1218
Anal. Chem. 1985, 57, 1216-1219
LITERATURE CITED Orth, R. G.; Dunbar, R. C. J. Am. Chem. SOC.1978, 100, 5949. Orth, R . G.; Dunbar, R. C. J. Am. Chem. SOC. 1982, 104, 5617. Benz, R. C.; Dunbar, R. C.; Claspy, P. C. J. Am. Chem. SOC. 1981, 103, 1799. Dunbar, R . C.; Teng, H. H. J. Am. Chem. SOC. 1978, 100, 2279. Kramer, J. M.; Dunbar, R. C. J. Chem. Phys. 1973, 5 9 , 3092. van Tilborg, M. W. E.; van Thljl, J.; van der Hart, W. J. Org. Mass Spectrom. 1984, 19, 149. van Zalen, P. N. T.; van der Hart, W. J. Chem. Phys. 1981, 6 1 , 335. Gooden, R.; Brauman, J. I.J. Am. Chem. SOC. 1982, 104, 1483. Dunbar, R. C. I n “Molecular Ions: Spectroscopy, Structure, and Chemistry”, Miller, T. A., Bondybey, V. E., Eds.; North Holland PubllshIng Co.: Amsterdam, 1983. Mukhtar, E. S.;Griffiths, I.W.; March, R. E.; Harrls, F. M.; Beynon, J. H. Int. J . Mass Spectrom. Ion Phys. 1981, 4 1 , 61. Griffiths, I. W.; E. S. Mukhtar; Harris, F. M.; Beynon, J. H. Inf. J. Mass Spectrom. Ion Phys. 1981, 3 8 , 333. Mukhtar, E. S.;Grlfflths, I.W.; Harris, F. M.; Beynon, J. H. Int. J . Mass Spectrom. Ion Phys. 1982, 4 2 , 77. Mukhtar, E. S.;Grlfflths, I. W.; Harris, F. M.; Beynon, J. H. Org. Mass Spectrom. 1981, 76, 51. Rosenstock, H. M.; Draxl, K.; Steiner, B. W.; Heron, J. T. J. Phys. Chem. Ref. Data 1977, 6 , Suppl. 1. Cooks, R. G.; Beynon, J. H.; Caprloll, R. M.; Lester, R. G. “Metastable Ions”; Elsevier: New York, 1973. Dunbar, R. C. Anal. Chem. 1978, 4 8 , 723. Weger, E.; Wagner-Redeker, W.; Levsen, K. Int. J. Mass Spectrom. Ion Phys. 1983, 4 7 , 77.
(18) Bieri, G.; Burger, F.; Heilbronner, E.; Maier, J. P. Heiv. Chim. Acta 1977, 60, 145. (19) Kalller, R. E.; Russell, D. H. Org. Mass Spectrorn., in press. (20) Lifshitz, C.; Gefen, S.; Arakawa, R. J. Phys. Chem. 1984, 88, 4242. (21) Kraliler, R. E.; Russell, D. H.; Jarrold, M. F.; Bowers, M. T. J. Am. Chem. SOC.,in pres$. (22) Dass, C.; Sack, T. M.; Gross, M. L. J. Am. Chem. SOC. 1984, 106,
5780. (23) Hoffman, M. K.; Bursey, M. M.; Winter, R. E. K. J. Am. Chem. SOC. 1970, 9 2 , 727. (24) Gross, M. L.; Russell, D. H. J. Am. Chem. SOC. 1979, 101, 2082. (25) Dass, C.;Gross, M. L. J. Am. Chem. SOC. 1983, 105, 5724. (26) Woodward, R. B.; Hoffman, R. J. Am. Chem. SOC. 1985, 8 7 , 395. (27) Wolkoff, P.; Holmes, J. L. Can. J. Chem. 1979, 5 7 , 348. (28) Jarrold, M. F.; Bass, L. M.; Kemper, P. R.; van Koppen, P. A. M.; Bowers, M. T. J. Am. Chem. SOC. 1983, 78, 3756.
RECEIVED for review December 13,1984. Accepted February 15,1985. This work was supported by the U.S. Department of Energy, Office of Basic Energy Sciences (DE-AS0582ER13023), and the Robert A. Welch Foundation. Some of the equipment used for this work was purchased from funds provided by the Texas A&M University Center for Energy and Mineral Resources.
Beam Geometry Optimization in Dual-Beam Thermal Lensing Spectrometry Thierry Berthoud,* Nathalie Delorme, and Patrick Mauchien DCAEAISEAISEACC, Centre d’Etudes NuclBaires, 92260 Fontenay-aux-Roses, France
Thls paper descrlbes the first systematic study of the Influence of beam geometrlc posltlons In dual-beam thermal lenslng spectrometry. Intenslty and slgn of thermal lens slgnals are studied varylng both beam waist and measurement cell locations. A theoretical model, Involving beam sizes in the cell, Is proposed to explain the experlmental data. These results Indicate that differentlal dual-beam thermal lensing should be possible with either pulsed or contlnuous excltatlon. Usually llmlted by nonspeclflc solvent absorption, very low level concentratlon determlnatlon can therefore be considered.
Several recent studies of weak absorption measurements have been realized by using thermal lensing spectrometry and optoacoustic spectroscopy. These techniques have been demonstrated to be very powerful tools for low-level determination of nonfluorescent molecular species in solution. Thermal lensing spectrometry reaches limits of detection 100-1000 times lower than classical absorption spectrophotometry LOD (1-7). The first experimental thermal lensing studies have been made with single-beam arrangements where the beam both creates and probes the thermal lens in the sample. Afterward, dual-beam configurations have been employed where another beam (usually He-Ne) probes the thermal lens formed in the sample by the excitation beam. In dual-beam schemes, most investigators have used a single lens to focus both excitation and probe laser beams. The sensitivity of the thermal lens signal has been investigated in regard to the locations of the pump and probe beam waists
in the sample cell. Assuming that the two beam sizes are nearly the same in the sample, when the single- and dual-beam setups are compared, the signal magnitudes only depend on their respective wavelengths (8-13). Although Fang and Swofford have suggested that mismatched waists of the two beams give enhanced sensitivity, no systematic study has been published about geometry of dual-beam arrangements (16). This work proposes a theoretical interpretation of experiments relating the probe beam waist to its position toward the excitation beam waist. Such experiments and their explanations are determinant for further investigations for an eventual extension of the differential thermal lensing technique, studied for single-beam measurements by Dovichi and Harris ( I I ) , to dual-beam measurements with either continuous or pulsed excitation.
EXPERIMENTAL SECTION The experimental setup is schematically depicted in Figure 1. The excitation laser beam is produced by a tunable dye laser (Spectra-PhysicsModel 380 C)pumped by a 5-W argon ion laser (Spectra-Physics Model 165) working in the light control mode. Experiments are performed with a neodymium solution of 5 X mol/L in distilled water. The dye laser is tuned to the maximum absorption wavelength of neodymium at 575 nm and adjusted to a power of 500 mW. The dye laser beam, chopped at a frequency of 1Hz,is focused by a 20-cm focal length lens. The probe beam is focused, prior to the beam splitter, with a 33-cm focal length lens, mobile on a 30-cm path. After passing through the absorbing solution in a quartz cell, the excitation beam at 575 nm is filtered out and the He-Ne beam is directed through a pinhole of 1-mm diameter after an optical path of 2.5 m. The photodiode current is amplified with a current amplifier (Keithley Model 427) and sent to a digital
0003-2700/85/0357-12 16$0 1.50/0 0 1985 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 57, NO. 7, JUNE 1985
1217
%M
