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Anal. Chem. 1982, 5 4 , 1236-1237
spectra are noise limited in wavelength regions where the output power of the xenon arc lamp and monochromator is low, i.e., between 200 and 250 nm, 750 and 800 nm, and above 2000 nm. In testing the performance of our DR attachment, we were interested in how much the reflection of light by the walls of the cell contributed to the observed signal. T o answer this question, we made two measurements with a magnesium oxide sample, first covering the entire inner cell surface (machined aluminum or white Teflon) with black paper and then leaving it exposed. The reflectivity of the black paper relative to magnesium oxide was determined to be less than 2%. Somewhat surprisingly, the difference between the two measurements was smaller than 3 % throughout the visible wavelength range, indicating that the DR signal is almost entirely produced by light reflected directly off the sample. This observation can partly be ascribed to the fact that the sensor element of the pyroelectric detector is recessed 2.5 mm from the detector’s front edge and thus has a limited “field of view”. In conclusion, this study has shown that the described DR attachment is a valuable addition to our PA spectrometer. Applied to opaque solids, diffuse reflectance and photoacoustic spectroscopy are competing techniques, but more importantly, they are also complementary in that somewhat different sample properties affect the observed analytical signals. While
both techniques are sensitive to the absorbing and light scattering properties of the materials studied, PAS is alsc affected by sample thermal properties and DR spectra can be degraded by specularly reflected light (7). We are non using the newly acquired capability of recording DR and PA spectra under nearly identical experimental conditions tc further characterize the differences between the two types of spectroscopy.
LITERATURE CITED Parker, J. G. Appl. Opt. 1973, 12, 2974. Rosencwaig, A. Opt. Commun. 1973, 7 , 305-308. Rosencwaig, A. Anal. Chem. 1975, 47, 592 A-599 A. Adams, M. J.; King, A. A.; Kirkbright, G. F. Analyst (London) 1978, 10 I , 73-85. (5) Gray, A. C.; Fishman, V. A.; Bard, A. J. Anal. Chem. 1977, 4 9 , 897-700. (8) Tiigner, R.; Luscher, E. Z . Phys. Chem. 1978, 1 1 1 , 19-29. (7) Freeman, J. J.; Friedman, R. M.; Reichard, H. S. J . Phys. Chem. 1980, 84, 315-319. (8) Henderson, G.; Bryant, M. F. Anal. Chem. 1980, 52, 1787-1790. (9) KortDm, G. “Reflexionsspektroskopie”; Springer-Verlag: Berlin, 1969. (10) LochmOiler, C. H.; Rohi, R.; Marshall, D. E. Anal. Lett. 1981, 14, 41-45. (11) Vidrine, D. W. Appl. Spectrosc. 1980, 3 4 , 314-319. (1) (2) (3) (4)
RECEIVED for review October 29, 1981. Resubmitted and accepted March 22, 1982. This research was supported by NSF Grant CHE 78-03001, North Carolina Board of Science and Technology Grant 1004, and EPA Grant CR 807407.
Simultaneous Thermoluminescence and Differential Scanning Calorlmetry Emanuel P. Manche” Department of Natural Sciences, York College of the City University of New York, Jamaica, New York 11451
Benjamin Carroll Department of Chemistry, Rutgers, The State Unlversity of New Jersey, Newark, New Jersey 07102
The simultaneous determination of thermoluminescence (TL) and differential scanning calorimetry (DSC) on the same sample has not been previously presented in the literature, although there appears to be one case where T L and thermal procedure (DTA) have been reported (1). However, in the latter case which involved lunar material, the two analyses were run concurrently on separate samples. Data based on the various thermal methods obtained for a given substance generally contain different yet complementary features which aid in the interpretation of the thermally stimulated processes. A problem arises when the substance is of a heterogeneous character, rather than a well-defined single compound from a nonvarying radiation environment. In such cases, the simultaneous use of two thermal methods on the same sample is desirable. In this paper an instrument is presented for simultaneous T L and DSC. Analytical data indicating the reliability of the instrument are given.
EXPERIMENTAL SECTION The Apparatus. A differential thermoluminescence (DTL) instrument (2) and the Perkin-Elmer DSC-1B instrument were both modified so as to form an integrated unit. A two-channel potentiometric recorder was used one channel to measure the thermoluminescent signals and the other channel, with a center scale zero, to measure enthalpy changes. Figure 1is the schematic of the TL-DSC apparatus. Briefly, the DSC part of the apparatus consisted of two matching calorimeter cups, 0003-2700/82/0354-1236$0 1.25/0
each containing a heater and a temperature sensor. Two containers of high thermal conductivity, one containing the sample material and the other a reference material, were placed within the “sample” and the “referencen cups. In Figure 1, these are designated as D1 and D2,respectively. The two cups were designed for stable and reproducible steady-stateheat flow conditions. Both temperature sensors were kept at nearly identical temperatures through an electronic sensing circuit that measured the instantaneous signal difference and apportioned the electric power to their heaters. This difference in power was amplified and read out as an endothermic or exothermic trace on a recorder. The total electrical power to the heaters was increased or decreased to keep the average temperature of the sample and reference cups in correspondence with the programmed temperature. It is this average temperature that formed the basis for the abscissa of the recorder (3). The furnace enclosure was modified to be lightproof and to accept a pair of matched fiber optics, the ends of which were mounted directly above each calorimeter cup. The assembly also had provisions for permitting experiments to be carried out under a controlled gas flow rate. For this work, an RCA No. 6217 photomultiplier tube, with an S-10 spectral response was used. Experimental Procedure. As a test of the analytical reliability of the TL-DSC instrument, mixtures of LiF (Harshaw Chemical Co., TLD-100powder) and KN09 (Fisher Scientific Co., reagent grade) were used as samples. Selection of LiF was suggested by the fact that it is a well-known and extensively studied thermoluminescent material, while KNOBis certified jointly by the International Confederation for Thermal Analysis and the U S . National Bureau of Standards as a dynamic temperature 0 1982 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 54, NO. 7, JUNE 1982
1237
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Figure 1. Block diagram of instrument for simultaneous TL-DSC analysis: (Al, A,) lock-In ampllflers tuned to frequencies REF, and REF,, respectively; (A3) differentlal ampllfler; (B) temperature programmer and differential calorimeter; (C,, C,) choppers rotating at two different frequencies; (Dl, 0,)calorimeter cups with "sample" and "reference" materiials; (HV) regulated hlgh voltage power supply; (L) lamps for reference slgnals; (F,, F,) matched fiber optics; (PI, P,) photocells for genieratlng reference signals REF, and REF,; (PM) photomultiplier tube; (Si, S,) light signals orlglnatlng from D, and D, respectively; (S,-I- S,) signal output from the photomultiplier; (SIS,) thermoluminescent signal; (Hi, H,) power input to calorimeter cups; (Ti, T,) temperature signals from D, and D, at time t ; (T,) average temperature of calorimeter cups at time t ; (HI - H,) differential heat input, dHldt; (REC) TwoGhannel recorder for the simultaneous dlspby of the glow curve and thermogram.
reference standard in thermal analysis work (4). Potassium nitrate exhibits a crystallographic transition (5) phase I1 (orthorhombic)
c 3
-
phase I (trigonal)
with an enthalpy change, AH, of 11.6 f 0.5 cal g-.l. This choice of KN03was further dictated by the fact that KN03 does not ordinarily exhibit thermoluminescence (6) which was experimentally confirmed by irradiating a sample with X-rays to a fluence 1order of magnitude greater than that used in this study. A series of samples with varying proportions of dry LiF and KNO, were prepared, ground together in an agate mortar, and then stored in glass vials. By use of an analytical balance, 0.0205 g of each of these mixtures was individually transferred into a specially built pellet die and sample disks approximately 5 mm X 0.8 mm were pressed out using a pressure of 2.2 X lo6 kg cm-2. Each sample was riubsequently mounted in a specially constructed Lucite sample holder, aligned on the exit port of an X-ray generator and irradiatd by a beam of X-rays generated from a Dunlee X-ray diffraction tube having a copper target. Exposure was carried out for 1.100 min at 36.0 kV and 16.0 mA, after which it was stored in a glass vial and kept in a dark place. After 11days, the samples were individually investigated. The sample pellet was placed in a gold pan which was then put in the "sample" cup, and a pellet made of LiF (TLD-100) that had been preheated to 500 "C to remove any thermoluminescence was placed in another gold pan and put in the "reference" cup. In Figure 1,these are represented as D1 and D2,respectively. The furnace was linearly programmed at a heating rate of 80 O C mi& from 20 "C to 270 "C. The furnace chamber was flushed with 02-free dry nitrogen at u flow rate of 20 mL mi&.
RESULTS AND DISCUSSION The T L and enthalpy signals were fed to the appropriate Y axis channel of the recorder. A representative trace of the simultaneously generated signals is shown in Figure 2 for a pellet with a composition of 40% (w/w) LiF in KNOB. [t should be noted that the samples were subjected to consid-
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200
TEMPERATURE,
300
400
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Flgure 2. Simultaneous TL-DSC trace of a 20 mg sample pellet consisting of 40% (wlw) LiF, Harshaw (TLD-loo), in KNO,. Sample was irradiated with X-rays for 1.00 min at 36 kV and 16 mA. The temperature was IinearUy programmed at 80 "C min-' and the sample was flushed with N, flowing at 20 mL min-'. The KNO, was also used
as an Internal temperature standard. erable handling. For this reason, at the conclusion of this work, all sample pellets were weighed. The data used were selected from those samples that maintained a mass of 0.0200 g i5%. The areas under the glow curves and endotherms were obtained with a polar planimeter. The measured areas represent the relative thermoluminescence intensity and enthalpy and were plotted vs. composition. The resulting straight lines were fitted to the data by the method of least squares. The coefficient of correlation for the enthalpy changes was calculated to be 0.996, while that for the glow curves was 0.985. The results show that the intensity of luminescence of LiF and the enthalpy of transition of KN03 in the mixtures are a linear function of the weight fraction of the individual salts.
ACKNOWLEDGMENT The authors thank Steven Roth and Klaus Ulbrich for their valuable assistance in this project. LITERATURE CITED (1) Hoyt, H. P., Jr.; Mlyajima, M. W.; Walker, R. M.; Zimmerman, D. W.; Zlmmerman, J.; Britton, D.; Kardos, J. L. "Proceedings of the Second Lunar Science Conference"; The M.I.T. Press: Boston, MA, 1971; Vol. 3, pp 2245-2263. (2) Manche, E. P. Rev. Sci. Instrum. 1978,4 9 , 715-717. (3) Manche, E. P.: Carroll, B. "Physical Methods in Macromolecular Chemistry"; Carroll, B., Ed.; Marcel Dekker: New York, 1972; Vol. 2, Chapter 4. (4) McAdie, H. G. I n "Thermal Analysls" (Proceedings of the Third Internatlonal Conferehce on Thermal Analysis, Davos, 1971); Wiedemann, H. G., Ed.; Birkhauser Verlag: Basel, 1972; Vol. 1, pp 591-608. (5) Rossinl, F.; Wagrnian, D. D.; Evans, W. H.; Levlne, S.; Jaffe, I. Selected Values of Chemlcal Thermodynamic Properties", Circular 500; National Bureau of Standards, U.S. Government Printing Office: Washington, DC, 1952. (6) Rleke, J. K. Phys. Chem. 1957,6 1 , 633-635.
RECEIVED for review January 4, 1982. Accepted March 25, 1982. Bell Telephone Laboratories, IBM Corporation, and Texas Instruments, Inc., generously provided material assistance. This work was supported, in part, by a Professional Staff Congress-City University of New York, Grant No. 12294, awarded to E.P.M.
