembles the absorption spectrum. Features with high emittance or absorbance, such as the broad band between 1000 and 1200 cm-I are reduced in size in the, TIRES spectrum relative to the absorption spectrum, giving the TIRES spectrum a more saturated appearance. This apparent saturation is the feature truncation caused by the residual self-absorption still present under the experimental conditions. Despite this saturation, even small features are still readily observable (e.g., the small peak at 1200 cm-' and the shoulder at 1047 cm-'1. Coal provides a second example of the improvements attained by observing transient rather than steady-state emission. Coal was chosen both because of its industrial importance and because its irregular surface would test the applicability of TIRES to a less-than-ideal sample. The coal disk consisted of a series of 4- to 7-mm-thick pieces of Illinois No. 6 coal glued to an aluminum disk so that the laser beam illuminated a continuous path of coal. Because of the roughness of the coal, its surface varied in and out from the spectrometer entry port by about 2 mm when the disk was rotating. This roughness caused fluctuations in the signal intensity observed by the spectrometer, but it did not appreciably increase the noise in the resulting spectrum. These fluctuations were principally caused by the 45' angle of incidence of the laser beam on the coal; as the coal surface moved in and out, the angle of illumination caused the laser spot to move up and down. Using a different geometry with an il-
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Flgure 3. Emittance spectra of coal when the sample is (A) rotating at 75 rpm and (B) stationary. (C) Reference photoacoustic absorption spectrum of the coal.
lumination angle more nearly normal to the sample surface would reduce such fluctuations, but it might require adding an optical filter to prevent reflection of the laser beam into the spectrometer. Figure 3 compares the emittance spectra resulting from 2.2 W of laser light being focused on the sample disk rotating at 75 rpm and 0.11 W being focused on the stationary disk with a reference photoacoustic absorption spectrum. The laser powers were adjusted so that the total emission intensity observed by the spectrometer was the same in the two cases. As with the phenolic plastic, self-absorption obscures features in spectrum 3B for the stationary disk sufficiently to make the spectrum analytically worthless, while spectrum 3A, the TIRES emittance, is comparable to the reference absorption spectrum. The TIRES spectra for the plastic and the coal demonstrate the only two significant ways in which TIRES spectra are inferior to absorption spectra; the signal-bnoise ratio degrades markedly at higher wavenumbers, and the TIRES spectra appear more saturated. The reduced signal-to-noise ratio at high wavenumbers is due to the black-body-liketailing off of the emission spectrum in that region, and so is intrinsic to emission spectroscopy. Our system is not optimized for emission, and numerous changes could be made to improve the signal-to-noise ratio. Since emission intensity is proportional to the fourth power of the temperature, increasing the heating of the sample by raising the laser wattage or by reducing the sample speed through the laser beam will significantly increase the signal size, as long as the damage threshold for the sample is not exceeded. The TIRES spectra in Figures 2 and 3 were recorded under conditions that produced the same emission intensity as the accompanying stationary-sample spectra and not under conditions producing optimum signal-to-noise ratios. For the stationary sample, the emitting area is a circular spot, while for the moving sample the emitting area is stretched into a physically larger band. The larger emitting area allows TIRES to produce a stronger signal than stationary-samplelaser-induced emission without exceeding the damage threshold of the material. For example, spectrum 3B of coal was recorded with a laser power of 0.11 W, just below the level that would produce smoke from the coal, while spectrum 3A at 2.2 W is well below the TIRES
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ANALYTICAL CHEMISTRY, VOL. 81, NO. 7, APRIL 1, 1989
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Figure 4. Emission spectra of phenolic plastic at different sample speeds (A through D) without cooling and (E through G) with cooling to reduce the total emission to the same level as spectrum 0. t
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damage threshold for 75 rpm. This threshold exceeds the 3.5-W capability of our laser. The greater apparent saturation in the TIRES spectra than in the absorption spectra is of course the result of self-absorption still present under our experimental conditions. The 75-rpm rotation of the sample for spectra 2A and 2D corresponds to a h e a r velocity of 31 cm/s at the laser-beam focus. With the laser focus at the center of the 8-mm-wide spectrometer field of view, this means that the irradiated portion of the disk was observed for 13 ms after passing through the focus. Using data and equations from the Introduction gives a maximum emitting-layer thickness of 100 pm. Increasing the rotation rate decreases the observation time and thus the maximum emitting-layer thickness, resulting in less self-absorption and less apparent saturation. The left half of Figure 4 shows the emission from the phenolic-plastic disk at four different rotation rates, increasing in speed from bottom to top, and Figure 5 shows the emittance curves calculated from these emission spectra. The laser power was adjusted in proportion to the rotation rate so that the absorbed energy density ( E = P/(ud)) was the same for all. It is obvious from spectra 4A through 4D that the structural features increased in size with each increase in rotation rate such that the structural features became a larger fraction of the total emission. This increase also appeared in the resulting emittance spectra. Each doubling of the rotation rate increased the size of the emittance spectrum by 1.55 f 0.10. The spectra in Figure 5 have been scaled so that the difference between the minimum near 1900 cm-' and the peak at 1125 cm-' is the same size in all four. Even after this scaling, the spectra still show evidence of the decrease in self-absorption with higher sample speed, especially in the band of high emittance between 1000 and 1500 cm-'. The smaller structure atop this band
increases in contrast as the sample speed rises. Overall, then, increasing the sample speed by a factor of 8 in the series of spectra in Figure 5 decreased the maximum emitting-layer thickness from 100 to 36 pm and the resulting reduction in self-absorption caused the size of the emittance spectra to increase by a factor of 3.7 while the spectral contrast within the spectra increased as well. The left side of Figure 4 shows that the black-body-like component of the emission also rose with increasing rotation rate. This black-body increase occurred because our moving sample was a rotating disk instead of a linear stream of material. This meant that each sampled region on the disk was repeatedly irradiated and sampled and that heat from previous passes through the laser beam could accumulate and raise the steady-state temperature of the sample along the beam path. The sample speed of 75 rpm used for spectrum 4D allowed each sampled point on the disk to cool for 0.8 s before resampling. For spectrum 4A, with a speed of 600rpm, this time was cut to only 0.1 s, and so a larger fraction of the heat from previous passes through the laser beam was still present when a point was again irradiated and sampled. This remnant heat had diffused to deeper layers of the sample by the time the resampling occmed, and so it contributed mainly to the highly self-absorbed, black-body-like portion of the emission. Such remnant heat can be removed by actively cooling the irradiated region shortly after it passes through the beam focus. In our experimentsa cooling jet was located just outside the spectrometer field of view and thus could only affect the remnant heat from previous excitations; it could have no effect on the transient emission response immediately after irradiation that our experiments were designed to observe. On removal of the remnant heat, the cooling allowed our experiments to simulate a single-pass arrangement in which the target material moves in a linear stream. The right half of Figure 4 shows emission spectra from the phenolic plastic under the same conditions as in the left half of the figure except that enough cooling was used to reduce the total emission intensity of spectra 4E through 4G to the same value as that of spectrum 4D. A comparison of the two halves of Figure 4 illustrates that the cooling had only a modest effect on the structured emission while greatly reducing the blackbody portion of each curve. The emittance spectra calculated from the emission curves of the cooled sample also reflect the modest effect of cooling. The emittance spectra of the cooled sample have signal-to-noise ratios and spectral contrast similar to the analogous uncooled-sample spectra in Figure 5. The absolute size of the cooled-sample emittance spectra are reduced somewhat from their uncooled counterparts, increasing by only 1.32 f 0.07 for each doubling of the rotation rate. Figure 6 compares spectra of blue-green baked-enamel paint on 3-mm-thick aluminum. The paint was examined for two reasons. First, being a thin film,the paint would test whether TIRES can improve on the conventional results from a sample sufficiently thin to produce a better conventional emission spectrum than the materials so far discussed. Second, the paint was expected to produce a weaker emission signal than the other samples and thus it would test how well TIRES could handle a low-signal sample. The low signal should result both because the blue-green color of the paint made it a good reflector of the laser light and because it was a thin layer on aluminum, whose high thermal conductivity should diffuse the laser-deposited heat faster. Spectrum 6A at the top of the figure is the emittance of the stationary sample and spectrum 6B directly below it is the TIRES emittance taken under conditionsthat produced the same total emission signal strength as the stationary-sample spectrum had. These may be compared to the reference absorption spectrum at the bottom of the figure. The differences between spectra 6A and
ANALYTICAL CHEMISTRY, VOL. 61, NO. 7, APRIL 1, 1989
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Figure 6. Emittance spectra of blue-green paint on aluminum when the sample is (A) stationary, (B) rotating at 750 rpm, and (C) rotating at 75 rpm. (D) Reference photoacoustic absorption spectrum of the paint.
Figure 7. Emittance spectra of electrical tape when the sample is (A) stationary, (B) rotating at 600 rpm, and (C) rotating at 187 rpm. (D)
6B are not as great as those between the analogous matched stationary sample and TIRES spectra in earlier f i e s because of the thinness of the paint. Nevertheless, spectrum 6B still shows less self-absorption and better spectral contrast than spectrum 6A. As previously discussed, the TIRES technique produces an equivalent emisston intensity at a lower sample temperature than stationary-sample laser-induced emission spectroscopy does. This is graphically illustrated by spectrum 6B, which is missing the two bands at 2857 and 2930 cm-‘ that are present in the other paint spectra. The temperature was too low for observable emission at such short wavelengths. Another point previously mentioned is that the TIRES spectra recorded under conditions chosen to match the stationarysample spectra were necessarily not the optimum TIRES spectra we could observe. Spectrum 6A was made just below the damage threshold for the paint, but spectrum 6B was not. Spectrum 6C is included to demonstrate the improvement possible under more favorable conditions. The higher laser power and slower sample speed of spectrum 6C greatly increase the signal-to-noise ratio, as well as produce the features at 2857 and 2930 cm-’, with only a slight loss in spectral contrast due to the slower sample speed. Even spectrum 6C has a poorer signal-to-noise ratio than the TIRES spectra in Figures 2 and 3. This is the result of the high reflectivity of the paint at the laser wavelength and the high thermal conductance of the aluminum substrate. The final sample to be discussed is electrical tape, which is a 0.18-mm-thick (excludingadhesive), pigmented, plasticized poly(viny1chloride) sheet. The tape was attached by its own adhesive to a 1.6-mm-thick aluminum disk. The tape was chosen because it has a lower thermal-decomposition threshold than the other samples. (The maximum service temperature for plasticized poly(viny1 chloride) is typically 80 to 105 “C (IO)). Spectra 7A and 7B (Figure 7) are the stationary-sample emittance and the TIRES emittance recorded under conditions of identical emission intensity. The results are similar to those in Figure 6 for the paint. Spectrum 7B shows much less self-absorption than spectrum 7A. Even though spectrum 7A was recorded just below the damage threshold of the
plastic, spectrum 7B barely shows the prominent features at high wavenumbers because of the low effective temperature of the sample. The tape can withstand a much higher energy density than in spectrum 7B, and spectrum 7C shows the improvements a higher density brings. Spectrum 7C compares very well to the photoacoustic absorption spectrum, having a similar signal-to-noise ratio at low wavenumbers and similar spectral contrast. The spectra discussed in this paper demonstrate that the TIRES technique effectively reduces the saturation in the emission from optically thick samples to levels comparable to photoacoustic absorption spectra. This does not mean that the TIRES spectra have negligible saturation since the reference photoacoustic spectra presented are themselves partly saturated; however, they do demonstrate that TIRES provides results comparable to a well-known, widely used technique. The variety of less-than-ideal samples presented shows that TIRES is potentially widely applicable and can be used on materials with high reflectivity, irregular surfaces, and moderate thermal lability. Our TIRES results have excellent signal-to-noiseratios below 2000 cm-’ but marginal ones above 2000 cm-’. In our experiments the spectrometer was not modified beyond replacing the normal source with the irradiated sample. There are numerous changes that could be made to optimize the spectrometer. Perhaps the most obvious improvement would be the addition of light collection optics analogous to those in infrared microscopes. These would allow observation of a greater portion of the emission from a smaller area of the sample and would thereby increase the signalto-noise ratio while simultaneously reducing self-absorption (because of the reduction in r a n d thus in l-). Experiments are presently under way to improve the moving-sampleform of TIRES and to develop the stationary-sample, pulsed-laser approach.
Reference photoacoustic absorption spectrum of the tape.
ACKNOWLEDGMENT The authors wish to thank Joseph Shinar of Ames Laboratory, Iowa State University, for the generous loan of the laser used in this study, and Robert Hoult of the Perkin-Elmer Corp. for helpful discussions.
Anal. Chem. 1989, 6 1 , 656-660
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LITERATURE CITED (1) Griffiths, Peter R. Appl. Spectrosc. 1072, 26, 73-76. (21 Lin. L. T.: Archibald. D. D.:Honlas. D. E. A,m . / . Soectrosc. 1088. 42. 477-483. (3) Chase, D. B. Appl. Spectrosc. 1981, 3 5 , 77-81. (4) Kember, D.;Chenery, D H.; Sheppard, N.; Fell, J. Specb-ochim. Acta, Part A 1070, 35A, 4551459. (5) Handke, M.; Harrlck, N. J. Appl. Spectrosc. 1986, 4 0 , 401-405. (6) Chen, Inan; Lee, Sanboh J . Appl. Phys. 1083, 5 4 , 1062-1066. (7) Hill, James M.; Dewynne, Jeffrey N. Heat Conduction; Blackwell SCIentific: Oxford, U.K., 1987; pp 8-1 1. (8)Chang, Shu-Sing In Thermal Analysis in Polymer Characterization; Turi. E. A.. Ed.: Hevden and Son: Philadelahia. PA. 1981: DO 98-113. (9) Erk, S.; Keller, A.;*Poltz, H. Phys. 2. 1937, 38, 394-402: 1
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(10) Handbook of Chemistryand Physics, 62nd ed.;Weast, Robert C.. Ed.; Chemical Rubber: Cleveland, OH, 1981; p C-753.
RECEIVED for review August 16, 1988. Accepted December 22, 1988* This work was funded in part by the Center for New Industrial Materials, which is operated for the U.S.Department of Commerce by Iowa State University under Grant No. ITA 87-02, and in part by AmeS Laboratory, which iS Operated for the U.S. Department of Energy by Iowa State University under Contract No* supported by the Assistant Secretary for Fossil Energy. W-7405-ENG-829
Silver-Coated Fumed Silica as a Substrate Material for Surface-Enhanced Raman Scattering A. M. Alak’ and T. Vo-Dinh* Advanced Monitoring Development Group, Health and Safety Research Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831 -6101
A new and simple type of substrate for surface-enhanced raman scatterlng (SERS) was lnvestlgated. This new substrate Is based upon sliver-coated fumed slllca on surfaces such as glass plates or microscopic slides. Fumed slllca, which has submkrometer size structure provides the surface roughness necessary for SERS process. The effects of several experlmental factors, lncludlng the type of fumed siilca, the concentratlon of the mlcropartlcles, and the sllver layer thickness were Investlgated. The SERS spectra of various organk species were used to demonstrate the efficiency and applicabHity of the new substrate. Fumed-slllca-based substrates offer many advantages, since they are slmple to prepare and easy to handle. The bask material, fumed slllca, Is also commercially avallable at very low cost.
INTRODUCTION Raman spectroscopy has proved its usefulness as a practical tool for organic analysis (1,2). A major disadvantage of the Raman technique,however, is the small scattering cross section that often requires the use of powerful and costly laser sources for excitation. Recently, a renewed interest has developed in Raman spectroscopy as a result of discoveries of “giant enhancement” in the Raman scattering efficiency when an analyte molecule is adsorbed on metal surfaces or metal particles in solutions (3-8). Many theoretical models have been developed to explain the large Raman enhancement and to account for both the physical and chemical effects associated with the surfaceenhanced Raman scattering (SERS) (5). More than one mechanism appears to contribute to the SERS phenomenon. One major contribution to Raman enhancement is associated with amplified local electromagnetic fields a t the surface. These fields originate from roughness-induced excitation of surface plasmons (5)and from the concentrationof the electric field lines near high-curvature points of the surface (8,9). If the adsorbate molecule is relatively close to the surface, image ’Present address: M e r r e l l Dow Pharmaceuticals, Inc., 2110 Galbraith Rd.,Cincinnati, OH 45215.
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dipole and the modulated reflectance mechanism may also contribute to Raman enhancement (5). This electromagnetic enhancement contribution is not molecule-specificand does not require physical contact between adsorbate molecule and metal surface. Enhancement is also attributed to the modification of molecular polarizability and, hence, the Raman cross section. This modification of the molecular polarizability is caused by interaction of molecules and metal surface through chemisorption, a process that consists of the formation of a complex between analyte molecules and the atoms of the metal surface (10). The chemical mechanism implies the presence of “active sites” or structures on the surface capable of forming particular molecular configurations. In contrast to the electromagnetic model, in which the enhancement mechanism has a long-range character, the “active site” model involves an enhancement mechanism limited to the molecular layer in direct contact with the metal surface. Another chemical concept called “adaatom”, which involves chemical adsorption between the analyte molecule and the substrate ( I l ) , may also be involved in the surface enhancement process. Generally, the observation of SERS of molecules adsorbed on metal surfaces requires that the metal surface be roughened. The specific nature of this roughness, and its exact role in the enhancement, has been a subject of considerable debate (5). The three common types of substrates used in SERS were chemical electrodes, colloidal sols, and island films prepared by vacuum deposition of silver. Repeated oxidation-reduction cycles were used to dissolve and redeposit silver to provide a rough surface. The repeated oxidation-reduction process gives a rough surface, which will induce enhanced Raman scattering via the electromagnetic enhancement mechanism ( 4 , 5 ) . The use of silver sol for obtaining SERS has attracted considerable interest among analytical chemists because of its experimental simplicity. Other types of SERS-active solid substrates involving metal-covered surfaces having submicrometer structures, such as microspheres, quartz post, etc., have been reported (12-1 6). Previous studies in this and other laboratories (12-20) have demonstrated the effectiveness and potential of SERS as a powerful spectrochemicaltechnique to identify and quantify different compounds such as nitropolyaromatichydrocarbons, drugs, organophosphorus chemicals, and pesticides using
0003-2700/89/0361-0656$01.50/00 1989 American Chemical Society
