Anal. Chem. 1998, 70, 4636-4639
Technical Notes
Resonant Cavity Gas-Phase Polarimeter Je´roˆme Poirson,* Marc Vallet, Fabien Bretenaker, Albert Le Floch, and Jean-Yves The´pot†
Laboratoire d’Electronique Quantique-Physique des Lasers, Unite´ Mixte de Recherche du Centre National de la Recherche Scientifique 6627, Universite´ de Rennes I, Campus de Beaulieu, F-35042 Rennes Cedex, France
A high-sensitivity polarimeter is demonstrated for application to gas-phase chirality measurement. This device is based on the physics of the eigenstates of a Fabry-Perot cavity, permitting improvement in the sensitivity with respect to the usual polarimeters. Typical measurements of rotations of 50 ((1) × 10-5° induced by the optical activity of (R)-(+)-limonene and (S)-(-)-limonene in the vapor phase are shown. A noise level corresponding to a rotation of 10-6° is experimentally demonstrated. Application to the polarimetric monitoring of an enantiomer mixing racemization of limonene in the gas phase is also presented. The optical activity of organic liquids is known to be due to the dissymmetry of molecules. Consequently, vapors obtained from these liquids possess similar characteristics, as first observed by Biot1 in the case of turpentine oil and confirmed and generalized by Gernez2 and Guye and do Amaral.3 However, the observable effect is very small in vapors and even today, such measurements are seldom used. In particular, to our best knowledge, no commercial gas-phase polarimeter is available.4 This lack of commercial apparatus is first due to the insufficient sensitivity of the usual devices, which is of the order of 10-4°. To improve this sensitivity, several liquid-phase polarimeters have been reported, such as chiroptical detectors with increased sensitivity used in high-performance liquid chromatography and built with carefully selected optical components,5 heterodyne detection-based devices,6 and devices based on helicoidal waves.7 In particular, this latter method has been shown to lead to a high sensitivity using the properties of an anisotropic Fabry-Perot * Corresponding author: (fax) +33 2 99 28 67 50; (e-mail)
[email protected]. † Organome ´talliques et Catalyse: Chimie et Electrochimie Mole´culaires, Unite´ Mixte de Recherche du Centre National de la Recherche Scientifique 6509, Universite´ de Rennes I, Campus de Beaulieu, F-35042 Rennes Cedex, France. (1) Biot, J.-B. Me´ m. Acad. Sci. 1817, 2, 114-133. (2) Gernez, D. Ann. Sci. Ec. Norm. Sup. 1864, 1, 1-38. (3) Guye, P.-A.; do Amaral, A.-P. Arch. Sci. Phys. Nat. 1895, 33, 513-529. (4) Recently, chiral discrimination using piezoelectric and optical gas sensors has been demonstrated: Bodenho¨fer, K.; Hierrlemann, A.; Seeman, J.; Gauglitz, G.; Koppenhoefer, B.; Go¨pel, W. Nature 1997, 387, 577-580. Bodenho¨fer, K.; Hierrlemann, A.; Seeman, J.; Gauglitz, G.; Christian, B.; Koppenhoefer, B.; Go¨pel, W. Anal. Chem. 1997, 69, 3058-3068. (5) Yeung, E. S.; Steenhoek, L. S.; Woodruff, S. D.; Kuo, J. C. Anal. Chem. 1980, 52, 1399-1407. (6) Mitsui, T.; Sakurai, K. Jpn J. Appl. Phys. 1996, 35, 4844-4847. (7) Le Floch, A.; Le Naour, R. U.S. patent 4,305,046, 1981. Le Grand, Y.; Le Floch, A. Opt. Lett. 1992, 17 (5), 360-362.
4636 Analytical Chemistry, Vol. 70, No. 21, November 1, 1998
cavity sandwiched between crossed polarizers. Let us also recall that Kastler showed, as early as in 1962, that, in the case of small absorption measurements, the use of an isotropic Fabry-Perot interferometer enhances the absorption signal by a factor of the order of the finesse.8 Consequently, the aim of this paper is to explore the potentialities of a resonant cavity polarimeter, built to detect and measure the specific rotations of chiral compounds in the gas phase. The application of this polarimetric device to monitor the racemization of chiral compounds is also investigated. EXPERIMENTAL SECTION Polarimeter Principle. The polarimeter scheme is shown in Figure 1. The source is a homemade 200-µW laser emitting at λ ) 633 nm. It is an intensity- and frequency-stabilized He-Ne laser, protected against spurious reflections by an optical isolator OI. A lens L (30-cm focal length) is used to match the laser beam to the TEM00 mode of the 50-cm-long resonant optical cavity, which is sandwiched between two standard crossed Glan prism polarizers P1 and P2 (extinction ratios better than 10-5). The confocal cavity is built with two identical mirrors M1 and M2 (98.5% intensity reflection coefficients; 50-cm radii of curvature). The polarimetric vapor cell VC (length L ) 30 cm) is located inside the cavity. To make the cavity eigenstates sensitive to optical activity, two high-quality antireflection coated quarter-wave plates L1 and L2 sandwich the cell. Their fast axes make angles θ1 and θ2, respectively, with the input light polarization fixed by the polarizer P1. Moreover, L1 and L2 are used as closing windows for the polarimetric cell. Thanks to the cumulative effect of the Fabry-Perot cavity, we expect the presence of an optical activity, leading to a single-pass rotation R, to yield an intensity transmitted through the crossed output polarizer P2 given by
It )
4KF2 (θ1 - θ2 + R)2I0 π2
(1)
I0 is the intensity of the laser beam matched to the cavity TEM00 mode, and K and F are the transmission coefficient at resonance and the finesse of the Fabry-Perot cavity, respectively. Compared with usual single-pass polarimeters, we thus expect an enhancement factor of the sensitivity equal to 4KF2/π2. However, for eq 1 to be valid, we must take care to keep the Fabry-Perot cavity at resonance. (8) Kastler, A. Appl. Opt. 1962, (Suppl. 1), 67-74. 10.1021/ac980286e CCC: $15.00
© 1998 American Chemical Society Published on Web 10/02/1998
Figure 1. Experimental arrangement for the gas-phase polarimeter: OI, optical isolator; P1, P2, Glan prisms; M1, M2, cavity mirrors; L, matching lens; L1, L2, quarter-wave plates; PM, photomultiplier tube; PD, photodiode; LIA, lock-in amplifier; R, recorder; LFG, low-frequency function generator; HVA, high-voltage amplifier; PZT1, PZT2, piezoelectric transducers; VC, polarimetric vapor cell.
Cavity Stabilization. The resonance frequency of the passive cavity is locked to the stabilized frequency of the He-Ne laser by the servoloop schematized in Figure 1. The cavity length is sinusoidally modulated at ν1 ) 2.5 kHz by applying an ac voltage to a piezoelectric transducer PZT2 carrying mirror M2. After detection, the induced modulation of the intensity Ir rejected by the output polarizer P2 is demodulated. The component at ν1 is filtered and amplified in order to monitor the position of mirror M1, carried by a piezoelectric transducer PZT1. Hence, cancellation of the intensity modulation component at ν1 permits one to servolock the cavity length to resonance during the whole measurement process. Signal Modulation and Linearization. To improve the signal-to-noise ratio, we modulate at a frequency ν2 ) 10 Hz the orientation of the quarter-wave plate L2, by use of a piezoelectric transducer (Physik Instrumente, model P-830-40). Indeed, eq 1 proves that a modulation θ2 ) Θ cos(2πν2t) is equivalent to a modulation of the optical activity itself. In our experiment, Θ is equal to 10-2°. Demodulation at ν2 of the signal transmitted by P2 and detected by the photomultiplier tube PM thus leads to the following intensity component:
It2 )
8KF2 Θ(θ1 + R)I0 π2
(2)
With R ) 0°, i.e., no chiral compound inside the polarimetric cell, parts a and b of Figure 2 reproduce the PM output intensity modulation for θ1 ) 0° and θ1 ) +7 × 10-4°, respectively. Once demodulated, Figure 3 reproduces the experimental recording of the demodulated signal of eq 2, corresponding to the evolution from Figure 2a to Figure 2b (θ1 ) 0° to θ1 ) + 7 × 10-4°). Calibration and Sensitivity Test. The signal of Figure 3, corresponding to a rotation of the first quarter-wave plate L1 of θ1 ) +7 × 10-4°, will provide a calibration for polarimetric measurements of the following section. Moreover, it permits us to evaluate
Figure 2. Output signal from the photomultiplier tube versus time with R ) 0 ° and θ2 ) Θ cos(2πν2t): (a) θ1 ) 0 °; (b) θ1 ) +7 × 10-4°.
the noise level of our polarimeter. The inset of Figure 3 shows the typical noise level obtained for an integration time of 1 s. For a 1-Hz bandwidth, this noise level corresponds to a rotation of 10-6°, which we take equal to the detectability of our apparatus. The experimentally measured values of the finesse F and of the cavity transmission at resonance K are equal to 120 and 0.3, respectively. From eq 1, they yield an expected enhancement factor 4KF2/π2 equal to 1750. This is confirmed by the fact that, for θ1 ) 2.6 × 10-2°, we measure an output intensity It equal to 69 nW, leading to an enhancement factor equal to 1700, in good agreement with the theoretical value. RESULTS AND DISCUSSION Observation of Vapor Chirality. To explore the potentialities of our polarimeter to observe chirality in the gas phase, we choose Analytical Chemistry, Vol. 70, No. 21, November 1, 1998
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Figure 3. Calibration of the polarimeter with a mechanical rotation θ1 ) +7 × 10-4° of the quarter-wave plate L1. The inset exhibiting the magnified noise level permits us to estimate the noise level of the device at 10-6°.
Figure 4. Detected signals corresponding to the optical activity of (a) (R)-(+)-limonene and (b) (S)-(-)-limonene in the gas phase. The arrows labeled + and - indicate when limonene is introduced inside the polarimetric cell. Inset c shows the output signal recorded when a nonchiral compound (cyclohexene, for instance) is poured inside the polarimetric cell. The arrow indicates when the sample is introduced. The small transient signal is due to an alteration of the cavity transmission during the introduction of the liquid. Notice that no overall signal is obtained.
limonene as a test molecule. In the liquid phase, specific rotations of (R)-(+)-limonene (Fluka, Sigma-Aldrich Co., purity 98% (GC, sum of enantiomers)), and (S)-(-)-limonene (Fluka, Sigma-Aldrich ) +(115 Co., purity 97% (GC, sum of enantiomers)), are [R+] 20°C D -1°‚cm2‚g-1. To ( 5) 10-1°‚cm2‚g-1 and [R+] 20°C ) -(90 ( 5) 10 D realize the observation, we pour 0.5 mL of liquid limonene at room temperature in the bottom of our polarimetric cell. Then, we just observe the flux through the light beam of the vaporizing limonene escaping from our nonhermetic cell. Parts a and b of Figure 4 correspond to the results obtained with (R)-(+)-limonene and (S)(-)-limonene, respectively. As can be seen from these polarimetric signals, after a few minutes, a stationary gas flux has settled through the cell. Optical activity angles of +6 × 10-4° and - 4.6 × 10-4° are measured for (R)-(+)-limonene and (S)-(-)-limonene, respectively, for a 30-cm-long optical path. The amplitudes of these signals are converted into angles with the calibration curve of Figure 3. As expected, these two measurements have opposite 4638 Analytical Chemistry, Vol. 70, No. 21, November 1, 1998
Figure 5. Monitoring of a racemization process in the gas phase with the resonant cavity polarimeter. The arrows labeled - and + indicate when (S)-(-)-limonene and (R)-(+)-limonene are respectively introduced inside the polarimetric cell.
signs, and their ratio corresponds to the ratio of the given specific rotations of the two stereoisomers. During the measurement time of 10 min, the observed stability of our polarimeter is better than 10-5°. Finally, we have checked that the obtained output signals are well due to the optical activity of the intracavity compound by pouring a few milliliters of a nonchiral liquid (cyclohexene for example) at the bottom of the cell (inset c of Figure 4). In this case, there is no overall evolution of the output signal when cyclohexene evaporates inside the cell. Gas-Phase Racemization Measurement. Good stability and high detectability are required for measurement of gas racemizations exhibiting slow kinetics. To illustrate this potentiality, we realize a simple racemization experiment with limonene in the following manner. First, we inject 0.8 mL of (S)-(-)-limonene at the bottom of the cell. We then obtain a typical (S)-(-)-limonene polarimetric signal, as shown in the beginning of Figure 5. After 7 min, we add 1.2 mL of (R)-(+)-limonene. Then, the molecules of (R)-(+)-limonene progressively compensate for the molecules of (S)-(-)-limonene in the vapor, leading to the racemization signal of Figure 5. This result illustrates a racemization process whose rate would be equal to 5.7 × 10-3 s-1. The end of the experimental recording of Figure 5 corresponds to the expected optical activity of the final gas mixture of the two species. Discussion. To quantify the amount of limonene detected in Figure 4, one needs to suppose that the polarimetric cell is hermetically sealed. A typical cell would be, for example, a 30cm-long tube with a 5-mm-bore diameter. The rotation of 6 × 10-4° shown in Figure 4a then corresponds to 10 µg of limonene inside the cell. A detectability of 10-6° leads to a detection limit of around 20 ng of limonene. In gas-phase chromatography (GC), the quantities to be detected on column are in the range of 1 ng.9 To couple our polarimeter to GC, the enhancement factor would thus have to be increased by 1 or 2 orders of magnitude. This could be performed using quarter-wave plates with high-quality antireflection coatings (transmission better than 99.9%) and mirrors with reflectivities of 99.9%. In this case, finesses in the range of 1000 could be expected. Finally, by improving the mechanical support of L2, we could expect the modulation frequency of L2 to reach a few hundred hertz, leading to an (9) This value is actually in the range of the sensitivities reached by liquidphase polarimeters. See, for example: Lloyd, D. K.; Goodall, D. M. Chirality 1989, 1, 251-264, and therein.
enhancement of the signal-to-noise ratio and to a detection time compatible with typical GC gas flows (a few cm3‚mn-1). Such a gas-phase polarimetric detector could then be coupled at the output of gas chromatographs, leading to complementary stereospecific information with respect to chiral recognition structures in stationary phases. In conclusion, we have built and tested a compact highsensitivity device for detecting and measuring the optical activity of chiral compounds in the gas phase. Applications to chiral gas sensing and to the analytical study of essential oils10 in the vapor phase can be considered. Moreover, the sensitivity of our device
could be improved by using carefully selected optics in the cavity, thus leading to new potentialities in the detection of chiral compounds11 and in the study of racemization of chiral compounds exhibiting slow racemization kinetics.
(10) de Oliveira, C. M.; Ferracini, V. L.; Foglio, M. A.; de Mejeire, A.; Marsaioli, A. J. Tetrahedron 1997, 8 (11), 1833-1839. (11) Eliel, E. L.; Wilen, S. H.; Mander, L. N. Stereochemistry of Organic Compounds; Wiley-Interscience: New York, 1994; pp 424-440.
Received for review March 12, 1998. Accepted August 23, 1998.
ACKNOWLEDGMENT We thank A. Collet and P. Guenot for fruitful discussions. The Socie´te´ de Secours des Amis des Sciences and the Conseil Re´gional de Bretagne are acknowledged for financial support. This work has been performed within the framework of the Centre Laser et Applications a` la Chimie.
AC980286E
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