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Anal. Chem. 1989, 6 1 , 1810-1815
whereas single record concentration increases were hard to distinguish from background (6E). As well as demonstrating DA detection in the tens of nanomoles range, diffusion analysis (23) provided a relevant evaluation of the response time of the untreated CFVMs to DA concentration change. The value of D determined by using these CFVMs was about 5.0 X lo+ cm2& (17 "C) (Figure 5C,E and Figure 6), which, when temperature corrected (35),agreed well with previously reported values determined by using a flow injection method (24). The accurate monitoring of the 30-9 rising portion of the concentration profile suggests that even shorter intervals could be evaluated if such data were needed. Smaller values for D ((3-4) x 10%cm2.s-') seen with some electrodes suggested that adsorption and slow desorption of DA were increasing the CFVM time constant. Similar slow responses were seen when the CFVM was exposed to the air after measuring DA in agarose and was not rinsed before continuing measurements. In that case, a thin layer of dried agarose could be acting as a diffusion barrier to the CFVM surface. In conclusion, the rational design of low-noise CFVMs can be acomplished by addressing possible noise sources that are the result of the VM fabrication process. The features of the electrodes described here have been optimized for high-speed cyclic voltammetry; however the design principles are generally applicable for other materials and techniques. Concern about possible interferences, including pH changes, is also pertinent to all voltammetric techniques used to study changes of neuroactive compounds in brain and mandates access to cyclic voltammograms throughout the progress of the experiment.
ACKNOWLEDGMENT We are grateful to Dr. F. G. Gonon for his gift of 8-pm Le Carbone Lorraine fibers and to Dr. R. M. Wightman for his gift of 8-10-gm Thornell fibers. We also thank Mr. A. Benedek for electronic support and Ms. S. Sleet for assistance with photography and graphics. LITERATURE CITED (1) Kissinger, P. T.; Hart, J. 8.; Adams, R. N. Brain Res. 1073, 55, 209-312. (2) Ponchon, J. L.; Cespuglio. R.; Gonon. F.; Jouvet, M.; Pujol, J. F. Anal. Chem. 1079, 5 1 , 1483-1486.
(3) Armstrong-James, M.; Millar, J. J . Neurosci. Methods 1070, 1 ,
279-287. (4) Dayton, M. A.; Brown, J. C.; Stutts, K. J.; Wightman, R. M. Anal. Chem. 1080, 5 2 , 946-950. (5) Maidment, N. T.; Marsden, C. A. Brain Res. 1085. 338, 317-325. (6) Meulemans, A.; Poulain, B.; Baux, G.; Tauc, L.; Henzei, D. Anal. Chem. 1086. 58, 2088-2091. (7) Brazell, M. P.; Kasser, R. J.; Renner, K. J.; Feng, J.; Moghaddam, B.; Adams, R. N. J . Neurosci. Methods 1087, 22, 167-172. (8)Rice, M. E.; Nicholson, C. J . Neurochem. 1087, 49, 1096-1104. (9) Nicholson, C.; Rice, M. E. Neuromethods Vol. 9 : NewonalMicroenvironment; Boulton, A. A., Baker, G. B., Walz, W., Eds.; Humana Press: Clifton, NJ, 1988; pp 247-361. (10) Gerhardt, G. A.; Oke, A. F.; Nagy, G., Moghaddam, 8.; Adams, R. N. Brain Res. 1084, 290,390-395. (11) Kristensen, E. W.; Wightman, R. M. Anal. Chem. 1087, 59, 1752-1 757. (12) Crespi, F.; Martin, K. F.; Marsden, C. A. Neuroscience 1088, 27, 885-896. (13) Armstrong-James, M.; Fox, K.; M i l k , J. J . Neurosci. Methods 1080, 3 . 37-48. (14) Gonon, F. G.;Fombariet. C. M.; Buda, M. J.; Pujol, J. F. Anal. Chem. 1081, 53, 1386-1389. (15) Gonon, F. G.;Buda, M. Neuroscience 1085, 74, 765-774. (16) Gonon, F. G. Neuroscience 1088, 2 4 , 19-28. (17) Feng, J.-X.; Brarell, M.; Renner, K.; Kasser, R.; Adams, R. N. Anal. Chem. 1087, 59, 1863-1867. (18) ArmstrongJames, M.; Kruk, Z. L.; Millar, J. Nature 1080, 288, 18 1 7 83. (19) Millar, J.; Stamford, J. A.; Kruk, Z. L.; Wightman, R. M. €or. J . Pharrnacol. 1085, 109, 341-348. (20) Kuhr, W. G.;Wightman, R. M. Brain Res. 1086, 381, 168-171. (21) Stamford, J. A.; Kruk, Z. L.; Millar, J. Brain Res. 1086, 381, 351-355. (22) May, L. J.; Kuhr, W. G.; Wightman, R. M. J . Neurochem. 1088, 51, 1060-1069. (23) Nicholson, C.; Phillips. J. M. J . Physlol. 1081, 321,225-257. (24) Gerhardt, G. A.; Adams, R. N. Anal. Chem. 1082, 54, 2618-2620. (25) Kelly, R. S.;Wightman, R. M. Anal. Chim. Acta 1086. 187, 79-87. (26) Fox, K.; Armstrongdames. M.; Millar, J. J . Neurosci. Methods 1080, 3,37-48. (27) Rice, M. E.; Nicholson, C. Soc. N w o s c i . Abstr. 1088, 14, 742. (28) Stamford, J. A. J . Neurosci. Methods 1088, 17, 1-29. (29) Dayton, M. A.; Ewing, A. G.: Wightman, R. M. Anal. Chem. 1080, 52, 2392-2396. (30) Kraig, R. P.; Ferreira-Filho. C. R.; Nicholson, C. J . Nemphysiol. 1083, 49, 831-850. (31) Chesier, M.; Chan, C. Y. Neuroscience 108ap27, 941-948. (32) Rice, M. E.; Nicholson, C. Brain Res. 1088, 461, 328-334. (33) bnsen, A. J. Physiol. Rev. 1885, 8 5 , 101-148. (34) Nagy, G.;Gerhardt. G. A.; Oke, A. F.; Rice, M. E.; Adams, R. N.; Moore, R. B., 111; Szentlrmay. M. N.; Martin, C. R. J . Nectroanal. Chem. 1085, 188, 85-94. (35) Meltes, L. Polarographic Techniques; 2nd ed.; Interscience: New York, 1965; p 139.
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RECEIVED for review April 17,1989. Accepted June 12,1989. This work was supported by USPHS Grant NS-13742.
Transient Infrared Emission Spectroscopy by Pulsed Laser Excitation Roger W. Jones and John F. McClelland* Center for Advanced Technology Development, Iowa State University, Ames, Iowa 50011
Translent Infrared emlsslon spectroscopy (TIRES) produces analytically useful emission &tra fr&-ohkaily thick samples by reduckrg sell-absorption of the emitted radlatbn. Use of a pulsed laser in the TIRES technlque allows the observed emlsslon from samples to be ilmlted to a sufficiently thin surface layer of the material that self-absorption is nearly eliminated and the resulting spectra are close to belng saturation free. Pulsed-laser TIRES emittance spectra are presented for varlous plastlcs and Inorganic materials, and these spectra are compared to conventional emittance spectra and photoacoustlc absorbance spectra. 0003-2700/89/0361-18lO$OlSO/O
INTRODUCTION Applications of conventional infrared emission spectroscopy have long been limited to optically thin materials because of the phenomenon of self-absorption. When a material is heated to a uniform elevated temperature, all parts of it both emit and reabsorb infrared radiation, and this self-absorption of previously emitted light severely truncates and alters features in the emission spectra of optically thick samples. The resulting emission spectra closely resemble blackbody spectra and contain very little structure characteristic of sample composition ( I ) . Recently a new technique called transient 1989 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 61, NO. 17. SEPTEMBER 1, 1989
infrared emission spectroscopy (TIRES) that produces analytically useful emission spectra from optically thick samples was introduced (2). In TIRES a laser is used to rapidly heat a thin surface layer of the sample and emission from this layer is gathered before it thickens and cools by thermal diffusion. This effectively minimizes self-absorption by limiting the emitting and reabsorbing to an optically thin volume. In the previous paper on TIRES (2) the background and characteristics of performing TIRES using both continuous-wave (CW) lasers with moving samples and pulsed lasers with both moving and stationary samples were discussed, but only results obtained by using a CW laser were presented and discussed. This paper reports TIRES experiments involving a pulsed, KrF-excimer laser and moving samples. Since emission induced with a pulsed laser is itself pulsed, TIRES done with a pulsed laser and a Fourier-transform infrared (FT-IR) spectrometer requires both the addition of an integrator circuit into the spectrometer signal path and the synchronization of laser firing with signal sampling by the spectrometer, as described in the Experimental Section. CW-laser TIRES has the advantage of simplicity since no synchronization is required and any M'-IR spectrometer fitted with a HgCdTe (or similarly sensitive) detector and adaptable to emission spectroscopy may be used without modification. Pulsed-laser TIRES, however, produces spectra with less self-absorption than CW-laser TIRES does under typical conditions. The amount of self-absorption in a spectrum is related to the emitting-layer thickness, 1, in the sample while it is being observed by the spectrometer. As was described previously ( 2 ) ,a reasonable estimate for 1 is (4Dt)'12,where D is the thermal diffusivity of the sample and t is the time since laser heating occurred. Accordingly, how long an area of the sample is observed after heating governs how thick the observed emitting layer becomes. It was also previously shown that the maximum observed layer thickness, l,, is (4Dr/u)'12 for CW-laser TIRES, where r is the width of the sample region behind the laser beam observed by the spectrometer and u is the sample velocity. For pulsed-laser TIRES the absolute maximum limit on t is the period between laser shots, which would give 1,- = (4D/R)'12, where R is the repetition rate of the laser. When an integrator is inserted into the circuitry, however, the maximum t a t which observations are made is controlled by the integrator gating, and so I, = (4Dt,,)'Iz, where t,, is the time between the laser pulse and the end of the observation gate of the integrator. This is the advantage of pulsed-laser TIRES over CW-laser TIRES-the size of I , can be changed simply by altering t,, and so smaller values of l,, are readily achieved. In CW-laser TIRES, reducing l,, requires either raising the sample speed or reducing the spectrometer field of view, or both, but neither adjustment may be practical in a given application. For example, all of the spectra reported here were observed with a t,, of 110 ps. If D is taken to be 0.002 cm2/s, which is typical of organic solids, this results in an l,, of 9 pm. Such a small l,, can be difficult to produce by CW-laser TIRES. If D were the same and r were 0.4 cm, as it was for the earlier CW-laser TIRES work (2),a sample velocity of 40 m/s would be needed The small values of I , that are readily to attain a 9-pm Im,. achieved with pulsed-laser TIRES result in spectra suffering from only minimal self-absorption. Such low-1, spectra have a larger fraction of the total emission in the structured portion of the emission (at the expense of the blackbody-like portion) and have better spectral contrast (that is, less apparent saturation) in the resulting emittance spectra.
EXPERIMENTAL SECTION Figure 1 is a diagram of the setup used for observing pulsedlaser TIRES spectra. The physical and optical arrangements were
, SPECTROMETER
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very similar to those used for CW-laser TIRES (2), but the electronic arrangement and signal treatment were modified in a manner similar to that used by Leone and co-workers ( 3 , 4 )to study chemical kinetics by infrared emission. 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 a Lambda Physik EMG 105 excimer laser operating on KrF (248 nm) at 10 mJ/pulse was focused onto the sample disk at a 45' angle within the 8-mm-wide field of view of the spectrometer. A NaCl window covered the entry port of the spectrometer, which viewed the sample normal to the sample surface. The size of the laser beam spot on the sample was optimized for each sample with a single 25-cm-focal-length lens. No other optics were used to better match the emitting area to the spectrometer field of view. The sample disks were spun at 1000 rpm so that the area irradiated by a single laser pulse had left the spectrometer field of view before the next laser pulse. The spectrometer used a wide-band HgCdTe detector (D*= 1 X 1O'O crn.H~"~/W) and accumulated 256 interferograms (128round-trip cycles of the interferometer mirror with an interferogram collected in each direction of mirror travel) at a 0.09 cm/s optical-pathdifference velocity and 8 cm-' nominal resolution. Each TIRES spectrum appearing in this paper required about 13 min to record. Because of the pulsed nature of the emission produced by the laser irradiation, it was necessary both to synchronize the laser firing with the signal sampling by the spectrometer analog-todigital (A/D) converter and to insert an integrator between the output of the detector preamp and the A/D converter. The pulse used internally by the spectrometer to trigger its A/D converter was fed through a trigger interface to f i e the laser. At a 0.09 cm/s optical-path-difference velocity, the spectrometer sampled at a 2845-Hz rate, but the laser had a maximum repetition rate of lo00 Hz. The trigger interface prevented overdriving of the laser by firing the laser only on every fourth signal sampling. The output signal of the detector preamp showed a sharp onset (accompanied by some rapidly damped ringing) when the laser fired, that then decayed away over typically a few hundred microseconds. This output was fed to a boxcar integrator (SRS Model SR250) which was modified to produce a 100-ws sampling gate that started 10 ps after the laser fired to avoid the initial ringing. The integrator was adjusted so that each laser shot was integrated independently, with no averaging among successive shots. In experiments where the emission strength was not strongly sensitive to pulse-to-pulse fluctuations in the laser energy, the output of the integrator was fed directly to the A/D converter of the spectrometer. The integrator was triggered by the same pulse as the laser and so the four spectrometer samplings between successive laser shots all recorded the same output value from the integrator. In effect, this lowered the Nyquist frequency; however, under these conditions the Nyquist wavenumber was still 3951 cm-'. The pulses from the laser fluctuated in energy by approximately the &15% specification of the laser. Usually this fluctuation caused the emission from the sample to vary by a similar amount, resulting in excessive noise. In such cases additional signal processing was done before the signal was fed to the spectrometer A/D converter. A quartz flat was placed in the laser beam to reflect a small portion of the light onto a photodiode, as shown in Figure 1. The output of the diode was fed to a second integrator adjusted and triggered like the first to integrate each laser pulse separately, without averaging successive pulses. The outputs of
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the two integrators were then ratioed together to reduce the effect of laser fluctuations. This ratioed signal was then fed t o the spectrometer A/D converter. This two-integrator method was used for all of the pulsed-laser TIRES spectra presented here except for that of the polycarbonate. The emission spectra fall off in intensity with increasing wavenumber in the same manner as blackbody emission curves. To compensate for this, the emission spectra were converted to emittance spectra by ratioing them against blackbody spectra in the same manner as was used previously (2). Since the samples did not have single, uniform temperatures when the TIRES spectra were recorded, temperature could not be used as the criterion for selecting which blackbody curve was to be used with a particular emission spectrum. Instead, since the emittance of an object cannot exceed one, the blackbody curve was chosen in each case so that the resulting apparent emittance of the strongest feature in the spectrum was between 0.8 and 0.95. The blackbody curves were produced by placing a plastic plate covered with carbon black at the source position of the spectrometer and heating it to a series of temperatures in the range 30-80 "C. The spectra were recorded with the same signal processing as was used for the pulsed-laser TIRES emission spectra (with only one integrator). Since carbon black acts as a "gray body" with an emittance less than one and since the selection of a blackbody spectrum described above is somewhat arbitrary, only the relative emittances that result are valid; the absolute values of the emittances are not meaningful. Accordingly, no ordinate scales are provided for any of the TIRES spectra. As the size of the laser-beam spot on a sample is reduced by more sharply focusing the beam, the intensity of the structured portion of the emission increases substantially due to the higher transient-layer temperature while the blackbody-likeportion from the cooler bulk increases more slowly. The signal-to-noiseratios of the emittance spectra increase with decreasing spot size, while the degree of saturation is usually not affected. Accordingly, optimum focusing in most cases corresponded to an irradiation intensity slightly below the threshold for rapid damage to the sample. Slate is the one exception. The emittance spectra of slate, which has a high damage threshold, show increasing saturation as the threshold is neared. Optimum focusing for slate corresponded t o a point having both sufficient spectral contrast and a sufficient signal-to-noise ratio. Conventional emittance spectra are included in the figures for comparison purposes. For these spectra the sample disks were placed at the source position of the spectrometer and heated from behind with heating tape. Because the non-blackbody structure was very weak in such emission spectra, they were recorded under the conditions described elsewhere for CW-laser TIRES spectra ( 2 ) ,which result in reduced noise. Briefly, no integrator was used and 256 cycles were accumulated at a 1.50 cm/s optical-pathdifference velocity and 4-cm-' nominal resolution. Blackbody spectra required to calculate emittances from these emission spectra were recorded under the same conditions. Also for comparison with the TIRES results, infrared absorbance spectra were recorded by means of photoacoustic detection. An MTEC Model 200 photoacoustic detector was mounted in the FT-IR spectrophotometer (with its normal light source) and 32 cycles were accumulated at a 0.05 cm/s optical-path-difference velocity and 8-cm-' resolution.
RESULTS AND DISCUSSION Figure 2 shows the emission from a 3.2-mm-thick disk of polycarbonate. At the top is the pulsed-laser TIRES emission spectrum resulting from a laser-beam fluence per pulse of 30 mJ/cm2. Below that is the conventional emission spectrum from the same disk heated to 45 "C. For comparison, a blackbody emission curve from carbon black at 44 "C is shown at the bottom. The measurement conditions for the two polycarbonate spectra were such that the spectrometer observed the same total emission intensity for both, and all three spectra were recorded with the same signal handling. (Only one integrator was used.) The spectra have not been corrected for the response function of the spectrometer and detector. In the pulsed-laser TIRES spectrum the strongest features
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Figure 3. Emittance spectra from a polycarbonate disk acquired by conventional means (top) and by pulsed-laser TIRES (middle). An absorbance spectrum of the polycarbonate is included for reference (bottom). have intensities comparable to the blackbody background above which they appear. In contrast, the conventional emission spectrum has very small features and is very similar in appearance to the blackbody spectrum. The difference between TIRES and the conventional technique is equally striking when the emittance spectra are compared. Figure 3 contains emittance spectra derived from the TIRES spectrum in Figure 2 and from a conventional emission spectrum of the sample disk a t 45 "C (recorded under better signalto-noise conditions than the spectrum in Figure 2, as described in the Experimental Section). The two emittance spectra are shown on the same vertical scale, except for an offset. Included for reference in Figure 3 is a photoacoustic absorbance spectrum of polycarbonate, which has been scaled so that small (and therefore low-saturation) features appear the same size in the absorbance and TIRES spectra. The conventional
ANALYTICAL CHEMISTRY, VOL. 61, NO. 17, SEPTEMBER 1. 1989
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emittance spectrum demonstrates the severe effects of selfabsorption. Only the strongest features are observable, and they correspond to small (less that 0.1) changes in emittance. In addition, they have taken on a derivative-like shape. The pulsed-laser TIRES spectrum, on the other hand, is so free of self-absorption that it appears even less saturated than the photoacoustic spectrum. All of the large features are sharper, and the broad feature at 1200 cm-' is much better resolved into its components in the TIRES spectrum than in the absorbance spectrum. The one characteristic in which the TIRES spectrum is lacking is its signal-to-noise ratio, although it is adequate for observing all but the smallest features. It should be noted that the surface of the irradiated area of the polycarbonate disk slowly turned dark brown while the TIRES spectrum was being taken. This damage, however, had no effect on the observed spectrum. During the recording of the TIRES spectrum the laser fired approximately 500000 times, which corresponds to roughly 7000 laser shots on every point within the sampled area of the disk. The observed damage, therefore, was the result of repeated irradiations and would not occur in situations where there is a continuous flow of new material, such as on a production line. We have not investigated whether the damage was photochemical or was thermally induced by the high instantaneous laser intensity (approximately 2 MW/cm2). Figures 4 and 5 show TIRES and conventional emittance spectra, as well as reference absorbance spectra, of white blackboard chalk and of slate, respectively. In each figure the two emittance spectra were recorded under conditions resulting in the same total emission intensity, and all three spectra are scaled in the same manner as those in Figure 3. These materials were chosen because they allowed higher laser fluences per pulse than the polycarbonate without observable damage (96 and 82 mJ/cm2 for chalk and slate, respectively) and because their irregular surfaces would test the effects on pulsed-laser TIRES of motion in space of the laser spot. The chalk sample disk consisted of 6-mm-thick sticks of chalk glued to an aluminum disk and planed flat so that the laser tracked over a continuous path of chalk. Variations in the surface caused the laser spot to move 1to 2 mm as the disk spun. The slate disk was a single piece of stone cut from approximately 4-mm-thick roofing slate. Its surface followed the natural grain of the stone and was therefore irregular,
600
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WAVEN UMBERS Figure 4. Emittance spectra of blackboard chalk acquired by conventional means (top) and by pulsed-laser TIRES (middle). An absorbance spectrum of the chalk is included for reference (bottom).
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Emittance spectra from a slate disk acquired by conventional means (top) and by pulsed-laser TIRES (middle). An absorbance spectrum of the slate is included for reference (bottom). Figure 5.
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Emittance spectra from a phenolic-plastic disk acquired by conventional means (top) and by pulsed-laser TIRES (middle). An absorbance spectrum of the plastic is included for reference (bottom). Figure 6.
causing the laser spot to shift by about as much as with the chalk disk. The TIRES spectra in Figures 4 and 5 again show better spectral contrast than the absorbance spectra (however, the structure atop the broad 1450-cm-' feature in the chalk spectrum is not reproducible), while the conventional emittance spectra are not related in an analytically useful way to the absorbance spectra. The poorer signal-to-noise ratios of these two pulsed-laser TIRES spectra are the results of their irregular surfaces. The pulsed-laser version of TIRES, at least as implemented here, is apparently more sensitive to surface irregularities than CW-laser TIRES which was not affected by modest surface features ( 2 ) . The earlier article on TIRES (2) presented results for several samples with a CW laser. Spectra for two of the same samples-witha pulsed laser are shown in Figures 6 and 7, again with emission intensities matched and scaling done as in the other illustrations. Figure 6 contains the spectra from a 3-
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ANALYTICAL CHEMISTRY, VOL. 61, NO. 17, SEPTEMBER 1, 1989
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mm-thick, red, filled-phenolic-plastic disk, and Figure 7 shows the results for electrical tape (O.l%rnm-thick,black, plasticized poly(viny1 chloride) sheet) attached by its own adhesive to an aluminum disk. The results are qualitatively the same as those in the other figures; the TIRES spectra show less saturation than the absorbance spectra, and the conventional emittance spectra show structure that either is not easily related to the absorbance spectra or is reduced in strength and has a derivative shape. Like the polycarbonate, the phenolic plastic darkened during the laser irradiation. The electrical tape being black could not darken, but it lost its glossy finish, turning dull. The TIRES spectrum of the phenolic plastic in Figure 6 has an extra peak at 1724 cm-' that is caused by the laser irradiation, probably because of the production of carbonyl species. No change definitely related to laser damage appears in the electrical tape spectrum, although some of the increase in the peak at 1735 cm-' may be the result of carbonyl production instead of reduced self-absorption. These last two pulsed-laser TIRES spectra may be compared to the CW-laser TIRES spectra appearing in the previous paper on TIRES (2). The spectra observed by using the two types of TIRES differ in only two important waysthe amount of feature saturation and the signal-to-noiseratio. The CW-laser TIRES spectra have roughly the same amount of saturation as the photoacoustic absorbance spectra, while the pulsed-laser spectra have less saturation. As discussed briefly in the Introduction and in more detail elsewhere ( 2 ) , the degree of self-absorption, and hence the amount of saturation, is governed by the maximum emitting-layer thickness, , ,I which in turn increases approximately as tms11/2,where t,, is the maximum time after irradiation that the spectrometer observes sample emission. For all of the pulsed-laser TIRES spectra presented in this paper, t , is 110 ps, which for phenolic plastic corresponds to an I , of 9 pm (D= 0.0020 cm2/s (5)). The previously published CW-laser TIRES spectra of the phenolic plastic (spectra 2D and 5F of ref 2) were recorded with a 13-ms ,t giving a 100-pm .,mI Another factor that may also reduce I, for the pulsed-laser TIRES spectra is the laser wavelength involved. The above estimates of I, assume that the laser light is fully absorbed in a negligibly thin layer at the sample surface, that is, that the absorption coefficient of the sample is effectively infinite at the
laser wavelength. The lower the absorption coefficient actually is, the deeper the laser light will penetrate, and the thicker the emitting layer will be. The CW-laser TIRES spectra were made with a multiline argon-ion laser operating in the blue and green regions of the visible spectrum, while the pulsed laser was operated on KrF at 248 nm. Since for many materials the absorption coefficient is greater in the ultraviolet region than in the visible region, the pulsed-laser light was normally absorbed within a thinner layer than the CW-laser light. The superior contrast in the pulsed-laser TIRES spectra is the direct result of the smaller l,, produced by the above factors. The signal-to-noise ratios of all emittance spectra, both TIRES and conventional, decrease with increasing wavenumber since the strengths of the original emission spectra decrease at higher wavenumbers, as Figure 2 shows. Beyond that, though, the signal-to-noise ratios of the pulsed-laser TIRES spectra are more than an order of magnitude less than those of the CW-laser spectra. These reduced ratios are the results of both less signal and more noise. The ease with which sample damage could be induced with the KrF laser necessitated limiting the average beam intensity to a maximum of roughly 0.4 W/cm2. The CW-laser experiments were limited to a maximum of about 2 W/cm2. The lower intensities allowed with the pulsed laser generally produced lower average sample temperatures and smaller signals. The increased noise in the pulsed-laser case comes from several sources, including the pickup of electromagnetic noise from the laser discharge, the increased bandwidth of the infrared-detector amplifier required in order to accommodate the pulsed emission signal, and the reduced Nyquist frequency that results in the folding of more noise into the spectrum. The principal noise source, however, is the shot-to-shot fluctuations in the laser-pulse energy. The fluctuations were partially compensated for by ratioing the observed signal against the laser-pulse energy, as described in the Experimental Section; however, this approach assumes the signal is linearly related to the pulse energy, which is at best only approximately true. Better compensation could be achieved if the signal were ratioed against the total observed emission intensity (prior to modulation by the interferometer) instead of the laser-pulse energy. This approach should also reduce the sensitivity of pulsedlaser TIRES to surface irregularities. The pulsed-laser TIRES spectra presented here show that nearly saturation-free emittance spectra can be obtained from optically thick samples by synthetically limiting emission to a sufficiently thin layer. For these experiments the spectrometer was modified to only the minimum extent necessary; the normal light source was replaced with the irradiated samples, an integrator was inserted in the signal path to capture the pulsed emission response, and certain band-pass filters were bypassed. Numerous further changes could be made to improve the modest signal-to-noise ratios observed. Better optical matching between the irradiated sample and the spectrometer and the use of an optimum laser wavelength (which would be sample dependent) that would permit higher average laser intensities would increase the signal, while better compensation for laser fluctuations, use of a laser with a higher repetition rate so that the Nyquist frequency could be raised, and better tailoring of the signal-path bandwidth to the TIRES technique would reduce the noise.
ACKNOWLEDGMENT We wish to thank Robert Hoult of the Perkin-Elmer Corp. for many discussions and much essential information. We gratefully acknowledge the generous loan by A. P. DSilva and Stephan Weeks of Ames Laboratory of integrators and other electronic equipment used in this study. We thank David
Anal. Chem. 1989, 6 1 , 1815-1821
Eckels of Ames Laboratory for his advice and assistance.
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(5) Erk, S.; Keller, A.; PoRz, H. Phys. 2. 1937, 38, 394-402.
LITERATURE CITED (1) (2)
Grifflths, Peter R. Appl. Spechosc. 1072, 26, 73-76. Jones, Roger W.; McClelland, John F. Anal. Chem.
1989, 67,
650-656. ... ....
(3) Donaldson, D. J.; Leone, Stephen R. Chem. W s . Lett. 1986, 132, 240-246. (4) Fletcher, T. Rick; Leone, Stephen R. J . Chem. Phys. 1988, 88, 4720-4731.
RECEIVED for review March 10,1989. Accepted May 12,1989. This work was funded by the Center for Advanced Technology Development (formerly 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.
Element Selective Detection after Supercritical Fluid Chromatography Using a Radio Frequency Plasma Detector R. J. Skelton, Jr.,l P. B. Farnsworth,* K. E. Markides, and M. L. Lee*
Department of Chemistry, Brigham Young University, Provo, Utah 84602
A radlo frequency plasma was evaluated as an element selective detector for caplliary supercrltlcal fluid chromatography. Atomic emission from S and CI was detected at 921.3 and 837.6 nm, respectively. The analytical performance of the detector was evaluated by monitoring its response to components of several mlxtures Introduced chromatographically, and to test solutes Introduced by uslng an exponented dllutlon flask. Minimal spectral Interferenceswere found for the CO, and N,O doped plasmas. Detectlon limits and sensltlvltles were dependent on the mass flow of CO, into the detector. The detection iimlts ranged from 50 to 300 pg/s.
INTRODUCTION Element selective detectors, such as the flame photometric and thermionic ionization detectors, have been important to the development of gas chromatography as an analytical technique. These detectors are known to be very selective and relatively easy to operate on a routine basis. In addition, the thermionic ionization detector is among the most sensitive detectors used for chromatographic detection. Although these detectors are well established, they are not without shortcomings. The investigation of element selective detectors has continued, with much attention given to the use of atomic emission as a basis for detection. Detection based on atomic emission offers many potential advantages. First, atomic emission lines are narrow and often intense, affording high selectivity and sensitivity. In addition, the atomic-emission-based detectors are tunable. A single detector can be used for several elements, instead of only one or two as with conventional selective detectors. Also, all emission lines are present in the plasma simultaneously, and either single or multichannel detection is possible. The most common device based on this principle is the microwave induced plasma detector (MIP), which was first reported by McCormack et al. ( I ) and Bache and Lisk ( 2 ) . The number of workers using MIPS is significant, and many excellent applications have been described (3-6). Other detectors based on plasmas have also been investigated. These include direct and inductively current discharge (7), glow discharge (8,9),
* Authors to whom correspondence should be addressed. *Current address: Shell Development Co., P.O. Box 1380, Houston, TX 77251. 0003-2700/89/0361-1815$01.50/0
coupled plasmas (10-15). Rice et al. have worked extensively with afterglow detectors (16),and they have recently reported some encouraging results from a helium afterglow detector based on a primary discharge generated by a radio frequency (rf) power source (17). More recently, we reported the use of a helium radio frequency generated plasma as a gas chromatographic detector (18). This detector is based on an rf discharge formed directly between a pointed metal electrode and a ground, and it has shown promise as a simple, tunable multielement selective detector for gas chromatography. This detector differs from that described by Rice et al. (17) in that emission is observed from the interelectrode region through the wall of a quartz tube. No attempt is made to divide the discharge into a primary discharge and afterglow. Small quantities of oxygen are doped into the plasma to create an ideal environment for sample decomposition and atomic emission. Intense emission lines for the nonmetals of chromatographic interest exist in this portion of the spectrum,and background emission from the plasma arising from molecular species is much less intense than in the UV region. Because of this spectral simplicity, low-resolution light sorting can be used. The combination of a doped helium plasma and the use of the near-infrared emission allows for a simple, low-cost detector. Because of the good operational characteristics of this plasma for gas chromatographic detection, its use as a detector for supercritical fluid chromatography (SFC) was investigated. Other element selective detectors, such as the thermionic ionization and the flame photometric detectors, have been employed with SFC (19,20),but their use has not yet become popular because of problems of detector incompatibility with the SFC mobile phases. Recently, the use of surface-wavesustained microwave induced plasma system was evaluated for the detection of sulfur-containing compounds after SFC (21,22). In like manner, this paper is intended as an evaluation of the potential use of the radio frequency plasma detector with SFC, including its inherent advantages and limitations.
EXPERIMENTAL SECTION Supercritical Fluid Chromatography. The supercritical fluid chromatograph consisted of a Hewlett-Packard 5890 gas chromatographic oven (Hewlett-Packard, Avondale, PA) equipped with a flame ionization detector. A varian 8500 syringe pump (Varian Associates, Palo Alto, CA) controlled by an Apple IIe computer was used to deliver the supercritical fluid. The COz and N20 used were of SFC grade (Scott Specialty Gases, Plum0 1989 American Chemical Soclety
