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Several modified piezoelectric quartz crystals for determination of omethoate
Talanta 49 (1999) 253 – 259
Several modified piezoelectric quartz crystals for determination of omethoate Fang Cheng, Wang Xianbao, Zhang Wuming, Zhou Xingyao * Department of Chemistry, Wuhan Uni6ersity, Wuhan 430072, People’s Republic of China Received 26 January 1998; received in revised form 24 August 1998; accepted 18 November 1998
Abstract Determination of omethoate by modified PQC (piezoelectric quartz crystal) sensor is described. Several modified films were studied and compared. Omethoate of 0.1 – 10 ppm can be determined directly and sensitively with PVC-dioctyl sebacate film modified PQC. The modified crystal has good reversibility and reproductivity. The lifetime is about 1 month. © 1999 Elsevier Science B.V. All rights reserved. Keywords: PQC; PQC sensor; Organophosphorus compounds; Omethoate
1. Introduction In the last few decades, much interest has been directed towards the analytical applications of the PQC (piezoelectric quartz crystal) sensor. King was the first who employed PQC in an analytical apparatus [1]. After Sauerbrey gave the relationship between the mass changes on the electrode surface of the piezoelectric crystal and the corresponding frequency shifts [2], much attention has been paid to the PQC sensor [3 – 5]. The utilization of the PQC as a sensor depended, almost entirely, on the advantage of high sensitivity in measuring very minute mass changes. The expending application of the PQC sensor is also due to its simplicity and portability, its latent commercial benefits, and its ease of operation. * Corresponding author. Fax: +86-27-87882661. E-mail address: [email protected] (Z. Xingyao)
Organophosphorus compounds have become environmental pollutants because of their wideranging utilization as pesticides or insecticides. In addition, these compounds have also been developed for chemical warfare (CW). Therefore, the detection of organophosphorus compounds is of great practical importance and has received intensifying interest. A large number of papers have been published in this field [6–8]; the works of Guilbault and his co-workers are of great value [9–11]. Gas chromatography is a very suitable method for the detection of organophosphorus compounds [12], but it is difficult to make it portable. Despite the shortcoming in selectivity, PQC is an ideal method for in situ monitoring of the organophosphorus compounds. One way to increase the selectivity is to find a selective film coating on the surface of the electrode, which
0039-9140/99/$ - see front matter © 1999 Elsevier Science B.V. All rights reserved. PII: S 0 0 3 9 - 9 1 4 0 ( 9 8 ) 0 0 3 7 5 - 0
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F. Cheng et al. / Talanta 49 (1999) 253–259
means to modify the PQC. In previous studies on selective films for detection of organophosphorus compounds, modified PQC has made considerable headway [13,14]. The modifying materials include not only transition metal complexes [15], organic compounds [16], but also enzymes and immunity agents [17]. Some authors [13] have pointed out that long-chain oxime complex could selectively respond to the organophosphorus compounds. This complex is a kind of imitated enzyme. Instruments have been built to carry out microgravimetric immunoassays, using microbial assays and gas phase immunosensor [18]. When used for the specific detection of the organophosphorus compounds, excellent results have been achieved [19]. Sometimes, however, the sensitivity is more important than the selectivity, either because the task is to determine the concentration of a known agent in the absence of interfering substance, or because the main interference could be discounted or avoided. Under these conditions, attention may be focused on how to improve the sensitivity. In this paper, several common materials were selected to modify the PQC electrode for the sensitive detection of organophosphorus compounds. These materials were selected for the following reasons: (1) they were usually employed as absorbents or surface active agents; (2) if practical application is possible, the modified reagent must be economical and easily-preserved. In order to form the film, PVC (polyvinyl chloride) was used as film carrier due to its excellent film forming characteristics. Omethoate was utilized as a model compound in our investigation for a sensitive, reversible and reproducible PQC sensor for
organophosphorus compounds. There are two reasons to choose omethoate as a representative of organophosphorus compounds. First, omethoate is a type of insecticide that is applied commonly and extensively. Second, we purpose to detect organophosphorus compounds in gas phase; omethoate is volatile enough to obtain very minute concentration vapor without heating.
2. Experimental
2.1. Apparatus A home-made TTL oscillator was employed. The piezoelectric crystals are 10 MHz AT-Cut quartz crystals with silver electrodes plated on both sides. The crystal diameter is about 1.0 cm and each silver electrode diameter is about 0.4 cm. All crystals were purchased from Wuhan Radio Component Factory (P. R. China). The frequency output from oscillator was recorded with a frequency counter (made in Taiwan, GFC-8010G). The experiment set-up for the evaluation of PQC sensor is shown schematically in Fig. 1. Argon is selected as the carrier gas to avoid the interference of water vapor in air and other substances. A desiccator is used, which contains P2O5 as drying agent to absorb the water vapor in argon. Omethoate was dissolved in acetone. A microsyringe was used to inject the acetone sample containing the omethoate insecticide into detector cell. We selected acetone as solvent not only because acetone changes into vapor readily, but also
Fig. 1. Experimental apparatus: (1) argon pump; (2) desiccator (containing P2O5); (3) buffering bottle; (4) detector cell (100 ml); (5) oscillator; (6) frequency counter; (7) power supply; (8) microsyringe; (9) switch; (10) thermostat.
F. Cheng et al. / Talanta 49 (1999) 253–259
because acetone interference can be removed easily.
2.2. Reagents o-Dimethyl benzene, dioctyl phthalate, dibutyl phthalate, dioctyl sebacate, butyl acetate, o-nitrotoluene, 4-methylpentanone, palmityl trimethyl ammonium bromide and omethoate (Hubei Provincial Agriculture Institution) were all analytical grade. They were used without further purification. All solvents used for preparation of coating solutions were of analytical reagent grade.
2.3. Crystal modification PVC (0.10 g) was dissolved with cyclohexanone firstly, then diluted with acetone to 0.50% (m/v). Omethoate of 2.0% (v/v) was prepared with acetone and preserved at 0°C. All coating materials of 0.50% (m/v or v/v) were dissolved with acetone. The coating solutions were achieved when 1.0 ml acetone containing 0.50% coating material was added into1.0 ml PVC solution. The modified crystals were prepared by dropping 1.0 or 2.0 ml coating solution onto the central of each electrode with a microsyringe separately. Then the crystals were placed in an oven at 60 – 80°C for 3 h or in a desiccator overnight. After acetone was evaporated, a thin film was left on the crystal surface. The thickness of the modified film can be detected through the frequency shift.
255
using the same method. The temperature was maintained at 109 0.5°C.
3. Results and discussion
3.1. Calibration cur6es Calibration curves corresponding to the different modified crystals are shown in Figs. 2 and 3. The calibration graphs are similar to those published previously [13]. In our study, a saturation effect was not observed as the crystal stopping oscillation. From these studies, PVC-dioctyl sebacate film was chosen as a model modified film.
Fig. 2. Calibration curves for different modified films with omethoate. The modifying material is as follows: (1) odimethyl benzene; (2) dioctyl phthalate; (3) dibutyl phthalate; (4) dioctyl sebacate. All modified crystals were fresh.
2.4. Procedures The measurement was conducted in argon. The volume of detection cell was 100 ml. Hence, when 5.0 ml acetone sample (2.0%) was injected into the cell and vaporized, 1.0 ppm omethoate vapor was obtained. After each measurement, pure argon was pumped in to purge the organophosphorus compounds and refresh the cell. At the same time, the crystal was regenerated, as verified from the digital changes in the frequency counter. Interferences were evaluated
Fig. 3. Calibration curves for different modified films with omethoate. The modifying material is as follows: (1) odimethyl benzene; (2) dioctyl phthalate; (3) dibutyl phthalate; (4) dioctyl sebacate; (5) butyl acetate; (6) o-nitrotoluene; (7) 4-methylpentanone; (8) palmityl trimethyl ammonium bromide. All modified crystals were fresh.
F. Cheng et al. / Talanta 49 (1999) 253–259
256
sitivity as shown in Fig. 4. However, the crystal fails to oscillate if the modified film is too thick. For example, crystal no. 4 (c ) (PVC-dioctyl sebacate film), 5 (c ) (PVC-butyl acetate film), and 6 (c) (PVC-nitrotoluene film) stop oscillating after the sixth injection of the sample (about 6.0 ppm) due to the large frequency shifts of about 40 000 Hz during the modification, which can be seen from Table 1.
Fig. 4. Effect of the thickness of the PVC-dioctyl sebacate modified film. The coating solutions were: 2.0 ml (a) (Df= 11 216 Hz during modifying) or 1.0 ml (b) (Df= 6016 Hz during modifying). The response is to omethoate. All modified crystals were fresh.
The exact mechanism of the interaction between vapor and film is complicated and not known exactly. From the sorption isotherm, a linear relationship can be obtained between the logarithm of concentration and the frequency shift. Furthermore, saturation phenomenon seems not to appear. Hence, the sorption belongs more to the Freundlich type than to the Langmuir type or Brunauer-Emmett-Teller (BET) type, which means that the sorption is not monolayer but multilayer. However, if the mass on the surface of the electrode is too large, whether this results from the sorption or the film itself, the crystal will cease to oscillate. Due to the interference of acetone, a concentration of 0 – 6 ppm of omethoate was determined in order to remove the frequency shift resulting from acetone readily.
3.3. Response cur6e The frequency varied with time after each injection. It takes some time for the frequency to stabilize, which can be seen in Figs. 5 and 6. The time required for stabilizing is determined by the following: (1) the speed that the sample changed into vapor, as determined by the volatility and the sample volume; (2) the interaction between the vapor and the film, which can be influenced not only by the sorption mechanism but also by the thickness of the modified
3.2. Modified film The film thickness is important for a good measurement. A thicker film can increase the sen-
Fig. 5. PVC-dioctyl sebacate modified film responses to 0.20 ppm omethoate. The coating solution is 1.0 ml. The modified crystal was fresh.
Table 1 The frequency reading during the modifying (Hz)a No. of crystal ( c )
1
2
3
4
5
6
7
8
Before modifying After modifying Df
2462 19 237 16 775
3155 15 005 11 850
3030 35 135 32 105
1766 43 985 42 219
1231 39 466 38 235
3300 46 362 43 062
1453 26 966 25 513
3505 39 340 35 835
a
No. of the crystal is as the same as in Figs. 2 and 3.
F. Cheng et al. / Talanta 49 (1999) 253–259
257
Sometimes, the frequency reading kept changing for a long time, as shown in Fig. 6. When this occured, it was difficult to obtain a constant reading. In our experiment, if the changing reading is below 0.5% (frequency drift to frequency shift) per min, we think the sorption on the crystal has arrived at balance. That is to say, the standard deviations for frequency shift is below 0.5%. Fig. 6. PVC-dioctyl sebacate modified film responses to 5.0 ppm omethoate. The coating solution is 2.0 ml. The modified crystal was fresh.
film. So, the more readily the sample vaporizes, the faster the crystal arrives at a balance. For instance, it took only 5 – 10 s for an unchanging frequency to be achieved after 1.0 ml ether was injected into the detector cell, much less time than acetone which took 1 – 2 min. In addition, the greater the sample volume and the thicker the film, the longer time it took before a steady frequency reading could be obtained. Moreover, the sorption process can be different (Figs. 5 and 6). In Fig. 6, a peak is observed. The possible reason is that vaporization causes a non-uniform distribution because the crystal was located too close to the injection position. The peak disappeared when the injection location was changed to a more distant point from the crystal or there was an agitator during the determination.
3.4. Interference Several organic vapors were expected as interferences in our study. These are summarized in Table 2. From the Table 2, a little interference was observed for all three kinds of materials, ether, acetone and ethyl alcohol (about 99%) for concentrations up to 100 times as that of omethoate. However, the water vapor disturbs the sample detection critically; hence each experiment was done in argon. In addition, Neither CO2 nor N2 has any interference. Although ether has less interference and faster response, it is too volatile, which easily leads to changes of the sample concentration. Furthermore, the purchased omethoate sample is acetone solution. Therefore, we selected acetone as the solvent.
3.5. Measurement The determination was carried out using PVC-dioctyl sebacate film as a model film. Fig. 7 shows the calibration curve. Each point is the
Table 2 Responses of the PVC-dioctyl sebacate modified film to acetone, ethyl alcohol and ethera,b No. of crystal
1
2
3
4
5
6
7
8
Acetone (500 ppm) Ethyl alcohol (500 ppm) Ether (500 ppm)
182 805 42
149 289 31
482 562 26
726 157 25
424 531 72
470 115 56
229 113 43
595 159 78
a b
No. of the crystal is as the same as in Figs. 2 and 3. The modified crystal was fresh.
F. Cheng et al. / Talanta 49 (1999) 253–259
258
Fig. 7. Calibration curve of PVC-dioctyl sebacate modified crystal to methoate. The modified crystal was preserved for half a month with 2.0 ml coating solution.
mean value of two measurements obtained by deducting the interference of acetone of the same concentration. The determination employs the ‘addition and recovery method’. The background in the detector cell is air (after desiccating). When the omethoate sample was added into the detector cell, a responding reading was obtained, which could be converted into concentration from the calibration curve. The determined concentration is compared with the added concentration. The results are shown in Table 3.
Fig. 8. The lifetime of the PVC-dioctyl sebacate modified crystal. (1) fresh; (2) after 3 days; (3) after half a month.
Table 4 The repeating experiment data of the PVC-dioctyl sebacate modified crystal in the determination of omethoatea Concentration (ppm)
0.2
0.4
0.6
0.8
1.0
Df1 (Hz) Df2 (Hz)
63 63
142 148
250 249
362 370
501 511
a The modified crystal has been preserved for half a month with 2.0 ml coating solution.
Table 5 The repeating experiment data of the PVC-dioctyl sebacate modified crystal in the determination of acetonea
3.6. Lifetime Satisfactory reversibility, reproductivity and lifetime are obtained, as seen in Tables 4, 5 and Fig. 8. A longer lifetime can be achieved if the crystal is washed with argon immediately after each detection and preserved in a desiccator. During washing the crystal regenerates, which can be monitored from the frequency shift. The
Concentration (ppm)
10
20
30
40
50
Df1 (Hz) Df2 (Hz)
12 11
29 29
49 49
70 68
90 88
a The modified crystal has been preserved for half a month with 2.0 ml coating solution.
Table 3 The determination data of the addition and recovery methoda Addition (ppm)
Df (Hz)
Df (removing acetone) (Hz)
Detection (ppm)
Recovery (%)
0.300 0.500
111 225
81 165
0.294 0.508
98.0 102
a
The modified crystal was preserved for half a month with 2.0 ml coating solution.
F. Cheng et al. / Talanta 49 (1999) 253–259
sorption is reversible. The modified crystal has an excellent reproductivity for omethoate and acetone (Tables 4, 5 and Fig. 8). The crystal lifetime is about 1 month. However, the sensitivity decreases during preservation as indicated in Fig. 8. The dropping of the sensitivity was notable in the first few days. The crystal can be re-used after immersion in acetone overnight. Acknowledgements This work was supported by the Foundation of Chinese-France Cooperation Program on the Advance Research. References [1] [2] [3] [4]
W.H. King Jr, Res. Dev. Port 1 20 (4) (1969) 28. G.Z. Sauerbrey, Z. Phys. 155 (1959) 206. J. Janata, A. Bezegh, Anal. Chem. 60 (1988) 64R. H.T. James, L.G. Robin, Anal. Chem. 67 (1995) 3372.
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[5] B.L. Wu, H.W. Lei, C.S. Cha, J. Electroanal. Chem. 374 (1994) 97. [6] Y. Wang, Y. Sun, J. Huang et. al., Wuhan University J. Nat. Sci., in press. [7] J. Ngeh-Ngwainbi, A.A. Suleiman, G.G. Guilbault, Biosens. Bioelectron. 5 (1990) 13. [8] L.V. Rajakovic, S. Strbac, Anal. Chim. Acta 315 (1995) 83. [9] Y. Tomita, G.G. Guilbault, Anal. Chem. 52 (1980) 1484. [10] G.G. Guilbault, J. Affolter, Y. Tomita, Anal. Chem. 53 (1981) 2057. [11] G.G. Guilbault, J. Kristoff, Anal. Chem. 57 (1985) 1754. [12] M. Agruilar, A. Farran, C. Serra, et al., J. Chromatogr. 778 (1997) 201. [13] Q. Dai, Q. Huang, Y. Deng, et al., Chem. Sensor (in Chinese) 16 (1996) 107. [14] J.M. John, Analyst 114 (1989) 1173. [15] J. Janata, M. Josowicz, D.M. Devancy, Anal. Chem. 66 (1994) 207R. [16] S. Milanko, S.A. Milinkovic, Anal. Chim. Acta 269 (1992) 289. [17] W. Wei, R.H. Wang, L.H. Lie, S.Z. Yao, Instrum. Sci. Techol. 25 (1997) 157. [18] M. Minunni, M. Mascini, G.G. Guilbault, B. Hock, Anal. Lett. 28 (5) (1995) 749. [19] A. Suleiman, G.G. Guilbault, Anal. Lett. 24 (1991) 1283.
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Several modified piezoelectric quartz crystals for determination of omethoate Fang Cheng, Wang Xianbao, Zhang Wuming, Zhou Xingyao * Department of Chemistry, Wuhan Uni6ersity, Wuhan 430072, People’s Republic of China Received 26 January 1998; received in revised form 24 August 1998; accepted 18 November 1998
Abstract Determination of omethoate by modified PQC (piezoelectric quartz crystal) sensor is described. Several modified films were studied and compared. Omethoate of 0.1 – 10 ppm can be determined directly and sensitively with PVC-dioctyl sebacate film modified PQC. The modified crystal has good reversibility and reproductivity. The lifetime is about 1 month. © 1999 Elsevier Science B.V. All rights reserved. Keywords: PQC; PQC sensor; Organophosphorus compounds; Omethoate
1. Introduction In the last few decades, much interest has been directed towards the analytical applications of the PQC (piezoelectric quartz crystal) sensor. King was the first who employed PQC in an analytical apparatus [1]. After Sauerbrey gave the relationship between the mass changes on the electrode surface of the piezoelectric crystal and the corresponding frequency shifts [2], much attention has been paid to the PQC sensor [3 – 5]. The utilization of the PQC as a sensor depended, almost entirely, on the advantage of high sensitivity in measuring very minute mass changes. The expending application of the PQC sensor is also due to its simplicity and portability, its latent commercial benefits, and its ease of operation. * Corresponding author. Fax: +86-27-87882661. E-mail address: [email protected] (Z. Xingyao)
Organophosphorus compounds have become environmental pollutants because of their wideranging utilization as pesticides or insecticides. In addition, these compounds have also been developed for chemical warfare (CW). Therefore, the detection of organophosphorus compounds is of great practical importance and has received intensifying interest. A large number of papers have been published in this field [6–8]; the works of Guilbault and his co-workers are of great value [9–11]. Gas chromatography is a very suitable method for the detection of organophosphorus compounds [12], but it is difficult to make it portable. Despite the shortcoming in selectivity, PQC is an ideal method for in situ monitoring of the organophosphorus compounds. One way to increase the selectivity is to find a selective film coating on the surface of the electrode, which
0039-9140/99/$ - see front matter © 1999 Elsevier Science B.V. All rights reserved. PII: S 0 0 3 9 - 9 1 4 0 ( 9 8 ) 0 0 3 7 5 - 0
254
F. Cheng et al. / Talanta 49 (1999) 253–259
means to modify the PQC. In previous studies on selective films for detection of organophosphorus compounds, modified PQC has made considerable headway [13,14]. The modifying materials include not only transition metal complexes [15], organic compounds [16], but also enzymes and immunity agents [17]. Some authors [13] have pointed out that long-chain oxime complex could selectively respond to the organophosphorus compounds. This complex is a kind of imitated enzyme. Instruments have been built to carry out microgravimetric immunoassays, using microbial assays and gas phase immunosensor [18]. When used for the specific detection of the organophosphorus compounds, excellent results have been achieved [19]. Sometimes, however, the sensitivity is more important than the selectivity, either because the task is to determine the concentration of a known agent in the absence of interfering substance, or because the main interference could be discounted or avoided. Under these conditions, attention may be focused on how to improve the sensitivity. In this paper, several common materials were selected to modify the PQC electrode for the sensitive detection of organophosphorus compounds. These materials were selected for the following reasons: (1) they were usually employed as absorbents or surface active agents; (2) if practical application is possible, the modified reagent must be economical and easily-preserved. In order to form the film, PVC (polyvinyl chloride) was used as film carrier due to its excellent film forming characteristics. Omethoate was utilized as a model compound in our investigation for a sensitive, reversible and reproducible PQC sensor for
organophosphorus compounds. There are two reasons to choose omethoate as a representative of organophosphorus compounds. First, omethoate is a type of insecticide that is applied commonly and extensively. Second, we purpose to detect organophosphorus compounds in gas phase; omethoate is volatile enough to obtain very minute concentration vapor without heating.
2. Experimental
2.1. Apparatus A home-made TTL oscillator was employed. The piezoelectric crystals are 10 MHz AT-Cut quartz crystals with silver electrodes plated on both sides. The crystal diameter is about 1.0 cm and each silver electrode diameter is about 0.4 cm. All crystals were purchased from Wuhan Radio Component Factory (P. R. China). The frequency output from oscillator was recorded with a frequency counter (made in Taiwan, GFC-8010G). The experiment set-up for the evaluation of PQC sensor is shown schematically in Fig. 1. Argon is selected as the carrier gas to avoid the interference of water vapor in air and other substances. A desiccator is used, which contains P2O5 as drying agent to absorb the water vapor in argon. Omethoate was dissolved in acetone. A microsyringe was used to inject the acetone sample containing the omethoate insecticide into detector cell. We selected acetone as solvent not only because acetone changes into vapor readily, but also
Fig. 1. Experimental apparatus: (1) argon pump; (2) desiccator (containing P2O5); (3) buffering bottle; (4) detector cell (100 ml); (5) oscillator; (6) frequency counter; (7) power supply; (8) microsyringe; (9) switch; (10) thermostat.
F. Cheng et al. / Talanta 49 (1999) 253–259
because acetone interference can be removed easily.
2.2. Reagents o-Dimethyl benzene, dioctyl phthalate, dibutyl phthalate, dioctyl sebacate, butyl acetate, o-nitrotoluene, 4-methylpentanone, palmityl trimethyl ammonium bromide and omethoate (Hubei Provincial Agriculture Institution) were all analytical grade. They were used without further purification. All solvents used for preparation of coating solutions were of analytical reagent grade.
2.3. Crystal modification PVC (0.10 g) was dissolved with cyclohexanone firstly, then diluted with acetone to 0.50% (m/v). Omethoate of 2.0% (v/v) was prepared with acetone and preserved at 0°C. All coating materials of 0.50% (m/v or v/v) were dissolved with acetone. The coating solutions were achieved when 1.0 ml acetone containing 0.50% coating material was added into1.0 ml PVC solution. The modified crystals were prepared by dropping 1.0 or 2.0 ml coating solution onto the central of each electrode with a microsyringe separately. Then the crystals were placed in an oven at 60 – 80°C for 3 h or in a desiccator overnight. After acetone was evaporated, a thin film was left on the crystal surface. The thickness of the modified film can be detected through the frequency shift.
255
using the same method. The temperature was maintained at 109 0.5°C.
3. Results and discussion
3.1. Calibration cur6es Calibration curves corresponding to the different modified crystals are shown in Figs. 2 and 3. The calibration graphs are similar to those published previously [13]. In our study, a saturation effect was not observed as the crystal stopping oscillation. From these studies, PVC-dioctyl sebacate film was chosen as a model modified film.
Fig. 2. Calibration curves for different modified films with omethoate. The modifying material is as follows: (1) odimethyl benzene; (2) dioctyl phthalate; (3) dibutyl phthalate; (4) dioctyl sebacate. All modified crystals were fresh.
2.4. Procedures The measurement was conducted in argon. The volume of detection cell was 100 ml. Hence, when 5.0 ml acetone sample (2.0%) was injected into the cell and vaporized, 1.0 ppm omethoate vapor was obtained. After each measurement, pure argon was pumped in to purge the organophosphorus compounds and refresh the cell. At the same time, the crystal was regenerated, as verified from the digital changes in the frequency counter. Interferences were evaluated
Fig. 3. Calibration curves for different modified films with omethoate. The modifying material is as follows: (1) odimethyl benzene; (2) dioctyl phthalate; (3) dibutyl phthalate; (4) dioctyl sebacate; (5) butyl acetate; (6) o-nitrotoluene; (7) 4-methylpentanone; (8) palmityl trimethyl ammonium bromide. All modified crystals were fresh.
F. Cheng et al. / Talanta 49 (1999) 253–259
256
sitivity as shown in Fig. 4. However, the crystal fails to oscillate if the modified film is too thick. For example, crystal no. 4 (c ) (PVC-dioctyl sebacate film), 5 (c ) (PVC-butyl acetate film), and 6 (c) (PVC-nitrotoluene film) stop oscillating after the sixth injection of the sample (about 6.0 ppm) due to the large frequency shifts of about 40 000 Hz during the modification, which can be seen from Table 1.
Fig. 4. Effect of the thickness of the PVC-dioctyl sebacate modified film. The coating solutions were: 2.0 ml (a) (Df= 11 216 Hz during modifying) or 1.0 ml (b) (Df= 6016 Hz during modifying). The response is to omethoate. All modified crystals were fresh.
The exact mechanism of the interaction between vapor and film is complicated and not known exactly. From the sorption isotherm, a linear relationship can be obtained between the logarithm of concentration and the frequency shift. Furthermore, saturation phenomenon seems not to appear. Hence, the sorption belongs more to the Freundlich type than to the Langmuir type or Brunauer-Emmett-Teller (BET) type, which means that the sorption is not monolayer but multilayer. However, if the mass on the surface of the electrode is too large, whether this results from the sorption or the film itself, the crystal will cease to oscillate. Due to the interference of acetone, a concentration of 0 – 6 ppm of omethoate was determined in order to remove the frequency shift resulting from acetone readily.
3.3. Response cur6e The frequency varied with time after each injection. It takes some time for the frequency to stabilize, which can be seen in Figs. 5 and 6. The time required for stabilizing is determined by the following: (1) the speed that the sample changed into vapor, as determined by the volatility and the sample volume; (2) the interaction between the vapor and the film, which can be influenced not only by the sorption mechanism but also by the thickness of the modified
3.2. Modified film The film thickness is important for a good measurement. A thicker film can increase the sen-
Fig. 5. PVC-dioctyl sebacate modified film responses to 0.20 ppm omethoate. The coating solution is 1.0 ml. The modified crystal was fresh.
Table 1 The frequency reading during the modifying (Hz)a No. of crystal ( c )
1
2
3
4
5
6
7
8
Before modifying After modifying Df
2462 19 237 16 775
3155 15 005 11 850
3030 35 135 32 105
1766 43 985 42 219
1231 39 466 38 235
3300 46 362 43 062
1453 26 966 25 513
3505 39 340 35 835
a
No. of the crystal is as the same as in Figs. 2 and 3.
F. Cheng et al. / Talanta 49 (1999) 253–259
257
Sometimes, the frequency reading kept changing for a long time, as shown in Fig. 6. When this occured, it was difficult to obtain a constant reading. In our experiment, if the changing reading is below 0.5% (frequency drift to frequency shift) per min, we think the sorption on the crystal has arrived at balance. That is to say, the standard deviations for frequency shift is below 0.5%. Fig. 6. PVC-dioctyl sebacate modified film responses to 5.0 ppm omethoate. The coating solution is 2.0 ml. The modified crystal was fresh.
film. So, the more readily the sample vaporizes, the faster the crystal arrives at a balance. For instance, it took only 5 – 10 s for an unchanging frequency to be achieved after 1.0 ml ether was injected into the detector cell, much less time than acetone which took 1 – 2 min. In addition, the greater the sample volume and the thicker the film, the longer time it took before a steady frequency reading could be obtained. Moreover, the sorption process can be different (Figs. 5 and 6). In Fig. 6, a peak is observed. The possible reason is that vaporization causes a non-uniform distribution because the crystal was located too close to the injection position. The peak disappeared when the injection location was changed to a more distant point from the crystal or there was an agitator during the determination.
3.4. Interference Several organic vapors were expected as interferences in our study. These are summarized in Table 2. From the Table 2, a little interference was observed for all three kinds of materials, ether, acetone and ethyl alcohol (about 99%) for concentrations up to 100 times as that of omethoate. However, the water vapor disturbs the sample detection critically; hence each experiment was done in argon. In addition, Neither CO2 nor N2 has any interference. Although ether has less interference and faster response, it is too volatile, which easily leads to changes of the sample concentration. Furthermore, the purchased omethoate sample is acetone solution. Therefore, we selected acetone as the solvent.
3.5. Measurement The determination was carried out using PVC-dioctyl sebacate film as a model film. Fig. 7 shows the calibration curve. Each point is the
Table 2 Responses of the PVC-dioctyl sebacate modified film to acetone, ethyl alcohol and ethera,b No. of crystal
1
2
3
4
5
6
7
8
Acetone (500 ppm) Ethyl alcohol (500 ppm) Ether (500 ppm)
182 805 42
149 289 31
482 562 26
726 157 25
424 531 72
470 115 56
229 113 43
595 159 78
a b
No. of the crystal is as the same as in Figs. 2 and 3. The modified crystal was fresh.
F. Cheng et al. / Talanta 49 (1999) 253–259
258
Fig. 7. Calibration curve of PVC-dioctyl sebacate modified crystal to methoate. The modified crystal was preserved for half a month with 2.0 ml coating solution.
mean value of two measurements obtained by deducting the interference of acetone of the same concentration. The determination employs the ‘addition and recovery method’. The background in the detector cell is air (after desiccating). When the omethoate sample was added into the detector cell, a responding reading was obtained, which could be converted into concentration from the calibration curve. The determined concentration is compared with the added concentration. The results are shown in Table 3.
Fig. 8. The lifetime of the PVC-dioctyl sebacate modified crystal. (1) fresh; (2) after 3 days; (3) after half a month.
Table 4 The repeating experiment data of the PVC-dioctyl sebacate modified crystal in the determination of omethoatea Concentration (ppm)
0.2
0.4
0.6
0.8
1.0
Df1 (Hz) Df2 (Hz)
63 63
142 148
250 249
362 370
501 511
a The modified crystal has been preserved for half a month with 2.0 ml coating solution.
Table 5 The repeating experiment data of the PVC-dioctyl sebacate modified crystal in the determination of acetonea
3.6. Lifetime Satisfactory reversibility, reproductivity and lifetime are obtained, as seen in Tables 4, 5 and Fig. 8. A longer lifetime can be achieved if the crystal is washed with argon immediately after each detection and preserved in a desiccator. During washing the crystal regenerates, which can be monitored from the frequency shift. The
Concentration (ppm)
10
20
30
40
50
Df1 (Hz) Df2 (Hz)
12 11
29 29
49 49
70 68
90 88
a The modified crystal has been preserved for half a month with 2.0 ml coating solution.
Table 3 The determination data of the addition and recovery methoda Addition (ppm)
Df (Hz)
Df (removing acetone) (Hz)
Detection (ppm)
Recovery (%)
0.300 0.500
111 225
81 165
0.294 0.508
98.0 102
a
The modified crystal was preserved for half a month with 2.0 ml coating solution.
F. Cheng et al. / Talanta 49 (1999) 253–259
sorption is reversible. The modified crystal has an excellent reproductivity for omethoate and acetone (Tables 4, 5 and Fig. 8). The crystal lifetime is about 1 month. However, the sensitivity decreases during preservation as indicated in Fig. 8. The dropping of the sensitivity was notable in the first few days. The crystal can be re-used after immersion in acetone overnight. Acknowledgements This work was supported by the Foundation of Chinese-France Cooperation Program on the Advance Research. References [1] [2] [3] [4]
W.H. King Jr, Res. Dev. Port 1 20 (4) (1969) 28. G.Z. Sauerbrey, Z. Phys. 155 (1959) 206. J. Janata, A. Bezegh, Anal. Chem. 60 (1988) 64R. H.T. James, L.G. Robin, Anal. Chem. 67 (1995) 3372.
259
[5] B.L. Wu, H.W. Lei, C.S. Cha, J. Electroanal. Chem. 374 (1994) 97. [6] Y. Wang, Y. Sun, J. Huang et. al., Wuhan University J. Nat. Sci., in press. [7] J. Ngeh-Ngwainbi, A.A. Suleiman, G.G. Guilbault, Biosens. Bioelectron. 5 (1990) 13. [8] L.V. Rajakovic, S. Strbac, Anal. Chim. Acta 315 (1995) 83. [9] Y. Tomita, G.G. Guilbault, Anal. Chem. 52 (1980) 1484. [10] G.G. Guilbault, J. Affolter, Y. Tomita, Anal. Chem. 53 (1981) 2057. [11] G.G. Guilbault, J. Kristoff, Anal. Chem. 57 (1985) 1754. [12] M. Agruilar, A. Farran, C. Serra, et al., J. Chromatogr. 778 (1997) 201. [13] Q. Dai, Q. Huang, Y. Deng, et al., Chem. Sensor (in Chinese) 16 (1996) 107. [14] J.M. John, Analyst 114 (1989) 1173. [15] J. Janata, M. Josowicz, D.M. Devancy, Anal. Chem. 66 (1994) 207R. [16] S. Milanko, S.A. Milinkovic, Anal. Chim. Acta 269 (1992) 289. [17] W. Wei, R.H. Wang, L.H. Lie, S.Z. Yao, Instrum. Sci. Techol. 25 (1997) 157. [18] M. Minunni, M. Mascini, G.G. Guilbault, B. Hock, Anal. Lett. 28 (5) (1995) 749. [19] A. Suleiman, G.G. Guilbault, Anal. Lett. 24 (1991) 1283.
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