Piezoelectric Sensor for Mercury in Air Eugene P. Scheide* and John K. Taylor Air Pollution Analysis Section, Analytical Chemistry Division, National Bureau of Standards, Washington, D.C. 20234
A quartz piezoelectric crystal detector with gold evaporated onto the electrode as the sensor substrate has been developed for the detection of small mass changes caused by the selective adsorption of mercury vapor from a n air sample. Incorporation of the crystal into a variable oscillator circuit and measurement of the change in frequency of the crystal due to the increase in mass allows a highly sensitive indication of the amount of mercury present in the air sample down to the subpart-per-billion level. Thus, the selectivity of mercury adsorption onto gold films and the sensitivity of the piezoelectric sensor are combined in this instrument. Calibration curves are obtained from part-per-million to subpart-per-billion concentrations of mercury. Reversibility is achieved by placing the sensor in an oven, raising the temperature to 150°C, and flow switching a stream of clean, dry air over the detector. This detector has potential use both as an air pollution sensor and in industrial hygiene applications.
Air pollution is an ever-growing problem in our environment, and the need for monitoring devices that are small, simple, and rugged, yet sensitive and selective is urgent. King (1-4) has shown that the piezoelectric detector is such a device. This detector consists of a vibrating quartz crystal coated with a substrate capable of interacting selectively with a component of interest in a static system or flowing gas stream. The frequency of vibration of the crystal is dependent on the weight of the coating and the weight of the vapor adsorbed onto the coating. The concentration of the pollutant in the atmosphere is therefore measured by detecting changes in the frequency of the coated piezoelectric crystal detector. Mercury Pollution. The problem of mercury pollution in the environment is well known, and in recent years much attention has been devoted to the development of methods for the detection of low concentrations of mercury in air. Ambient atmospheric concentrations of mercury vapor on a regional scale are always below toxic levels for man, but the cumulative contribution to mercury levels in soil, vegetation, and water around polluting industries may be important. Because of its highly toxic nature, mercury contamination must be avoided and strict monitoring of industrial work sites must be done. A portable instrument capable of rapid, inexpensive analysis is highly desirable. Most instruments in use in industrial hygiene analysis are variations of the nonflame atomic absorption technique for mercury. These instruments suffer, however, from interferences and lack of precision in the analysis of low levels of mercury. Piezoelectric Sorption Detectors. In 1959, Sauerbrey (5, 6). developed a relationship between the weight of metal films deposited on quartz crystals and the change in frequency. The relationship which he derived is:
A where
AF= frequency change, Hz F = fundamental frequency of the piezoelectric crystal, MHz Amf = mass of coating deposited, grams A = area coated, cm2 This equation predicts that a commercially available 9-MHz crystal would have a mass sensitivity of about 400 Hz per microgram. It is therefore apparent that the vibrating quartz crystal can be an extremely sensitive mass indicator. In 1964, King ( I ) used the idea of a coated piezoelectric crystal to construct a sensitive and selective detector for gas chromatography. King coated the crystals with substrates used in gas chromatographic columns. Since these substrates had proved useful in partitioning various gases on a column, King proposed that they would be capable of interaction with the same components of a gas stream while on the crystal surface. The frequency of the crystal depended on the mass of the coating which, in turn, depended on the mass of the vapor taken u p by the coating. Using Equation 1, King estimated that detection limits of 10-12 gram could be realized. Furthermore, this detection limit was independent of carrier gas provided the carrier gas did not partition in the substrate. King called his device the “piezoelectric sorption detector,” since the interaction “is probably a combination of adsorption and absorption.” The most useful feature of the “sorption” detector is the ability to detect gases selectively. By applying polar substrates, such as polyethylene glycol, selective detection of polar compounds was possible. On the other hand, a nonselective hydrocarbon detector was devised by coating the crystal with squalene. In 1969, King (3) reviewed his work and described the utility of the moisture analyzer he developed in detecting air pollutants by selective combustion of hydrocarbons to carbon dioxide and water. The high sensitivity, selectivity, and ruggedness of the sorption detector make it an ideal candidate for certain air pollution and industrial hygiene applications. If hydrocarbons, moisture, and hydrogen sulfide can be continuously and sensitively monitored by using coated piezoelectric detectors, then why not other contaminants? It appears that the sorption detector could play an important role in the future of air pollution and industrial hygiene control and monitoring. The mercury sensor described here utilizes as the detector substrate a thin film of gold metal evaporated onto the piezoelectric crystal electrode. The ability of gold to adsorb and amalgamate mercury has been known for some time, and this sensor combines this selective adsorption with the sensitivity of the piezoelectric detector. The objective of this research was to develop the piezoelectric detector as a measurement technique and to show its feasibility in the analysis of mercury vapor in air.
Experimental A diagram of the experimental apparatus used is shown in Figure 1. The sensor is placed in an oven kept a t 25°C while sampling and then raised to 150°C for the desorption process. Various concentrations of mercury in air were produced using a mercury generation system based Volume 8, Number 13, December 1974
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on a saturation technique (7, 8). These samples were passed over the detector and out through a flowmeter and wet test meter to accurately measure the sample volume. The piezoelectric detector crystal is driven by a variable oscillator and the frequency is monitored by using a digital readout frequency counter with a readout capability of 8 digits with a precision of fl in the last digit. The piezoelectric crystals used are 9 MHz quartz crystals AT-cut in a HC 6/U holder. Gold films were evaporated onto the electrodes of these crystals by a vacuum deposition technique using a conventional vacuum system (10-6 torr) with a nickel underlayer and no substrate heating. During the desorption cycle, the oven temperature is raised to 150°C and air is passed first through a drying system, a filter, a heat exchanger located inside the oven (to ensure that the air temperature is also 150"C), and over the detector. The mercury is thereby desorbed from the gold detector surface and the sensor is thus regenerated. The hot air then passes through another heat exchanger a t 25°C and a flowmeter and wet test meter.
..r,.*
Figure 1. Block diagram of measurement system
Results Effect of Flow. Air can be passed over the sensor a t flow rates up to 300 cc/min without affecting the detector stability. Small frequency changes do occur, however, when the flow is started and stopped and this is probably due to desorption and adsorption of moisture from the sensor surface. Effect of Temperature. Since all frequency measurements are made at room temperature, and small variations in room temperature do not affect the stability of AT-cut piezoelectric crystals ( 3 ) , close temperature control of the sensor is not necessary. It is not recommended to use a much higher desorption temperature than 150°C because of the possibility of introducing fracture strains into the crystal and also because of the low-melting soldered electrical connections. Collection Efficiency. Figure 2 shows the effect of flow rate on detector response. At flow rates of 100 cm3/min or less, the response rate is constant, with decreasing sensitivity a t higher flow rates. For this reason sampling must be done a t flow rates of 100 cm3/min or less for greatest sensitivity. If sampling is done a t higher flow rates, the detector response is still linear, but with decreased sensitivity, and the measurements have less precision. Also, since a t higher flow rates the sensitivity is dependent on the flow rate, the flow rate must be more closely controlled. Detector Sensitivity a n d Range. Experiments were performed to determine the detector sensitivity, linearity, and range. Calibration curves were prepared by measuring the frequency response (IF) resulting from exposure t o a measured volume of air of known mercury concentration, at a flow rate of 100 cm3/min. Figure 3 shows linear response over the range of industrial hygiene interest. A similar calibration curve over a larger range is shown in Figure 4. Linear response falls off after collection of a 2 proximately 0.5 fig of mercury and a "saturation" con$ tion is approached after the collection of several milkgrams of mercury. Accordingly, reactivation of the crystal surface by the heating-desorption cycle is recommended after collection of 500 ng of mercury, corresponding to a frequency change ( I F ) of 50 Hz for the crystals used in this work. The analytical conditions shown for Figure 3 are based on 10-liter samples while those for Figure 4 are based on the collection of 1-liter samples. Even greater sensitivity can be achieved by taking larger air samples since the piezoelectric sensor responds to mass rather than concen1098
Environmental Science & Technology
I
I 50
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310
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Figure 2. Response vs. flow rate
30t
CONCENTRATION/AMOUNT
OF M E R C U R Y I
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liter
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Figure 3. Sensor response to low concentrations of mercury in air
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2
4
6
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14
MERCURY
Figure 4. Adsorption of mercury onto piezoelectric sensor
tration. On the other hand, higher concentrations of mercury will fall on the linear portion of the calibration curve if smaller samples are taken. Thus, by varying the sample size according t o concentration, a 10-fold increase in sensitivity and a 100-fold increase in linear range can be achieved. The range can also be increased by a factor of two by evaporating gold films onto both crystal electrodes thereby doubling the surface area available for sorption. Also, with samples of higher mercury concentration, the sample can be diluted prior to analysis. Another way of determining the mercury concentration of an air sample, instead of taking discrete samples and looking at the frequency change for each sample, is to monitor the frequency of the sensor as a function of sample volume during continuous monitoring and determine the mercury concentration from the slope of the line drawn through the data points (the rate of adsorption). Since the mass sensitivity of the detector is known (Equation l),the concentration can be easily calculated. Interferences. Table I shows the sensor response to various possible interferents compared with that for 0.06 ppm mercury under the same conditions. Water vapor a t 100% relative humidity causes a frequency shift in the sensor but this is probably due to small water droplets adhering to the crystal surface and could be eliminated by the use of an appropriate filter. Five parts per million of chlorine also cause a detector response, but this concentration will rarely be encountered in a real situation since this is five times the threshold limit value (TLV). If necessary, water and acidic gas vapors can easily be removed by a trap containing MgC104 and Ascarite (9). Conclusions
The quartz piezoelectric crystal sensor described is sensitive and selective for mercury vapor in air. The analytical reliability of the measurement system used in this work may be inferred from inspection of the data of Figure 3. Measurements of concentrations below 15 ppb have a precision of 10% with uncertainties of 5% above this level. However, the experimental procedure has not been optimized to give the ultimate accuracy and sensitivity. The sensitivity demonstrated here can be improved by a t least two orders of magnitude by the use of more sensitive frequency-measuring instrumentation and more sensitive piezoelectric crystals. If these steps are taken, ambient levels of mercury vapor in air (0.003-2.0 pg/M3) easily can be measured using this sensor. It has many possible uses mainly in air pollution and industrial hygiene analysis
Table I. Interferences Substance
H20
100% R H
Hz0
82% RH 67% RH
HzO
co CH4 CH4
H2S H2s
so2 so2 N0 2 NO2 CI2 CI2 Hg
Response, AF
Concentration
24 2 0 0
50 PPm 100% 4% 25 PPm 10 PPm 20 PPm 20 ppm
+ 75%
25 PPm 5 PPm 5 PPm 1 PPm 0.06 ppm
0 0 0 0 0 0 0
RH
0 9 0 50
and should be particularly useful for measurement of very small amounts of mercury in many substances in which the mercury present can be chemically separated, reduced to the elemental state, and released from the sample into a n air stream. Acknowledgment
The authors wish to thank P. A. Pella for his help in the evaporation of the gold films. Literuture Cited King, W. H., Jr., Anal. Chem., 36, 1735 (1964). King, W. H., Jr., Environ. Sci. Technol., 4, 1136 (1970). King, W. H., Jr., Res./Deuelop., 20,28 (1969). King, W. H., Jr., U S . Patent 3,164,004 (Jan. 5, 1965). Sauerbrey, G. Z., Z. Physik, 155,206 (1959). Sauerbrey, G. Z., ibid., 178,457 (1964). Nelson, G. O., “Controlled Test Atmospheres,” p 171, Ann Arbor Science Publishers, Inc., 1961. (8) Scheide, E. P.. R. Alvarez. B. Greifer. E. E. Hughes. J . K . Taylor, “A Mercury Vapor Generation and Dilution System,” NBSIR 73-254, Oct. 1973. (9) McNerney, J . J., Busack, P. R., Science, 178,611 (1972).
(1) (2) (3) (4) (5) (6) (7)
Receiued for review April 26, 1974. Accepted August 11, 1974. Presented at the 167th National Meeting of the American Chemical Society, Division of Environmental Chemistry, Los Angeles, Calif., March 31-April 5, 1974. Work supported by the Measures for Air Quality Program at the National Bureau of Standards.
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