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ANALYTICAL CHEMISTRY, VOL. 51, NO. 9, AUGUST 1979
Table IV. TCDD Determination in Control Trout Homogenate at Higher Mass Resolution
GC is an alternate method for both high sensitivity and selectivity.
TCDD,a ppt
ACKNOWLEDGMENT The authors acknowledge the support and advice of Warren Crummett, Nels Mahle, Curt Pfeiffer, and William Parker in the development and accomplishment of this work.
sample
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__
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examined in this manner, lower limits of detection were obtained due to the mass separation of the hydrocarbon background (with its much larger positive mass defect) in the presence of TCDD. Table IV shows the results obtained. As can be seen, 10-ppt concentrations (0.1-ng amounts) of TCDD are carried through this procedure with very good efficiency. As has been described by Buser (161,the use of GC-MS can provide a high degree of specificity and sensitivity in the determination of TCDD in environmental samples. For matrices with a high lipid content or those containing large excess amounts of other chlorinated aromatic compounds, considerable cleanup is generally required to achieve low detection limits. Although capillary column GLC can provide good resolution, this LC cleanup followed by packed column
LITERATURE CITED (1) Environmental Health Perspectives, No. 5, Sept. 1973; National Institute of Environmental Health Sciences,Research Triangle Park, N.C., George Lucier, Ed. (2) Ref. 1, W. 6.Crummett and R. H. Stehl, p 15. (3) R. A. Hummel, J . Agric. Food Chem., 25, 1049 (1977). (4) C. D. Pfeiffer, J . Chromatogr. Sci., 14, 386 (1976). (5) C. D. Pfeiffer. T. J. Nestrick, and C. W. Kocher, Anal. Chem., 50, 800 (1978). (6) L. A . Shadoff and R. A . Hummel, Biomed. Mass Specfro. 5 , 7 (1978). (7) N. H. Mahle, H. S. Higgins, and M. E. Getzandaner, Bull. Environ. Contam. roxico/.. 18,123 (1977). (8) R. H. Stehl and L. L. Lamparski, Science. 197, 1008 (1977). (9) L. L. Lamparski, N. H. Mahle, and L. A. Shadoff, J . Agric. Fow'Chem., 26, 1113 (1978). (10) P. W. O'Keefe, M. S. Meselson, and R. W. Baughman, J , Assoc. Off. Anal. Chem., 61,621 (1978). (11) K . Fukuhara, M. Takada, M. Uchiyama, and H. Tanabe, EiseiKagaku, 21, 318 (1975). (12) J. R. Hass, M. D. Frieson, and M. K. Hoffman, unpublished results. (13) L. A. Shadoff, W. W. Bhser, C. W. Kocher, and H. G. Fravel, Anal. Chem., 50, 1586 (1978). (14) A. S. Kende and J. J. Wade, Environ. Health Persp.. 5. 49 (1973). (15) T. H. Harris and J. G. Cummings, Res. Rev., 6, 104 (1964). (16) H. R. Buser, Anal. Chem., 48, 1553 (1976).
RECEIVED for review February 7 , 1979. Accepted May 3, 1979.
Surface Acoustic Wave Probe for Chemical Analysis. I. Introduction and Instrument Description Henry Wohltjen' and Raymond Dessy" Chemistry Department, Virginia Polytechnic Institute and State University, Blacksburg, Virginia 2406 7
Surface acoustic waves are easily propagated along a quartz or lithium niobate surface. Interdigitized finger transducers serve as transmitters of 30-60 MHz SAW waves toward an equivalent detector. Surface molecules predictably affect the wave propagation. Circuitry has been built to measure such interactions by changes in amplitude or phase-angle shift of the SAW, or the alteration of the resonance frequency when the device is part of a "tank" circuit. Amplitude response is proportional to the pressure of gaseous molecules in the environment. For a given gas, the response factor is proportional to (molecular weight)"*. Large linear frequency shifts were noted as ambient pressures changed. Amplitude measurements on quartz have the best S/N ratio (566). Limited data suggest that LiNbO:, devices have considerably better S/N ratios.
Modern instrumentation systems require a transducer to convert the physical property of interest into a form which Present address: IBM Watson Research Laboratory, Yorktown Heights, N.Y. 0003-2700/79/0351-1458$01 OO/O
is ultimately sensible to the scientist. The major advances in instrumentation art have been invariably preceded by major advances in the transducer art. This article describes the research and development of a new type of transducer: the Surface Acoustic Wave (SAW) Device. The principle of operation of this transducer is conceptually quite simple. An acoustic wave confined to the surface of some substrate material is generated and allowed to propagate. If matter is present on the same surface then the wave and the matter will interact in such a way as to alter the properties of the wave (e.g., amplitude, phase, harmonic content, etc). The measurement of changes in the surface wave characteristics is a sensitive indicator of the properties of the material present on the surface of the device. The convenient generation of a surface acoustic wave requires a substrate material which is piezoelectric. Applying a time varying electric field to the piezoelectric material will cause a synchronous mechanical deformation of the substrate with a coincident generation of an acoustic wave in the material. Proper selection of a single crystal orientation for the substrate will result in the acoustic wave propagation being constrained to the surface. While the generation of a number of different types of surface acoustic waves are possible ( I ) , this research utilized only Rayleigh wave propagation. The b 1979 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 51,
NO. 9,
AUGUST 1979
-T
( d S P A C E - F I X E D PARTICLE LOCATION FOR A TYPICAL ELASTIC SOLID T H E M A T E R I A L IS UNSTRESSED.
1459
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( b . ) T H E PARTICLE DISPLACEMENT OF AN E L A S T I C SOLID DUE TO A RAYLEIGH. OR A C O U S T I C SURFACE WAVE, I S COMPLEX AND FOUND ONLY NEAR SURFACE OF THE SOLID.
i NTERDlGlTAL ELECTRODES (500-2000 THICK ALUMINUM
CR GOLD
Figure 1, Rayleigh wave particle displacement
ELECTRODE SPACING V, (SURFACE WAVE VELOCITY = fo (SYNCHRONOUS FREQUENCY NUMBER OF ELECTRODE PAIRS N = I / PERCENTAGE BANDWIDTH
of
Figure 4. S A W interdigital transducer design PIEZOLECTRIC SUBSTRAT E THE ELECTROMAGNETIC SIGNAL INTERACTS WITH THE PIEZOLETRIC SUBSTRATE. THIS INTERACTION WILL CAUSE THE SUBSTRATE TO BECOME DISTORTED THAT I S , ALTERNATELY STRAINED AND THEN R ~ L A X E D
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Flgure 2. Interdigital transducer operation bPPLIED ELECTRICAL
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a two and one half finger pair interdigital transducer. The same type of transducer that is used to generate the surface wave can also be used to detect the surface wave. Thus, the construction of a simple surface acoustic wave delay line (as shown in Figure 5 ) provides a means for the generation of a surface wave, a means for interaction with matter on the surface of the delay line, and a means for subsequent monitoring of changes in the wave resulting from the wave/matter interaction. The purpose of the research described was to investigate the use of surface acoustic wave devices as physical probes for chemical analysis. Initially it was not clear which electronic detection mode (Le., amplitude, phase, frequency) would provide the optimum signal-to-noise ratio for a particular type of wave/matter interaction. A separate electronic system was designed and constructed t o monitor each of the three detection modes. Amplitude changes in the propagating surface wave were anticipated due to coupling of energy from the surface into the adjacent layer of matter (2). This phenomenon is known as mass loading. The wave/matter interaction was also expected to produce phase shifts in the wave
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Figure 3. Cutaway view of Rayleigh wave operation
particle displacement of the substrate supporting a Rayleigh surface wave is portrayed in Figure 1. An interdigital transducer (IDT) is used to apply the electric field to the piezoelectric substrate. This transducer consists of a thin metal foil which is deposited on the surface by a vapor deposition process and is subsequently etched to the desired geometry using photolithography. A source of radiofrequency (rf') excitation is then connected to the interdigital transducer and a surface wave is generated. This process is depicted in the cutaway views of Figures 2 and 3. The operating frequency is determined by the spacing between the interdigital electrodes. The effective bandwidth is determined by the number of electrode pairs. The aperture width of the array determines the rf impedance. Figure 4 illustrates in detail
-
ANALYTICAL CHEMISTRY, VOL. 51, NO. 9, AUGUST 1979
1460
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due to changes in the velocity of surface wave propagation caused by the adjacent layer. Thin metal films have been known to produce significant changes in surface wave velocity (3). Finally, the resonant frequency of a SAW device oscillator was expected to shift as matter came into contact with the surface. The evaluation of the three electronic detection modes required that a reproducible wave/matter interaction be generated. I t was decided that pressure changes of a gas in contact with the surface would provide excellent reproducibility, ease of monitoring, and flexibility of interaction. All of the experiments conducted were interfaced to an LSI-11 microcomputer which acquired the relevant data and controlled external parameters such as temperature. The experimental data were stored on a floppy disk from which subsequent data reduction and plotting could be performed. CONSTRUCTION O F APPARATUS S T - Q u a r t z S u r f a c e Acoustic Wave Device Fabrication. The ST-quartz surface wave device used in most of the studies conducted was not commercially available. The substrate material was obtained from the Valpey Fisher Corporation. Each ST cut, X propagating quartz substrate was one-half inch wide, two inches long, and 35 mils thick. One side was "SAW polished" (a surface roughness of less than 250 A). After cleaning, a 2300 f 50 A thick film of aluminum was rf sputtered onto the smooth surface in preparation for the fabrication of the interdigital transducers. Positive photoresist was applied to the aluminum film. A photolith mask with the 1:l scale transducer images on it was then placed in contact with the substrate and illuminated. The photoresist was developed, then baked, followed by etching. The photolith mask with the transducer images on it was also custom designed. Each transducer consisted of eight pairs of electrodes of 1-mil width spaced on 2-mil centers. The aperture of the fingers was 106 mils and the center to center spacing of the two transducers was 1.84 inches. The transducers had a resonant wavelength of 4 mils and with the ST quartz SAW velocity of 3158 m/s this meant that an operating frequency of about 34 MHz was expected. The insertion loss of the SAW delay line was over 60 db. This required that fairly high power be applied to the devices to obtain a usable signal at the output. The power transfer could be greatly improved by increasing the number of finger pairs. Lithium Niobate SAW Device Fabrication. The lithium niobate SAW device used for some of the GC detector experiments was commercially available from Plessey Semiconductors. The device actually contains two complete transversal filters on the same substrate. The interdigital transducer is fabricated from a chromium and gold alloy as determined by ESCA. In normal operation, both filter imputs
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were connected in parallel. Typical insertion losses were in the vicinity of 25 db. SAW Device Detector Block. A mounting fixture was required for the SAW device for three reasons. First, a heat sink with an appreciable thermal mass was needed for temperature studies. Second, a gas tight cell with a small internal volume was needed for GC studies. Finally, metal shielding was required to isolate the input and output connecting wires to the SAW delay line. The block is illustrated in Figure 6. The dead volume of this assembly was calculated to be approximately 200 gL. A hole in the center of the block accommodates a cartridge heater. The interface between the block and the SAW device was coated with silicone grease to ensure good thermal contact. Amplitude Measurement System. A block diagram for the Amplitude Measurement System can be seen in Figure 7 . A power leveled rf source was used to generate about 1 W of power. The output of the rf source was fed into a zero degree phase shift power splitter which provided excitation for the SAW device and a resistive attenuator which formed the two "arms" of a bridge circuit. Each detector module used a diode of different polarity. This provided a positive dc output level from the attenuator arm of the bridge and a negative dc output level from the SAW device arm. The outputs were then combined in a potentiometer which could be adjusted until a null was achieved. Slight attenuation of the amplitude of the surface wave would cause an easily measured positive change in the voltage output of the potentiometer. A low pass active filter was used to minimize the bandwidth of the system before amplification. A change of 1 mV in the rf level a t the detector input would produce a change of about 3 V a t the output of the instrumentation amplifier. The noise level of the electronics in the amplitude measurement system was over three orders of magnitude greater than the theoretical noise level of the SAW device. Reducing the insertion loss of the SAW device would permit the elimination of the rf preamp and allow tighter control of the output level of the rf source. P h a s e Measurement System. A block diagram for the phase measurement system is shown in Figure 8. This system used the same rf power source, power splitter, attenuator, rf preamplifier, and instrument amplifier that was described in the amplitude measurement system. The heart of the system is a double balanced mixer. If two rf signals of the same
ANALYTICAL CHEMISTRY, VOL. 51, NO. 9, AUGUST 1979
253 W K T -EATER
Figure 9. Frequency measurement system
frequency are applied to the local oscillator (LO) and radiofrequency (RF) ports of a double balanced mixer, then a dc potential corresponding to the phase difference between the two signals will appear at the intermediate frequency (IF) port of the device ( 4 ) . The mixer is a completely passive device made from transformers and diodes and does not generate a significant amount of noise. In normal operation, the rf level a t the output of the attenuator is adjusted to match the rf level of the SAW preamplifier output. The mixer is then connected into the circuit and the IF port is connected to the instrument amplifier through an RC filter. The calculated sensitivity of the device was in the range of O . O 2 O / V . Changes in the amplitude of the surface wave of several percent did not introduce significant error into the phase measurement. However, wave/matter interactions that involved substantial attenuation of the wave made data obtained from this phase measurement scheme difficult to interpret. Frequency Measurement System. The use of a SAW device as a resonator in an oscillator circuit requires an rf amplifier with gain significantly greater than the insertion loss of the SAW device. If the output of this amplifier is fed back into its own input through the SAW device, then the circuit will oscillate. The frequency will be determined by the SAW device resonant frequency. These oscillations can be counted very precisely using digital techniques and changes in the frequency of the SAW oscillator over an arbitrary time interval can be used as indication of some type of wave/matter interaction. Figure 9 illustrates the block diagram of a SAW device frequency measurement scheme. Because of the large insertion loss of the quartz SAW device, it was necessary to cascade three stages of rf amplification before oscillation would occur. The resulting oscillations were sinusoidal and the digital counting circuits required T T L compatible square wave pulses. A pulse shaping circuit performed this operation. The pulses were counted on a four-stage, 16-bit binary counter which could toggle a t speeds up to 100 MHz. The gating period of the counter was determined by a crystal controlled time base. The output of the frequency counter was interfaced to an LSI-11 using a standard DEC parallel interface module. The computer easily determined the quiescent frequency of the SAW oscillator and subtracted this value from all subsequent readings. Processed data was then displayed in real-time on an XY plotter to provide operator feedback. In addition, simple filtering algorithms were employed in real-time to enhance the signal-to-noise ratio of the data. The systems employed for the quartz and lithium niobate SAW devices were identical except for the oscillator circuitry. The lithium niobate resonated at a higher frequency (67 MHz) and has a lower insertion loss. Only two stages of amplification were ordinarily required to achieve oscillation. Pressure Monitoring System. Pressure changes on the SAW device were used as a reproducible stimulus for evaluating the performance of the detector systems. The task of monitoring the pressure and system response was handled by the LSI-11 computer. An electronic pressure monitoring system was constructed around a National Semiconductor LX1602 A absolute pressure transducer. An op amp buffer circuit permitted adjustment of the output voltage so that a pressure change of 0 to 760 Torr would provide an output
1461
Figure 10. Temperature control system
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change of 0 to 7.60 V. The transducer contained its own vacuum reference. Temperature, Control System. The thermomechanical analysis of polymers required that accurate control of the temperature of the SAW device be maintained. The use of a simple on-off controller in this application was unacceptable. A commercially available zero voltage firing proportional temperature controller was used in this system (RFL Industries model 72-115). Figure 10 illustrates the temperature control system. Temperature and Pressure Test Apparatus. The stainless steel detector block which accommodated the SAW device and the previously described temperature control equipment was incorporated into a system to perform thermal analysis of the SAW devices themselves and films in contact with them. This arrangement, shown in Figure 11,permitted measurements to be made over the temperature range of 0 to 200 "C. The equipment used to control the pressure around a SAW device is shown in Figure 12. Data Acquisition Software. All of the SAW device experiments used the LSI-11 microcomputer in some capacity. The computer permitted very rapid data acquisition and control, buffering of experimental data, real-time plotting
1462
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capability, and signal-to-noise ratio enhancement by digital filtering. The LSI-11 was not used for data reduction. As a result, all LSI-11 software was written on an assembler level. Subsequent data reduction was performed in a batch mode on a larger PDP-l1V/03 which could perform BASIC and FORTRAN manipulations on data files stored on a floppy disk. The PDP-l1V/03 was interfaced to a digital incremental plotter which was controlled by a FORTRAN plot package.
PRELIMINARY EVALUATION OF SYSTEM PERFORMANCE Pressure vs. Amplitude Response Profiles. A quartz SAW device was connected to the amplitude measurement system and pressure test apparatus described. The vacuum chamber was evacuated, purged, evacuated, and the test gas was allowed to bleed in at a slow rate while the relative amplitude was being monitored by the LSI-11. Helium, nitrogen, air, carbon dioxide, nitrous oxide, and Freon-13 were used as test gases. Multiple runs with each gas were performed. The results obtained for helium, air, and Freon-13 at room temperature are illustrated in Figures 13a, b, c. A plot of amplitude response (expressed in millivolts per 100 Torr) vs. molecular weight is shown in Figure 14. Discussion of SAW Amplitude Behavior. The ST Quartz SAW device used in these studies was capable of propagating a Rayleigh wave which is well understood and
known to contain two wave components; namely longitudinal and shear vertical ( 5 ) . The longitudinal component of the Rayleigh wave is characterized by particle displacements normal to the surface, causing density changes in the ambient medium and a subsequent emission of a compressional wave into the medium. This compressional wave causes energy to be lost from the Rayleigh wave and results in its attenuation (6). The shear component is characterized by particle displacements parallel to the surface which can result in frictional losses to the Rayleigh wave that are dependent upon the viscosity of the surrounding medium. Several investigators have proposed models to describe the effect of ambient gases on the attenuation of Rayleigh surface waves (6-8). All of the proposals focus on the emission of compressional waves as the dominant loss mechanism. The angle at which the compressional wave is launched depends upon the ratio of the velocity of sound propagation in the gas to that in the substrate material. These models predict a qualitative dependence of attenuation on molecular weight of the ambient gas since sound waves usually propagate more slowly in gases of high molecular weight. Theory 3 predicts a linear dependence of attenuation on operating frequency and pressure and a square root dependence on the molecular weight of the gas. A good qualitative agreement exists. Theoretical treatments of other surface wave interactions have not progressed very far owing to the excessive mathe-
ANALYTICAL CHEMISTRY, VOL. 51, NO. 9, AUGUST 1979
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response (expressed in millivolts per 100 Torr) vs. molecular weight is shown in Figure 15. There is no significant dependence of the phase of the wave on the molecular weight of the test gas. Pressure vs. Frequency Response Profiles. Pressure changes were applied to quartz and lithium niobate surface acoustic wave devices that were employed as resonators in an oscillator circuit. The results of an air pressure change on quartz and lithium niobate are illustrated in Figures 16 and 17, respectively. Base-Line Noise Profiles. Evaluation of the detector signal-to-noise ratio required measurement of the noise levels typically generated by each detection system (Le., amplitude, phase, and frequency). Figure 18a-d shows a composite profile. Comparison of Detection Methods. The base-line noise data were statistically analyzed to determine the root mean square noise level and base-line drift. Table I summarizes the performance of the electronic detection modes investigated. The RMS base-line noise indicated for the quartz amplitude measurements reveals that the rf signal processing electronics are significantly more noisy than the SAU' device itself. The noise generated by the SAW device a t room temperature with a 10-Hz system bandwidth is approximately 0.16 nV. The rf amplifiers and signal processing electronics had to generate over 3.2 pV of noise in order to observe the base-line noise recorded in Table I. Thus, the signal-to-noise
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Figure 17. Lithium niobate frequency shift vs. air pressure
matical difficulty and lack of interest. Models describing the amplitude behavior of a Rayleigh wave with a thin organic layer interacting with a gas (i.e., a GC detector) will prove to be a significant challenge. Pressure vs. Phase Response Profiles. The measurement of phase shift in a surface wave as a function of pressure was accomplished using the phase measurement system and pressure test apparatus previously described. A plot of phase N
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ANALYTICAL CHEMISTRY, VOL. 51, NO. 9, AUGUST 1979
1464
Table I. Typical SAW Detector Performance 500 Torr pressure signalrms change to-noise base-line base-line response ratio quartz noise drift
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quartz. On the other hand, the lithium niobate device provided an excellent signal-to-noise ratio using frequency shift measurements. Temperature Response Profiles. A quartz SAW device was intended to be used for polymer thermomechanical analysis. It was necessary to investigate the response of the device as its temperature was varied. The previously described temperature test apparatus was used to control the temperature over the range of 0 to 150 "C in ambient air. The results of amplitude, phase, and frequency measurements are illustrated in Figure 19a, b, c. ST-Quartz is a selected crystalline orientation which has a zero (or nearly so) temperature coefficient around room temperature (9). Temperature fluctuations between 0 and 30 "C will not produce a significant change of the crystal dimension in the direction of SAW propagation. (Figures 19 b-c). At higher temperatures the propagation distance changes more rapidly. The "bumps" observed in the response curves are believed to be caused by the excitation of bulk wave oscillations in the crystalline substrate (10, 11). The shift in amplitude with temperature is caused by the interdigital transducer efficiency changing as the substrate dimensions change. The transducer is resonant at a frequency determined by the spacing between the electrodes. If the dimensions change then so should the resonant frequency. The system is driven a t a fixed frequency, however, and the transducer is no longer optimum. The presence of bulk wave "plate modes" in the substrate would cause the bumpy appearance of the profile. The generation of spurious bulk waves in addition to surface waves can result in constructive or destructive interference of a bulk wave reflected from the bottom and a surface wave a t the transducer. LITERATURE C I T E D
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Figure 19. (a) Quartz amplitude vs. temperature response. ( b ) Quartz phase vs. temperature response. (c) Quartz frequency vs. temperature
White, R. M. "Surface Elastic Waves", R o c . I€€€. 1970, 58(8),1238. Slobcdnik, A. J. "Attenuation of Microwave Acoustic Surface Waves Due to Gas Loading", J . App/. f h y s . 1972, 43(6), 2565. Famell, G. W. "SAW Propagation in l?ezoekctric Solids", Wave Electronics, 1 9 7 6 , 2 , 15. "Mixer Applications Handbook", Mini Circuits Laboratories: Brooklyn, N.Y., 1977. Oliner, A. A. "Microwave Network Methods for Guided Elastic Waves", I€€€ Trans. Microwave Theory Techn. 1969, 17(11), 822. Dransfeld. K.; Sattzman, E. "Excitation, Detection, and Attenuation of High Frequency Elastic Surface Waves", "Physical Acoustics"; Thurston and Mason, Eds; Academic Press: New York, 1970; Vol. VII, p 259. Arzt, R. M.; Salzman, E.; Dransfeld, K . Appl. Phys. Left. 1967, 10, 165. Tiersten, H. F.; Sinha, B. K. "A Perturbation Analysis of the Attenuation and Dispersion of Surface Waves", J . Appl. f h y s . 1978, 49(1). 8 7 . Hauden, D.. et al. "Temperature Effects on Quartz Crystal Surface Wave Oscillators", Appl. Phys. Lett. 1 9 7 7 , 37(5), 315. Wagers, R. S. "Spurious Acoustic Responses in SAW Devices", E€€ Proc. 1976, 64(5), 699. Daniel, M. R.; deKlerk, J. "Temperature Dependent Attenuation of Ubasonic Surface Waves in Quartz", Appl. f h y s . Lett. 1970, 16(1), 30.
response
ratio of the amplitude measurement system could potentially be improved by more than four orders of magnitude. T h e frequency shift measurements appeared to afford poorer signal-to-noise ratios than phase measurements in
RECEIVEDfor review January 22, 1379. Accepted May I, 1979. The authors thank the Gillette Charitable and Educational Foundation whose funds helped support this research.
