Chiang Mai J. Sci. 2016; 43(4) : 782-792http://epg.science.cmu.ac.th/ejournal/Contributed Paper

Simple Laboratory-made Piezoelectric Sensors for Detection of Selected Gaseous and/or Vapor Sample Amnat Ruangchaiwat [a], Narabhats Rannurags [a], Sukon Phanichphant [d], Boonsom Liawruangrath [b,c] and Saisunee Liawruangrath*[a,c][a] Alpha Flow Analysis Group, Department of Chemistry and Center of Excellence for Innovation in

Chemistry (PERCH-CIC) Together with Material Science Research Center, Faculty of Science, Chiang Mai University, Chiang Mai 50200, Thailand.

[b] Department of Pharmaceutical Science, Faculty of Pharmacy, Chiang Mai University, Chiang Mai 50200, Thailand.

[c] Science and Technology Research Institute, Chiang Mai University, Chiang Mai 50200, Thailand.[d] Material Science Research Center, Faculty of Science, Chiang Mai University, Chiang Mai 50200,

Thailand.*Author for correspondence; e-mail: scislwrn@gmail.com

Received: 8 August 2014Accepted: 24 November 2014

ABSTRACT A very simple laboratory made prototype piezoelectric transducer (PZT) or sensor is described using inexpensive materials and equipment easily available in Chiang Mai and in the laboratory. Initially, experiments are carried out using a single cell device constructed in this laboratory afterwards a double-cell device is modified from the prototype device for measuring gaseous/vapour contaminants in air. The PZT detectors were tested for detection of SO2 on the basis of the best known SO2 detection using triethanolamine as coating material. It was found that the dual-cell device was more sensitive to SO2 (48 Hz ppm) than the single-cell one (6 Hz/ppm). Optimal conditions for the PZT detector were also investigated. The AT-cut quartz crystals with circular shape having thin film gold electrodes on both sides were employed. It was shown that the 10 MHz crytal coated with triethanolamine was more sentitive than the 6 MHz quartz crystal and hence the 10 MHz quartz crystal was selected as optimum. Selectivity of the PZT detector can be achieved by changing the coating materials. Under suitable conditions, choice of appropriate coating materials could be achieved for a number gaseous/vapour samples. The proposed PZT sensors using selective coatings were successfully applied for detection of SO2 and methanol with satisfactory results.

Keywords: piezoelectric, PZT sensors, methanol, sulphur dioxide

1. INTRODUCTION Sauerbrey’s pioneering work [1] on the theoretical aspects decribing the relationship between the weight of metal films deposited

on quartz crystals and the change in frequency was reported in 1959, but no analytical aspects were described by him. Until in 1964 King [2]

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was the first who followed Sauerbrey’s idea to develop such a device termed “piezoelectric sorption detector” for gas chromatography. The method is based on the change in frequency of vibration of the coated quartz crystal due to the amounts of gas of interest which are selectively and reversibly sorbed by the coating so that magnitude of the change is a measure of the amount of the analyte sorbed. Since then, the volume of literature concerned with the use of coated piezoelectric crystal detector for gases and vapours [3,4] including air pollutans has grown almost expotentially [3-7]. A brief review based on coated piezoelectric quartz crystal in analytical applications had been presented [5]. Selectively of the PZT sensor was improved by a wide choice of coating materials. Various methods for application of the coating on to the electrodes on the quartz plate and the sampling methods had been reported. The most widely used methods for coating the electrode surfaces and the sampling methods were using the tiny brush and syringe dilution methods respectively. [8] In recent years, coated piezoelectric sensor or piezoelectric transducers (PZT) have been increasingly used for chemical, pharmaceutical and environmental analyses together with process monitoring, ultrasonic imaging, etc. [6,7] High temperature piezoelectric aceler omuters are of particular interest owing to their simple structures, fast suspense time and easy integeation with other parts where the high temperature piezoelectric materials are the mainstay [9] Several types of piezoelectric materials for high temperature applications have been proposed such as quartz (SiO2), lithium niobate (LiNbO3·LN), gallium orthophosphate (GaPO4). Langasite (LGS) and aluminium nitride (AlN). Quartz is one of the most widely used piezoelectric materials in electronic devices due to its excellent electrical resistively (higher than 1017 Ωcm at room temperature), ultralow

mechanical loss (high mechanical quality factor), narrow bandwidth, high stability as soon as exposed to high temperature up to about 450°C above this high losses are observed. [10,11] Recently coated piezoelectric quartz crystals have been increasingly employed for quantification of certain gaseous and/or volatile compounds in environmental samples including air water and food samples. For examples Teresa et al., [12] described a procedure for quantification of CO2, SO2, NH3 and H2S using a coated piezoelectric quartz (with different coatings) as detection. A piezoelectric quartz crystal sensor array with optimized oscillator circuit for analyzing organic vapors mixtures has been reported. The sensor has been successfully applied for analysis of some binary and tertiary mixtures of environmentally harmful organic vapors; for binary mixtures such as 1,2-dichoromethane plus methanol and benzene plus cyclohexane; for tertiary mixtures such as a mixture consisting of dichloromethane, methanol and cyclohexane [13]. An amalgamated piezoelectric sensor has been proposed for determining free and total sulphur dioxide in wine sample using flow-based technique to combine sample treatment and detection on-line [14]. The method is rapid, cost effective, reproducible and accurate. Stress corrosion cracking of a lead-zirconate-titanate piezoelectric ceramics was studied in water, methanol and formamide. It was shown that this phenomenon could occur at constant load tests using single-edge notch tensile spenimens [15]. More recently, Trejo-Tzal et al., [16] have studied the photoactivation of titanium oxide Degussa P25 in ethanol-methanol suspension with varying molar ratios followed by monitoring at real using a PZT sensor. In the present work, a simple, cost effective, laboratory-made piezoelectric detector was developed and an electronic circuit was designed and constructed. The PZT sensor was tested for detector of SO2 and MEOH.

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2. MATERIALS AND METHODS2.1 Apparatus A home-made oscillator, A time convertor model 5382A Hewlett-Packard, Frequency meter, DC power supply, A frequency voltage convertor (RS component 307-070), A Digital frequency, Chart reader model 3000 (Oxford, UK), A personal computer.

2.2 Materials and Chemicals2.2.1 Materials Electronic component, PZT quartz crystals, AT-out (RS Component 13-17, Gpworth St, London, UK).

2.2.2 Chemicals Most chemicals used are of analytical reagent grade and used without purification unless otherwise specified. Liquid sulphur dioxide in a cylinder was obtained from BDH Chemicals Ltd., Poole, England. Dry gasous sulphur was prepared by passing sulphur dioxide vapor through a Drechsel bottle containing sulphuric acid (50 mL), then through a U-tube containing anhydrous calcium chloride and storing over phosphorus pentaoxide.

2.2.3 Coating materials All the coating materials were employed without further purification and prepared in chloroform (%V/V) unless otherwise stated.(i) Triethanolamine (Hopkin and Williams reagent for organic synthesis) 100 µg mL-1 in chloroform. (ii) The following chromatographic substances were obtained from Phase Separation Ltd., Deeside Industrial Estate Queensferry Flintshire. They were dissolved in chloroform. Diethylene glycol adipate (10%), Diethylene succinate (LAC 3R 728) (20%), Di-2-ethylhexyl sebacate (10%), carbowax 1000 (1% w/w), carbowax 20 M (1%w/w). (iii) Saturated solutions of sulphanilic acid (Laboratory Chemical, May and Baker, Dagenham, England) and Dimedone (BDH Laboratory Reagent) were

prepared in aceteone. (iv) 2% (w/v) solutions of sucrose octa-acetate and sorbitol were prepared in methanol. (v) 5% (w/v) solutions of sodium sulphite (AR), sodium hydrogen sulphite was prepared by passing an excess of SO2 into a NaOH solution, crystal from EtOH and allowing the crystals to dry under vacuum at room temperature. A 1000 µgS mL-1 solution was prepared by dissoluring 0.3250 g of NaHSO3 in distilled water; 2 mL of 0.1 mol L-1 NaEDTA were added and made up to 100 mL with water. 2.3 Construction of Home-made PZT Sensors2.3.1 A single cell instrument This instrument consisted of a single piezoelectric sensor. The experimental set-up together with an all glass gas-tight piezoelectric crystal cell used in this investigation were rather similar to those previously reported in the literature [17] (Figure 1a). The coated piezoelectric quartz crystal was connected to the oscillator for the single cell instrument via two tungsten leads. The oscillator was built with a transistor crystals oscillator (1-20 MHz) kit (the Quartz Crystal Co. Ltd., Wellington Crescent, New Malden, Surrey, England). The electronic diagram of the transistor crystals oscillator was shown in Figure 2. A Hewlett Packard Model 5382A Timer-Counter which has a read-out capability of 8 digits with a precision of ±1 Hz in the last digit and a resolution of 1 Hz was used to monitor the crystal frequency. This oscillator can be used together with either AT-cut 6 MHz or 10 MHz quartz crystal.

2.3.2 A double cell instrument This instrument was a dual-cell instrument constructed based on the modified design from Figure 1(a) which was shown in Figure 1(b) and the electronic diagram of the differential crystal oscillator is shown in Figure 3. The detector consists of two coated piezoelectric AT-cut

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(a) (b)Figure 1. Experimental set-up of home-made piezoelectric sensors (not to scale). (a) a single cell instrument (b) a dual instrument

Figure 3. Electronic diagram of a ifferential crystal oscillator for a double cell PZT sensor.

quartz crystals with fundamental frequency of 10 MHz, which are driven by two variable oscillators A and B incorporated into a mixer circuit. The oscillators are powered by a D.C. power supply set at ±5 V. The difference in frequency between the two oscillators (reference and detector) is fed to a frequency-voltage

convertor (RS component 307-070). The output voltage is monitored by a mV recorder (Oxford 3000, response time = 0.2 sec for 90% f.s.d.) and / or a PC. The frequency of the two oscillators is monitored by a digitals frequency counter (Hewlett Packard Model 5382A) as above.

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to the flattened tube which enables the change or removal of the detector cell. The crystal is placed in the flattened position of the cell, the parallel cell walls of which are separated by a distance of 1.0 cm. The parallel walls are 1.8 cm long and 1.8 cm wide, thus the effective cell volume is 3.2 mL (1.8x1.8x1.0 mL). The column effluent is equally divided by two 0.5 cm (o.d.) Pyrex tubes, which are connected two through the two parallel walls in such a way that their tapered ends (0.2 cm o.d.) reach as close to the centre of the opposite faces of crystal as possible. The gas escapes through an exit tube (0.5 cm o.d.) connected at the bottom of the cell. 2.4 Choice of PZT crystal Three types of commercially available piezoelectric quartz crystals having AT-cut and metal electrode (e.g., gold or silver) on both sides were used (Figure 5).

2.5 Procedure2.5.1 Methods of coating the crystals Two methods were employed for the application of the coating to the electrodes. (i) Dropping with a syringe. [18] An appropriate amount of the coating material was dissolved in a volatile solvent (e.g., chloroform) and a small droplet of this solution was applied to the electrode by means of a microliter syringe (Scientific Glass Engineering PTY Ltd., 111 Arden S7, North Mellbourne Australia 3051). After the solvent was evaporated, the crystal was then placed in an oven at 60-70°C for several hours so that the coating spread uniformly. (ii) Smearing with a tiny brush. [17] Various coating materials were dissolved in suitable solvents, and the solutions thus obtained were coated over the entire electrode, on both side, with a tiny brush. Care must be taken to apply the coating as uniformly as possible; but the uniformity of the coating was not checked. The solvent was allowed to evaporate leaving

Figure 2. Electronic diagram of the transistor crystal oscillator 1-20 MHz.

Component valuesR1 = 10K R2 = 4.7K R3 = 1KRFC = 1.5 mH.R.F. Choke C1 = 3/30 pf. Air trimmer C2 = 100 pf. Silver micaC3 = 25 Mfd. Electrolytic C4 = 0.1 Mfd. Polyester C5 = 10 pf. Silver Mica

Figure 4. The piezoelectric crystal detector cell ( volume = 1.8 x 1.8 x 1 = 3.2 ml) (not to scale).

2.3.3 Construction of the sample cell The all glass gas-tight piezoelectric crystal cells were as shown in Figure 4. The cell is made from a flattened Pyrex tube (2.3 cm o.d.). A pyrex B24 ground joint is connected

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the coating material on the surface of the electrodes.

Figure 5. Typical piezoelectric quartz crystals used (a) a 10 MHz crystal (The Quartz Crystal Co. Ltd., New Malden, Surrey); (b) a 10 MHz crystal (RS Components, 13-17 Epworth St., London); (c) a 10 MHz (manufacture as (a) (not to scale).

2.5.2 Sampling method In the present work, a syringe dilution [11] method was used to obtain very low concentrations of gases in air. The method involved sucking 1 mL of pure gas sample into a 10 mL gas-tight syringe, the dried air from air pump was sucked so that the total volume became 10 mL. the tip of the syringe needle was closed by one’s finger. Various mixing times were investigated and it was found that after 30 second the mixture in the syringe became homogeneous by diffusion and 1 mL of this mixture was diluted to 10 mL resulting in a 100-fold dilution. When the above procedure was repeated for several times ppm or ppb of gas mixtures could be obtained.

2.5.3 General procedure Gas or vapour samples were introduced to the detector by injecting the samples through a silicon rubber septum of a chromatographic type injection port by means of a 10 mL syringe (PS pressure-lok gas syringe Series A, as above) into a stream of carrier gas (dry nitrogen). The sample was carried through the system by dry nitrogen in such a way that the gas sample is selectively and reversibly sorbed

onto the appropriate coating, and the frequency of vibration of the crystal is thus altered. The frequency changes due to the sorption of the gas sample on the coated piezoelectric quartz crystal were measured. The gas sorbed was completely removed by the stream of dry nitrogen within a few minutes so that the sensor could be regenerated for re-use. The injection of the sample was carried out directly in front of the detector in order to achieve a quick response. The empty column was included to prevent the injected sample flowing back into the flow meter.

3. RESULTS AND DISCUSSION3.1 Use of the Single Cell Instrument A. Preliminary determination of sulphur dioxide The Threshold Limit Value for sulphur dioxide [19-20] in air is 5 ppm by volume or approximately 13 mg m-3. According to New Jersey Department of Health, USA. Reported TLV for SO2 in 2010 [20] based Workplace Exposure Limit, The TLVs for SO2 are follows: (i) OSHA: The legal airone permissible

exposure limit (PEL) is 5 ppm averaged over an 8-hour workshift.

(ii) NIOSH: The recommended airborne exposure limit (REL) is 2 ppm averaged over a 10-hour workshift and 5 ppm, not to be exceeded during any 15-minute work period.

(iii) ACGIH: The threshold limit value (TLV) is 0.25 ppm averaged over an 8-hour workshift.

Sulphur dioxide is often used as an index of general air pollution because of its widespread sources and occurrences. Piezoelectric detectors have been successfully used as excellent sulphur dioxide monitors. In the present work, the performance of the single cell piezoelectric device was evaluated on the basis of published work for the monitoring of sulphur dioxide in the

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atmosphere. Triethanolamine, [17,21] Quadrol (N, N, N’, N’-tetrakis-(2-hydroxypropyl) ethylenediamine) and 2, 2’, 2’’, 2’’’-(ethylenedinitrilo)-(tetraethanol) [14] had been reported as sensitive substrates for sulphur dioxide. In this experiment, triethanolamine was chosen because it is readily available.

(i) Investigation of experimental parameters An AT-cut, 6 Hz piezoelectric quartz crystal, having square shape (15 x 15 mm2) with gold electrodes (9 mm diam.) on both sides was used. The surfaces of the entire electrodes were coated with triethanolamine using a tiny brush. The effect of various parameters on the detection of sulphur dioxide was investigated.

Effect of nitrogen flow rate, the optimal nitrogen flow rate for obtaining maximal adsorption of sulphur dioxide on triethanolamine was found to be 15 mL min-1. At greater nitrogen flow rates, the maximum frequency change (∆F) decreased.

Effect of variation of room temperature, the detector was stable when it was operated at room temperature. Experimentally, by variations of room temperature showed a negligible effect.

Effect of coated area, the sensitivity of the detector is inversely proportional to the area coated, according to the Sauerbrey equation.2 It was found that when 100 ng of triethanolamine was coated on both sides of the entire electrodes (coated area ≈ 0.64 cm2) by using a microlitre syringe, the frequency change due to 1 mL of dry SO2 was 7 Hz. Such a frequency change was 17 Hz when the same amount of coating was applied in the centre of the electrodes, having a coated area of 0.06 cm2. Although the sensitivity obtained by the use of a totally coated crystal was worse than that obtained by using a crystal with partly coated electrodes, it

is good to coat the entire electrode in order to obtain a repeatable coated area on both sides.

Effect of moisture (water vapour), water vapour caused a very strong irreversible interaction with the triethanolamine coating. It was found experimentally that moisture content in SO2 cause a positive error of 52.94%. By comparison the frequency change due to 1 mL of sulphur dioxide from a cylinder was 26 Hz whereas that obtained from dried sulphur dioxide (1 mL) was 17 Hz.

Effect of the coating weight, in order to investigate the optimal weight, it is necessary to determine the overload frequencies of the piezoelectric crystals. These could be determined by coating the crystal with various amounts of coating material (e.g., triethanolamine for SO2) and measuring the frequency of variation. It was evident that the frequency decreased with increasing weight of the coating according to Sauerbrey equation until the crystal was overloaded and then ceased to resonate. The crystal ceased to resonate when the coating weight was ≥ 20 µg. The coating might be applied onto the entire electrodes using a tiny brush and the increased weight due to the coating was calculated from Sauerbrey equation2. The amounts of sulphur dioxide sorbed could be calculated in the same manner. Using the circular 10-MHz crystal, the mass sensitivity was 1171 Hz/µg of triethanolamine and that obtained by the use of a 6-MHz crystal was 129 Hz/µg. Thus the 10-MHz crystal seems to be more sensitive than the 6-MHz crystal. Therefore, the 10-MHz crystal was selected and used throughout.

(ii) Detection of sulphur dioxide Selection of piezoelectric quartz crystal. Attemps were made to use the triethanolamine-coated crystals having fundamental frequencies of 6 and 10 MHz as SO2 sensor. The amounts of

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the coating applied corresponded to a decrease of 6 kHz for the 6-MHz crystal and 6.7 kHz for the 10-MHz crystal. Various dilutions of sulphur dioxide in air or nitrogen (previously dried by passing through a U-tube containing silica gel) are prepared by syringe dilution and monitored according to the procedure described in the experimental section. It was found that using a 10-MHz crystal the frequency decrease due to SO2 x 5 mL of 30 ppm of SO2 was 18 Hz as compared to 9 Hz for the 6-MHz crystal. Thus, the more sensitive 10-MHz crystal was used in the further studies since a 50% enhance in sensitively was obtained. The crystal was coated with triethanolamine to give a decrease of 1019 Hz using a tiny brush. The effect of nitrogen flow rate on the determination of 30 ppm of SO2 (by volume) was investigated (Table 1). The adsorption of sulphur dioxide on the coating increased with nitrogen flow rate up to 15 mL min-1 above which it decreased. The time taken for reaching maximal adsorption decreased with increasing nitrogen flows (Table 1). A flow rate of 15 mL min-1 was chosen as optimal because it gave a maximum signal of 78 Hz for 30 ppm of SO2. The effect of various amounts of coating material on the determination of 30 ppm of sulphur dioxide was also studied. The amount of triethanolamine coated corresponding to a decreased frequency of 3511 Hz was chosen

as optimal because it exhibited the greatest response to SO2.

(iii) Precision, calibration and sensitivity Under the above suitable conditions, the procedure was repeated for various sulphur dioxide concentrations in air (previously dried by passing air through a U-tube containing silica gel). The coefficient of variation for 6 determinations of 20 ppm of SO2 was 3%. A calibration graph over the range 0.6-30 ppm of SO2 (∆F after coating = 3511 Hz) was linear. The sensitivity (linear slop) was 6 Hz/ppm. It was found that the sensitivity depended not only on the fundamental frequency of the quartz plate and the coated area but also depended on the amounts of the coating applied. In addition the care taken in coating the crystal played an important role in achieving high sensitivity, since the sensitivity depended on the uniformity of the coating applied. It was reported that the coating method employed was not critical provided that the resulting coating was smooth. Since Sauerbrey equation [2] was derived for smooth thin film, the weight-sensitivity predicted by such an equation could be obtained only if a smooth substrate surface was applied on the quartz crystal. The simplified equation is represent as:

∆F = -2.3 x 106 F2 (∆Ms/A)

Where, ∆F = changes in frequency due to the coating (Hz), ∆Ms = mass of the deposited coating (g), F = the frequency of the quartz plate (MHz) and A = the area of the deposit (cm2) The analytical characteristics for the determination of SO2 using the various triethanolamine-coated crystals are presented in Table 2. It was seen that the TEA coated quartz crystal with different original frequencies (6, 10 and 10 MHz) after coating the three coated quartz crystals showing the decrease in resonating frequencies of (a) 345, (b) 1019 and (c) 3511 Hz respectively. The order of

Table 1. Effect of nitrogen flow rate on the response from 30 ppm of SO2.

N2 Flow (mL min-1)

∆F due to SO2 (Hz) t (sec)

5 64, 64 130

10 76.75 60

15 78, 78 49

25 60, 60 25

50 38, 38 24

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decreasing in sensitivity (defined as slope of calibration graph) was (c) 6.4 Hz/ppm > (b) 2.7 Hz/ppm ≥ (a) 2.2 Hz/ppm with respect to the sensitivity is the linear range of the calibration graphs the decreasing order was (c) 0.6-30.0 ppm > (a) 1.0 – 30.0 ppm ≥ (b) 1.0 – 28.0 ppm. The detection limit is influenced by the coating used after about 300 injections of samples because triethanolamine “bleeds” from the crystal, resulting in a sensitive change in the detector response. The solution to this problem is to improve the stability of the triethanolamine (TEA) coating by dissolving, triisopropanolamine (TIP) in the TEA so as to reduce the vapour pressure of TEA and thus reduce the coating bleed rate. A loss of sensitivity was observed of since TIP was less sensitive to SO2 than TEA. Alternatively, a thin layer of Teflon (tetrafluoroethylene) could be sprayed onto the TEA coating. Since Teflon membranes are permeable to SO2, very little loss in sensitivity was obtained. Interference from atmospheric moisture could be overcome by using a hydrophobic Millipore filter.

B. Detection of methanol Methanol is used to denature methylated spirit, as a solvent and in the chemical industry. Methanol is a toxic solvent; many casees of

fatal intoxication and blindness have resulted from the ingestion of methanol. Therefore, there is a need for rapid, sensitive and selective method for the detection of methanol. Coated piezoelectric detectors seem likely to be good methanol sensors. The detection of methanol vapour was based on the sorption of methanol on the surface of a piezoelectric quartz crystal coated with diethylene glycol adipate. The sorption of methanol vapourized from a vial of a volatilization system on the coating material was linear from 12.5 µg to 150.0 µg of methanol. At greater amounts, the slope decreased probably owing to the saturation of the coating material. The regression equation was Y = 0.124x + 2.775 (r2=0.9844). The reproducibility obtainable for 7 determinations of 50.0 µg of methanol was 3.5%. Further studied based on the single cell piezoelectric detector were not made; and the double cell instrument was used instead.

3.2 Use of a Double Cell Instrument The double cell piezoelectric sensor used was described in the experimental section. It was used in conjunction with two AT-cut, 10 MHz square quartz crystals (10 x 10 mm2) with circular gold electrodes (6 mm diam.) on both sides. The electrodes of both crystals were coated using a tiny brush, to obtain

Table 2. Analytical characteristics for the determination of SO2 by piezoelectric detector.

Frequency of

quartz plate (MHz)

∆F after coating

(Hz)

Linear range

(ppm)

Sensitivity

(Hz/ppm)

Regression equation

(a) 6 345 1-30 2.0Y = 6.391-0.432

r2=0.9914

(b) 10 1019 1-28 3.0Y = 2.723+1.090

r2=0.9906

(c) 10 3511 0.6-30 6.0Y = 2.149+0.286

r2=0.9997

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a decreased frequency of 25-27 kHz. The advantage of this instrument is that the blank signal may be compensated by the reference sensor as in a double beam spectrophotometer. Furthermore the signal can be displayed by a chart recoder in mV (1 mV=3Hz) and / or a personal computer. This allows sensitivity to be improved by increasing the sensitivity of the procedure. The performance of such an instrument was again evaluated on the basis of published work, for the monitoring of sulphur dioxide in the atmosphere. As describes earlier with the single cell instrument.

3.3 Preliminary Investigation In order to avoid the effect of pressure due to injection of the gas sample on the detector response, it is essential to determine the optimal volume for the gas to be injected, before attemping any quantitative analysis. Various volumes (1-5 mL), of dry air gave a peak corresponding to 7 Hz and that 1 mL of air gave a negligible signal under similar conditions. Therefore further investigations were restricted to 1 mL samples. An initial investigation showed that the dual-cell instrument was very sensitive to sulphur dioxide, e.g., a 1 mL air sample of 1 ppm SO2 gave rise to a desreased frequency of 16 mV (or 48 Hz) which is more sensitive than the single cell sensor (6 Hz/ppm). A linear calibration graph was established over the range of 0 – 1.5 µg SO2 having a regression equation of Y = 17.813x + 0.954 r2=0.9949 using the dual cell. By comparison with results obtained from single cell instrument showing the linear range of the calibration graph of 0 - 3.0 µg SO2 giving the regressing equation of Y = 5.691x + 0.728 r2=0.9886. It was seen that the sensitivity was enhanced by 3 times as high as those obtained by the single cell. Methanol gives a very strong irreversible reaction with the triethanoamine-coated detector. Other coating materials were therefore saught to develop

combined detector ethylene glycol succinate coated and/or ethylene glycol adipate coated crystal were tested as SO2 and MeOH detector.The sorption of SO2 on the diethylene glycol succinate-coated crystal) was slight at 0-0.7% SO2 above which such an adsorption increases linearly with increasing SO2 concentartion. Therefore SO2 in air could be determined by using the ethylene glycol succinate coated piezoelectric quartz crystal detector. Methanol gave a relatively strong interaction with the diethylene glycol succinate coated PZC. Its sorption on the coating was linear over the range 0-40% of methanol in air and 0-30 µg of liquid methanol. Under the same conditions but with the diethylene glycol adipate coating the sorption of methanol on such a detector was linear over the range 0.2-0.9% of methanol in air and 30-160 µg of liquid methanol. Under optimal conditions, a selective piezoelectric sensor for methanol may be devrloped by choice of appropriation sorbent on the crystal as well as diethanol amine. A combined PZT detector for quantification of binary mixture of gaseous and/or vapor sample would be possible. The proposed PZT device would be very useful in the laboratories in poor countries such as developing Countries including Thailand.

4. CONCLUSION It has been possible to construct a piezoelectric quartz device for measuring gaseous cataminants in air, both as a single-cell or double-cell instrument. Under suitable conditions, sensitivities can be well below ppm, andsome degree of selectivity can be achieved by choice of sorbent on the crystal. The devices constructed functioned reliably, and were not susceptible to normal variations of laboratory conditions. Although the design problems of the detector were evaluated on the basis of the well known SO2 detection, preliminary experiments showed that the same basic equipment could be used to monitor methanol and acetonitrile,

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the first and the second being typical polymer decomposition product, the third being a solvent used in fuel cells. Nevertheless, the technique requires further improvement in various aspects in order to achieve the best sensitivity. These include the design of suitable equipment for producing standard, dilute gas mixture, means of excluding water vapour from the detectors and methods of reproducibility coating the crystals with sorbents.

ACKNOWLEDGEMENT The authors would like to express their sincere thanks to The National Research University (NRU) Project Under Thailand’s Office of the Commission on Higher Education (CHE), Materials Science Research Center Faculty of Science, CMU and Rajamangala University of Technology Isan Surin Campus (RMUTI) for their very kind financial support. Specially partial support from Center of Excellence for Innovation in Chemistry (PERCH-CIC) and Science and Technology Institute CMU would be gratefully acknowledged. We also would like to express our sincere thanks to the Graduate School and the Department of Chemistry Faculty of Science Chiang Mai University for their partial support.

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