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Anal. Chem. 1982, 5 4 , 1234-1236
Diffuse Reflectance Attachment for a Photoacoustic Spectrometer Raimund Rohl,‘ Jeffrey William Childers, and Rlchard Alan Palmer * Paul M. Gross Chemical Laboratory, Duke University, Durham, North Carolina 27706
Since the rediscovery of the photoacoustic effect in solids about a decade ago (1, 2), the technique of photoacoustic spectroscopy (PAS) has undergone rapid development and has attracted the attention of researchers in a wide variety of fields (e.g., chemistry, biology, geology, and materials science). One feature of particular interest is the applicability of PAS to solids which are both light scattering and absorbing. The molecular spectroscopy technique previously most generally applicable to the analysis of such samples is diffuse reflectance spectroscopy (DRS). During the development of methods and instrumentation for PAS, diffuse reflectance (DR) spectra have often been used in comparison to demonstrate the superiority of PAS in obtaining “real spectra” (3-5). Recently several more serious methodological comparisons between the two techniques have been reported (6-8). However, even these comparisons have been somewhat indirect, since the two types of spectra were obtained with different instruments, i.e., different light sources, monochromators, and signal processing systems. For example, the use of a brighter light source and lock-in detection in recording PA spectra can lead to an overly optimistic assessment of the capabilities of PAS compared to DRS. In order to record DR and PA spectra under nearly identical conditions, we have designed and constructed a diffuse reflectance attachment for our commercial PA spectrometer (EG & G Princeton Applied Research Model 6001) which replaces the upper portion of the PAS sample holder. Thus, the sample is not disturbed by changing from one detection mode to another and is in the same geometry with respect to the light beam. Diffusely reflected light is monitored in a nominal ,&, geometry (9) with a pyroelectric detector. (Note that, owing to the close proximity of the detector to the sample, the 4 5 O angle applies strictly only to the angle between the axis of the incident light beam and a line drawn from the center of the sample to the center of the pyroelectric detector.) This type of detector is ideally suited for our purpose for several reasons. Similar to a PA cell, a pyroelectric device yields power spectra, is sensitive over a wide wavelength range, and only responds to modulated light. Because the detector output is directly compatible with the PAR 6001 microprocessor, the DR signal can be enhanced by the same lock-in amplification and multiple scan averaging as the PA signal. In addition, the small size of the detector allows it to be positioned very close to the sample, which obviates the need for additional optics to concentrate a portion of the diffusely reflected light onto the sensor element.
EXPERIMENTAL SECTION Instrumentation. The PAR 6001 is basically a microprocessor-controlled, single-beam UV-VIS-NIR grating spectrometer using standard sample-gas-microphone PA detection with a nonresonant cell. The standard instrument is equipped with an electronically modulated 1000-W xenon arc lamp and high throughput optics. Correction for temporal variations in lamp intensity is achieved by diverting a constant fraction (less than 10%) of the totalbeam intensity onto a pyroelectric detector with a quartz beam splitter, and dividing the PA signal by the output from that detector. This source compensation mode is used to record both the sample and reference spectra. The sample spectrum is then divided by the stored reference spectrum in order Present address: 14 Tennenloher Str., 8521 Uttenreuth, West Germany. 0003-2700/82/0354-1234$01.25/0
to correct for spectral variation of power incident on the sample. The photoacoustic cell of the PAR 6001 consists of a main body, housing the microphone and preamplifier, and a removable, two-piece sample holder. When screwed together, the two halves of the sample holder enclose the stainless steel sample boat and seal against each other with an O-ring (Figure 1). The upper portion contains a quartz window, while the lower portion has two narrow channels (not shown in Figure 1)for acoustic contact to the microphone. A positioning pin ensures that the sample boat is always mounted in the same orientation. In order t o convert the PAR 6001 into a DR spectrometer, we replaced the upper half of the sample holder by the DRS attachment described below. Because the bottom half of the PAR sample holder is used as a base, no optical realignment is necessary. DRS Attachment Design. The diffuse reflectance attachment consists of three main parts: (1)the cell body; (2) a top plate covering the cell body; and (3) the pyroelectric detector mounted in a Teflon holder. Figure 1 presents a cross section of the attachment along the cell body’s axis. The cylindrical cell body and top plate are machined from aluminum stock. The mount for the pyroelectric detector is attached to the top plate. This mount is cut from a cylindrical piece of white Teflon, which fits the inner diameter of the cell body; the side facing the inside of the cell is flat. A hole (8 mm diameter) drilled into the Teflon piece at a 45“ angle holds the pyroelectric detector. The detector is a Laser Precision Corp. Model kT-2230 which features a 3 X 3 mm sensor element and an integral J-FET voltage mode preamplifier. The voltage responsivity as specified by the manufacturer is 62 V/W at 20 Hz. When the DRS cell is fully assembled, the bottom surface of the cell body rests firmly against a circular ridge on the lower portion of the PAR sample holder, thus ensuring that the detector is always positioned at a right angle to the long side of the rectangular ( 5 X 10 mm) sample area. In this fixed position, the sensor element is at a minimum distance of 12 mm from the sample surface. Considering the size of the incident beam, the distance from the sample to the pyroelectric element, and the size and geometry of the element, the detection angle (nominally 45O) actually varies from 35 to 57’. With our present design, a 2 mm thick spacer is inserted below the sample boat for DRS work, in order to bring the sample as close to the detector as possible. It may be mentioned that the DR attachment can also be employed for the analysis of air-sensitivesamples if the top plate is sealed with a quartz window. The cell can then be flushed with an inert gas, using the gas exchange system of the PAR cell body. In addition, measurements at moderately low temperatures (to -20 “C) can be performed by exchanging the standard bottom part of the PAR sample holder with an alternate piece which contains a Peltier-type element for cooling the sample. Procedures and Materials. The output of the pyroelectric detector is directly compatible with the PAR 6001 microprocessor as long as sensitivity settings of 10 or lower are used. Higher settings result in overloading of the sample signal channels. All reported spectra were recorded at a modulation frequency of 20 Hz, since the output signals of both the pyroelectric detector and the PA cell generally decrease with increasing modulation rate. PA spectra were normalized against a carbon black reference prepared from Norit A decolorizing carbon, which was sieved to isolate particles smaller than 50 pm (IO). The DR reference sample consisted of magnesium oxide, which was smoked onto a sheet of aluminum and transferred to the sample boat. At 20 Hz with the standard reference sample in place and using 2 mm slits the output of the pyroelectric detector was 0.31 V/W (Le., 1.24 mV/4 mW incident intensity) at 530 nm. For routine purposes, a piece of solid white Teflon was also found to be a satisfactory DRS reference, Holmium oxide powder of better than 99% purity was obtained from Alfa Ventron and consisted of particles smaller than 50 pm. In the measurement of the preliminary data reported 0 1982 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 54, NO. 7, JUNE 1982
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Flgure 1. Diffuse reflectance attachment for PAR 6001 photoacoustic spectrometer: (B) bottom of PAS cell; (C) DRS cell body; (D) detector holder (Teflon); (P) pyroelectric detector; (S)sample; (T) top plate; (Bn) BNC connector; (Or) O-ring; (Pp) positioning pin; (Sb) sample boat; (Sp) spacer. 300
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Figure 3. Comparison of PAS and DRS of Ho,03 powder recorded under identical optical end electronic conditions using the normal configuration and DR attachment for the PAR 6001, respectively, 200-700 nm.
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Figure 2. Comparison of PAS and DRS of Ho203powder recorded under identical optical and electronic conditions using the normal configuration and DR attachment for the PAR 6001, respectively, 200-2600 nm.
here no effort was made ta reject specularly reflected light by use of polarizing fdten. However, the specular component is relatively minor for powdersd samples.
RESULTS AND DISCUSSION One of the first experiments with our PA instrument converted to the DRS modo was to record a diffuse reflectance spectrum of holmium oxide powder, which has become the classical test material for PAS of solids. Similar to other lanthanide oxides, Ho& gives rise to a multitude of strong and sharp absorption bands in the UV-VIS-NIR wavelength region. A comparison of the DR spectrum to a PA spectrum obtained under nearly identical conditions (Figures 2 and 3) yields three important results. First, the spectral resolution achieved by both analytical techniques is very similar. In this context, particular attention may be called to the shape of the absorption bands near 450 nm, which are shown in more detail in Figure 3. Secondly, the signal-to-noise ( S I N ) ratio of the DR spectrum is superior to that of the PA spectrum, especially in the near-infrared. Finally, the relative intensity of absorption lines in the DR spectrum is greater than that of
Flgure 4. Root-mean-square noise spectra: (a) PAS of carbon black sample divided by carbon black reference. (b) DRS of magnesium oxide sample divided by magnesium oxide reference. Source compensation used in both PAS and DRS modes.
corresponding lines in the PA spectrum (signal from carbon black = 1.0). Spectra obtained with other lanthanide oxides, namely, Ndz03,Tm203,and Er203,yielded completely analogous results and are therefore not shown here. The comparison of the spectra in Figure 2 also illustrates an artifact in the PA spectrum just below 800 nm which does not occur in the DR spectrum. Briefly stated, this feature is due to a steep drop in lamp intensity in that wavelength region, which is followed by the source compensation detector, but not by the PA cell. Because the sample is weakly absorbing, the output signal of the PA cell is very small; i.e., it is close to the relatively constant noise level. The same artifact is responsible for th.e slow base line increase in the PA spectrum from 1500 t o 2400 nm. This type of artifact does not affect the DR spectrum because the DR signal level is generally high when the PA signal is low. The S I N ratio obtainable with identical illumination conditions for both spectroscopic techniques was investigated in more detail by recording root-mean-squarenoise “spectra” (11) with magnesium oxide and carbon black, respectively. The results, shown in Figure 4, confirm the earlier observation that DR spectra are less noisy than PA spectra. Both types of
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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
