Anal. Chem. 1984, 56,925-929 (25) Buffet, C. E.; Morris, M. D.Anal. Chem. 1983. 55, 376-378. (26) Whinnery, J. R. Acc. Chem. Res. 1974, 7,225-231. (27) Sheldon, S. J.; Knight, L. V.; Thorne, J. M. Appl. Opt. 1982, 21, 1663- 1689. (28) Carter, C. A,; Harris, J. M. Appl. Opt. 1984, 23, 476-481. (29) Gordon, J. P.; Leite, R. C. C.; Moore, R. S.; Porto, S. P. S.; Whinnery, J. R. J . Appl. Phys. 1985, 36,3-8. (30) Carter, C. A.; Harris, J. M. Appl. Spechosc. 1983, 37,166-172. (31) Brannon, J. H.; Magde, D. J . Phys. Chem. 1878, 82, 705-709. (32) Abbate, 0.; Attanasio, A,; Bernini, U.; Kagozzlno, E.; Somma, F. J . Phys. Chem. 1978, 9 , 1945.
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(33) Mason, H. L. Trans. ASME 1954, 76,817. (34) Currle, L. A. Anal. Chem. 1988, 4 0 , 586-593. (35) Jenkins, F. A.; White, H. E. “Fundamentals of Optics”; McGraw-Hill: New York, 1957.
RECEIVED for review October 7,1983. Accepted January 24, 1984. This is based upon work supported by the National Science Foundation under Grant CHE 82-06898.
Quantitative Determination of Acetaldehyde by Pulsed Laser Photoacoustic Spectroscopy M.A. Leugers’ and G . H.Atkinson*2 Department of Chemistry, Syracuse University, Syracuse, New York 13210
A technlque based on photoacoustlc spectroscopy and utillzlng pulsed laser excltatlon has been developed for quantitatlvely detecting acetaldehyde In purlfled air at total pressures of 1 atm. Results are reported which demonstrate a llmltlng sensltlvlty of 25 ppbv for acetaldehyde and a llnearlty of photoacoustic signal wlth acetaldehyde partlal pressures over 6 orders of magnltude. The lowest acetaldehyde concentratlon detected corresponds to the observatlon of an absorptlon coefflclent of 9 X lo-@cm-‘. The methods for optlmlrlng the photoacoustic slgnal for the case where excitation occurs by a short duratlon optlcal pulse are dlscussed.
The availability of versatile laser systems during the last
2 decades has fostered a resurgence of research interest in photoacoustic spectroscopy (PAS) especially for quantitative measurements of small molecular concentrations in atmospheric environments (1-3). The degree to which these analytical applications have been successful has remained critically dependent on the mode and characteristics of laser excitation. Early workers, utilizing excitation from continuous wave (CW) lasers operating in the infrared region, observed high detection sensitivities for a wide range of atmospheric pollutants (1). Recently, PAS generated by modulated, CW laser excitation in the visible region was shown to have sensitivites comparable while interto those found in the infrared experiments (4), ference effects due to intense C02 and HzO absorptions are absent. The primary limitation associated with visible PAS is its restriction to molecular species absorbing within the spectral range covered by CW dye lasers. An alternative approach derives from pulsed laser excitation which encompasses a wider tuning range for excitation extending throughout the visible and well into the ultraviolet. Such pulsed PAS can be used to monitor a much larger group of atmospheric species. Pulsed PAS, however, requires a completely different methodology for recording photoacoustic signals, especially if the sensitivity and selectivity found for CW PAS in the visible (4) is to be maintained. The development of a pulsed PAS methodology utilizing laser excitation Current address: Union Camp Corp., Research and Development Division, P.O.Box 412, Princeton, NJ 08540. Current address: Departments of Chemistry and Optical Sciences, University of Arizona, Tucson, AZ 85721.
in the visible and ultraviolet and its application to the quantitative detection of atmospheric acetaldehyde are addressed in this paper. Although the characteristics of pulsed PAS as it pertains to detection in the condensed phase were recently presented (5),it has not been used widely for gaseous samples (pulsed PAS measurements associated with vibrational overtones (6) and Raman spectrometry (7), however, have been reported). Siebert and co-workers (8) reported the first application of pulsed photoacoustic Raman spectrometry to the detection of trace gases. The maximum sensitivities obtained from their experiments, however, were approximately lo2to lo3less than those reported here and elsewhere (1-4). The results reported here represent an application of pulsed PAS derived from electronic transitions to the quantitative detection of gas phase species at trace levels. The general advantages of pulsed PAS derive from three characteristics: (1)Pulsed laser excitation can be obtained over a wide tuning range extending from the vacuum ultraviolet to the near-infrared. Much of this range is covered by tunable pulsed laser systems which in some cases utilize nonlinear optical techniques. This versatility not only permits one to excite molecules which had previously been energetically inaccessible, but also makes it feasible to utilize spectral “windows”by which a high degree of molecular selectivity can be achieved. (2) The optical excitation of the molecule by rapid (10 ns) pulsed laser excitation permits the time-resolved detection of the acoustical signals. Since there is a large difference in the time dependence of the acoustical signals originating from the nonradiative decay of the absorbing molecule and those generated by background cell noise, most of these signals can be effectively separated in time-resolved measurements. As a result, time resolution removes the need for measuring the signal from a reference cell in order to account for the cell noise. (3) Pulsed PAS can be efficiently measured in acoustically nonresonant sample cells which are much easier to construct and optically align than their acoustically resonant counterparts. By use of these characteristics of pulsed PAS, the varying concentration of gas-phase acetaldehyde in purified air at total pressures of 1 atm was measured quantitatively. Data are reported here for the observation of acetaldehyde concentrations as low as 25 parts per billion by volume (ppbv). The amplitude of the photoacoustic signal was found to be linear over 6 orders of magnitude in acetaldehyde concentration.
0003-2700/84/0356-0925$01.50/00 1984 American Chemical Society
026
ANALYTICAL CHEMISTRY, VOL. 56, NO. 6,MAY 1984
The smallest photoacoustic signals reported here (signal-tonoise ratio of 1)correspond to an absorption coefficient (a) of 9.0 X lo-@cm-l. This sensitivity level is approximately a factor of lo2 better than that observed for NO2 in air by modulated CW laser excitation in the visible (4). The value of CY observed in these studies compares favorably to the high sensitivity achieved by resonant acoustical pumping of an intracavity photoacoustic cell using a ring dye laser where an absorption coefficient of 1 X cm-' was observed (9). The applicability of this relatively simple, ultrasensitive technique to the detection of atmospheric species is discussed.
7
I
l
!
THEORY The theoretical expression for the photoacoustical signal generated by the pulsed excitation of a gas in a cylindrical cell has been derived by Rosengren (10). We merely state the result in expression 1 so that the important experimental parameters may be noted. The time-dependent pressure change is given by 2pwuN1(1- exp[-t(TL1 T;~)]) Pr(t) = (1) 3V(1 7 , ~ ; ~ )
+
+
where @ is the thermal expansion coefficient for the gas (E (3/2)[(Cp/Cv) - 111, W is the total energy per pulse, u is the absorption cross section for the wavelength of radiation, N is the number of absorbing molecules present in the illuminating cylinder, 1 is the length of the PAS cell, t is the time after the pulse of radiation, T~ is the collisional relaxation time, T , is the radiative relaxation time, and V is the volume of the PAS cell. In the derivation of expression 1,it is assumed that (1)the pulse of radiation having a temporal width, T~ is much shorter than the thermal relaxation time, Tt, (2) the absorbed radiation is transferred rapidly into translational energy, and (3) the molecular transition is not optically saturated. Two types of relaxation processes from the excited state are considered, namely, collisional with the relaxation time, rc and radiative with the relaxation time, 7,. The value of Tt for thermal conduction in a real photoacoustic cell can be estimated by (10) Tt N
r2 -
to16 where r is the cell radius, tO1 is the first root of the Bessel function, J&) = 0, and 6 is the thermal diffusivity of the gas. For these experiments (where the gas is greater than 99.7% air at 303 K in a cylindrical cell of radius 11mm), we calculate rt to be approximately 0.74 s. Since the XeCl laser produces a radiation pulse of 10 ns duration (fwhm), it is clear that radiative energy is deposited in the molecule on a time scale much faster than the thermal relaxation of the gas (Le., rt >> T ~ and ) that the first assumption is fulfilled. In examining the second assumption, we consider first the rate at which excited-state energy is transferred into translation by collisions. For a gas a t standard temperature and pressure, molecular collisions occur approximately once every s. (11),and therefore, with a T~ of 0.74 s, rt >> 7,. The deactivation of vibronically excited molecules can also occur radiatively. The radiative decay lifetime, T , was found for acetaldehyde to be independent of pressure above 10 torr,but dependent on the excitation wavelength varying from 3.5 ns for excitation a t 325 nm to 0.8 ns for 275-nm excitation (12). Even with a maximum r, value of 3.5 ns, (7;' + T;')-'
