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Anal. Chem. 1904, 56,2336-2338
Thermal Lens Spectrometry Based on Single-LaserlDual-Beam Configuration Yen Yang Department of Chemistry, Loyola University of Chicago, Chicago, Illinois 60626
A new dual-beam thermal lens photometer based on the utilization of a single laser is reported. The probe beam Is Isolated from the pump beam by utlllzlng differences In polarization. Optical interference between the pump and probe radiations is observed and discussed. A mlnlmum measurable is found by using 2-mW available absorbance of 2.5 X power of a He-Ne laser.
The laser-induced thermal lens effect, just as laser-induced fluorescence (I),has proven to be a sensitive spectroscopic tool for measurements of small absorbances (2-6). While fluorescence is associated with radiative decay of excited molecules, thermal lens is associated with the heat created through the nonradiative processes. Consequently, thermal lens spectrometry is potentially useful for ultratrace analysis of weakly or nonluminescent samples (6, 7). In addition, the thermal lens method appears to be attractive to analytical chemists because of its experimental simplicity. In the past, two types of experimental arrangements, e.g., single beam and dual beam, have been developed to acquire lens signals. In the single-beam configuration, a TEM, laser beam is employed to traverse the sample. When the beam is unblocked, the time-dependent defocusing of the beam, resulting from the formation of a “divergent” lens inside the sample, is monitored via the intensity change (drop) at the beam center by an oscilloscope (2-5). For sensitive detection of small absorbances, a computer is utilized to evaluate the transient lens signals (7-17). Using a 160-mW Ar+ laser, Dovichi and Harris have demonstrated a minimum measurable absorbance of 7 X (10). The functions of the laser in the single-beam configuration are both to create and to probe thermal lens production. In the dual-beam configuration, however, a second laser, usually a small He-Ne, is introduced to probe the formation/defonnation of the thermal lens when the pump (heating) laser is modulated. The pump and the probe beams are combined collinearly, propagating either in the same (4, 6) or in opposite directions (6,18) through the sample. The probe beam is then isolated from the pump beam with fiiters or/and dispersive optics. A significant advantage of introducing a second beam (probe) is that the change in the probe intensities, hence the thermal lens signal strength, can be detected directly with a lock-in amplifier (4, 6, 18-21). Recently, a comparison of the single- and dual-beam configurations has been reported (22). Improvements in detection sensitivity have also been reported with image detection of the probe beam (23) and with pulsed laser excitation (24). In principle, in a dual-beam configuration, both the pump and the probe beams may be derived from the same source, provided that the probe beam can be isolated from the pump beam by either optical or electronic means. Such a one-laser/dual-beam system would preserve the convenience of lock-in detection while maintaining the simplicity of the technique. Moreover, any variations in the probe and pump beam intensities may be simultaneously corrected for through 0003-2700/84/0356-2336$0 1.50/0
laser power monitoring. This paper describes a new dual-beam configuration in which only one laser is required. The pump beam is rejected from the probe beam based on their differences in polarizations. The feasibility of this approach is demonstrated by using a discrete wavelength, low-power laser.
EXPERIMENTAL SECTION Instrumentation. The single-laser/dual-beam thermal lens photometer developed in this study is shown in Figure 1. The optical system is constructed on a 2 X 4 f t x 21/4 in. optical breadboard (National Research Corp., Model LS 24). The laser is a He-Ne (Coherent, Model 80-5H) which produces a 4-mW randomly polarized beam at 632.8 nm. The laser beam is split by a polarizing beam splitter (Oriel, Model 2630) into two linearly polarized beams having polarizations which are orthogonal to each other. The horizontally polarized beam is used as the pump beam, with the vertically polarized beam as the probe beam. The beam-splitting cube, which has an extinction ratio of better than for the transmitted beam, is slightly tilted with respect to beam incidence to avoid optical feedback to the laser resonator. The pump beam is modulated, usually at about 70 Hz, by using a mechanical chopper (Opt. Eng.). The frequency of the chopper is controlled with a laboratory variable autotransformer. The probe beam is unchopped and folded by two mirrors before being recombined collinearly with the pump beam via a glass-plate beam splitter. The combined beam thus consists of (a) a chopped, horizontally polarized pump beam and (b) an unchopped, vertically polarized probe beam whose intensity is approximately 4% of the pump beam. The laser beam is then focused by an achromatic biconvex lens, f = 120 mm. The sample cell, which is a standard 1-cm quartz cuvette, is tilted slightly with respect to normal incidence to avoid interference effeds (8). The position of the cell is adjusted to give the most optimum thermal lens signal. The laser power at the sample is only about 1.5 mW due to reflective losses. The ”pump” portion of the laser beam is subsequently absorbed by a dichoric sheet polarizer (Melles Groit, Model 03-FG-003; extinction ratio which is placed some distance away from the laser-beam waist to reduce the thermal lens effect induced in this polarizer. The intensity of the center portion of the transmitted probe beam is detected by a photodiode (EG & G, Electro-Optics, Model FND-100) through a 500-fimdiameter, 15-cm long optical fiber (AMP). The optical fiber also serves as the limiting aperture (19). The photodiode is reversebiased with a 9-V battery through a 1-kCl load resistor, all of which are enclosed in a commercial RFI shielded box (Pomona, Model 3602). The reference photodiode detector is a PARC fast photodiode edge trigger (Model 2100/99), which detects the reflected pump-beam radiation from the beam splitter. A lock-in amplifier (Princeton Applied Research, Model 5101), operated with 1-8 filter time constants, is used to demodulate the thermal lens signal. The reference phase on the lock-in is normally set at 90’ to reject any signals that originate from residual chopped pump radiation which is not blocked by the sheet polarizer. On occasion, the time-dependent lens signal is also monitored with a digital storage oscilloscope (Nicolet, Model 206). The outputs of both the oscilloscope and the lock-in amplifier are displayed on an x-y recorder. Reagents and Procedure. All chemicals are ACS reagent or spectrograde. LD-690 is a laser dye and is obtained from Princeton Applied Research. The aqueous Cu-EDTA solutions, which are buffered at pH 7, are prepared by procedures described by Dovichi et al. (8)except the copper stock solution is prepared from cupric sulfate (CuSO4.5H2O).The bromophenol blue/ethanol solutions 0 1984 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 56, NO. 13, NOVEMBER 1984
P
s
L
SP
V
LASER
REF. PD
SIG.
LOCK
a=J
- IN
0S CI LLOSCOPE
Figure 1. Thermal lens experimental arrangement. P, polarizing beam splitter; C, chopper; BS, beam splitter; L, lens; S,sample cen;SP, sheet polarizer; OFC, optical fiber cable; PD, photodiode; M, mirror.
Table I. Analytical Results of Detection Limits and Minimum Measurable Absorbances
sample solution Cu-EDTA/water LD-69O/chloroform bromophenol blue/ethanol
molar absorptivity min at 632.8 nm, detection measurable* M-' cm-' limit," M abs, Amin 47
3.0 x 104 1.2 x 104
"At SIN = 2. *Amin= detection limit
6.0 X 10" 2.6 x 10-8 2.1 x 10-8 X
2.8 X 7.8 x 10-4 2.5 x io+
molar absorptivity.
are prepared by adding several milliliters of 1.0 X M stock or previously prepared bromophenol blue/ethanol solutions (successive dilutions) and 1.5 mL of a 0.1 M aqueous potassium hydroxide solution before being diluted to 25 mL with ethanol. The additions of the potassium hydroxide solutions keep the pH of all the solutions at about 12. No attempt is made to filter out any suspended solids in the solutions prepared in this study. The thermal lens signal is usually optimized by using the LD-69O/chloroform dye solution at a concentration of 2.6 X lo4 M. The fluorescence of the dye along the laser beam inside the solution also helps in aligning the system optics. Molar absorptivities at 632.8 nm are obtained at a 0.1-0.01 absorbance range by using a laboratory spectrophotometer (Perkin-Elmer,Model 575). The wavelength of the spectrometer is calibrated with a He-Ne laser interference filter (Oriel, Model 5274).
RESULTS AND DISCUSSION One of the most important experimental parameters for the thermal lens photometer is the extinction ratio of the polarizers. This ratio determines the extent to which the pump beam of improper polarization could be rejected. Any transmitted pump radiation would constitute the background. Although such background signals, being in phase with the
reference, can be suppressed by the lock-in amplifier through a 90° phase adjustment, the transmitted pump radiation causes additional detector shot noise that is detrimental to sensitive detection. The intensity of the transmitted pump beam, hence the extinction ratio of the dichroic sheet polarizer utilized in this study, was found to vary significantly, depending on the location of beam incidence. The effect of the residual transmitted pump beam on the thermal lens signals is shown in Figure 2. Figure 2a represents the oscilloscope tracing of the modulated reference signal, showing the pump "on" and "off' times. Parts b and c of Figure 2 show the tracings of the time-dependent probe beam signals obtained in the absence (b) and presence (c) of a sample solution. The random signal fluctuations observed only during the "on" periods indicated that they were associated with the pump beam. Further experimentations revealed that these fluctuations were not caused by the electrical ringing phenomenon that might occur in the detetor circuit in response to an instantaneous intensity or current swing. It is very likely that such signal variations were originated from the optical interference exhibited by the probe beam and the pump beam. Such interferences coupled with mechanical vibrations gave rise to large fluctuations in the signal. In fact, a similar interference phenomenon, induced by sample cell windows acting as a Fabry-Perot interferometer, was also observed by Dovichi and Harris (8). In principle, if these signal fluctuations were caused by optical interference, such interference should disappear upon elimination of one of the participanta-the pump beam. Therefore, by judicious selection of a "good" location on the sheet polarizer where the pump beam can be rejected most effectively, Figure 2d was obtained, which shows the interference-free thermal lens signal. Another source of noise, that may be present in the current system, is the variation in the polarization content of the He-Ne laser beam employed in this study. Because a "randomly polarized" laser beam has no preferable direction of polarization, the polarization content of the beam will change with time. Consequently, the intensities of the pump and probe beams, which derive from such a source, would also vary. Evidently, such polarization noise, if it poses problems, can be avoided by using a linearly polarized laser source. To evaluate the detection sensitivity of current photometer, three types of solution systems were studied. They are CuEDTA/water, LD-69O/chloroform, and bromophenol blue/ ethanol. Cu-EDTA/water can be regarded as a less favorable system for thermal lens production because of the less favorable thermooptical properties of water (7). LD-690/ chloroform is also a less favorable system due to the high fluorescence quantum efficiency of LD-690. Bromophenol blue/ethanol can be considered to be the most favorable of the three. The analytical results of the minimum detectable
Table 11. Comparison of the Detection Sensitivities of Various Thermal Lens Photometers signal processor
laser power P, mW 2
(He-Ne)
lock-in
detection sensitivity, solvent water chloroform
4
(He-Ne)
dual-channel boxcar/computer
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ethanol water acetone:water, 3:l
Amin
PAmin
2.8 x 10-3" 7.8 x 10-4' 2.5 X 10""
5.6 x 10-3 1.6 x 10-3
5.1 x 10-3b 1.0 x io+* 6.3 x 10-7c 7.0 X 5.7 x 10-3e
200 (Art) computer CC14 160 (Ar+) computer CCl, 100 (Art) oscilloscope water 800 (Art) lock-in chloroform 3.4 x l0-6f 600 (Ar') photodiode array chloroform 6.0 X 10-'8 "This work (Table I). *Reference 8. CReference9. dReference 10. eReference 25. fReference 26. gReference 23.
5.0 X lo4 2.0 x 10-2 4.0 x 10-3 1.3 X 10" 1.1x 10-5 5.7 x 10-1 2.7 x 10-3 3.6 x 10-4
2338 (a)
ANALYTICAL CHEMISTRY, VOL. 56, NO. 13, NOVEMBER 1984
OFF ON
4 4
isms
(b)
(although the most important criterion for the probe laser is the stability, not the power of the laser, a state-of-the-art low-power laser with a stability of better than 0.01 % can be moderately expensive); (3) Elimination of the beam walk-off problem which may occur in the conventional two-laser system as a result of the different refractive index dispersions of solvent toward the laser beams when the sample cell is tilted to avoid interference effects (8, 22); (4) Easy laser power variation corrections (in the singlelaser/dual-beam system any variations in the probe and pump beam intensities may be simultaneously corrected for through laser power monitoring, whereas in the conventional two-laser system these variations occur independently and may require separate corrections).
ACKNOWLEDGMENT The author acknowledges the assistance of Robin Hairrell and Thanh Viet Ho with the sample solution preparations. Figure 2. Tlmedependent signals o f (a) reference from the chopped pump beam, (b) blank without a sample cell In place, and (c) and (d) a 2.6 X IO-' M LD-690/CHC13 dye solution.
concentrations and the minimum measurable absorbance are presented in Table I. It can be seen that a minimum measurable absorbance of 2.5 X was achieved for a "favorable" solution by using 2-mW available laser power (half of the 4-mW laser power is used for the probe beam). Further improvements may be expected by using a higher power and more stable laser, a polarizer pair of higher extinction ratios, a quieter chopper, and a narrower band-pass filter on the lock-in and by using a more effective system vibration isolation and solvent background subtraction (9). In Table 11, the minimum measurable absorbances from various thermal lens systems are listed and the detection sensitivities in terms of the power-Ami, product are compared. The detection sensitivity of our system is comparable, in some cases superior, to those of others, which is considered excellent in terms of present capabilities.
CONCLUSIONS The optical configuration reported here offers a new and simple alternative to the conventional single-beam and dual-beam arrangements. Several obvious advantages can be realized: (1) Elimination of the utility of a computer to evaluate thermal lens signals as commonly employed in a single-laser/single-beam system; (2) Elimination of an additional laser for the probe beam which is required in the conventional dual-beam system
LITERATURE CITED Yang, Y.; D'Siiva, A. P.; Fassei, V. A. Anal. Chem. 1981, 5 3 , 894. Gordon, J. P.; Leite, R. C. C.; Moore, R. S.; Porto, S.P. S.;Whinnery, J. R. J. Appl. Phys. 1965, 3 6 , 3 . Hu, C.; Whinnery, J. R. Appl. Opt. 1973, 72, 72. Long, M. E.; Swofford, R. L.; Aibrecht, A. C. Science (Washington, D C . ) 1978, 191, 183. Whinnery, J. R. Ace. Chem. Res. 1974, 7 , 225. Kliger, D. S . Acc. Chem. Res. 1980, 13, 129. Harris, J. M.; Dovlchi, N. J. Anal. Chem. 1980, 5 2 , 695A. Dovichi, N. J.; Harris, J. M. Anal. Chem. 1979, 5 1 , 728. Dovichi, N. J.; Harris, J. M. Anal. Chem. 1980, 52, 2338. Dovichi, N. J.; Harris, J. M. Anal. Chem. 1981, 5 3 , 108. Dovichl, N. J.; Harrls, J. M. Anal. Chem. 1981, 53, 689. Leach, R. A.; Harris, J. M. J. Chromatogr. 1981, 278, 15. Carter, C. A.; Brady, J. M.; Harris, J. M. Appl. Spectrosc. 1982, 3 6 , 309. Carter, C. A.; Harris, J. M. Appl. Spectrosc. 1983, 3 7 , 166. Carter, C. A.; Harris, J. M. Anal. Chem. 1984, 56, 922. Fujiwara, K.; Uchiki, H.; Shimokoshi, F.; Tsunoda, K.4.; Fuwa, K.; Kobayashl, T. Appl. Spectrosc. 1982, 36, 157. Fujiwara, K.; Lei, W.; Uchiki, H.; Shimokoshl, F.; Fuwa, K.; Kobayashi, T. Anal. Chem. 1982, 54, 2026. Higashi, T.; Imasaka, T.; Ishibashi, N. Anal. Chem. 1983, 55, 1907. Buffett, C. E.; Morris, M. D. Anal. Chem. 1982, 54, 1824. Haushaiter, J. P.;Morris, M. D. Appl. Spectrocs. 1990, 3 4 , 445. Buffett, C . E.; Morris, M. D. Appl. Spectrosc. 1983, 37, 455. Carter, C. A.; Harris, J. M. Anal. Chem. 1983, 55, 1256. Miyaishi, K.; Imasaka, T.; Ishibashi, N. Anal. Chem. 1982, 54. 2039. Mori, K.; Imasaka, T.; Ishibashi, N. Anal. Chem. 1982, 5 4 , 2034. Imasaka, T.; Miyaishi, K.; Ishibashl, N. Anal. Chim. Acta 1980, 775, 407. Mlyaishl, K.; Imasaka, T.; Ishibashi, N. Anal. Chlm. Acta 1981, 124, 381.
RECEIVED for review May 4,1984. Accepted June 11,1984. This research was supported by grants from Loyola University of Chicago Summer Research Award, Research Stimulation Fund, and Small Research Grant.
