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(34) Solimini, D. J. Appl. Phys. 1966. 37, 3314. (35) Appleby, R.; James, D. W.; Bowie, C. A. Spectrochlm. Acta, Part A 1904, 40A. 785. (36) Jam!eson, D. T.; Tudhope, J. S. Desalination 1970, 8 , 393. (37) Jamleson, D. T.; Tudhope, J. S.; Morris, R.; Cartwright, G. Desalination 1970, 7. 23. (38) Boltz, D. F.; Howell, T. A. I n Chemical Analysis; Wiley: New York. 1978; Vol. 8,p 340. (39) Proctor, C. M.; Hood, D. W. J . Mar. Res. 1954, 13, 122. (40) Stephens, K. Limnol. Oceanogr. 1963, 8 , 361.
(41)
Henriksen, A.
Analyst (London) 1965, 9 0 , 29.
RECEIVED for review January 21,1986. Accepted March 10, 1986. This work was supported in part by National Science Foundation Grants CHE-8320620 (SRC) and CHE-7921319 (GEL). A summary of the results was presented at the 12th Annual Meeting of FACSS, Philadelphia, PA, Sept 1985.
Helium-Neon Laser Intracavity Photothermal Beam Deflection Spectrometry Chieu D. T r a n '
Department of Physics and Atmospheric Sciences, Jackson State University, Jackson, Mississippi 3921 7
paratus be developed. One possibility is to replace the inA novel, compact, inexpensive He-Ne laser lntracavlty p h s t ~ a l b e a m d e t l e c t k n ~ w h s s b e e n d . v ~ .terferometer with a laser cavity resonator, Le., measuring photothermicity of a sample placed inside a cavity of a laser. I n this Instrument, a sample Is placed M d e the cavity of the Morever, the advantages of this method are not only limited He-Ne laser, which serves as a probe. An argon ion exclto its compact size but also include its inherent higher sentatlon laser is traversed in the horizontal plane collinear with sitivity due to the intracavity enhancement effect. the He-Ne laser beam Inside the sample cell. The apparatus It is well-known that when an absorbing sample is placed is 28 thnes more sensltlve than the conerpawlhg extracavity inside the laser resonator, the so-called intracavity enhanceinstrument. The detection lbnit for cyanine dye aqueous sois generated from such effects as multiple passes, ment that lution ls 1.0 X M. A mechanism for the lntracavity mode competition, and threshold can become as large as lo7 enhancement Is discussed.
Applications of laser to chemical analysis have increased significantly in recent years (1,2). Laser absorption technique.3 are the most widely used among those applications. Measurement of small absorption such as those in trace analysis requires an ultrasensitive laser spectrometer. The two most widely used techniques to enhance the sensitivity of laser absorption spectrometry are based upon photothermal and intracavity effects. When a laser beam passes through a material of finite optical absorption, the heat generated by the absorption of light increases the temperature within the sample, which changes the index of refraction, which in turn affects the optical beam. The results include a defocusing or focusing of the beam (photothermal lensing), a change in phase delay as the refractive index changes, and a deflection of the beam (photothermal deflection). Certain of the effects are observalbe for beams in the power range of only milliwatts in samples normally thought to be transparent, and are thus useful for measurement of low absorptivity. In fact, absorhave been measured by using photothbances as low as ermal techniques (3-7). Recent attempts to improve the sensitivity of photothermal measurement have utilized Mach-Zehnder interferometers to detect the change in the phase delay as refractive index changes (8-10). These methods were able to detect part-per-trillion levels (i.e., a = cm-') of SF, in nitrogen. Unfortunately, inherent disadvantages of the interferometric photothermal apparatus such as its bulk and the strict requirements for its accurate alignment make this apparatus impractical for general analytical use. Thus, it is important that a compact system having the same or better sensitivity than the interferometric photothermal apPresent address: Department of Chemistry, Marquette University, Milwaukee, WI 53233. 0003-2700/86/0358-1714$01.50/0
(12).In spite of its great potential, intracavity photothermal deflection spectrometry has not been exploited. All but two studies were reported (13,14).Although these systems hold great promise, they are still inferior compared to the interferometric photothermal techniques because: (1) The sample that was a gas in the former study and developed chromatogram in the latter was placed inside a cavity of a dye laser or an argon ion laser, which is complicated and bulky compared to interferometer. (2) The necessity for a sample to absorb dye laser radiation restricts applications of the apparatus to only a few gases (13). (3) The perpendicular overlap between the excitation and probe beams in the latter decreases the sensitivity of the system (14). Such considerations prompted inititation of the present study, which aims to develop for $he first time a compact, simple, inexpensive, ultrasensitive He-Ne laser intracavity photothermal deflection spectrometer. It will be demonstrated in this communication that such an intracavity photothermal beam deflection spectrometer (IPD) can be constructed by placing a sample inside the cavity of a simple He-Ne laser, which serves as a probe laser. The argon ion excitation laser beam will be collinear with the probe beam inside the sample cell. By use of the indicated apparatus, cyanine dye at a M was detected. concentration of 1.7 X
EXPERIMENTAL SECTION Typically, a commercial H e N e laser has its high reflector and output coupler sealed into its plasma tube. The laser used in this work, which was specially constructed according to our requirements, was purchased from the Particle Measurement Systems, Inc., Boulder, CO. It consists of a plasma tube with one end sealed with a concave high reflector ( H E R = 100 cm) while the other end is a Brewster window. Ita output couple (OC) was a concave mirror with R = 100 cm and dielectrically coated for 1% transmission at 632.8 nm. The output coupler was mounted on a translational stage to provide tilt, vertical and horizontal adjustments for the coupler, and a horizontal adjustment for the cavity length. The distance from the Brewster window to the output coupler was fixed at 20 cm during this work. A schematic 0 1986 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 58, NO. 8, JULY 1986
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to the external photothermal beam deflection technique, the
Sample
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Figure 1. Schematic diagram of the intracavity He-Ne laser photothermal beam deflection spectrometer. MI and M2are dlelectrlcelly coated mirrors to reflect the 514.5-mm laser beam; P.D. is a photodiode; O.C. is an output coupler; and H.R. is a high reflector for the He-Ne laser. In this experiment, the H.R. was permanently sealed to the He-Ne laser plasma tube. diagram of the intracavity photothermal deflection apparatus is shown in Figure 1. A sample cell placed inside the He-Ne laser cavity was constructed from glass windows of 0.1 mm thickness. It was placed on a cell holder titled to provide a Brewster angle between its windows and the He-Ne laser beam. Intensity of the He-Ne laser was optimized by aligning the O.C. with the aid of a Spectra Physics Model 404 power meter (P.D.). A Coherent CR-2 argon ion laser operated at 512.5 nm was used as an excitation source. The 514.5-nm excitation beam, which was amplitude modulated by a mechanical chopper (EG&G Princeton Applied Research Model 196variable-speed chopper), was aligned in the horizontal plane collinear with the He-Ne laser beam inside the sample cell. MI and M2 are dielectrically coated mirrors to reflect the 514.5-nm excitation beam and to transmit the 632.8nm probe beam. A pinhole was placed in front of the Brewster window to avoid having the 514.5-nm beam entering the plasma tube of the He-Ne laser. The intensity of the He-Ne laser beam was measured with a Spectra-Physics Model 404 power meter (photodiode, P.D.). The signal component from the P.D., which was modulated at the chopping frequency, was synchronously detected in a lock-in amplifier EG&G Princeton Applied Research Model 5207). The lock-in amplifier time constant was maintained at 1 s. Signal intensity was obtained from the digital reading on the lock-in amplifer. The whole system was vibrationally isolated by means of an optical table. The arrangement for external cavity photothermal beam deflection was identical to the intracavity setup except that the sample cell was placed outside the He-Ne laser cavity and the pinhole was placed in front of the photodiode. Each measurement was performed at least 3 times, and their average values were reported. Absorption spectra were measured on a Beckman spectrophotometer Model UV 5230. 1,1’,3,3’-tetramethyL2,2’-cyanine methosulfate was from the same source as cited in ref 15 and was a gift from the Royal Institution of Great Britain, London, England. Solution was made up by dissolving the cyanine dye in a small amount of methanol and then diluting it with water to the required volume. The concentration of the dye was checked spectrophotometrically (e = 3.5 X lo4 M-’ cm-l at 528 nm) (16).
RESULTS AND DISCUSSION The output power of the He-Ne laser with a 1% transmission output coupler was 7 mW, which corresponds to approximately 0.7 W intracavity power. Introducing a 0.96cm-path-length sample cell containing lo4 M cyanine dye into the cavity substantially decreased the output power to 1mW. The reduced power is probably due to the reflection loss by the two cell windows and also the induced lens effect by the sample solution. Because the He-Ne laser has very low gain, it is very important to reduce the loss introduced by the intracavity cell as much as possible. This was achieved somewhat by preparing a sample cell with windows as thin as possible and placing them at a Brewster angle. Pyrex glass with 0.1 mm thickness was found to be adequate for cell windows as it was not too thick to have too much loss for the laser to lase and not too thin to lose its rigidity. Compared
intracavity method provided much improved signal-to-noise ratios. The signal-to-noiseratio of the same sample, measured under the same conditions, is 28 times better for the intracavity than the extracavity. There have been several theories proposed to account for the intracavity enhancement. Generally, there are three effects responsible for the enhancemenk (1) a gain in optical path length, or multipass effect, arises as the photon in a cavity of a laser passes through the sample many times; (2) a threshold effect arises because near threshold a small change in loss in the laser cavity makes a large change in the number of photons in a mode; and (3) mode competion occurs between those modes that are affected by the loss and other modes of the laser (17). The mode competion is not, however, important in this work as the cyanine dye is a broad-band absorber with respect to the laser bandwidth, so all modes are quenched equally. The contribution of the threshold effect is relatively unimportant as the H e N e laser was operated well above its threshold and, in addition, the sample concentrations as well as the cel length were kept in such a way that there was not so much heat generated from absorption, which may bring the He-Ne laser closer to the threshold. Consequently, multipass seems to be a major reason for the intracavity enhancement. It is well-known that the intracavity enhancement, q, due to the multipass effect relates to the laser output compler by q = 1/T wher T i s the transmission of the output coupler (17). In the presence case, T = 1% therefore q is 100. The enhancement that we obtained is relatively lower (28) than the calculated value. Effects such as reflection losses from cell windows and induced lens effects by the sample solution are probably responsible for the low intracavity enhancement. The modulation frequency dependent of the IPD signal of 2.0 X 10-8M cyanine dye solution was measured over the range 4-120 Hz. Plots of signal against the period in which the excitation beam was on; i.e., the reciprocal of the excitation modulation frequency exhibited a linear relationship. I t indicates that the average temperature increases of the sample is proportional to the time the excitation beam is on. A lower chopping frequency, i.e., increase the excitation time, will increase the IPD signal intensity as more heat is generated. The reverse is true when the chopping frequency is increased. A similar relationship was also found on an external photothermal beam deflection apparatus (2,19).Noise increased substantially as the chopping frequency decreased to lower than 4 Hz. This is probably due to the inherent instability of the mechanical chopper a t lower frequencies and also the He-Ne laser (2, 20). Quantitative treatment of the frequency response of thermooptical experiments has been delineated (21,22). Generally, the time constant for the formation of a temperature rises within a sample for a Guassian beam is often described in terms of a thermal time constant, t, = w2/4D, where w is the pump spot size and D is the thermal diffusivity of the sample (23). Signal amplitude is inversely proportional to the modulation frequency for chopping periods short compared to t,. When the modulation periods are relatively long compared to t,, the signal reaches a steady state. The pump beam spot size used in this experiment was about 2.5 mm, which corresponds to t, = 11s for water (23). Thus,the chopping period used in this experiment is short compared to t,, and the observed linear relationship between signal and modulation period is in agreement with the theory. A potential improvement which stems from this result is that the sensitivity of the apparatus can be enhanced substantially by using a lens to focus the pump beam as smaller spot size leads to shorter t, and, consequently, makes it feasible to operate at a higher
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ANALYTICAL CHEMISTRY, VOL. 58, NO. 8, JULY 1986
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i Power, m W Flgure 2. Excitation laser power dependence of signal of 5.0 X lo-’ M cyanine dye aqueous solution: chopping frequency, 15 Hz.
chopping frequency without decreasing the signal. Furthermore, the strength of the deflection is predicted to increase inversely with the pump beam spot size, a more tightly focused pump beam produces a more sensitive measurement of sample absorbance (24). The relationship between excitation laser power chopped at 15 Hz and IPD signal of 5.0 X lo-* M cyanine aqueous solution is shown in Figure 2. The noise level was not constant throughout the excitation power range. Increasing the pump power not only increases the signal but &o decreases the noise as well. The instability of the old argon ion laser used in this experiment at low power probably accounts for the variation in the noise level. Furthermore, there is a possibility that the pump laser heats the M2 mirrors and the output coupler of the probe laser, which in turn generates background signal. By keeping the excitation power larger than 35 mW, one can achieve a fairly good linear relationship. The dependence of signal to cell length was investigated for 5.0 x M cyanine dye solution, excited by a 50-mW laser a t 1 5 - H chopping ~ frequency, using a sample cell with length ranging from 0.96 to 2.08 cm. A good linear relationship was obtained, which ensures the application of this technique for trace analysis. A calibration curve for 1,1’,3,3’-tetramethyL2,2’-cyanine methosulfate aqueous solution was measured with 50-mW excitation laser power chopped a t 15 Hz and 0.96-cm cell length. The calibration plots exhibited a linear response (correlation coefficient, r = 0.998) over a concentration range of 1.0 X lo-* to 2.5 X M. Reasons for choosing 1,1’,3,3’-tetrmethyl-2,2’-cyanine methosulfate include its high absorptivity at the excitation laser wavelength (e = 3.17 X lo4 M-l cm-’ at 514.5 nm) and its low fluorescence quantum yield (16). The photo-to-heat conversion of this dye, therefore, is high as the major photophysical pathway of the excited molecule is internal conversion to the ground state in which the excited-state energy is efficiently dissipated as heat (15). Furthermore, the lack of the absorption of the dye at the He-Ne laser wavelength eliminates any undesired complications ( 2 ) . The limit of detection, LOD, defined as the amount of the sample that yielded a signal twice the standard deviation of
the blank, is estimated to be 1.7 X M for a 0.96-cm cell length, which corresponds to an absorbance of 6.3 X This detection limit was for pump laser power of 50 mW and 1 5 - H ~ chopping frequency. The detection limit reflects the objective of this work, which is to demonstrate the feasibility of the technique. Therefore, the apparatus is far from its optimized conditions. As mentioned previously, the intracavity enhancement obtained in this work was only 28, whereas it could be ar large as lo7 (12). The enhancement in this work was only a small part of the multipass effect. Further enhancement can be achieved by fully utilizing the multipass effect as well as threshold effect; i.e., force the He-Ne laser to operate near its threshold. These improvements can be performed by (1)shortening the He-Ne laser cavity as the intracavity enhancement in inversely proportional to the cavity length; i.e., the cavity acts as a multipass cell (24);(2) decreasing the transmison of the output coupler as well as varying its radius of curvature; and (3) increasing the excitation laser power and cell length and decreasing the modulation frequency. These possibilities are now under investigation. It has been demonstrated that an ultrasensitive spectrometer can be developed by using a rather simple and inexpensive He-Ne laser intracavity photothermal beam deflection technique. The high sensitivity and versatility of this technique enable it to be used for trace analysis. Experiments are now in progress to explore these potential applications and also to gain insight into the mechanism of intracavity enhancement.
ACKNOWLEDGMENT I am grateful to Ken Sample (PMS) for the gift of the output couplers. Registry No. He, 7440-59-7; Ne, 7440-01-9. LITERATURE CITED Hieftje, G. M.; Travis, J. C.; Lytle, F. E. Lasers in Chemical Analysis; Humana: Clifton, NJ, 1981. Kliger, D. S. Ultrasensitive Laser Spectroscopy; Academic Press: New York, 1983. Jansen, K. L.; Harris, J. M. Anal. Chem. 1085, 57, 2434-2436. Pang, T. K. J.; Morris, M. D. Appl. Specfrosc. 1085, 39, 90-93. Harris, J. J.; Dovichi, N. J. Anal. Chem. 1080, 52, 695A-706A. Gordon, J. P.; Lelte, R. C. C.; Moore, R. S.; Porto, S . P. S.; Whinnery, J. R. J . A@. PhyS. 1085, 36,3-8. Grisko, V. I.; Ludelevich, I. G.; Grisko, V. P. Anal. Chim. Acta 1084,
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RECEIVED for review February 3,1986. Accepted March 11, 1986. Financial support was provided by the donors of the Petroleum Research Fund, administered by the American Chemical Society.
