Applications of the piezoelectric crystal detector in analytical chemistry

Applications of the Piezoelectric Crystal Detector in Analytical Chemistry J. Hlavay' and G. G. Guilbault' Deparfment of Chemistry, University of New Orleans, New Orleans, Louisiana 70 722

Applications of the piezoelectric quartz crystal In Merent areas of analytlcai chemistry are reviewed. One of the most useful analytlcal methods Is in detectlon of different gases In ambient alr In the ppm and ppb concentratlon range. The usefulness of the plezoeiectric crystal In gas chromatography and ilquld chromatography as a sorption detector, in polymer research, for the determination of water and for mlcro weighlng wlth a quartz crystal, are also detalled. The commerclal availabillty of piezoelectric devices is discussed.

The phenomenon of piezoelectricity was first observed in 1880 by Pierre and Jacques Curie in a number of substances including natural quartz ( I ) ; they discovered that a pressure exerted on a small piece of quartz caused an electrical potential between deformed surfaces. The following year, the Curies demonstrated the converse of this by applying a voltage to a quartz crystal and measuring the physical distortions. A crystal activated by a concise electrical charge vibrates mechanically for a short time until a physical equilibrium is attained. Alpha quartz is the most widely used material for piezoelectric crystal detectors. The properties which make it suitable are its water insolubility and resistance to temperatures up to 579 "C with no loss of piezoelectric properties. The frequency of the quartz crystal is dependent upon the physical dimensions of the quartz plate and the thickness of the electrodes on it. For use as a piezoelectric crystal detector, only AT or BT cut quartz plates are useful (2) (AT and B T refer to the orientation of the plate with respect to the crystal structure). These are two high-frequency mode plates which vibrate in a shear mode about an axis parallel to the major surface; these plates have low or zero temperature coefficients a t the temperature of use, and have surfaces that are antimodal in displacement. Temperature coefficients are critical functions of the angle of cut. The AT cut is superior in temperature coefficient and in mass sensitivity to the BT. The 9-MHz crystals used most frequently are 10- to 16-mm disks, squares and/or rectangles which are approximately 0.15 mm thick (Figure 1). The metal electrodes are 3000 to 10000 8, thick, 3 to 8 mm in diameter and are made of gold, nickel, silver, or aluminum. In a well-behaved resonator, the vibration is only in the direction of the plate plane; no vibration occurs in the y axis at an angle 90° to x in the plate plane and through the thickness (2 axis) of the plate. The resonant frequency of a crystal is normally obtained using a frequency meter attached to the output of an oscillator circuit. Piezoelectric devices have been used for many years for controlling frequency in the communications field and as selective filters in electronic networks and as sensors to measure temperature accurately. Recently, they have been used in analytical chemistry for detection and determination Present address, Institute of Analytical Chemistry, Veszprem University of Chemical Engineering, 8201 Veszprem, P.O. Box 28, Hungary. 1890

ANALYTICAL CHEMISTRY, VOL. 49, NO. 13, NOVEMBER 1977

of micro concentrations of substances. In this review the applications of piezoelectric crystal detectors in analytical chemistry will be summarized.

QUANTITATIVE RELATIONSHIP BETWEEN FREQUENCY CHANGES OF THE QUARTZ CRYSTAL AND DEPOSITED MASS In 1959, Sauerbrey ( 3 , 4 )developed a relationship between the weight of metal films deposited on quartz crystals and the change in frequency. This relationship was derived for quartz (AT-cut) crystals vibrating in the thickness shear mode. The applied electric field is along the Y-axis. The frequency (F) can be expressed as:

F = V t , / 2 t= N / t

(1)

where utr = velocity of propagation of a transverse wave in the plane of the crystal. t = thickness of the crystal. N = frequency constant = vtJ2. From Equation 1with appropriate algebraic manipulations, Sauerbrey obtained an expression for the change in frequency (AF) when the thickness ( t ) of a quartz plate is increased by a differential amount (At):

AF/F = A t / t

(2)

A relationship of density and mass:

t = M/Ap (3) where M = mass of the quartz plate, A = surface area of the quartz plate, p = density of the quartz plate. Substituting a differential form of Equation 3 into Equation 2, yields,

AF/F = - AM/tAp

(4)

In the case of pure shear mode vibrations, the strains (derivatives of elastic displacement along the X-axis) are all zero at the principal faces. Sauerbrey assumed that if the plate were divided into an infinite number of parallel planes along the x-z plane, those planes very close to the surface would affect the frequency only through their mass and not through their elastic character. Thus, the change in frequency resulting from the deposition of a thin, uniform film of any foreign substance would be equal to that resulting from a layer of quartz of the same mass. Sauerbrey altered Equation 4 to give the more general equation,

AF/F = - AMs/tAp

(5)

where h M s = mass of film of any substances coated on the quartz plate. Substituting t = N / F

A F / F = - AMs F / A p N

(6)

The frequency constant (N) for AT-cut quartz crystals is 0.1679 MHz/cm ( 3 ) . Substituting values for the other constants, a final equation is obtained:

A F = - 2 . 3 X lo6 F 2 ( A M s / ( A ) (7 1 where AF = the change in frequency due to the coating (Hz), F = the frequency of the quartz plate (MHz), AMs = mass

I

U

I

J

Figure 1. Typical piezoelectric crystal (60)

of deposited coating (g), A = area coated (cm2). This equation predicts that a commercially available 9-MHz crystal would have a mass sensitivity of about 400 Hz/pg or a 15-MHz crystal a sensitivity of 2600 Hz/pg. It is therefore apparent that the vibrating quartz crystal can be an extremely sensitive weight indicator. The detection limit is estimated to be about g (5). Sauerbrey successfully tested Equation 7 by evaporating thin metal films on quartz crystals, weighing them on a microbalance and monitoring the change of frequency electronically. Metals and many other solids change the frequency, but do not greatly affect the crystal‘s ability to vibrate. However, when liquids are deposited on the crystal surface, the ability to vibrate is often impaired, because the vibrating crystal surface dissipates energy in the liquid. If a gas is allowed to absorb onto the liquid coating, the amplitude of vibration can be used to detect the gas composition. Another use of amplitude changes is the sensing of crystalline and other phase changes in solids and liquids. Sauerbrey studied the amplitude of vibration in great detail (5). “SORPTION DETECTOR” In 1964, King (6, 7) used a coated piezoelectric crystal to construct a sensitive and selective detector for gas chromatography. Since the frequency of a vibrating crystal can easily be measured to f1.0 Hz, any frequency change greater than this can be easily detected. King coated the crystals with different substrates used in gas chromatographic columns, and proposed that they would interact with the chromatographable components of a gas stream while on crystal surface. The frequency of the crystal depended on the mass of the vapor taken up by the coating. Applying Equation 7 , King estimated that detection limits of g 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 high sensitivity, selectivity, and ruggedness of the sorption detector makes it suitable for use in analytical chemistry, especially in detection of air pollutants. PIEZOELECTRIC CRYSTAL DETECTOR FOR WATER King has developed a coated crystal analyzer sensitive to 0.1 ppm water (6-8). Since the adsorption isotherms of many materials are often known, the performance of a sorption detector is readily predicted. The relative adsorption of water on crystals coated with molecular sieves, hygroscopic polymer, silica gel, and polar liquids was investigated. The liquid coated crystals were rapid and linear response detectors, but the solid

adsorbents were outstanding for assay of water at low concentrations. Since the adsorption characteristics of uncoated crystals are of only limited usefulness, the response of gold, nickel, and aluminum crystals to water vapor was investigated (8). Assuming a 1 cm2 surface area, the gold electrode picked up a 10-8, layer of water at 50% relative humidity. This response was probably due to surface adsorption since the crystals had been washed in methanol before testing. The amount adsorbed was small on clean crystals compared to the adsorption on a coated crystal. The coatings used normally account for frequency changes of 5 to 50 KHz; thus 1% adsorption amounts to frequency changes of 50 to 500 Hz. When deliquescent salts were used as coatings for crystals, interesting water detectors resulted. The response of the detectors is rapid a t normal conditions, but at low temperatures and iow humidities the response can be very slow due to a lack of driving force for the reaction, slower diffusion, and existence of a solid state reaction. A crystal coated with 500 8, of lithium chloride for example, had a time constant of 18 min at -32 “C and less than 1 s a t room temperature. The adsorption isotherms of melt salts are well known, but the hysteresis effects are not. The coated crystal is a sensitive and rapid way to study this. As it was established, hysteresis does not occur in LiCl or CaClz over a 11% change in relative humidity or in LiBr over 6% relative humidity (8). When the liquid hydrate is formed, the hysteresis loop is closed. Water detectors can be constructed from a wide variety of materials and each has its own range of usefulness (11). A selective water detector based on a hygroscopic polymer coated crystal has been commercially available since 1964 (9). The unique qualities of the instrument are ppm detection in 30 s, high selectivity, and long life time. Gjessing et al. (10) developed a radio-sonde humidity element consisting of an SiOx film evaporated on a crystal. N o hysteresis was found between 15 and 95% relative humidity. Several studies have demonstrated conclusively that the piezoelectric crystal detector can be used to measure moisture in the Martian atmosphere. The atmospheric environment on the Martian surface differs significantly from the Earth’s atmosphere. For example, a t least 80% CO2 can be found in the Martian atmosphere. Recent Mariner flybys have shown that the total pressure of the Martian atmosphere is approximately 6 to 8 millibars. In addition, evidence suggests that atmospheric water vapor pressure may range from 0.05 to 900 pm (12). A solution to these problems were found by using a King piezoelectric sorption hygrometer with several different polymer coating systems (13,14). Since the quartz crystal detector is extremely sensitive to weight changes and the water vapor is detected by measuring the change in weight of an hygroscopic f ilm on the crystal, and since the wind-blown Martian dust could accumulate on the crystal, and interfere with water vapor measurements, a need developed for an electronic method of removing dust particles from the crystal surface. Water was quantitatively determined a t different experimental conditions (temperature, pressure, etc.).

PIEZOELECTRIC CRYSTAL DETECTOR FOR DETECTION AND DETERMINATION OF AIR POLLUTANTS Detector for Sulfur Dioxide. The analysis of SOz in air has been a problem for about 60 years. Oil refineries, pulp mills, and effluents from a number of other industrial stacks constitute a prime source of discharge of SO2 into the atmosphere. Burning of high sulfur fuels in automobiles is another cause of pollution at the ground level. Therefore, an ever-increasing demand has resulted for new, simple and inexpensive methods for measurement and control of sulfur dioxide pollution. ANALYTICAL CHEMISTRY, VOL. 49, NO. 13, NOVEMBER 1977

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Table I. Summary of Coating for SO, Coating material

Detection limit

Sampling method

Styrene-dimethyl0.1 ppm aminopropyl maleimide 1:1 copolymer Tridodecylamine 5 PPm Igepal CO-880 Dially lamine Phenyldiethanolamine PP-20 40 Tripropy lamine SDM polymer

Gas tight

Sodium tetrachloromercurate Apiezon Silicone SE-30 Silicone QF-1 Carbowax 20M Versamid 900

5 ppm

Gas tight

p-Toluidine Amine 220 Triethanolamine Quadrol Armeen 2 s

1.0 ppb

Quadrol

1 PPb

syringe

Coating method

Effect of temp.

Interferences

Dropping with syringe

...

NO,,

...

...

NO*

Gas tight syringe or gas sampling valve

Spraying

moisture

...

From 25 to 40 "C. No considerable change

syringe

Remarks Portable device

...

Ref. 15

16

Effect of changes of voltage was investigated

17

New cells design

18

Syringe dilution

Smearing with a tiny brush

NO,,

Syringe dilution


NO,

Hydrophobic membrane used t o eliminate interfere

19

Quadrol

20-50 ppm Syringe in auto dilution exhaust. Up to 300 in stack gases


NO,

Portable device

20

Carbowax 400, 20M Dinonyl phthalate Polyphenyl ether p ,p -0xidipropionitrite SAIB (sucrose acetate isobutyrate) Triethanolamine Amine 220 Squalane

5 PPm

Gas handling apparatus

Dropping

Response time was investigated

21

Triethanolamine

25 ppm

Gas handling

...

apparatus. EPA method

Ethylenedinitrilotetraethanol

1 PPm

Flow system in N,

ANALYTICAL

CHEMISTRY, VOL. 49, NO. 13,NOVEMBER 1977

...

0-22.5 C was investigated

10-35 C for uncoated crystal

Smearing 25-40 with cotton swabs

There have been several research reports describing the use of coated piezoelectric quartz crystals as highly sensitive detectors for SO2 (15-24). The different coating materials and detection limits as well as other conditions of the measurements are summarized in Table I. Many coating materials have been investigated for SO2. Trace amounts of SO2 can be detected by a new detector cell design using triethanolamine and quadrol as coating materials (18) (Figure 2). The important feature of this design is that the column effluent is split into two equal streams which directly and simultaneously fall on the opposite faces of a coated crystal. This arrangement is expected to improve the sensitivity since the amount of the sample gas reacting 1892

moisture

C

NO,

Response 22, timeand 23 nature of absorption were investigated

NO,

Position 24 of coating material was investigated

1 _CZLJNh 5 F C L E U -

Figure 2.

New

design for the piezoelectric crystal detector cell ( 78)

with the coating at any moment is appreciably increased. Effects of the change of the temperature were investigated in several experiments. Guilbault et al. (17) pointed out that the frequency of the crystals increased with increases in temperature, especially from 100 to 200 OC. From 25 to 40 “C the effect of the temperature is very small, an increase of about 40 Hz. These experiments show that temperature must remain constant during the reading, but a change of 10 “C is not critical. Triethanolamine was used as coating material by Cheney et al. (22,231. They measured the temperature dependence of an uncoated 9-MHz crystal and found 71 Hz of change while the temperature increased from 10 to 35 OC. Adsorption and desorption of the SOz on the coating material at different temperatures were also investigated. The method of coating the crystal with various substrates has been shown to be very important. In application of the substrate to the piezoelectric crystal, several different techniques were evaluated. Among these were: dropping, dipping, and spraying methods. Earp (2.5) in a previous study, showed that the sensitivity of the sorption detector is inversely proportional to the area coated and the distance from the center of the crystal. As the area of coating on electrode surface decreases, the sensitivity increases to a maximum and then decreases again because of less surface area being available for sorption. It is essential, therefore, that the coating be placed on the crystal in the exact same manner each time. In the drop method, the substrate is dissolved in a volatile solvent and then a drop of this solution is placed at the center of the crystal electrode using a microsyringe or glass bar: the solvent is then allowed to evaporate. In the dip method, the crystal is dipped into a solution of the substrate in a volatile solvent and then allowed to dry. In the spray method the crystal is coated by spraying a solution of the different compounds in a suitable solvent onto a crystal surface. The critical factor involved in coating the crystal is not so much the amount of coating as it is the ability to reproduce the coating operation, as established by Hartigan (26). Cheney et al. (24) used cotton swabs to coat a crystal with ethylenedinitrotetraethanol for detection of SO2. The technique of the coating of the crystal was checked for repeatability of both coating amount and sensitivity. A greater sensitivity to SOz was obtained using a center-coated 9-MHz crystal (340 Hz) than a totally coated crystal (260 Hz). The authors also found that the frequency change due to a varying coating is predictable and consistent for a center-coated crystal, but not for a fully coated crystal. It can be seen from Table I that considerable interferences were caused by NOz and moisture in the assay of SOz using all coatings. An accurate quantitative determination cannot be obtained for SO2 in the presence of NOz by coated piezoelectric crystals. Guilbault et al. (19) showed that a hydrophobic membrane filter (pore size 0.45 fim) was successful in greatly reducing response of a quadrol coated crystal to atmospheric moisture. Each additional filter layer further reduced the moisture response, and with 4 layers the response for mositure was completely suppressed. Detector for Ammonia. Detection of NH, in the ppb range was carried out by coated piezoelectric crystal detectors. When Ucon 75-H-90,000 and Ucon-LB-3OOX were used (27) as coatings, good sensitivity to ammonia was obtained, and nitrogen dioxide reacts only following an exposure of the coating to nitrogen dioxide for about 5 min. The nitrogen dioxide reacted with Ucon coatings and new compounds were formed on the crystal showing a great sensitivity for both ammonia and nitrogen dioxide. Forming of new compounds was evident from the appearance of new bands and shifts of some bands observed in the infrared specta of these sub-

3SC1 L A T O R 1 -l&l2

N2

Flgure 4. Experimental apparatus for detection of organophosphorus compounds and pesticides (29)

stances. Some problems were caused by atmospheric moisture, and high concentrations of organic compounds which dissolve the coatings. New Coating Materials. Capiscum annuum pods, ascorbic acid, and ascorbic acid with silver nitrate, were applied as coatings for the selective detection of ammonia in the atmosphere (28). The experimental setup is shown in Figure 3. These specific coatings were not dependent on activation, and could be used a t ambient conditions. The ascorbic acid and an extract of Capiscum annuum react reversibly with ammonia in such a way that this type of coated crystal can be used to detect ammonia over a wide concentration range. In order to determine the type of reaction which occurs between ammonia and the coating materials, infrared spectra were obtained under various conditions. These spectra suggest that the reaction of ammonia with ascorbic acid is a simple acid-base reaction and this was assumed to be mechanism of the Cupiscum-ammonia reaction also. The nature of the increased sensitivity caused by silver nitrate is not known; however, it undoubtedly results from some type of interaction between ascorbic acid and silver. This interaction is probably an oxidation of ascorbic acid by silver ion, which is in turn reduced. Detector for Organophosphorus Compounds and Pesticides. In recent years great concern has arisen concerning the effect of pesticides on the environment. Pesticides are powerful cholinesterase inhibitors and as such are toxic to wildlife and humans, as well as to pests. The organophosphorus insecticides are all structurally related and undergo similar reactions. Since almost all of the organophosphorus pesticides contain either phosphoryl or thiophosphoryl groups, and the thiophosphoryl pesticides readily undergo oxidation reactions to produce phosphoryl-containing compounds, diisopropylmethyl phosphonate (DIMP) was chosen as a model compound in studies by Guilbault and Scheide (29). The experimental apparatus used is shown in Figure 4. Applying different inorganic salts (FeCl,, CuClZ,NiClZ, CdClZ)to coat the crystal, the detection of DIMP in the ppm range was effected. It was established that the FeCl,-DIMP complex, which was used as the substrate in the determination ANALYTICAL CHEMISTRY, VOL. 49, NO. 13, NOVEMBER 1977

1893

of low concentrations of DIMP, could not be useful in detection of other organophosphorus compounds of similar structure. Therefore, a detector was built using a FeC13paraoxon complex as the substrate on the piezoelectric crystal for the specific determination of paraoxon. A special instrument was developed from a modified design of Karasek's apparatus (30) in Shackelford's experiments (31). The instrument consisted of two modified Clapp oscillators, the signals from which were mixed, and the resultant frequency difference was fed through a pump diode circuit to either an ammeter or a recorder. A cobalt-isonitrobenzoylacetone (1BA)-diethyl-p-nitrophenylphosphonate (Paraoxon) complex was evaluated for use as a substrate on the quartz crystal. Three different compounds, Parathion, DDVP (dimethyl-dichlorovinyl phosphonate) and DIMP, were studied and Parathion showed the strongest interaction with the detector used. The recovery time for the crystal when Parathion was the pesticide detected was longer than that for DDVP in the ppb range, but recovery to the original frequency was complete. Various interferences were also investigated, and water vapor was compensated for by use of a mixer circuit in the instrument. Several inorganic salts, HgC12,HgBr2,MoCl5,CuC12,MnC12, and ZnClz,exhibited a strong chemical interaction with DIMP, as evidenced by a frequency shift to lower wavenumber of the phosphoryl stretching vibration in the infrared spectra (32). Guilbault pointed out that salts of most transition metals should prove useful as coatings in a piezoelectric crystal detector. Several experiments were done in order to determine the most optimum condition for detection of pesticides with a coated crystal (291. Among these, the cut-off points and the optimization of coatings were investigated in detail. The cut-off point is important in determining the optimum amount of substrate to be applied to the crystal. The frequency of oscillation of a crystal decreases with increasing mass according to the Sauerbrey equation, until the crystal is overloadedand ceases to resonate. It was established that a slightly larger amount of substrate can be added before cut-off occurs, if the crystal was coated on two sides instead of one. Detector for Hydrogen Sulfide. Hydrogen sulfide is a dangerous gas which has presented a safety problem to a number of American industries. This is especially true since hydrogen sulfide at a dangerous level is not noticed by workers and levels can increase suddenly. A method for selective detection of hydrogen sulfide in the atmosphere has been developed (33). This method is based on the adsorption of hydrogen sulfide on the surface of a quartz crystal coated by an acetone extract of various soots resulting from the burning of several organochlorine compounds. The extract of a soot prepared from chlorobenzoic acid provided the best substrate, and the method is most useful in the concentration range 1 to 60 ppm. Other coatings materials, such as lead acetate, metallic silver, metallic copper, and anthraquinone-disulfonic acid for detection of hydrogen sulfide with coated piezoelectric crystal were proposed by King (6). Detector for Hydrogen Chloride Gas. Hydrogen chloride gas is a noxious by-product of many industrial processes. The detection of this gas has been accomplished by the use of a crystal coated with different tertiary amines. A successful technique for placing these amines on the surface of a piezoelectric quartz crystal was to vapor deposit the amines on the crystal surface in a flask using vacuum and heating. The amount of amines deposited could be monitored during this process, and reproducible coatings were possible. T h e adsorption of hydrogen chloride on a crystal coated with triphenylamine was investigated as a function of time 1804


(34). The response in the field was demonstrated during the Titan-Centaur launch of the first US.Mars landing vehicle. Coupled with preliminary laboratory data, a concentration level of 2 ppb was found. Other amines were investigated for detection of hydrogen chloride gas in the ppb range. Among these the most sensitive was trimethylamine-HC1 (35). Detector for Aromatic Hydrocarbons. Since aromatic hydrocarbons are very common in gasoline and are naturally also found in auto exhausts, a fast and selective method for detection of the aromatic hydrocarbons in air is needed. A Nujol mixture of trans-chlorocarbonyl-bis(tripheny1phosphinelirridium (I) [trans-IrCl (CO) (PPh,),] was used as a coating material by Karmarkar et al. (36) in a piezoelectric device. A benzene solution of the Nujol Ir-complex (1% w) was prepared and the crystal was coated on both sides with a tiny brush. It was found that this crystal coating was reactive to aromatic hydrocarbons but was not as sensitive to aliphatic and ordinary olefines. Hence, aromatic compounds such as xylenes, benzaldehyde, 1,3,5-trimethylbenzene,anisole, n-butyl benzene could be detected a t low concentration (1-120 ppm), but compounds such as hexane, heptane, octane, cyclohexane, etc., could be detected only a t high concentrations. No pronounced effect of atmospheric moisture on the detection limit was observed. The method seems to be fast, reproducible, and inexpensive. Detector for Mercury in Air. In recent years, considerable attention has been devoted to the development of methods for detection of low concentrations of mercury in air because the presence of this substance in the environment is very dangerous for human beings. A quartz piezoelectric crystal detector with gold evaporated onto the crystal surface was shown to be a selective and sensitive substrate by Scheide et al. (37,38). Placing of the crystal into a variable oscillator circuit, measurements of the change in frequency of the crystal due to the increase in mass permits a sensing of the amount of mercury vapor present in air down to subpart-per-billion concentrations. Calibration curves were obtained from ppm to sub-ppb concentrations of mercury. Reversibility was achieved by placing the detector in an oven held at a temperature of 150 "C and flow switching a stream of clean as well as dry air over the detector. The effect of flow rates of air,temperature, and interferences on the detection of mercury was investigated. The detector designed by Scheide has potential for use both as an air pollution sensor and in industrial hygiene application.

+

DETECTOR FOR GAS CHROMATOGRAPH Traditional gas chromatographs utilize the hot-thermal conductivity detector or more sensitive flame ionization detector. Recently, even more sophisticated detection methods have been developed, using electron capture and mass spectrometry techniques. A new, relatively inexpensive gas chromatograph apparatus applying a piezoelectric crystal detector was developed by King (6,7) and Karesek (30,39,40). The quartz crystal was coated with the same partitioning liquid used in the gas chromatography columns. The separated compounds are detected as they leave the column by passage over a coated crystal surface, where they partition into the liquid coating changing the original resonant frequency of the oscillating piezoelectric quartz crystal. The change in frequency can be used as a detector response by converting it to a voltage. The piezoelectric detector can operate at room or higher temperature using air, nitrogen or helium carrier gas (Figure 5). Table I1 presents the applications of the piezoelectric crystal detector for qualitative and quantitative separation of different compounds. Even though the chromatograph with piezoelectric detector

Table 11. Various Coatings Used in Gas Chromatography with a Piezoelectric Crystal Detector Crystal coating

Column packing

Temperature "C

Time, min

Carrier gas, mL/min

Carbowax 400 (7 pg)

5% Carbowax 400 on Chromosorb W

40-100

..,

Carbowax 400 ( 7 pg)

5% Carbowax 400

74

...

Air ( 5 )

Nitrogen (10)

on Chromosorb W Ucon LB 550X (4 p g )

5% Ucon LB550X on Chromosorb W

25

8

Air (60)

Carbowax 400 ( 4 pg)


25

14

Air (50)

OV-17 ( 4

5% OV-17 on

25

14

Air ( 3 0 )

pg)

Chromosorb W Rubber cement


25

...

Carbowax 400 (4 pg)

5% Carbowax 400 on Chromosorb W Carbowax 400 on Chromosorb W

25

1

Air (60)

22

'i

Dry air (60)

5% OV-17 on

22

4

Helium (50)

Carbowax 400 OV-17

9

.

.

Chromosorb W Carbowax 20M


22

6.5

Helium (40)

Carbowax 20M

10%Carbowax 400

22

5.5

Helium (40)

on Chromosorb W Carbowax 20M


22

2

Helium (55)

OV-17

5% OV-17 on Chromosorb W

22

2.5

Helium (62)

Squalane

Squalane

27

4

Helium (60)

1,2,3-Triscyanoethoxy-propane (TRIS)

TRIS

23

28

Helium (45)

Squalane

Carbowax 1540 dinonyl phthalate (DNP)

62

. . ,

Helium (10-15)

has a limited range, it can be applied to a large number of compounds present in the boiling range up t o 200 "C. The response of a piezoelectric detector for a compound eluted from a gas chromatographic column can be described by the equation (39):

A = C W/yP°F where A = area of response curve, W = total weight of the eluent, y = activity coefficient of the eluent in crystal coating, Po = vapor pressure of the eluent a t the operating temperature, F = carrier gas flow rate, C = a constant which is

Samples

Ref.

Benzene, -ndecane, -nbutanol n-Hexane, noctane, n-decane, n-dodecane Ethyl acetate, propyl acetate, butyl acetate, amyl acetate, hexyl acetate Ethyl-, propyl-, butyl-, amyl-, alcohol n-Hexane, benzene, noctane, toluene, n-decane n-Nonane, ndecane, bundecane, dodecane SO, in nitrogen 53.4% methanol, 40.0% ethanol, 6.6% propanol Benzene, chlorobenzene, mdichlorobenzene Acetone, diethylketone, 2-methylpent-2-enol, nbutanol, cyclohexanone Acetone, ethanol, n-propanol, water, cyclohexanone Normal breath, breath containing ethyl alcohol n-Hexane: 35.5 wt %; n-heptane: 17.9 wt %; n-octane: 18.4 wt %; n-nonane: 18.7 wt %; n-decane: 9.5 wt % n-Pentane, nhexane, benzene, n-heptane, toluene, n-octane, ethylbenzene, o-xylene Benzene, toluene, ethylbenzene, propyl-benzene, butylbenzene, pentylbenzene, hexylbenzene, heptylbenzene n-Pentane, nhexane, noctane, benzene

39 39 40

40 40

40

40 41 30 30

30 30 30

6

8

21

characteristic of the detector temperature, the crystal, and the liquid phase used to coat. This equation points out many of the salient characteristics of the piezoelectric detector, when i t is used in conjunction with the gas chromatograph. Detector temperature is an important parameter in piezoelectric detection. The detector used at above ambient temperatures was found to have essentially the same characteristics (i.e., sensitivity, linearity, etc.) as those with piezoelectric detectors used at room temperature. The relative partition coefficients of components vary with temperature so that there is a difference in separation between compounds ANALYTICAL CHEMISTRY, VOL. 49, NO. 13, NOVEMBER 1977

1805

YiLENCiD

F'JHE

HOOD

I

%P%L

INLET

P A C K A X :RYCTAL

0% I L A T O R

TO 5ocEN31D VALVES

'A

Flgure 6. GPC crystal mass detector used by Schultz ( 4 2 ) Figure 5. Schematic drawing of the chromatograph with piezoelectric detector used by Karasek (30)

detected a t high temperatures. Absolute detector response for a given compound decreases with increasing temperature. This effect was found not to be as pronounced as is predicted from theoretical considerations. For optimum detector conditions, the column and detector temperatures should be kept as low as practical, yet sufficiently high to elute the component of interest in a reasonable time (39). Since all chromatographic partitioning solvents have a definite vapor pressure; the lifetime of the coated piezoelectric detectors depends on the solvent type and whether a carrier contains the substrate vapor from the column. Polymer and adsorbent coated crystals appear to be immortal. Water analyzer crystals have been in service 5 years. If an accident occurs, a fouled detector can be completely reconditioned by solvent washing, or placing a new coating on the crystal. In practice, therefore, the detector lifetime does not become an important consideration in instrument performance.

DETECTOR FOR LIQUID CHROMATOGRAPHY A universal mass detector based on the resonating piezoelectric crystal for liquid chromatography was developed by Schulz et al. (42) Figure 6. Effluent from a liquid chromatograph was sprayed on the crystal surface, the solvent was evaporated, and the mass of residual solute determined from the change in crystal frequency. Sampling of the liquid stream and solute deposition can occur at rapid intervals. Spraying, drying, and measuring was accomplished in 10 s and the sensitivity attained was similar to that observed by commonly used liquid-liquid chromatographic detectors. The piezoelectric detector was connected to the effluent from a differential refractometer on a gel permeation chromatograph. Butyl rubber and a mixture of polystyrenes were run. Differential distribution curves of polymer mass vs. retention volume compared closely to results obtained with the differential refractometer detector, and the piezoelectric crystal detector was found to be equal to and, in some ways, superior to the refractometer detector. The advantages of the crystal mass detector we-L e summarized by Schulz (42): mass detector, universal detector, high sensitivity, large dynamic measuring range, nondestructive, independent of temperature, pressure, or flow rate variations of chromatograph, volatile impurities do not interfere, any available solvent or solvent mixture can be used, digital response compatible with digital processing equipment. MICROWEIGHING WITH THE QUARTZ CRYSTAL DETECTOR The piezoelectric quartz crystal detector may also be used for the quantitative measurement of thin films deposited on its surface ( 2 , 4 3 , 4 4 ) . The advantage of the microweighing technique over conventional techniques is the very thin film, 1896


which resulted in a high speed of performing experiments. In addition, the high sensitivity permits observation of the initial stages of reaction. Among others, an oxidation test of uninhibited styrenebutadiene rubber was carried out, and the result was attained in a few minutes by King (8). Commercial SBR rubber was dissolved in hexane and a weight corresponding to 5.3 kHz was coated on a 10 MHz crystal, and then purged with nitrogen at 150 "C. At time zero, pure oxygen replaced the nitrogen. At the beginning of the experiment, the frequency can either increase or decrease depending on whether oxygen uptake is slower or faster than the competitive cracking and volatilization reactions. Because the film was thin and the technique sensitive, the very initial phases of oxidation were observed. This use of the crystal detector permits a rapid quantitative study of inhibitors, of the effect of light, as well as of polymer type and blends thereof. Another interesting application described by Haller et at. (45) is the study of the polymerization of butadiene. Fisher et al. (46) reported on the oxidation of elastomers, and King et al. (47) studied the oxidation and volatilization of asphalts. King (8) conducted a study of monolayers.

OTHER APPLICATIONS Piezoelectric Crystal for Polymer Research. Polymer-coated crystals were used routinely for measuring vapor solubilities of a wide variety of plastics and elastomers. Equilibrium between vapor and rubber is almost instantaneous, mainly because the film is so thin; further the data so produced are highly precise, allowing the observation of subtle changes in the polymer. Studies of a film of butyl rubber exposed to hexane vapor, n-pentane, n-hexane, n-heptane, n-octane, and chloroform were carried out by King (48). Transitions in polymers can also be examined using vibrating piezoelectric crystals. Conclusions derived from the frequency changes were confirmed by differential thermal analysis (DTA) (49). The density-temperature profiles of normal paraffins were investigated by Vand (50). Piezoelectric Crystals for Residues in Liquid and Hydrogen. The measurements of residues in water and solvent have been readily accomplished with simple portable battery-operated equipment. Solvents were tested for purity before use in making solutions for applying thin films to crystals. A micropipet was cleaned, flushed, and filled with the solvent; 0.5 WLincrementa were deposited on an electrode and allowed to evaporate. After 0.05 mL was evaporated, the frequency change was checked. A change of 100 Hz corresponds to 10 ppm impurity and 0.1 ppm was easy to detect. Hydrogen and hydrocarbons can be continuously analyzed by the Du Pont 510 moisture analyzer, which uses two coated crystals and can detect less than 1 ppm. An integral part of the analyzer is a heatless dryer used to provide a dry reference air sample (51). To analyze for hydrocarbons, the reference

air is burned by a simple hot platinum wire in a quartz combustion tube and introduced back into the analyzer. In this way, a water signal is obtained continuously, representative of hydrocarbons in the air. Miscellaneous Applications. A special, integral heater crystal is ideal for use as a reactive gas detector. These measurements become simpler because the reaction occurs only on the hot crystal and the rest of the environment can remain a t ambient conditions. Sensitive and selective hydrogen detectors were made by evaporating platinum and palladium on crystals and heating to 150 "C. Both detectors could reach the ppm range without difficulty; however, high H2S levels fouled the catalyst, reducing sensitivity (48). A piezoelectric device was developed to measure the size and concentration of particles produced in the cloud generated by an underground nuclear explosion with the formation of a crater (52). The measurements procedure can be adapted to measure the granulometry of any type of particle in a fluid suspension. In the calibration, the diameter and velocity of the particles were varied as well as the dimensions of the piezoelectric material used. Measurable signals were obtained for 5-ym particles having masses of the order of 3 X g ejected at velocities of about 210 m/s. Aerosol mass (concentration) determination using piezoelectric crystal sensors was investigated by Daley, et at. (62). The effect of five areas of influence were studied: temperature, humidity, particle collection characteristic, response linearity, and mass sensitivity. Neither air stream temperature nor humidity fluctuations were compensated by the use of a reference crystal. The temperature-induced error was satisfactorily reduced by minimizing the rate of inlet temperature change. The humidity-induced error resulted principally from moisture absorption and desorption by the aerosol deposit. The observed linear response limits ranged from 0.2 to 6 yg/mm2 for various aerosols and instrument designs. The mass sensitivity was a function of the deposit size and location. The mass sensing ability decreased for a particle size beginning at -2-ym diameter reaching essentially zero to 20 ym. The use of viscous crystal coatings appeared to improve the sensing ability in the 2- to 20-ym size range, Olin et al. (63) pointed out that a piezoelectric quartz crystal, can be effectively used a a microbalance for monitoring the mass concentration of suspended particles. A collector, such as an impactor or an electrostatic precipitator, deposits the suspended particles onto the electrode surface of the vibrating crystal and the resonant frequency decreases linearly with the added mass of the particles. The high sensitivity and time resolution of the instrument was studied. With an electrostatic-precipitator collector sampling at 1L min-' the total mass concentration of suspended atmospheric particles (100 was measured to within f 5 % in only 41 s. Chuan (65) reported a portable direct-reading instrument which can measure particulate mass concentrations from 50 to 5000 4 m 3 ;the detector is not sensitive to vapors (including water vapor) normally present in air. Examples of measurements of spatial and temporal variations of particulate mass concentration caused by specific short-term effectssuch as the startup of a machine tool, the use of an aerosol can, and the entry into an otherwise clean room of a smoker-were shown. Results of periodic sampling of air automatically in a shop area over a 25-h period were also shown. A piezoelectric quartz crystal was used by Mieure (61) for electrogravimetric metal trace analysis. Linear calibration curves showing frequency changes as function of the initial metal ion concentrations were found to be suitable for quantitative analysis. Determinations of very low concentrations of cadmium, nickel, zinc, indium, and lead in aqueous solutions were carried out. Cadmium was used as a model

for the most extensive study. Cadmium concentrations covering the range from 5.0 X M to 5.0 X M were determined and similar results should be obtainable for other metals. The accuracy of the method for cadmium varied from 0.42% a t larger concentrations to 8.7% at lower amounts. Chemisorption reactions of mono-, di-, and trimethylamine were studied at room temperature hy means of piezoelectric crystals in a vacuum system (53). Thin films of various metal salts (FeC13,ZnC12,HgBr,, CoCl,, and ZnIJ were used as solid substrate coatings on the crystals. These reactions were investigated for the purpose of determining the best coatings for detection and determination of these toxic amines using the piezoelectric crystal detector. Coatings of iron(II1) chloride show some promise. The application of a piezoelectric quartz crystal for detection and determination of H(D) in an inert gas and for determination of the deuterium content in a gaseous (H2+ D&mixture was reported by Bucur (64). The measuring ranges for H2 and D2 in nitrogen gas were 0-16% and 0-2570, respectively, with a relative error of 4'%. The measuring range for D content in isotropic mixtures was within 4% and 96% with a relative error of less than 2 % . The workirig temperatures were 45.8 and 50 OC, respectively. The effect of the adsorption of hexane and argon on a piezoelectric quartz crystal was investigated and some approximate isotherms and related thermodynamic quantities were reported by Slutsky et al. (54). A method for measurement of film thickness and deposition rate was developed by Behrndt et al. (55) and Oberg (56)by means of piezoelectric crystals. A thickness shear mode AT-cut crystal with a 2.5-MHz oscillating frequency has a frequency change on the order of 1 Hz/A thickness of metal deposited on the exposed crystal face. In general, the crystal can accumulate about 20 OOO A of metal before removal of the deposit is required or a new crystal used. A simple portable detector was made by King (57) using the solvent sorption detector for investigation of the detection limit of some compounds. A small aquarium pump sucked the air through the detector and the signal was observed by deflection of a galvanometer. A very low sensitivity was obtained for low molecular weight materials, but very high sensitivities were found for high molecular weight materials. Finally, other applications include the quartz crystal thermometer, quartz pressure transducer (58),and thin film thermocouples (59).

CONCLUSIONS A piezoelectric coated crystal analyzer sensitive to 0.1 ppm water vapor was developed by King (66). This sorption hygrometer has been commercially available since 1964 (Du Pont Instruments, Wilmington, Del., Model 560). The excellent qualities of the instrument are: high selectivity, fast response, long life-time with accuracy of about 5% (9). Detection of different gases in ambient air (SOz, NH3, H2S, HC1, NOz) and organophosphorus compounds, pesticides, as well as aromatic hydrocarbons has been already accomplished by a piezoelectric device. A portable instrument operating on a car battery for monitoring sulfur dioxide in auto exhausts and refinery stack gases was successfully used by Guilbault (20). It is not yet commercially available. Considerable interferences were caused by nitrogen dioxide and moisture on most of the coating materials used for detection of gaseous pollutants. A hand-calculator sized instrument with digital readout for measuring mercury vapor concentration in air and/or personal exposure to mercury vapor was developed and evaluated by Scheide (67). Measurements of concentrations below 15 ppb have a precision of 10% with uncertainties of 5% above this level. ANALYTICAL CHEMISTRY, VOL. 49, NO. 13, NOVEMBER 1977

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A gas-chromatograph apparatus using a piezoelectric crystal detector was designed and developed for determination of compounds in boiling range up to 200 “C. The qualitative and quantitative performance of this chromatograph (P/Z Chromatograph, Laboratory Data Control, Riviera Beach, Fla.) was investigated by Karasek (40). Janghorbani (21) pointed out that piezoelectric detectors in chromatography can be interfaced to a large, general purpose, time sharing computer for on-line data acquisition and processing with simple and inexpensive equipment. A prototype piezoelectric crystal mass detector for GPC was compared with the refractometer detector (42). The experience gained on the new type of sensor indicated that more measurements are needed to develop a generally useful detector. Two commercially available piezoelectric aerosol mass concentration measurement devices were used by Daley (62) for investigation of the conditions for determination of aerosol: Thermo-Systems (TSI) Particle Mass Monitor Model 3200 (Thermo-Systems, Inc. St. Paul, Minn.) and Celesco Quartz Microbalance, Model 37A (Celesco Industries, Costa Mesa, Calif.). The instruments are both portable, relatively rugged, and operate on ordinary commercial power. Chuan (65) also reported a typical commercial instrument based on the piezoelectric quartz crystal microbalance for rapid assessment of particulate mass concentration in the atmosphere: Particulate Monitor, Model P M 40, Celesco Industries, Inc., Costa Mesa, Calif. The adhesive coating used in the device is nonhygroscopic and nonreactive t o the usual concentrations of polluting gases in the atmosphere, such as carbon monoxide, sulfur dioxide, nitrogen oxides, and hydrocarbons. As indicated above, the piezoelectric crystal detector can be used in many areas of analytical chemistry. Some of the piezoelectric instruments described are now commercially available, but in the area of detection of gaseous pollutants (SO*,NH3, HC1, etc.) no commercial instruments are yet available. Some possible reasons for this are: it is very difficult to find coatings which have long life-times, are selective and sensitive only for the gas of interest, give a measurable response, do not suffer considerable interferences from other gases, and which are completely reversible. Experiments to solve these problems are in progress, and instruments that are portable for field use are already in the development stage. It appears that the piezoelectric crystal detector will play an important role in the near future of air pollutant control monitoring. LITERATURE CITED F. W. Maarsen, M. C. SmR, and J. Matze, Rec. Trav. Chim. Pays-Bas, 76, 713 (1957). R. A. Heising, “Quartz Crystal for Electrical Circuits”, Van Nostrand, New York, N.Y., 1946,p 24. G. 2 . Sauerbrey, 2. Phys., 155,206 (1959). G. 2 . Sauerbrey, 2. Phys., 178, 457 (1964). A. W. Warner, and C. D. Stockbridge, “The Measurements of Mass using Quartz Crystal Resonators,” Symposium on Vacuum Microbalance Techniques, Los Angeles. Calif., 1962

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W. H. King, Jr., Anal. Chem., 38, 1735 (1964). W. H. King, Jr., U S . Patent 3,164,004(Jan. 5, 1965). W. H. King, Jr., Res.lDev.. 20 (4),28 (1969). Moisture Analyzer Model 560,Du Pont Instruments, Wilrnington, Del. D. T. Gjessing, C. Holm, and T. Lanes, Electron. Lett., 3 (4),(1967). W. H. King, Jr., Ger. Offen 1,901,845(August 13, 1970). W. H. King, Jr., and G. M. Varga, Jr., Final Report for Martin-Marietta Corp.. Contract No. RCO-446067. W. H. King. Jr., F. E. Steidler, and G. M. Varga, Jr., Phase 1-11 Report for Martin-Marietta Corp., Contract No. RC1-2011337. W. H. King, Jr., F. E. Steidler, and G. M. Varga, Jr., Phase 1-11Report for Martin-Marietta Corp.. Contract No. RC1-201107, M. W. Frechette and J. L. Fasding, €nvi.on. Sci. Teahnd.,7, 1135 (1973). M. W. Frechette, J. L. Fasching, and D. M. Rosie, Anal. Chem., 45,1765

(1973). G. G. Guilbault and A. Lopez-Roman, Environ. Left., 2, 35 (1971). K. H. Karmarkar and G. G. Guilbault, Anal. Chim. Acta, 71,419 (1974). K. H. Karmarkar, L. M. Webber and G. G. Guilbauk, Environ. Lett., 8, 345 11975). I -,-

k..h.Karmarkar, L. M. Webber, and G.G. Guilbault, Anal. Chim. Acta, 81. 265 (1976). M.’Janghorbani and H. Freund, Anal. Chem., 45, 325 (1973). J. L. Cheney and J. B. Homolya, Anal. Lett., 8, 175 (1976). J. L. Cheney and J. B. Homolya, Sci. Total Environ., 5 , 69 (1976). J. L. Cheney, T. Norwood, and J. B. Homolya, Anal. Left., 9, 361 (1976). R. B. W. Earp, Ph.D. Dissertation, University of Alabama (1966). M. J. Hartigan, Ph.D. Dissertation, University of Rhode Island (1970). K. H. Karmarkar and G. G. Guilbautt, Anal. Chim. Acta, 75, 1 1 1 (1975). L. M. Webber and G. G. Guilbault, Anal. Chem., 48, 2244 (1976). E. P. Scheide and G. G. Guilbault, Anal. Chem., 44, 1764 (1972). F. W. Karasek and K. R . Gibbins, J. Chromatogr. Sci., 9, 535 (1971). W. M. Shackelford and G. G. Guilbault, Anal. Chim. Acta, 73, 383 (1974). G. G. Guilbault, Anal. Chim. Acta, 39, 260 (1967). L. M. Webber, K. H. Karmarkar, and G. G. Guilbault, Anal. Chem., in preparation. NASA Tech. Brief, MFS-23357. J. Hlavay and G. G. Guilbautt, Anal. Chim. Acta, in preparation. K. H. Karmarkar and G. G. Guilbauit, Environ. Lett., 10, 237 (1975). E. P. Scheide and J. K. Taylor, Envlron. Sci. Techno/., 8, 1097 (1974). E. P. Scheide and J. K. Taylor, Am. Chem. Soc., Div. Envlron. Chem., Prepr.. 14, 329 (1974),presented Los Angeles, Calif., April 1974. F. W. Karasek, P. Guy,H. H. Hill, and J. M. Tiernay. J. Chromatogr.. 124. 179 (1976). F. W. Karasek and J. M. Tiernay, J. Chromatogr., 89, 31 (1974). H. R. Smith, Am. Lab., 49, October (1972). W. W. Schulz and W. H. King, Jr., J. Chromatogr. Sci. 11, 343 (1973). A. W. Werner. Bell Svstem Tech. J.. 39. 1193 (1960). A. W. Werner, I€€€ b a n s . Sonics Ultrason., su-12 ( 2 ) (1965). L. Haller and P. White, J. Phys. Chem., 87, 1784 (1963). W. F. Fisher and W. H. King, Jr., Anal. Chem., 39, 1265 (1967). W. H. King, Jr.. and L. W. Corbett, Anal. Chem., 41, 580 (1969). W. H. King, Jr., ReslDev., 20, 28 (1969). W. H. Kina. Jr.. C. T. Camilli. and A. F. Findeis. Anal. Chem.. 40, 1330

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V. Vand, Acta Crystallcgr., 8 , 797 (1953). C. W. Skarstrom, U.S. Patent 2,944,627(July 12, 1960). A. Chabre. from Nucl. Sci. Abstr.. 27. 22601 (1973). G. G. Guilbautt. A LoDezRoman. and S. M. B & li&. A‘&. Chim. ActB, 58, 421 (1972). L. J. Slutsky and W. H. Wade, J. Chem. Phys., 38, 2668 (1962). K. H. Behrndt and R. W. Love, Vacuum, l(1962). P. Oberg and J. Logenslo, Rev. Scl. Insbum. 30, 1053 (1959). W. H. Kina. Jr.. Bull. N . Y . Acad. Med.. 48. 459 (1972). D. L. HaGmond and A. Benjaminson, I€€€ S p e c h m , ’ 8 , 53 (1969). W. H. King, Jr., C, T. Camilli. and A. F. Findais, Anal. Chem., 40, 1330

(1968). E. P. Scheide, Ph.D. Dissertation, Univ. of New Orleans, 1972. J. P. Mieure, PhD. Dissertation Texas ABM University, 1968. P. S. DaleyandD. A. Lundgren, Am. I d . Hyg. Assoc. J., 38, 516(1975). J. G. Olin and G. J. Sem, Atm. Environ., 5, 653 (1971). R . V. Bucur, Rev. Roum. Phys., 19, 779 (1974). R. L. Chuan. ISA J. 620 (1975). W. H. King, Jr., U.S. Patent No. 3,154,004(1964). E. P. Scheide, Presentation at the ACS Conference, New Orleans, La., Mar. 20-25, 1977.

RECEIVED for review March 21,1977. Accepted July 25,1977.

Applications of the piezoelectric crystal detector in analytical chemistry

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