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Anal. Chem. 1986, 58, 758-761
9.0
3.0
6.0
0.0
TIME (MIN.)
Flgure 4. Chromatogram of a l-kL injection of motor gasoline with electrochemical detection (+ 1-90 V) at a glassy carbon electrode using 90: 10 acetonitrile-water (0.05 M Et,NCIO,) as the eluent.
have recently been summarized in a report in ref 9. B. Detection at Glassy Carbon Electrodes. Figure 3 shows cyclic voltammograms for TEL and TML at a glassy carbon electrode in a static cell containing acetonitrile (0.1 M Et4NC104). The oxidation processes are well separated in acetonitrile. Dc amperometric detection at +1.40 V should be specific for TEL while detection at +1.60 V should be suitable for both TEL and TML when coupled to HPLC. Kochi et al. proposed the following general mechanism for tetravalent alkylmetal compounds (5):
R4M RdM+ R.
-
--
R4M++ e-
(3)
RSM+ + Re
(4)
R+ + e-
(5)
In this work, R3M+was identified as a product of the oxidation process using cyclic voltammetry and scanning immediately after oxidation to negative potential regions and noting a reduction peak consistent with that expected for trialkyllead compounds (approximately -1.2 V for R,Pb+).
While the electrochemical resolution is excellent and chromatograms for standard mixtures are well-defined, chromatograms for gasoline samples using acetonitrile as the eluent contain numerous peaks (almost one broad band) that obscure the analytical responses expected for tetraalkyllead compounds. Figure 4 demonstrates that numerous peaks are observed in chromatograms of lead-free gasoline even when water is added to improve the resolution of gasoline constituents. Under the same conditions no background responses are observed at a mercury electrode. Further, the need to apply very positive potentials has deleterious effects on the long-term stability of the glassy carbon electrode and no analytically usable responses are observed for TEL or TML in gasoline samples where well-defined responses are found with the mercury electrode as a detector. There are many constituents of gasoline containing organic functional groups that can be oxidized at glassy carbon electrodes at the positive potentials required for the oxidation of tetraalkyllead compounds. The much lower positive potentials required at mercury electrodes and the highly specific nature of the electrode response involving direct interaction with mercury alleviate this problem, clearly making this the electrode material of choice. Registry No. TEL, 78-00-2; TML, 75-74-1.
LITERATURE CITED (1) Grandjean. P.; Nielsen, T. Resldue Rev. I97g, 72, 97-146. (2) Frigerio, I. J.; McCormick, M. J.; Symons, R. K. Anal. Chlm. Acta 1982, 143, 261-264, and references cited therein. (3) ASTM, Standards, Part 24, D1949, "Separation of Tetraethyllead and Tetramethyllead In Gasollne". (4) Messman, J. D.; Rains, T. C. Anal. Chem. 1981, 5 3 , 1632-1636. (5) Kochi, J. K.; Kllnger, R. J. J . Am. Chem. Soc. 1980, 102, 4790-4796, and references cited therein. (6) Bond, A. M.; McLachlan, N. M. J . Electroanal. Chem. 1985, 182, 367-382. (7) Bond, A. M.; McLachlan, N. M. J. Electroanal. Chem. 1985, 194, 37-48. (8) MacCrehan, W. A. Anal. Chem. 1981, 53, 74-77, and references clted therein. (9) Anderson, E. Chem. Eng. News 1985, April 8, 17-18.
RECEIVED for review August 26, 1985. Accepted October 22, 1985.
Pump/Probe Thermal Lens Spectrometry with Oppositely Propagated Beams for Liquid Chromatography Yen Yang,* Steven C. Hall, and Marijo S. De La Cruz Department of Chemistry, Loyola University of Chicago, Chicago, Illinois 60626 A new slngle-laser/dual-beam thermal lens photometer Is developed for liquid chromatographic detection. The pump beam propagates In a dlrectlon opposite to the probe beam. By use of appropriate beam splltters, this optical scheme enables effective spatial separation of the pump/probe without uslng a relectlon optic or losing the beam colllnearity. Wlth a 10-mW He-Cd laser operatlng at 442 nm, the system Is shown to have a noise level of 3 X lo-' absorbance root mean square with a 1-s time constant and a 1 mL/mln mobile-phase flow rate. The signal-to-noise ratlo Is also demonstrated to be superior to that of other single-laser/lock-in detection systems.
Since the advent of thermal lens spectrometry, there have 0003-2700/86/0358-0758$01.50/0
been significant advances in developing it into a practical analytical technique (1-16). A recent development is in the area of new and simple experimental arrangements. Two distinct CW laser-based approaches have evolved, both of which are based on the utilization of a single laser and lock-in detection. The first approach involves a single-beam technique in which the thermal lens strength is extracted from the optical signals through lock-in detection referenced at the second harmonic modulation frequency ( I 7). The second approach is a dual-beam method in which both the pump and probe beams are derived from the same laser source. Separation of the two beams is accomplished by taking advantage of their differences in polarization (18,19). Both arrangements appear to be experimentally simple and provide a fast, real-time response that is well-suited for liquid chromatographic applications. 0 1986 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 58, NO. 4, APRIL 1986 759 The performance of both systems, however, depends largely on the performance of the appropriate rejection devices employed. For instance, in the single-laser/dual-beam arrangement, a “notch” polarizer of high extinction ratio is required to extinguish the intense pump radiation. In a somewhat analogous fashion, the second harmonic approach relies on an electronic notch filter of high impedence to suppress the huge fundamental signal induced by the “pump portion” of the laser beam. The residual transmissions, whether optical or electrical, constitute the system background that is detrimental to sensitive detection. Moreover, in the dual-beam system the transmitted pump radiation also interferes optically with the probe beam, causing additional system noise (18). In our earlier report, we demonstrated a crossed-beam approach for the circumvention of these problems (20). The principal drawback of this system is that crossing the laser beams reduces the beam interaction length. This approach, evidently, would not be very desirable in a chromatographic situation where the detector path length is more than a few millimeters long. As a consequence, there is a continuing need in developing alternative single-laser approaches, especially those that may allow more effective pump/ probe separation, or fundamental/subharmonic isolation, without degradation of the signal-to-noise ratio of the system. We present here a novel single-laser/dual-beam optical scheme that is inherently free of optical interference noise. The pump beam is allowed to propagate collinearly, in the opposite direction to the probe beam. By use of appropriate optics, this arrangement enables complete spatial separation of the pump from the probe radiation without recourse to a rejection device or loss of beam collinearity. The efficacy of this approach is demonstrated by comparing the analytical performances of the system to those of other single-laser/ lock-in approaches. To distinguish between the two singlelaser/dual-beam configurations developed previously and currently, we adopt the terms ”polarization-encoding” (19)and “propagation-encoding”, respectively, throughout this paper.
EXPERIMENTAL SECTION Thermal Lens Photometers. Figure l a shows the schematic diagram of the propagation-encoded beam arrangement. The entire optical system is constructed on a 2 X 4 ft X 2l/, in. optical breadboard (National Research Corp., Model LS-24). The excitation source is a He-Cd laser (Liconix, Model 4210NB), which produces a -10-mW vertically polarized beam at 442 nm. In order to generate two laser beams with a polarizing beam splitter, PB1 the laser is rotated (Oriel, Model 2630, extinction ratio physically about the beam axis by approximately 1 2 O and is mounted rigidly on the breadboard. As a result, the emergent beams, which are 90’ apart, have an intensity ratio of approximately 20:l. The reflected beam, which is vertically polarized and more intense, as represented by the heavy line drawings in the figure, is used as the pump beam. The transmitted beam is horizontally polarized and serves as the probe. The half-wave plate, X/2, shown in the figure is presently not available in our laboratory and will be discussed in a later section. The pump beam is modulated by a mechanical chopper (Opt. Eng.) normally at 50 Hz. The frequency of the chopper is controlled with a laboratory variable autotransformer. The pump laser is then focused by an achromatic plano-convex lens, f = 120 mm, into a flow cell, FC, after being reflected 90° by a second polarizing beam splitter, PB2 (Oriel, Model 2630, extinction ratio 10-3). The probe beam from PB, is not modulated and is folded by two mirrors before being combined collinearly with the pump radiation that propagates in the opposite direction. An achromatic biconvex lens, Lz,f = 200 mm, is used to focus the probe beam before the flow cell. The flow cell is a 12-fiLand 8-mm-path-length conventional HPLC cell obtained from Kratos (Model SFA-280). To achieve maximum thermal lens sensitivity, the cell is placed at the focal
H8-Cd
A
GP
h/2
PBI
A
PBp
FC
PBp
M
FC
Lz
M
Figure 1. Propagation-encoded beam (a) and polarizatlon-encoded beam (b) thermal lens experimental arrangements: X/2, half-wave
plate; PB, polarizing beam splitters; M, mirrors: C, chopper: L, lenses, FC, flow cell; A, aperture: GP, Glen-Thompson prism polarizer. point of L, and also at the most optimum position with respect to Lz (or the probe beam waist), which is about 120 mm from L1 and 220 mm from Lz. The resulting pump laser power in the sample cell is only about 7 mW, and the probe power less than 1mW. The sample cell, polarizers, and focusing lenses are slightly tilted with respect to beam incidences to avoid possible reflective feedback to the laser cavity and fiber optic aperture (A). The intensity of the center portion of the probe beam is then detected by a photodiode (EG&G, Electro-Optics, Model FND100) through a 500-wm-i.d. optical fiber (AMP). The photodiode detector is the laboratory-constructed unit described previously (18). The thermal lens signals from the detector are demodulated with a lock-in amplifier (Princeton Applied Research, Model 5101). The pre- and postfilter time constants on the lock-in are set at 1s and the reference phase at W9O0depending on the modulation frequencies. Concurrently, the lens signals are also monitored with an oscilloscope (Tektronix, Model 5440). Outputs of the lock-in amplifier are displayed on a stripchart recorder (ColeParmer, Model 8373-20). Figure l b shows the polarization-encoded beam system employed in this study. This arrangement is basically a modification of the propagation-encoded system. Changes made are (a) the second polarizer is rotated 90’ about the axis perpendicular to the optical table surface, thereby permitting both the pump and probe radiation to propagate collinearly in the same direction, and (b) a Glan-Thompson prism polarizer (Ealing, Model 34-5207) is placed beyond the flow cell. This polarizer, GP, has an extinction ratio of 1 x and a transmission of about 94%. In addition, the probe beam focusing lens, Lz, is readjusted and the flow cell replaced so that the two focusing lenses and the flow cell still bear the same distance relationships as before, In order to make parallel comparisons, the experimental arrangement used for the second harmonic experiments is also modified from the propagation-encoded beam setup (Figure la). In this case, the probe beam from the first polarizing beam splitter is blocked, and the mirrors and the probe beam focusing lens are removed. The flow cell is readjusted along the beam axis for optimum position, which is about 10 mm from the beam waist or 130 mm from the focusing lens. In the lock-in amplifier, an active notch filter, Q = 5, from PARC (Model 5101/98) is installed to suppress the fundamental and other subharmonic signals (17). The notch filter is carefully tuned to the modulated frequency just prior to each experiment. Confirmation of whether the filter is properly tuned is made by observing the filter output on the oscilloscope. With a square
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ANALYTICAL CHEMISTRY, VOL. 58, NO. 4, APRIL 1986
wave input the wave form of the output should correspond to the difference between the square wave and a sine wave of equal amplitude and phase. Normally, the output of the filter is monitored with an oscilloscope during each experimental run. In addition, the lock-in amplifier is operated in its second harmonic (2fl mode and the reference phase set at Oo. Liquid Chromatography. The chromatographic system is a Spectra-Physics,Model SP-8100, liquid chromatographequipped with a 10-pL sample loop and an autoinjector. A standard stainless-steelanalytical column (4.6 mm X 250 mm) packed with 10-pm Spherisorb ODS (Spectra-Physics,A2351-040) is employed. The mobile phase, methanol/water (80:20), is pumped through the system usually at a flow rate of 1mL/min. The entire liquid chromatographic system is placed on a separate table to avoid system vibration. Reagents and Procedures. Reagent grade o-nitroaniline (Aldrich) is used as the test compound,and HPLC grade methanol (Fisher)is used for mobile-phase preparation. Nitroaniline sample solutions are prepared in methanol/water with the same ratio as the mobile phase. The system is aligned, optimized, and calibrated with the flow cell filled with a standard o-nitroaniline solution of known absorbance. The molar extinction coefficient of o-nitroaniline at 442 nm, Le., 3.2 X lo3 M-’ cm-l, is obtained with a laboratory spectrophotometer (Perkin-Elmer, Model 575). The wavelength of the spectrometer is calibrated with an Ar+ laser interference filter (Oriel, Model 5263). R E S U L T S AND DISCUSSION Design Considerations. Although presently the laser is rotated physically to achieve proper polarization orientation, this technique would obviously not be feasible with more powerful lasers. An appropriate approach to acquire two orthogonally polarized beams without having to move the laser itself is to employ a half-wave (X/2) plate as depicted in Figure 1by the dotted line drawings. By rotation of the half-wave plate about the beam axis, the laser plane of polarization, and thus the pump/probe intensity ratio, can be altered at will. This added flexibility may be desirable when system optimization via pump/probe intensity ratio adjustment is needed. As can be seen from Figure l a , a direct result of aligning two oppositely propagated beams from the same laser is the formation of an optical loop. The possibility of round-trip optical feedback to the laser resonator was a major concern. In the present design, however, the laser cavity is protected against such a feedback via two different means. First, because the plane of polarization of the round-trip pump radiation is in improper orientation, i.e., 90° out of phase, with respect to the first polarizer, PB,, and also with respect to the laser Brewster window to some extent, this polarizer, in essence, acts as a “light blocker” that deflects away any incoming feedback radiation. Second, both focusing lenses in the loop also serve as “light attenuators” that diminish the beam center intensities substantially via beam divergence. Our observations have indicated that possible feedback is undetectable and does not pose any problems. One of the most unique features, resulting from the present design, is that the emergent probe beam from the system is completely devoid of pump radiation. As indicated earlier, any residual transmitted pump radiation in the probe beam can be detrimental. For instance, the extinction ratio of a high quality commercial polarizer of the Glan-Thompson (GT) type If a pump/probe intensity ratio of 10’ is is typically utilized, the GT polarizer in the polarization-encoded configuration can only extinguish the pump radiation to an inor 0.1%, of the probe beam. consequently, tensity that is the emerging probe beam could conceivably exhibit an intensity fluctuation of as much as 0.1% due to the presence of the residual pump radiation and its optical interference. In contrast, the propagation-encoded system would not be subject to such interference. Because the beams travel in opposite directions, the emergent probe from the polarizing
beam splitter (PB,) is absolutely free of pump radiation. Further, pump/probe isolation is accomplished without the need of using a rejection optic. To achieve the same level of rejection with the polarization-encoded system, on the other hand, requires a polarizer pair with extremely small extinction ratios. It is interesting to note that both configurations in Figure 1 bear many resemblances, but differ in principle. For example, PB1 and PB2 in both cases encode the beams with polarization. In the propagation-encoded system, however, the polarization characteristic of the beams is not utilized for pump/probe separation. While the use of polarizing optics is necessary in the polarization-encoded arrangement, conventional reflective beam splitters may suffice for the propagation-encoded system without any conceptual difference. Nevertheless, the use of polarizing optics (i.e., PB, and PB2) in the propagation system is advantageous for the following reasons. First, in addition to splitting the laser beam, PB, also serves as the laser cavity isolator as previously mentioned. The use of a conventional partially reflecting mirror instead would result in a substantial radiation feedback to the laser cavity. The amount of feedback obviously depends on the reflectivity of the mirror. Secondly, because PB2 is in proper orientation with the plane of polarization of the probe beam, it allows maximum probe radiation throughput without the concomitant intensity loss of using a partially reflecting mirror. It is also noteworthy that the probe beam, derived from the same source as the pump beam, also creates its own thermal lens in a single-laser/dual-beam arrangement. But since the probe beam is not modulated by the chopper, the steady-state probe lens signal is virtually blocked a t the lock-in amplification stage and is not detected. Moreover, when the system pump/probe intensity ratio is high, i.e., when the probe lens is much “weaker” than the pump lens, the probe-induced lens effect can essentially be ignored. Another different aspect of a single-laser/dual-beam arrangement, whether polarization-encoded or propagationencoded, is the dependency of the thermal lens signal strength on the laser power. Because the probe beam is derived from the pump radiation, any variation in the pump beam would automatically result in a probe intensity fluctuation. Therefore, any source-induced noise will be amplified in the lens signal in a single-laser/dual-beam system. According to Morris et al. (19), thermal lens signal strength exhibits a power-square dependency on laser intensity. We have also confirmed the observation by Berthoud et al. that maximum sensitivity occurs when the pump beam is focused in the sample cell and the probe is focused at about one confocal distance before the cell (21). When the probe beam is unfocused, however, the signal-to-noise ratio is degraded by a factor of about 3. The feasibility of using separate focusing lenses for the pump and probe undoubtedly provides additional flexibility and advantages for the dual-beam system from the sensitivity standpoint. System Performance. The performance of the system is summarized in Figure 2. Each figure represents the chromatogram of a 51-ng aliquot of o-nitroaniline, obtained under the same chromatographic conditions. Figure 2a shows the performance of the propagation-encoded system. The baseline noise level is about 3 X 10-5 absorbance root mean square. System response is linear over the concentration range tested, 0.1-5.1 ng/pL. The reproducibility of the peak heights for five consecutive injected samples a t a concentration of 5.1 ng/FL is 2.5% relative standard deviation. Figure 2b shows the performance of the polarization-encoded beam system. The signal-to-noise ratio is a t least a factor of 2-3 lower relative to the propagation-encoded system. Figure 2c shows the performance of the second harmonic
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Anal. Chem. 1986, 58. 761-765
polarization-encoded and crossed-beam thermal lens systems. It is simple, free from interference noise, applicable to short as well as long path length samples, and does not necessitate expensive polarizing optics. In comparison to the second harmonic approach, it is superior in performance in terms of signal-to-noise ratio. To achieve better performance with the second harmonic technique, more effective fundamental signal filtering is needed in the lock-in amplifier. Although the propagation-encoded system is somewhat more elaborate to construct, once set up and aligned, it should either remain in alignment indefinitely (19) or need only minor adjustments. 0
2
4
6
B
O
2 4 TIME I M I N I
6
8
0
2
4
6
8
Flgure 2. Comparlsorl of the liquid chromatograms of 51 ng of 0 nitroaniiine obtained from (a) propagation-encoded, (b) polarization-
encoded, and (c) second harmonic thermal lens detection systems: laser power, 7 m W modulation frequency, 50 Hz; flow rate, 1 mL/min. system. The loss of SIN ratio is about a factor of 4. The sloping base line observed in the figure is, in part, a result of the noisy chopper employed. Because the installed notch filter has a very narrow frequency bandwidth, any drift in the modulated frequency would result in a variation in the residual (fundamental) background signal and, consequently, a drift in the base line. Our chopper tends to stabilize somewhat at higher frequencies. Therefore, as the chopping frequency was increased to 150 Hz,which is the upper limit set by the available matched capacitor pair in the notch filter, the base-line drift was alleviated substantially; i.e., the base-line stability approaches those shown in Figure 2a,b. However, the noise levels in both the single-laser/dual-beam systems appear to remain invariant, indicating that the dual-beam system is somewhat less susceptible to chopping noise. Presently, the principal noise in the propagation-encoded system arises from turbulence in the cell and flow pulsation (20,22). The former can be reduced through refinement in cell design. The latter can be alleviated by using surgeless pumps or applying the differential thermal lens technique (23). CONCLUSIONS The arrangement described here combines the best of both
ACKNOWLEDGMENT We thank A. Keith Jameson for helpful discussions. LITERATURE CITED (1) Gordon, J. P.; Leite, R. C. C.; Moore, R. S.;Porto, S. P. S.;Whinnery, J. R. J. Appl. Phys. 1985, 36, 3. (2) Dovichi, N. J.; Harris, J. M. Anal. Chem. 1980, 52,2338. (3) bovichi, N. J.; Harris, J. M. Anal. Chem. 1981, 53, 106. (4) Leach, R. A.; Harris, J. M. J. Chromafogr. 1981, 278, 15. (5) Leach, R. A.; Harris, J. M. Anal. Chem. 1984, 56, 1481. (6) Miyaishi, K.; Imasaka, T.; Ishibashi, N..Anal. Chem. 1982, 54, 2039. (7) Mori, K.; Imasaka, T.; Ishibashi, N. Anal. Chem. 1982, 54, 2034. (8) Buffett, C. E.; Morris, M. D. Anal. Chem. 1982, 54, 1824. (9) Buffett, C. E.; Morris, M. D. Anal. Chem. 1983, 55,376. (IO) Nolan, T. G.; Weimer, W. A.; Dovichi, N. J. Anal. Chem. 1985, 56, 1984. (11) Higashi, T.; Imasaka, T.; Ishibashi, N. Anal. Chem. 1984, 56, 2010. (12) Long, M. E.; Swofford, R. L.; Albrecht, A. C. Science 1978, 797, 183. (13) Alfheim, J. A.; Langford, G. H. Anal. Chem. 1985, 57,861. (14) Long, G. R.; Bialkowski, S. E. Anal. Chem. 1984, 56, 2806. (15) Nakanishi, K.; Imasaka, T.; Ishibashi, N. Anal. Chem. 1985, 57, 1219. (16) Jansen, K. L.; Harris, J. M. Anal. Chem. 1985, 57, 1698. (17) Pang, T. K. J.; Morris, M. D. Anal. Chem. 1984, 56, 1467. (16) Yang, Y. Anal. Chem. 1984, 56,2336. (19) Pang, T. K. J.; Morris, M. D. Appl. Spectrosc. 1985, 39, 90. (20) Yang, Y.; Hairrell, R. E. Anal. Chem. 1984, 56, 3002. (21) Berthoud, T.; Delorme, N.; Mauchien, P. Anal. Chem. 1985, 57,1216. (22) Dovichi, N. J.; Harris, J. M. Anal. Chem. 1981, 53,689. (23) Pang, T. K. J.; Morris, M. D. Anal. Chem. 1985, 57, 2153.
RECEIVED for review August 5,1985. Accepted November 22, 1985. This research was supported by grants from Loyola University of Chicago Summer Research Grant, Research Stimulation Fund, and Small Research Grant.
Pulsed Photothermal Refraction Spectrometry Using an Elliptic Gaussian Excitation Beam Norio Teramae,' Edward Voigtman, Jose Lanauze, a n d J a m e s D. Winefordner*
Department of Chemistry, University of Florida, Gainesville, Florida 32611 The use of a one-dimensional heat source in pulsed photothermal refraction spectrometry has been studied both experimentally and theoretically. The one-dlmensional heat source is obtained by focusing the excltatlon beam with a cylindrical lens. The sensltlvlty Is shown to be superior to that obtained by uslng a two-dimensional heat source. A minlmum detectable absorptlvlty of 9 X IO-' cm-' was obtained for amaranth In methanol.
The application of lasers to analytical spectrometry has been one of the most exciting advances of recent years (1).Whole 'On leave: D e p a r t m e n t of I n d u s t r i a l Chemistry, F a c u l t y of Engineering, T h e U n i v e r s i t y of Tokyo, Bunkyo-ku, T o k y o 113, Japan.
new areas have been developed that would not be possible with conventional light sources. Since the first detailed description by Gordon et al. (2), the thermal lens effect has developed into a powerful analytical technique for high-sensitivity absorption measurements (3-5). The effect is related to the spatial variation of the refractive index that results from the localized temperature increase caused by absorption of the excitation source. The variation in the refractive index can be detected by phase fluctuation spectrometry (6, 7), thermal diffraction spectrometry (8,9),thermal lensing spectrometry (10-19), photothermal deflection spectrometry (20-22), and photothermal refraction spectrometry (23,24). The optical setup for photothermal refraction is similar to the one for transverse photothermal deflection, but the detection method of this technique can be related to the one for thermal lens
0003-2700/66/0358-0761$01.50/00 1966 American Chemical Society
