o0, and since the period of vibration of the scale is related to the mass,

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1 459 THE USE OF RESONATING DEVICES TO MAKE SMALL MASS MEASUREMENTS* W. H. KING, JR. Esso Research and Engineering Company Linden, N.J. W^i THEN we wish to weigh something most of us think of a scale or a balance as being the ideal tool. In the case of a scale the spring is extended by a weight, and the amount of spring extension is calibrated to read the weight or the mass; such systems are limited by our ability to measure the extension accurately. Other difficulties are the vibration and stickiness of the indicators. In this paper I propose to focus attention on a more precise way to weigh. Going back to the scale, where the mass being determined is extending a spring, we normally have to wait until the spring mass system stops bouncing up and down before we can take a measurement. However, in this modern age it is possible to measure the period or frequency of vibration to at least one part in o0, and since the period of vibration of the scale is related to the mass, the measurement of the period is an assessment of the mass. Of course, I am not suggesting that you use a scale to find blood clots; I use, instead, a resonant device to measure mass. Many forms of small resonant devices are available, and mass changes as low as Io-'2 gm. have been reported. Ultramicro weighing is of course, difficult to attain; however, measurements of io-9 or io-"0 gm. are entirely practical provided one selects the proper resonating device and measurement system. I shall discuss two resonant systems with which I have had experience over the past several years. The first system is that of the sorption detector. This is described in U.S. Patent No. 3,154,004 and elsewhere.1-3 The sorption detector was developed mainly to measure the composition of gas streams. The vibrating device is inserted into the fluid to be examined, and a material adsorbs onto a coating on the resonator which changes its frequency. In the second device, called a low-frequency vibrator, the fluid stream flows internally, and liquids may be accommodated. *Presented as part of a Symposium on Problems in Evaluating the Blood Compatibility of Biomaterfals held by the Section on Basic Medical Sciences and the Section on Biomedical Engineering of the New York Academy of Medicine February 18 and 19, Vol. 48, No. 2, February 1972

2 _460 W. H. KING, JR. Fig. 1. A recording of frequency change with time showing the ability to detect mass changes less than gmn with a resonant 9-MHz quartz crystal. Reproduced by permission of F. D. Thompson Publication, Inc., from: King, Jr., W. H.: Using quartz crystals as sorption detectors... Part I. Research/Development, April 1969, pp. 28 to RECORDING AF AT HIGH SENSITIVITY N I~- LLI 0.2 z (.) 0.1 z w LL X GRAMS B 1. I l,lil ll I TIME, MINUTES Fig. 2. Low frequency resonator capable of detecting moisture in a liquid stream. The pickup (18) senses vibration and the magnet (25) supplies the power to maintain resonance. Fluid flows in pipe (11) through the disiccant bed (15) and out pipe (12) As the absorbed moisture builds up the period of vibration increases. Reproduced by permission of F. D. Thompson Publications, Inc., from: King, W. H., Jr.: Using quartz crystals as sorption detectors... Part II. Research/Development, May 1969, pp Bull. N. Y. Acad. Med.

3 USE OF RESONATING DEVICES 4 6 I TABLE I. TYPICAL DATA OBTAINED WITH 9-MHZ. SENSORS Sensitivity, hz./mol % Temperature Coating material Hexane Benzene Water 0C. DC-200 silicone oil ,2,3-Tris-cyanoethoxv propane Silica gel , Hygroscopic polymer* , Rubber *Sensitivity is shown for the 10 ppm. range. In this particular device there are no shear forces developed within the liquid while measurement is made continuously. THE SORPTION DETECTOR The sorption detector employs a piezoelectric quartz crystal resonator. A typical device consists of a quartz disc 0.5 inch in diameter and inch thick. Electrodes deposited on its surface and an electronic circuit cause the crystal to vibrate or resonate at a frequency of io7 cycles per second (hz.). Such a device is extremely sensitive to the amount of mass deposited on its surface. The recording shown in Figure I indicates the ability to detect io-11 gm. changes. To make a sorption detector, the coating is placed on one of the electrodes. This, of course, causes a frequency shift. The amount of shift depends on the amount of coating and, normally, one will put on enough coating to shift the frequency about 25,000 cycles/sec. (hz.). The particular coating material is selected to interact with the desired vapor in the air that passes over the sensor. By selecting a hygroscopic polymer one can fashion a water sorption detector or, by use of a rubbery-like polymer one can make a solvent vapor detector. The performance of several different types of sorption detectors are shown in Table i. The table shows the coating material and the sensitivity obtained for hexane, benzene, and water as typical vapors. The first three materials make reasonably good gas chromatography detectors because they are extremely rapid in speed of response and they are sensitive to many different kinds of materials. The hygroscopic polymer, which shows zero sensitivity to solvents but an extremely high sensitivity to water, is presently available in a commercial water analyzer.4 The rubber-coated crystal is a very interesting sensor Vol. 48, No. 2, February 1972

4 W. H. KING, JR. TABLE II. PERFORMANCE OF A SOLVENT LEAK DETECTOR CONSISTING OF A POLYMER-COATED 9-MHZ CRYSTAL Leak detector meter deflection Solvent vapor Sensitivity for 1 Ag./cc. in air Hz./ppm. (v/v at 250 C.) % of full scale C C Ca nc, nc nc* Benzene nc Toluene nc Ethyl benzene f4. o-xylene 9.70 (42. ncv ,000. Trichlorobenzene ,600. Perchloroethylene Chloroform Tetrahydrofuran Water Ambient noise level is 0.1 sec. for trichlorobenzene. hz.; response time is less than 1 sec. for hexane and 5 to 10 because it is sensitive to many different types of organic and inorganic vapors. This sensor may have future use in the medical field because it is very sensitive to some of the anesthetic vapors used. A simple portable leak detector was made using the solvent sorption detector. The leak detector parts were housed in a small aluminum container having a handle on top to make it convenient to carry. A small aquarium pump inside the box sucked air through a sorption detector. The signal was observed by the deflection of a galvanometer. Table II shows the performance of the solvent detector. It can be seen that the sensitivity is very low for low molecular weight materials but very high for high molecular weight materials. All substances shown could be detected up to their saturated vapor points. While the sorption detector is a versatile and sensitive mass-measuring device, it would not be practical to place the io-megacycle crystal in a blood stream and look for blood clots, because the resonator will not vibrate in a liquid medium. It might be possible to coat the crystal Bull. N. Y. Acad. Med.

5 USE OF RESONATING DEVICES 46 3 TABLE III. Veight change mg. CALIBRATION OF VIBRATOR Period change Asec with the polymeric material under investigation, expose it to the blood medium and periodically remove the sensor, rinse it in water, and then measure its frequency in air. In this way, tiny weight increases could be detected. There are other piezoelectric resonators which can be made to vibrate in a liquid medium. These resonators are low-frequency devices, in the order of 20 khz. Such resonators have been used in the past to measure the viscosity of oil. The particular one used consisted of a quartz rod approximately ¼4 in. diameter and 2 in. long. The mode of oscillation was a torsional vibration where the ends of the rod twisted back and forth in opposite directions. If such a resonator were coated with a material to be examined, it could function in a blood stream, and the appearance of clots would greatly alter its frequency and damping characteristics. I estimate that masses as small as o.i mg. could readily be detected, provided they attach themselves near the ends of the rod where the vibrational amplitude is maximum. There may be some interaction between a vibrating surface and that of the clotting characteristics of the blood. I therefore suggest that the resonator be quiescent most of the time and turned on periodically to measure the influence of clotting. Fortunately, however, there is a way to observe mass continuously where no shear forces are involved in the liquid under examination. This device, called the low-frequency vibrator, appears in U.S. Patent No. 3,261,202. THE Low-FREQUENCY VIBRATOR Figure 2 depicts the important details of the low-frequency vibrator. A phonograph pickup needle senses the motion of the vibrating element. Vol. 48, No. 2, February 1972

6 464 W. H. KING, JR. This information is fed to a io-watt audio-amplifier whose output is connected to an electromagnet. The electromagnet attracts a tiny permanent magnet attached to the vibrator, and the electronic arrangement is such that the vibrator is maintained in an oscillation condition by this closed-loop system. A frequency counter attached to the amplifier measures the period or frequency of vibration. The fluid to be examined flows into Tube No. i i through the fixed support, No. 26. The fluid then enters a chamber at the end of the vibrator, where it passes through the material being examined, No. 15, and then goes out through Tube No. 12. The frequency of vibration is on the order of 4o hz., and this frequency is greatly influenced by the entire mass contained in the capsule at the end of the vibrator. Therefore, if the density of the fluid or the mass that may become trapped in the chamber changes, the frequency will change accordingly. The vibrator may be calibrated by simply adding weight to the order of io i-g. Table III shows the data obtained from a calibration of the lowfrequency vibrators. The data show that I mg. of mass changes the period of vibration I pasec. Since it was usually easy to observe I,usec. changes, the vibrator could easily detect i mg. mass increases. High sensitivities are indeed possible. By reducing the size of the vibrators and the mass load it is to carry to about I gm., a sensitivity for IO mg. is expected. SUMMARY AND CONCLUSION I have described the piezoelectric sorption detector and demonstrated its ability to measure extremely small mass changes in an air environment. I described the approach which should be taken to extend this knowledge to liquid systems. A low-frequency vibrator was described, and data presented where i mg. of weight change was easily detected in a liquid medium. The latter device has the advantage that no shear forces are imparted to the liquid from the vibrator. REFERENCES 1. King, W. H., Jr.: Anal. Chem. 36: , Fischer, W. F. and King, W. H., Jr.: Anal. Chem. 39: , King, W. H., Jr.: Using quartz crystals as sorption detectors. Res./Devel., April 1969, pp and May 1969, pp E. I. DuPont de Nemours, Instrument Division, Glasgow, Del. Bull. N. Y. Acad. Med.