Anal. Chem. 1980, 52, 1595-1598
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These features make it difficult to discriminate the signal from the background and adversely affect the analyzing power of AAS. In contrast with the absorption signals, the CFS signals look quite simple. Molecules do not contribute to CFS signals because of the very low creation rate of magnetooptically active molecules in the furnace atomization method, so they do not generate spurious signals. In this respect, CFS is superior t o AAS. This property of CFS, together with the high spectral radiance spectrum source excitation, serves to identify the trace elements in unknown samples.
ACKNOWLEDGMENT The author is grateful to S. Murayama and M. Yamamoto for helpful discussions and encouragement and M. Yasuda for helpful discussions and the experimental support. T h e author also thanks K. Kuga for his technical suggestions. 0 1 ppm
V AAS
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LITERATURE CITED OZppm
(1) A. Corney, B. P. Kibble and G.W. Series, Proc. R. SOC.London, Ser. A , 293, 70 (1966). (2) T. Hadeishi, Lecture Notes at University of Aarhus, Denmark, Nov. 5, 1973. (Present address: Lawrence Berkeley Laboratory, University of California, Berkeley, Calif. 94720). (3) D. A. Church and T. Hadeishi, Appl. Phys. Left., 24, 185 (1974). (4) D. Marcuse, "Engineering Quantum Electrodynamics", Harcourt, Brace World, New York, 1970. (5) M. Ito, K. Kayama, S. Murayama, and M. Yamamoto, J . Specfrosc. SOC.Jpn., 25, 299 (1976) (in Japanese). (6) M. Ito, S. Murayarna, K. Kayama. and M. Yamamoto, Spectrochim. Acfa, Part B , 32, 347 (1977).
TIME
Figure 7. Recorder traces of forward scattering signal (upper) and absorption signal (lower)of the V 318.4-nm line vs. time with varying concentration
analyzer axis to make the polarization directions of the analyzer and the polarizer coincide. In the absorption measurements, signals are observed on the background of the molecular absorption. The base line also fluctuates because of the thermal lens effect, which is caused by the thermal refractive index change along the optical path in the furnace.
RECEIVED for review January 3, 1980. Accepted May 30,1980.
Fourier Transform Photoacoustic Visible Spectroscopy of Solids and Liquids Lindsay B. Lloyd, Roger K. Burnham, Wayne L. Chandler, and Edward M. Eyring" Department of Chemistry, University of Utah, Salt Lake City, Utah 84 112
Michael M. Farrow Office Products Division, IBM Corporation, Boulder, Colorado 80302
Fourier transform photoacoustic spectroscopy (FTPAS) can be carried out in the visible using either a piezoelectric crystal or a microphone to detect the interferogram produced in a sample when the latter Is illuminated by whlte llght passed through a Micheison interferometer. Spectra of lanthanide oxldes and whole human blood are reported that illustrate benefits and limitations of visible FTPAS.
T h e earliest published experiments in photoacoustic spectroscopy (PAS), were those of Alexander Graham Bell (I, 2),and the recent popularization of the technique for studying solid and liquid samples owes a great deal to the efforts of A. Rosencwaig (3-6). In almost all condensed phase PAS experiments described to date, dispersive optics have been used to illuminate a sample through a window in an enclosed cell (7). The incident monochromatic light beam is chopped at a low audiofrequency. Also present in the typical PAS cell is a column of unreactive gas that carries a sound wave a t the chopping frequency from 0003-2700/80/0352-1595$01 .OO/O
the light absorbing sample to a sensitive microphone. Signal to noise in the output voltage from the microphone is enhanced by a lock-in amplifier. If most of the energy absorbed by the sample is dissipated in nonradiative decay, rather than fluorescence, and saturation effects (8) are avoided, the resulting photoacoustic spectrum closely resembles that obtained by transmission spectroscopy in those cases where the sample material is sufficiently transparent to be susceptible to spectral measurements by both techniques. Recent variants of the standard PAS experiment include the use of pulsed laser light sources (9), utilization of piezoelectric transducers (PZT) (10, 11) in lieu of microphonic detectors, and exploitation of ultrasonic microscope optics t o visualize microscopic samples by PAS (12). Use of an intense, monochromatic and wavelength tunable light source has been a fundamental feature of all these experiments because of the detector noise limitations of PAS. Dispersive optics are, in fact, deleterious to the production of a photoacoustic signal as they reduce drastically the amount of light incident a t any instant of time upon the sample and hence the signal to noise ratio. Thus, depending on surface roughness and other 1980 American Chemical Society
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ANALYTICAL CHEMISTRY, VOL. 52, NO. 11, SEPTEMBER 1980 LIGHT
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Figure 1. Schematic of a He-Ne laser referenced Fourier transform photoacoustic spectrometer
properties of the sample, times of up t o an hour may be required t o obtain dispersively broad band visible photoacoustic spectra in this detector noise limited experiment. Fourier transform-Michelson interferometer techniques provide a method for decreasing the amount of time necessary t o obtain a broad band spectrum [Jacquinot’s and Fellgett’s advantages ( 1 3 ) ] . T h e feasibility of obtaining a Fourier transform photoacoustic spectrum (FTPAS) has been demonstrated recently (13-15). Here we report FTPA spectra of several solids as well as of whole blood and analyze the advantages and disadvantages of FT and dispersive PAS a t visible wavelengths.
EXPERIMENTAL The instrument consists (see Figure 1) of a Michelson interferometer of local construction with visible wavelength optics through which the collimated radiation from a 500-W xenon arc lamp is passed. Infrared wavelengths from the lamp are removed with glass heat filters to reduce instrumental drift caused by heating and expansion of the interferometer components. One of the interferometer mirrors, mounted on a Dover Instrument 400-B air bearing, is moved by a computer interfaced Burleigh 555 Inchworm Translator. The other mirror, mounted in a Burleigh PZ-90 piezoelectric aligner-translator, is dithered by applying to the PZ-90 an ac voltage of selectable frequency (typically about 925 Hz) which produces a phase modulation of the interferometer output. This phase modulation is observable visually as a flickering of the interference fringes at low modulation frequencies and is analogous to chopping the light in dispersive PAS except that no light intensity is lost on chopping wheel blades. The phase modulated output of the interferometer is allowed to fall on a sample, either contained in a conventional microphone cell or glued directly to a PZT, while the mirror on the translation stage is moved. The difference in path length of the light between the two arms of the interferometer is referred to as the “retardation” of the moving mirror. The signal from the piezoelectric transducer or microphone is preamplified and introduced into a PARC 124A lock-in amplifier. The phase modulation frequency is set by the internal reference system of the lock-in amplifier, and 1024 equally spaced (in retardation) samples of lock-in amplifier output vs.
Figure 2. Double sided interferogram of powdered Ho,O, frorri which t h e Ho,O, spectrum (with Piezoelectric detection) shown in Figure 3 was calculated. The end-to-end len of the interfarograrn corresponds to a mirror motion of 8.10 X 10- cm
P
mirror position are fed into the 12-bit Data Translation 1751 analog-to-digital converter (ADC) interfaced to an LSI. 11 microcomputer (Terak Corp. 8510) as the mirror is swept from negative to positive values of retardation. The resulting double-sided interferogram is shown in Figure 2. Nearly equal spacing of data points with regard to retardation is achieved with a He-Ne laser interferometer linked to the interferometer described above by the common translation stage. The data sampling system imposes a lower wavelength limit on the spectra obtained to X > 316.4 nm. The dielectric coatings of the beam splitter further limit the bandwidth to the range X = 400 to 800 nm. The spectral resolution is limited by the travel distance of the mirror. The total mirror motion for 1024 data points is 0.0080998 cm giving an approximate spectral resolution of 123 cm-’ or 2 nm at X = 400 nm and 8 nm at 800 nm. The 1024 data point limit is imposed by the processing capabilities that in turn are set by the 28K microcomputer memory size. Sample preparation, for the case of piezoelectric detection, requires immersing the sample in a transparent epoxy or cyanacrylic matrix that attaches the sample to a front surface mirror. An Edo Western Corp. (Salt Lake City, Cltah) barium titanate or lead titanate zirconate disk PZT is then glued to the back of the mirror.
ANALYTICAL CHEMISTRY, VOL. 52, NO. 11, SEPTEMBER 1980
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Figure 3. Fourier transform photoacoustic spectrum of powdered Ho203 obtained using piezoelectric detection. The spectral resokuion is 2 nm at h = 400 nm and 8 nm at h = 800 nm. The FTPA spectrum (-) is compared with a dispersive PA spectrum (---) published by R. E. Blank and T. Wakefield 11, Anal. Chem., 51, 50 (1979). The FTPA spectrum is source normalized with a carbon black FTPA spectrum
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Figure 5. FTPA spectrum of liquid whole blood (-)
using microphonic detection compared with a hemoglobin spectrum (---) obtained with a conventional Beckman UV-visible spectrometer
of this interferogram. Source normalization, averaging with previous spectra (not necessary in the case of Figure 3), and video display of the resulting spectrum required a maximum t of 15 additional s. This -4-min total acquisition time is I faster than typical dispersive PA spectral collection times for I the same bandwith, signal t o noise, resolution, and sample composition a t visible wavelengths. There are analytical situations (for example, the determination of the concentrations of the various forms of hemoglobin present in whole blood under clinical conditions or the recognition of transient species present on the surface of a heterogeneous catalyst) in which the speedy acquisition of a broad band PA spectrum by Fr techniques is potentially advantageous. The cycle time of the minicomputer determines how long it takes to perform the FT of the interferogram. Faster minicomputers are I available and capable of further reducing the total time re-> . quired from - 4 min down t o barely more than 90 s. This - - _.-l-- --i-_____400 SO0 600 700 800 latter figure is dictated by the frequency with which the I Y A V t L E N G T H (""11 present interferometer mirror can be dithered. A further Figure 4. Source norrnalized FTPA spectrum of Nd303() compared important advantage of FT over dispersive spectroscopic wrth a dispersive PA spectrum of Nd203(-- -) from the same reference techniques is a more accurate wavelength calibration of the as cited in the caption to Figure 3 I n the region from -400 to 450 sample spectrum. nm aecreasing efficiency of the dielectric coated beam splftter causes Disadvantages of FT compared to dispersive PAS are the a io+fer signal to noise ratio in the FTPA spectra of Figures 3 and 4 greater expense of the instrumentation and the tedious interferometer alignment procedures. Since minicomputers are The magnitude of the photoacoustic signal in the microphone used in many dispersive spectroscopies t o acquire and reduce cell depends strongly upon the size of the sample-transparent gas interface. The interiace is large for powdered samples but much data, the need for a minicomputer in the FTPA experiment smaller for liquid samples such as whole blood. Hence it was is not a clear disadvantage. difficult to obtain a reasonable photoacoustic signal for liquid The ultimate worth of PAS in analytical chemistry has been blood without increasing the size of the sample-gas interface by questioned (18) a typical argument being that the technique placing small transparent particles of crushed glass coated with cannot provide unique information absolutely unattainable the liquid blood within the sealed microphone cell. The blood by other methods. In PAS, the S / N ratio improves with adhered well enough to the crushed glass that a 10-fold increase increasing sample surface roughness whereas reflectance in signal was obtained. The detector was an Archer 270-092A spectroscopies are more effective on smoother surfaces. Electret condenser microphone. Dispersive and FTPAS share equally in this advantage. PAS RESULTS AND DISCUSSION also permits the nondestructive measurement of the absorpBroadband wsible spectra of Hoz03,Ndz03,and hemoglobin tion spectrum of subsurface layers of an opaque sample by in whole blood (with citrate anticoagulant added) are shown varying chopping frequencies and collecting the signal at in Figures 3, 4,and 5. These spectra illustrate advantages different lock-in amplifier reference phases (19). In such and disadvantages of the F T photoacoustic method. experiments, the data deconvolution problem is distinctly As is generally true for FT spectroscopic techniques consimpler for dispersive as opposed t o FTPAS. trasted with dispersive spectroscopy, the FT measurement A more damning present criticism of PAS is t h a t it is not has the larger radiation throughput (Jacquinot's advantage) yet a reliable quantitative analytical tool. To obtain the true as well as Fellgett's multiplex advantage. The latter allows optical spectrum of a solid state sample, one must make the more rapid collection of spectra than in the dispersive meathermal diffusion length less than the mean optical absorption surement at the same signal to noise ratio (16). T h e interpath by increasing the PA chopping speed. Megahertz ferogram, Figure 2, leading to the FTPA spectrum, Figure 3 chopping frequencies appear desirable (20) and clearly preof holiniuni oxidk powder was collected in 90 s. T h e signal clude the use of microphonic detectors though certainly not t o noise ratio (1 7 ) of this spectrum is 30:l. An additional piezoelectric detection, particularly when the latter is per2 5 min was required to compute the Fourier transform (FT) formed a t resonant frequencis of the PZT (11). Acoustooptic
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Anal. Chem. 1980, 5 2 , 1598-1601
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techniques (22) for chopping a laser beam a t megahertz frequencies and high frequency lock-in amplification both exist, suggesting the practicability of a more quantitative dispersive PAS of solids. T h e corresponding high frequency FTPAS experiment probably can be performed using the train of nanosecond duration white light pulses from a synchrotron as t h e intense high frequency light source. Such an exotic FTPAS experiment offers little prospect of an advantage over megahertz dispersive PAS a t visible wavelengths unless speed of acquisition of a broad band spectrum assumes paramount importance.
C. K. N. Patei, A . C.Tam, and R. J. Kerl, Appl. Phys. Lett., 34, 467 (1979). A. Hordvik and H. Schlossberg, Appl. O p t . , 11, 101 (1977). M. M. Farrow, R. K. Burnham, M. Auzanneau, S . L. Olsen, N. Purdie, and E. M. Eyring, Appl. O p t . , 17, 1093 (1978). H. K. Wickramasinghe, R. C. Bray, V. Jipson, C. F. Quater, and J. R. Salcedo, Appl. Phys. Lett., 33, 923 (1978). M. M. Farrow, R. K. Burnham, and E. M. Eyring, Appl. Phys. Lett., 33, 735 (1978). G. Busse and B. Builemer, Infrared Phys., 18, 255 (1978). M. G. Rockley, Chem. Phys. Left., 88, 455 (1979). P. R. Griffiths, "Chemical Infrared Fourier Transform Spectroscopy", John Wiley and Sons, New York, 1975, p 40. C. Foskett and T. Hirschfeld, Appl. Spectrosc., 31, 239 (1977). Representative examples are: W. N. Delgass, G. L. Hailer, R. Keilerrnan, and J. H. Lunsford, "Spectroscopy in Heterogeneous Catalysis", Academic Press, New York, 1979, Chapter 4. R. Tilgner and E. Lushcer, Z . Phys. Chem. N.F., 111, 19 (1978). M. J. Adams. B. C. Beadle, A. A. King, and G. F. Kirkbright, Ana/yst (London), 101, 553 (1976). J. J. Freeman, R. M. Friedman, and H. C. Reichard, J . Phys. Chem., 84, 315 (1980). J. Sapriel, "Acousto-Optics", John Wiley and Sons, New York, 1979, Chapter 7.
ACKNOWLEDGMENT We have had numerous interesting discussions with Joel M. Harris, University of Utah, and also thank William A. Koldewyn, IBM-Boulder, for suggestions regarding the He-Ne laser interferometric referencing system.
LITERATURE CITED (1) (2) (3) (4) (5) (6) (7) (8)
RECEIVED for review December 26, 1979. Accepted June 20, 1980. T h e blood studies performed by LBL and WLC were made possible by Grant HL 23378-01 from the National Heart,
A. G. Bell, A m . J. Sci., 20, 305 (1880). A. G. Bell, Phil. Mag. 11, 510 (1881). A Rosencwaig, Opt. Commun., 7, 305 (1973). A. Rosencwaig, Anal. C h e m . , 47, 592 (1975). A. Rosencwaig and A. Gersho, J . Appl. Phys., 47, 64 (1976). A. Rosencwaig, Adv. Electron. Electron Phys., 48, 207 (1978) D. Cahen and H. Gam, Anal. Chem., 51, 1865 (1979). J. W.-P. Lin and L. P. Dudek, Anal. C h e m . , 51, 1627 (1979).
Lung, and Blood Institute. T h e applications of F T P A S to the study of solid samples were financed by a contract from the Department of Energy (Office of Basic Energy Sciences).
Dissociation Constants of Acetic Acid and Primary Phosphate Ion and Standards for pH in 10, 20, and 40 wt % EthanoVWater Solvents at 25, 0, -5, and -10 "C Roger G. Bates," H.
P. Bennetto,'
and Munessar Sankar2
Department of Chemistry, University of Florida. Gainesville, Florida 326 1 I
Earlier work leading to the standard potential of the Ag/AgCI electrode in 10, 20, and 40 wt YO ethanoVwater solvents at 25, 0, -5, and -10 O C has been extended to the study of an acetate buffer solution and a phosphate buffer solution in the same media at these four temperatures. Cells without liquid junction, Pt;H2(g, 1 atm)IHA,A,NaClIAgCl;Ag were used to determine the dissociation constants of acetic acid and H P 0 4 - In these ethanol/water media. Buffer solutions composed of (a) acetic acid (0.05 m ) , sodium acetate (0.05 m ) and (b) KH2P04(0.025 m ) , Na2HP04(0.025 m ) , where m Is molality, are useful as standards for pH measurements.
I n a recent contribution ( I ) , we have reported a determination of the standard potential of the silver-silver chloride electrode in 10, 20, and 40 wt % ethanol/water solvents a t four temperatures: 25,0, -5, and -10 "C. The results lay the groundwork for thermodynamic studies, by emf methods, of both weak and strong electrolytes in these solvent media a t temperatures below 0 "C. We have now investigated the On leave from Queen Elizabeth College, University of London, 1979. On leave from the University of Durban-Westville, South Africa, 1977-78. 0003-2700/80/0352-1598$01 OO/O
dissociation of two weak acids of different charge types, namely, acetic acid and H,PO,-, and have determined conventional p H values for an acetate buffer solution and a phosphate buffer solution in ethanol/water media at subzero temperatures. These solutions are useful as reference standards for p H measurements and control in such processes as the separation of the protein fractions of blood plasma by the Cohn method (2).
EXPERIMENTAL Glacial acetic acid was purified by repeated fractional freezing under nitrogen. The melting point of the product rose in five stages of purification from 14.5 t o 16.602 "C; this final value is to be compared with the literature value of 16.606 "C. Sodium acetate and sodium chloride were crystallized twice from doubly distilled water. The former was dried at 120 "C overnight and finally for 3 h under vacuum at 95 "C. The NaCl was dried at 130-150 "C in air. The KHzP04 and Na2HP04 were NBS Standard Reference Materials 186Ic and 186IIc; they were dried for 2 h under vacuum at 100 "C. Stock solutions containing equal molal ratios, HAc/NaAc and KH2PO4/Na2HPO4,in the appropriate mixture of water and absolute ethanol were prepared. The cell solutions were prepared by weight from these stock solutions, the ethanol/water solvents, and NaCl. Limited solubility in 40 wt % ethanol precluded the measurements of phosphate-NaCl solutions in this solvent at 0 "C and below. Likewise, some problems were encountered with freezing of the acetate buffers in 10 w t 70ethanol at -10 "C. One of the phosphate buffer solutions in 10% ethanol also froze after some time at -10 "C, but no difficulty was experienced in obtaining c 1980 American Chemical Society
