Anal. Chem. 1988, 60. 1925-1928
1925
Development of a Double-Beam, Dual-Wavelength Thermal-Lens Spectrometer for Simultaneous Measurement of Absorption at Two Different Wavelengths Mladen F r a n k o and Chieu D. T r a n * Department of Chemistry, Marquette University, Milwaukee, Wisconsin 53233
A novel dual-wavelength pump/probe conflguratlon thermal lens spectrometer that Is capable of slmuitaneously measuring thermal lens at two dlfferent wavelengths has been developed. I n this Instrument, the two excltatlon beams were derived from the same argon ion laser, which operated In a multlllne mode. The sample was excited by these two wavelengths aiternatlveiy and the correspondlng thermal lens signals were monitored by a He-Ne probe laser. Compared to the slngle-wavelength techniques, the advantages of thls dual-wavelength apparatus Include Its ablilty to correct for the solvent background absorption and Its improved selectlvlty. The solvent background absorption was allevlated because thls technique measures the difference between the two thermal lens signals at the two excitation wavelengths. The ratio of the two signals provides fingerprints and Mentiflcatlon of the analyte. Furthermore, It enables the determlnation of trace chemical specles In the presence of interference species at much higher concentratlons. With thls apparatus, the detectlon limit for praseodymium ions in the presence of 1.0 X IO-' M concentration of nlckei glyclnate complexes uslng 25mW exdtatlon beams modulated at 1 Hz Is estimated to be 4.7 X I O 4 M.
Applications of photothermal techniques such as thermal lens and photothermal deflection to trace chemical analysis have increased significantly in recent years (1-5). These techniques are based on the measurement of the temperature rises that are produced in an illuminated sample by nonradiative relaxation of the energy absorbed from a laser. Because the absorbed energy is measured directly, the sensitivity of photothermal techniques is relatively higher than that of the conventional transmission of reflection measurements. Absorbances as low as lo-' have been measured by using these techniques (1-5). Recent efforts are focused on the further enhancement of the sensitivity and introducing the selectivity into these promising techniques so that they can be used in the area of general trace chemical analysis. The detection limits of photothermal techniques are often governed by the background signals caused by absorption of solvent itself or of impurities in the solvent. The use of solvent with low absorptivity, such as supercritical fluids,can alleviate this problem somewhat (6, 7). I t is, however, not practical for real-time samples. Alternatively, background absorption can be corrected by using the differential absorption of the sample and that of the solvent (8-10). These techniques involve the measurement of the absorption of the two cells containing sample and blank solvent. They have proven to be useful in correcting the background absorption due to solvent. Unfortunately, restrictions such as precisely positioning the sample and reference cells (49)and the need for two identical detectors or cells (10) still impose on these promising techniques. Perhaps the simplest but most powerful method to correct for the background absorption and to increase the selectivity 0003-2700/88/0380-1925501.50/0
is to measure simultaneously absorption a t two different wavelengths. Any background absorption can be eliminated by taking the difference in the absorbance of these two wavelengths. There is no need to have different samples and blank cells, as the measurement can be performed on a sample cell only. In addition, the ratio of the absorption a t two wavelengths will provide fingerprints and identification of a substance in a mixture. In spite of its great potential, the dual-wavelength photothermal technique has not been exploited. Most apparatus operate with only one excitation laser wavelength. Such considerations prompted the present study, which aims to develop, for the first time, a pump and probe configuration, dual-wavelength thermal-lens apparatus capable of measuring the thermal lens signal at two different excitation wavelengths simultaneously. It will be demonstrated in this communication that such an apparatus can be constructed by using an argon ion laser operating simultaneously at two wavelengths, 514.5 and 457.9 nm, as an excitation source. The sensitivity and selectivity of the system as well as preliminary results on the determination of trace chemical species in a mixture will be described.
EXPERIMENTAL SECTION Figure 1shows the schematic diagram of the dual wavelength thermal lens apparatus. A Spectra-Physics (Model 164) argon ion laser operated in the multiline mode was used as an excitation source. The multiline output was separated into individual wavelengths by an Amici prism (Ealing Electro-Optics). The 514.5- and 457.9-nm lines were selected as the two excitation wavelengths for this work. Appropriate neutral density filters were used to provide equal intensity for the two beams. The two pump beams were modulated by a variable-speed mechanical chopper (Stanford Research System Model 540)whose blade was modified in order to modulate the two beams at the same frequency, but 90' out of phase to each other. A mirror and a beam splitter were used to align the two modulated excitation beams to propagate coincidentally. The two beams were then focused onto the sample by an achromatic lens having 60-mm focal length. The probe beam was provided by a Spectra-Physics H e N e laser (Model 105-1). The two pump beams and the probe beam were aligned to overlap at the sample cell by a dichroic mirror. The heat generated by the sample absorption of the pump beams changed the intensity of the probe beam. The intensity fluctuation of the probe beam was measured by a pin photodiode (United Detector Technology Pin 10 DP) placed 2.5 m from the sample and behind an interference fiiter (F) and a 2-mm pinhole. A lens with 50-mm focal length was used to focus the probe beam, and its relative distance from the sample was adjusted to give maximum thermal lens signals. The output of the photodiode was amplified and fed into a Heath digital memory oscilloscope (Model 4850). The output of the scope was connected to an AT&T personal computer (Mode PC 6300) to accumulate and average the signals. Typically, ten sets of thermal lens transients were collected and averaged. The thermal lens magnitude for each excitation beam was obtained from the appropriate peak height, and the ratio as well as the difference for the two values was calculated. Absorption spectra were taken on a Perkin-Elmer 320 spectrophotometer. 0 1988 American Chemical Society
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ANALYTICAL CHEMISTRY, VOL. 60, NO. 18, SEPTEMBER 15, 1988
Figure 1. Schematic diagram of the double-beam, dual-wavelength thermal-lens spectrometer: B.S., beam splitter; L, lens; S, sample; F, filter; pH, pinhole: PD, photodiode.
Nickel(I1) glycinate complexes were prepared from nickel carbonate and glycine according to the procedure described in methosulfate the literature (11). 1,1’,3,3’-Tetramethyl-2,2’-cyanine (TMCM), l,l’-ethylene-2,2’-cyanineiodide (ECI), and 3,3’-diethylthiacmbocyaninechloride (DTCC) and tris(bipyridyl)iron(II) complexes were obtained from the same source as cited in ref 12. (1,lO-Phenanthro1ine)ferroussulfate (Fisher) and other compounds and solvents were obtained from manufacturers and used as received. A 1M stock solution of Pr3+in water was prepared by dissolving praseodymium chloride hexahydrate (99.9%,Alfa Products) in water with a few drops of hydrochloric acid. Standard solutions for calibration curves were prepared by injecting the Pr3+aliquot into water or nickel(I1) glycinate aqueous solution. If necessary, the pH of the solution was adjusted by adding hydrochloric acid or sodium hydroxide solution. The mixture was then filtered through a Acrodisc membrane filter with 0.20-j~m pore size prior to any measurement.
RESULTS AND DISCUSSION Data acquisition for single-wavelength excitation, pump, and probe configuration, thermal-lens techniques is relatively simple because it can be measured by using phase-sensitive detection techniques such as a lock-in amplifier (3-5,13). The situation becomes more complex, however, when the sample is excited alternatively by two different excitation wavelengths. This is because the magnitudes of the induced thermal lens signals are different for different excitation wavelengths. As a consequence, the intensity of the probe laser beam that monitors for the thermal lens effect will reflect three different signals: the highest value that corresponds to the period in which the sample is not excited by any excitation beams, and two lower values having different magnitudes that correspond to the cases when the sample is excited by either one of the two pump beams. It is thus not possible to use lock-in amplifiers for detection. One possible solution for this problem is to measure a set of thermal lens transients by means of the probe beam intensity and average them to enhance the signal to noise ratio. The peak height of the transients corresponds to the thermal lens strength. It should be noted that this transient averaging method not only facilitates data acquisition for this dual wavelength technique but also provides a larger dynamic range compared to that obtained by using the lock-in amplifier. This is because the transient averaging method does not suffer the signal saturation that often plagues and, hence, shortens the dynamic range of the lock-in (14). In fact, as described later, a linear calibration curve was found over 3 decades of concentrations. By use of this transient averaging method, the excitation profile and thermal lens transient signals for 8 X M aqueous solution of 1,l’-ethylene2,2’-cyanine iodide excited by 20 mW of the 514.5- and 457.9-nm excitation beams modulated a t 1.5 Hz were measured and the results are shown in Figure 2. The timing of the excitation is shown in Figure 2a. The sample was excited alternatively by two excitation beams (A, = 514.5 nm and A2
.. .. .I
i
.. .... *. . t
400ms
Flgure 2. Excitation beams Intensity (a)and probe beam intensity (b-d) as a function of time: (a)excitation beams intensity profile, A, = 514.5 nm and X2 = 457.9 nm; (b) probe beam intensity measured on sample of 8 X IO-’ M l,l’-ethylena2,2’cyanineiodide aqueous solution excited by only the 514.5-nm excitation beam while the 457.9-nm beam was blocked; (c)same as In part b except the sample was excited by only the 457.9-nm beam while the 514.5-nm beam was blocked; (d) same as part b except the sample was excited by 514.5 nm beam as well as the 457.9 nm beam.
= 457.9 nm) having the same intensity (20 mW) and modulating at the same frequency (1.5 Hz). These conditions were achieved by orienting the two beams and chopping them with a mechanical chopper whose blade was modified to provide not only the same modulation frequency for the two excitations beams but also a closed period between the two consecutive open periods. Therefore, the sample was excited always by the two pump beams alternatively. There was a dark period that followed each excitation to provide adequate time for the sample to completely reach the steady state before being excited by the other excitation beam. To ensure the proper operation of the system, individual thermal lens signals induced by each excitation wavelength were measured (Figure 2b,c). This was accomplished by blocking the 457.9-nm beam by a beam blocker and measuring the thermal lens signals of the sample excited only by the 514.5-nm beam (Figure 2b). Similarly, the thermal lens obtained by single 457.9-nm excitation beam is shown in Figure 2c. As shown in the figures, the intensity of the probe beam decreased gradually when the chopper opened to allow irradiation of the sample by the pump beam. This was because the heat that was produced by the sample absorption of the pump beam acted as a diverged thermal lens to decrease the beam center intensity of the probe beam. The intensity of the probe beam is recovered to its original value when the
ANALYTICAL CHEMISTRY, VOL. 60, NO. 18, SEPTEMBER 15, 1988
Table I. Ratio of Absorbance at 514.5 nm to That at 457.9 nm Determined by Conventional Absorption and the Thermal-Lens Technique A614.6/ A451.0 by
absorp-
compound nickel phthalocyaninetetrasulfonic acid,
tion’
A461.0
180 mV
by
t
1927
(4
thermal lensb
0.83
0.80
1.27 1.97 3.25 3.61
1.27 1.99 3.24 3.63
10.48
10.00
tetrasodium salt (1,lO-phenanthro1ine)ferroussulfate
tris(bipyridyl)iron(II)
l,l’-ethylene-2,2’-cyanineiodide 1,1’,3,3’-tetramethyl-2,2’-cyanine
methosulfate
3,3’-diethylthiacarbocyaninechloride
‘Estimated errors, *0.5%. *Estimated errors, fl%. chopper is closed because the generated heat was dissipated to the whole system. Similar thermal lens transients have been observed (3) and quantitative treatments of the effect have been reported (5). The thermal lens signals for each excitation wavelength correspond exactly to the excitation period as shown in Figure 2a. Furthermore, after each excitation, the sample completely relaxed back to its original thermal state before being excited again. The thermal lens signals of the sample excited by two beams alternatively are shown in Figure 2d. As expected, the signals are the sum of the two individual signals (parts b and c of Figure 2)) which were taken when the sample was excited by only one excitation beam. One of the advantages provided by this two-color thermal lens technique is its ability to provide additional information on the identification of the analyte. This selectivity factor is achieved by taking the ratio of the thermal lens signals at the two excitation wavelengths. In this apparatus, the two pump beams were modulated at the same frequency and their intensities were adjusted, taking into account the difference in their frequencies, so that the corresponding excitation energies provided by the two beams were the same. In addition, the instrument was carefully designed so that other parameters that are known to affect the thermal lens were the same for the two beams (5). Such considerations include the use of an achromatic lens to focus the two beams to the same spot sizes at the sample and a probe laser to monitor the thermal lens signals in order to cancel any excitation chromatic effect due to the two different color pump beams (5). It is, therefore, anticipated that the ratio of the two thermal lens signals should be the same as the ratio of the absorbances at these two wavelengths. As expected, the ratio of the thermal lens signals at 514.5 and 457.9 nm for the ECI cyanine dye shown in Figure 2 is 3.24, which is in good agreement with the 3.25 value determined on a conventional absorption spectrometer. Good agreement has also been found for compounds whose absorbances at these two wavelengths are much different. For instance, the ratio values for nickel phthalocyaninetetrasulfonate complexes, whose absorbance at 514.5 nm is much lower than that at 457.9 nm, were found to be 0.80 and 0.83 by thermal lens and conventional absorption techniques, respectively. For 3,3’-diethylthiacarbocyanine chloride, which has much higher absorbance at 514.5 nm than at 457.9 nm, values of 10.00 and 10.48 have been found by using these two techniques. In fact, as listed in Table I, excellent agreement has been found for a variety of different compounds whose absorbance ratios at these two wavelengths range from 0.80 to 10.00. The use of a two-color excitation also facilitates the detection and identification of a two-component mixture. A demonstration of this capability is seen in Figure 3, which shows the thermal lens signals of 3 X M praseodymium
I
180 mV
t
Flgure 3. Thermal-lens signals of (a) 3 X M praseodymium chloride aqueous solution, (b) 0.175 M nickel(I1) glycinate aqueous solution, and (c) the mixture of the Pr3+ and nickel complexes.
chloride aqueous solution (Figure 3a), 0.175 M nickel(I1) glycinate aqueous solution at pH 6.2 (Figue 3b), and the mixture of these two complexes, which has the same concentrations of the Pr3+and nickel glycinate as in parts a and b of Figure 3 (Figure 3c). These two metal complexes were selected because of their unique optical properties which manifest the potential of the instrument: the P P ions absorb only the 457.9-nm excitation light but not the 514.5-nm light while the nickel complexes have equal absorbances at 457.9 and 514.5 nm. As expected, the thermal lens signals of the Pr3+ion show a strong peak (180 mV) at the 457.9-nm excitation (Figure 3a). Close inspection of this figure also revealed a small signal (24 mV) at the 514.5-nm line. On the basis of the fact that the magnitude of this signal is independent of the Pr3+concentrations and signals having the same magnitude have also been observed on blank samples at the 514.5-nm excitation as well as the 457.9-nm excitation, it is not unreasonable to suggest that these signals are probably due to the solvent absorption. The absorption by impurities in the solvent or by the solvent itself via overtone absorption is probably responsible for these signals (13). The thermal-lens signal of the nickel(I1) glycinate aqueous solution at pH 6.2 is shown in Figure 3b. The thermal lens signal at 457.9 nm is the same as that at 514.5 nm because these complexes have equal absorbance at these two wavelengths. Mixing the Pr3+ and nickel(I1) complexes resulted in a mixture whose thermal lens signals are shown in Figure 3c. It is pleasing to see that Figure 3c is the exact sum of parts a and b; in Figure 3c, the signal magnitude at the 457.9-nm excitation is 180 mV higher than that of the signal at 514.5-nm excitation. This 180-mV difference corresponds exactly to the signal of the Pr3+ions as shown in Figure 3a. This example demonstrates that a small amount of a trace chemical species in the presence of a large background can be determined accurately by using this technique. Furthermore, it provides a more effective way to correct for the background absorption than the one-wavelength thermal-lens technique. The background correction is possible because in this technique, the thermal-lens signals were measured as the difference in the signal intensities at two different excitation wavelengths. Therefore, if the analyte absorbs only one of these two wavelengths, the difference in the signals at the two
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ANALYTICAL CHEMISTRY, VOL. 60, NO. 18, SEPTEMBER 15, 1988
wavelengths will eliminate any contributions from solvent background absorption. The situation has been highlighted briefly above and illustrated in Figure 3a. In this example, the thermal lens signal of Pr3+has two peaks: a larger one a t 457.9-nm excitation (180 mV) and a smaller one (24 mV) a t 514.5-nm excitation. The former is attributed to the absorption of the lanthanide ion while the latter is from the solvent background signal because the ions absorb only excitation light a t 457.9 nm but not the 514.5-nm light. In addition, two signals having the same magnitude (24 mV) were observed at 457.9- and 514.5-nm excitation lights for the blank sample. It is, therefore, clear that solvent absorption contributes to a broad background that can be eliminated by taking the difference between the two signals at the two different excitation wavelengths. Calibration curves for praseodymium aqueous solution with single-wavelength (457.9 nm) and two-wavelength (457.9 and 514.5 nm) excitation beams of 25 mW modulated at 1Hz were measured. In both cases, the calibration plots exhibit linear response (correlation coefficient r = 0.998 for a single wavelength and for two wavelengths as well) over a 3-decade concentration range, Le., 1 x to 0.1 M. As expected, the slope for the two straight lines is the same, which indicates that the sensitivity of the two techniques is the same. The background correction ability of the dual wavelength technique is again reflected from the fact that the straight line for the two-color technique goes through the origin while that of the single wavelengths has an intercept of 24 mV. This intercept, as explained above, is due to the solvent background absorption and was eliminated in the two-color technique because in this technique, signals were obtained as the difference between the two signals a t the two wavelengths. The limit of detection, LOD, calculated as twice the standard deviation of the background signal divided by the slope of the calibration line is estimated to be 4.7 X M for the single and as well as for the two-wavelength technique. This corresponds to the absorbance of 2.8 X The background discrimination ability of this technique enables it to be used for the determination of trace chemicals in the presence of a large background. In fact, similar LOD has been found for Pr3+ion aqueous solution prepared from tap water. Furthermore, the same LOD has also been found for Pr3+ions in an aqueous mixture containing nickel glycinate complexes whose concentration can be as high as 1.0 x
M. The Pr3+ions can still be determined accurately at higher concentrations of nickel complexes, but the LOD values were deteriorated somewhat. For example, the LOD increased to 6.4 X M when the nickel complex concentration was 0.1 M. This is due to the fact that the LOD depends on the standard deviation, u, of the background signal, which in this case is the aqueous solution of the nickel complexes, and the u values were found to be proportional to the background signals at higher nickel complex concentrations. The limitation in the resolution of the digital oscilloscope may also contribute to the increase in u value at higher background signals. Similar observation was also found by other workers (6).
It has been demonstrated that a novel dual-wavelength thermal-lens spectrometer can be developed with the use of an argon ion laser as the only excitation source. This was accomplished by deriving the two excitation wavelengths from the ion laser, which operated in a multiline mode. The sample was excited alternatively by these two excitation beams. Compared to the conventional thermal lens apparatus, this dual-wavelength technique provides an effective way to correct for the solvent background absorption and improve the selectivity. Experiments are now in progress to apply this technique to the area of general trace chemical analysis for the determination of two-component mixtures.
LITERATURE CITED (1) (2) (3) (4) (5) (6) (7) (8)
(9) (10) (11) (12) (13) (14)
Harris, J. M.; Dovichi, N. J. Anal. Chem. 1980, 52, 695A-704A. Tran, Chieu D. Anal. Chem. 1986, 58, 1714-1716. Tran. Cheu D. Appl. Spectrosc. 1988, 40, 1108-1 110. Tran, Chieu D. Appl. Spectrosc. 1987, 4 7 , 512-516. Kliger, D. S. Ulrrasensitive Laser Spectroscopy; Academic: New York, 1983. Leach, R. A.; Harris, J. M. Anal. Chem. 1984, 56, 2801-2805. Leach, R. A.; Harris, J. M. Anal. Chem. 1984 56, 1481-1487. Dovichi, N. J.; Harris, J. M. Anal. Chem. 1980, 52. 2338-2341. Teramae. N.; Winefordner, J. D. Appl. Spectrosc. 1987, 4 7 , 164-165. Berthoud, T.; Delorme, N. Appl. Spectrosc. 1987, 4 7 , 15-19. Wood, J. L. and Jones, M.; J . phvs. Chem. 1963, 67, 1049-1051. Tran, Chieu D. Anal. Chem. 1984, 56, 824-826. Dovichi, N. J. CRC Crit. Rev. Anal. Chem. 1987, 77, 357-423. Piepmeier, E. H. Analytical Application of Lasers; Wiley: New York, 1986.
RECEIVED for review February 2,1988. Accepted May 12,1988. Acknowledgment is made to the donors of the Petroleum Research Fund, administered by the American Chemical Society, for financial support of this research.
