Anal. Chem. 1988, 60, 1027-1032 (21) Bourne, A. R.; Taylor, J. L.; Watson, T. G. Gen. Comp. Endocrinoi. 1985, 58, 394. (22) Bourne, A. R.; Taylor, J. L.; Watson, T. G. Gen. Comp. Endocrinol. 1988, 6 1 , 278. (23) Kime, D. E. In Steroids and Their Mechanism of Action in Nonmammalian Vertebrates; Delno, G., Brachet, J., Eds.; Raven: New York, 1980; p 17. (24) Ozon, R. In Steroids in Nonmammailan Vertebrates; Academic: New York, 1972; Chapter 6. (25) Huf, P. A.; Bourne, A. R.; Watson, T. G. Gen. Comp. Endocrinoi. 1987. .... , 66. .. 364. .. (26) Brkina, M.; Voikov6, V.; Volke, J. Collect. Czech. Chem. Commun 1954, 19, 694.
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(27) (28) (29) (30) (31) '(32) (33) (34)
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Cauquis, G.; Genik, M. Tetrahedron Lett. 1988, 32, 3537 Cauquis, G.; Genies, M. Tetrahedron Lett. 1970, 33, 2903. Eisner, U.; Zemer, Y. J . Nectroanal. Chem. 1972, 38, 381. Eisner, U.; Glleadi, E. J . Nectroanal. Chem. 1970, 2 8 , 81. Eisner, U.; Zommer, N. J . Electroanai. Chem. 1971, 30, 433. Gllcksman, R. J . Nectrochem. SOC. 1981, 198, 922. Cohen, S. G.; Nicholson, J. J . Am. Chem. SOC. 1984, 86, 3892. Nicholson, J.; Cohen, S. 0.J . Am. Chem. SOC. 1968. 88, 2247.
RECEIVED for review June 29,1987. Accepted December 14, 1987.
Design, Characterization, and Applications of a Photoacoustic Cell for Temperature and Atmosphere Control Meg M. Thompson' and Richard A. Palmer*
P. M . Gross Chemical Laboratory, Duke University, Durham, North Carolina 27706
A photoacoustk cell designed for controlled temperature o p eratlon up to 500 O C wlth a fbwlng coupllng gas Is descrlbed. The compact dedgn and the easy access to the cel windows permlt use of the cell Interchangeably between ultravloletvlslble-near-Infrared and mid-Infrared spectrometers. The separation of the sample and mlcrophone compartments of the cell, which Is necessary to protect the microphone from high temperatures, creates a Helmholtz resonator conflguratlon for which the resonance Is In the region of useful photothermal modulation frequencies. The performance characteristlcs of the cell, lncludlng the effects of the resonance, are Illustrated, and Its applcatlon to the study of thermal decomposltlon reactlons, phase transltlons, and solld-gas reactions is demonstrated.
The ability of photoacoustic spectroscopy (PAS) to provide absorption data for condensed media has led to the application of this technique to a wide range of investigations in physics, chemistry, and biology (1). The reasons for the rapid development and attention that this technique has experienced are self-evident: (I) PAS requires little to no sample preparation, (2) PAS involves direct detection of the radiation absorbed by the sample and as a result facilitates the detection of weakly absorbing species in the presence of strongly absorbing species, (3) PAS measurements are relatively insensitive to scattering and particle size effects, and (4) PAS may be used to probe the subsurface of the sample or to obtain "depth profile" information. Although most previous condensed phase photoacoustic investigations have involved examinations of samples at ambient temperatures, several laboratories have extended the conditions of the photoacoustic experiment to temperatures above and below ambient (2-1 7). However, of these studies, only two investigators have reported measurements above 200 "C (7, 14-17). There is also increasing interest in using photoacoustic detection for the study of solid-gas reactions in situ. In this application the cell must be compatible with a reactive coupling gas and should also provide for gas flow during spectral measurements. Current address: Lorillard Research Center, 420 North English St., Greensboro, NC 27405.
This paper describes an elevated temperature photoacoustic cell with atmosphere control, which has been developed in our laboratory for use in monitoring solid-gas reactions. Results using carbon black as a broad wavelength, strong absorber illustrate the performance characteristics of the cell with respect to modulation frequency and sample temperature in both dispersive and interferometric instrumentation. In addition, the use of the cell to examine samples a t elevated temperatures, as well as samples in a flowing gas stream, is demonstrated with ultraviolet-visible (W-vis) spectral studies of the thermal decomposition of CoCl2-6Hz0 and of the melting of indium, and a Fourier transform infared (FTIR) spectral investigation of NaZCO3reacting with SOz. EXPERIMENTAL SECTION Cell Construction. The high-temperature photoacoustic (HT-PA) cell consists of four main parts: (1)window holder; (2) cell body; (3) microphone holder; and (4) cooling system. Figure 1 presents cross-sectional views of the cell. The stainless steel in. 0.d.) is designed to hold two window holder (11/2in. high, 15/8 windows (32 mm diameter X 2 mm thick, 25 mm diameter x 2 mm thick). The windows are easily changed and may be selected for the spectral region of interest. They are held in place -1 in. apart by threaded retainer rings and sealed with Teflon gaskets. The volume between the windows may be evacuated in order to provide added insulation (7) and temperature stability. The entire window holder screws into the stainless steel body of the cell such that the bottom window is 0.08 in. from the top of the sample. For operation at temperatures e400 OC all seals are made with Teflon gaskets. For temperatures above 400 "C, copper is substituted for the Teflon. The cell body houses the sample chamber, gas and microphone channels, and heater. A removable sample holder with a copper plug at its base (1/2 in. diameter X in. thick) fits closely into the sample chamber and rests on another copper plug at the center of the cell body. These copper plugs (1/4 in. diameter X in. thick) enhance the transfer of heat from the heater coil located directly below the sample chamber to the sample. In addition, the base of the sample holder is made with slanted sides (7) to prevent jamming of the holder in place due to expansion from in. square grooves at 90" intervals around the heating. Four sides of the sample holder correspond to the positions of the gas inlet, gas outlet, and microphone channels and a positioning pin. The gas inlet and outlet channels are made from in. 0.d. stainless steel tubing. The tubing is press fitted into holes in the cell body which are open to the sample chamber. At in. from in. the sample chamber, the holes in the cell body reduce to
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remaining stainless steel areas of the cell. The temperature of the sample is regulated with a PID temperature controller (Rika Kogyo Co.1 and a typeJ (iron-constantan) thermocouple (0.010 in. diameter; Cole Parmerl which is poxitinned in. from the sample holder. Although insulation at least '1, in. thirk surrounds all stainless steel arras of the rell body, with the exception of the area directly over the sample, a cooling system is required to prevent the eventual heating of the spectmmerer. For this purpose. a cylindriral aluminum water jarket surruunds the body of the HT-PA cell. Befnre wawr flows through the water jacket it paves through in. wpper ruhing wrapped around the sample-micruphone acoustic coupling tube and the two wpport 1mlu to reduce the conductive heat transferred to the mirrophone. Consequently, the incoming water cools the samplemirrophonr acoustir roupling tube tirst hefore removing heat from the oulside ot' the cell msultltion. Ultraviolet-VisibleNear-Infrared Spectral Measurements. UV-vis spertral measurements were made with a Princeton Applied Research ultraviolet-visible-near-infrared ( U V vis -near-IRJspectrometer (PAR 6001) (181. This is a u/4.21 dixperuive instrument equipped with an electronically modulated IUUU-W Xe arc lamp and a pyroelectric detector fnr R O U T C ~w m . pensation. Sample spectra were divided hy the 25 O C $pertrum of Norit-A carbon blark CvlCBl to yield rS, RI ploh vs energy (cm 'I; othemise plots of single heam intensity (SJare presented to illustrate temperature eflrrts. The upticii of the PAR fi0Ol d u m the tightly f u r w d output uf the monwhrumator downward to the sample position. and thus no additional transfer optics are necessary for UM of the HTPA reU. However. a wparate amplifier (PAR 60058 is applied to the microphone signal belure it is fed to the standard PAR 6UJ1 elertronics. When pertinent, rpertra recorded with the HT-PA rell were compared to thosr obtained with the PAR 6003 standard samplegas-microphone nonresonant pholoacou~ircell. Samples of Norit-A carhon hlack and Ho,O, (Alfa) were examined to evaluate the performanre of the cell. PA spectra ot' carhon hlark were collected as a function of sample temperature and modulation t'requency. The PA spectra of Ho,O, were recorded as a function of mdulation frequency from Mi tu 750 nm at a scan raw uf IO0 nm, min and slit width of 2 mm. Elevated temperature spectra of CoCI,6HI0 were recorded as four roadded scans from 275 to 760 nm at a scan rate of I00 nm min and slit width oi 4 nun. Spectra were measured only aiter the wmple hnd been held at ronstant temperature for at least 60 min and was at apparent equilibrium. The melting uf indium was observed by moniloring the signal at IO00 nm from carbon hlack dispersed in the metal sample, since indium itself is reflertive to near-IH radiation and thus gives a very weak PA response. 12). FTlR Spectral Measurements. All IR measurements were made with [he IBM FTIR-95 spectrumeter (19). Rrcause the converging beam from [he interferometer enters the sample chamher horimndy. transfer optics (Spertra-Tech)are used with the rell tu direct and more tightly focus the beam downward to a 4 mm diameter spot size on the sample. These optirs consist of two flat and one off-axis ellipsoidal mirrors whirh are mounted to an optical plaw s u p p o r d 5' in. shove [he haw of the sample rhamber. The output of the HT.PA cell is amplified with the PAR 6005 preamplifier hefore being feed intn the signal input of the FI'IR spectrometer. Spectra were normally acquired using 8 cm-' resolution and a mirror velority of u.059 r m s (optical velocity, 0.2Rfi cm, SI. yielding Fourier frequenrirs of 141-9.14 H a over the spectral range of 600-4000 cm '. For modulation frequency experiments, the mirror velocity was incrementally increa3ed from 0.059 to 0.236 cm, s (optical velucity, 0.9.l.lcm Y, yielding Fuurier frequencies of 566-2776 Hz over the same spectral range at the highest velocity. Standard measurements were made with 512 to 201d coadded interferograms tranxiormed using standard instrument software. When appropriate, E1'IIt HI'-P.4 swrrra were compared to spertra rerorded using the IHM Instruments nonresnnant PA cell (Model AGIZOdSU). PA sprrtrn of carbun hlark were roUened as a function ot'saniple temperature and mndulatiun frequenry (mirror velorityl. Spectra of Nn.CO1 expwed to SO, at elevated temperatures were nurmaliaed tu the 25 "C spertrum of Norit-A carhon black.
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Flgure 1. Cross-sectional view of the HT-PA (1) front, (A) window holder, (b) stainless steel rings, (C) window, (D) gasket, (E) ample holder, (F) stainless steel cell body, (G) copper plug, (H) gas channels. (I) Swagelok fiing. (J) water-cooling jacket. (K) coil heater with insulated wire adaptor, (L) Stainless steel frit. (M) insulation, (N) foam: (2) side, (A) microphone. (B)cylindrical microphone holder, (C) microphone encasing. (D) stainless steel bok. (E) Mylar insulation. (F) O-ring.
diameter. The sample ends of the gas channel tubing are fitted with 2-mn stainless steel frits (Alltech) and seat against this in. shoulder. As demonstrated by Royce et al. (14-16) although the frits allow the gas to flow through the sample chamber, they do not significantly reduce the acoustic integrity of the cell. The tubing of the gas channels is further secured in place with stainless steel Swagelock fittings threaded into the body of the cell. The microphone channel, or acoustic coupling tube (2.1 in. long), is made of thin-walled in. stainless steel tubing (1 mm id.). The thin-walled tubing limits the conduction of heat from the sample region to the microphone chamber. The microphone (Bruel and Kjaer 4165) is threaded into a stainless steel cylinder 3 / r in. a d . which in turn is sealed with two O-rings into the cylindrical cavity of the aluminum microphone chamber, with the microphone cylinder flush against the end of the cavity, a t the end of the acoustic coupling tube. The weight of the microphone chamber is borne hy two stainless steel holta which also connect it to the cell body. The side of the microphone chamber exposed to the cell body is faced with 'I4 in. thick Mylar insulation. The sample is heated by a 400-Whendable cartridge heater (0.062 in. ad.; 31 in, long; ARi Industries, Inc.) coiled into a 2 in, diameter spiral. This heater is held firmly against the bottom of the sample chamber (0.08 in. below the sample) hy a 1in. thick block of machinable mineral fiber insulation (Moldatherm 11, Lindherg). A '1, in. thick layer of this insulation is also placed around the cell body. Furthermore, a cylindrical ring of insulation ('I4 in. thick) is placed around the window holder and over the
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Flgure 2. Carbon black photoacoustic amplitude at 1000 nm versus
modulation frequency, f, using the HT-PA cell.
RESULTS AND DISCUSSION Determination of the Helmholtz Resonance Frequency. As in other PA cells designed for nonambient temperatures, the HT-PA cell described here is constructed in the form of a Helmholtz resonator (14). This design enables the microphone to be maintained at room temperature regardless of sample temperature, while still keeping the gas volume of the cell small (
