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Discrimination of halitosis substance using QCM sensor array and a preconcentrator
Sensors and Actuators B 99 (2004) 431–436
Discrimination of halitosis substance using QCM sensor array and a preconcentrator Junji Ito a,∗ , Takamichi Nakamoto b , Hiroshi Uematsu a a b
Department of Gerodontology, Tokyo Medical and Dental University, 1-5-45 Yushima, Bunkyo-ku, Tokyo 113-8549, Japan Department of Physical Electronics, Tokyo Institute of Technology, 2-12-1 Ookayama, Meguro-ku, Tokyo 152-8552, Japan Received 19 August 2003; received in revised form 8 December 2003; accepted 12 December 2003
Abstract An organoleptic test for the assessment of halitosis is usually performed by a dental clinician, although it is a subjective test. Thus, a halitosis test for objective evaluation is required. Halitosis is due to volatile sulfur compounds (VSCs) produced by bacterial metabolism. In this study, we proposed a method for discriminating sulfur compounds using a quartz crystal microbalance (QCM) sensor array combined with a preconcentrator. The adjustment of the temperature profile, the optimization of the flow path sequence and the use of a dehumidification filter enabled VSC measurement under a high-humidity condition. Moreover, the result of principal component analysis indicated that three types of VSC were separated using the sensor array output pattern. © 2004 Elsevier B.V. All rights reserved. Keywords: Halitosis; QCM; Preconcentrator; VSCs; Odor sensing system
1. Introduction Recently, an increase in the market sale of mouth washing suggests that the awareness of body odor including halitosis is increasing. In particular, young Japanese tend to become sensitive to their own bad breath as well as to others. In such a social situation, the consultation about halitosis has become more frequent in dental clinics. However, dental clinicians have not yet established a standard assessment of halitosis. Halitosis is due to volatile sulfur compounds (VSCs), such as hydrogen sulfide, methyl mercaptan, and dimethyl sulfide produced by bacterial metabolism. Oral pathologic odor is also caused by disease, pathologic conditions and oral tissue malfunction. These sulfides are mainly derived from the tongue coating, modified by pathologic conditions. In the dental field, it is well known that these substances mainly govern the halitosis [1]. Although hydrogen sulfide is the main compound causing halitosis, the occurrence of halitosis is also correlated with the amount of methyl mercaptan even when present at low concentrations [2]. Nowadays, an organoleptic test is mainly used by a medical practitioner for the evaluation of halitosis although this
∗ Corresponding author. Tel.: +81-358035559; fax: +81-358030208. E-mail address: [email protected] (J. Ito).
0925-4005/$ – see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.snb.2003.12.012
test is not objective. Since the final decision about halitosis treatment is based on the subjective evaluation of the patient, problems often occur due to the difference in the treatment evaluation between the patient and the doctor. Thus, the development of halitosis measurement system for objective and reliable evaluation is required. To date various methods of halitosis measurement have been developed for general use and for use in dental clinics. Gas chromatography is often used to measure halitosis. However, this method is not practical in dental clinics because of the specialized skill required for the operation. Commercially available portable sulfide monitors using a gas sensor have insufficient sensitivity and selectivity to measure the VSCs. In this study, a method for discriminating VSCs using a quartz crystal microbalance (QCM) sensor array combined with a preconcentrator is proposed. The QCM works as a gas sensor when its surface is coated with a sensing film. The identification of the sensor array output pattern enables odor recognition when the array includes several QCMs with different sensing films [3]. Although a QCM sensor coated with an appropriate sensing film is sensitive to VSCs, its sensitivity to humidity should be reduced. Thus, we used the preconcentrator to overcome this problem. Although the preconcentrator is commonly used to increase the sensitivity of sensors, it can also be used to reduce the influence of humidity when the appropriate temperature profile of the preconcentrator during the desorption process is adopted [4,5]. In the present
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study, we report the result of the measurement of VSCs using the QCM sensor array combined with the preconcentrator.
2. Experimental setup The experimental setup of the preconcentrator with variable temperature is shown in Fig. 1. It consists of the vapor supply system, the preconcentrator and the QCM sensor array. After the supplied vapor accumulates in the preconcentrator, the thermally desorbed vapor is exposed to the QCM sensor array, followed by the recognition of its output pattern. The shifts in the oscillation frequencies of the QCMs measured using the multichannel frequency counter implemented into a programmable logic LSI and the data are transferred to the computer via a serial interface. The temperature of the liquid samples is maintained using a thermobath. Two mass flow controllers (MFCs) with different flow rates were used in this study. One was used for the carrier gas and the other at a lower flow rate was used for the drying phase. The stainless preconcentrator tube (o.d.: 6.00 mm, i.d.: 5.00 mm, length: 100 mm) packed with 480 mg active carbon (30/60 mesh, GL Sciences) was used. Active carbon was held in place with glass wool. The active carbon with large-grain sizes was used to reduce the pressure difference between the inlet and the outlet of the preconcentrator tube. The tube was heated using an insulated nichrome coil wound around it when VSCs were thermally desorbed. The operation sequence is shown in Fig. 2. An appropriate measurement sequence was selected to suppress the humidity influence. (a) Cleaning: the preconcentrator tube is heated to clean out the tube. (b) Sampling: the sample gas is introduced into the preconcentrator (flow rate: 50 ml/min, time: 30 s).
Fig. 2. Close-up of flow path in each sequence.
(c) Drying: on the basis of the difference in desorption temperature, VSCs trapped in preconcentrator are dried by heating this tube at the intermediate temperature where only water vapor desorbs (temperature: 80 ◦ C, time: 300 s). The water vapor flows through the bypath without going to the sensors. Then, the flow rate in the preconcentrator is kept at one-half of the initial rate (25 ml/min) using the mass flow controller to prolong the drying time. (d) Heating: thermally desorbed vapor from the preconcentrator flows into the sensor cell. The preconcentrator is further heated from 80 to 130 ◦ C. Since the preconcentrator enhances sensitivity and reduces humidity, it is suitable for the measurement of a low-concentration sample. Because humidity effect cannot be completely eliminated, the dehumidification filter is placed immediately prior to a sensor cell to remove the remaining humidity. A Teflon tube packed with 900 mg Na2 SO4 (Wako Pure Chemical Industries Ltd.) was used as the dehumidification filter in this study. Three types of VSC (hydrogen sulfide, methyl mercaptan, and dimethyl sulfide) diluted to 0.1% (w/w) were used as samples. The odor samples were generated by putting carrier gas into the head space over the liquid sample (4 ml) in a test tube (volume: 15 ml). The sample and dry air were alternately introduced to the sensor cell by switching the solenoid valves. In our experimental setup, the test tubes were placed in the thermobath to maintain the sample temperature at 27 ◦ C. The samples are shown in Table 1. The Table 1 Odor samples
Fig. 1. Flow-type odor sensing system with a preconcentrator.
Name
Molecular weight
Boiling point (◦ C)
Hydrogen sulfide Methyl mercaptan Dimethyl sulfide
34.08 48.11 62.10
−60.3 5.95 37.0
J. Ito et al. / Sensors and Actuators B 99 (2004) 431–436
VSCs were diluted with water or nonvolatile liquid such as octyl decyl oil (ODO).
Table 3 Relationship between temperature and breakthrough volume of each sample Sample
3. Result
Various materials, such as lipids, polar GC stationary phase materials, and cellulose were used as sensing films. We first measured the VSCs using QCMs (AT-cut, 20 MHz) with various coatings. The spray-coating method was used to fabricate the sensors. The resonance frequency change of each sensor except the PAH/PAA sensor due to the film coating was about 20 kHz. The frequency change of the PAH/PAA [6] sensor due to the coating was about 66.0 kHz. Eight sensors listed in Table 2 were selected after eliminating the sensors with responses less than 50 Hz or unstable responses. The mutual self-assembly and polyion complex films were found to be sensitive to the VSCs. 3.2. Breakthrough volume and retention time of an adsorbent material Breakthrough volume is the volume of carrier gas per gram of adsorbent required to cause the analyte molecules to migrate from the front of the adsorbent bed to the back. The breakthrough volume may change depending on the sample temperature. The vapor compound with a small molecular size and a low boiling point generally tends to be desorbed easily. On the other hand, a large amount of carrier gas is required to desorb the vapor compound with a large molecular size and a high boiling point [7]. The breakthrough volume (B (l/g)) decreases as temperature increases. Retention time (R (s)) required for the desorption is given as, R=
BW + D . F
Here, W (g) is the weight of the adsorbent in the preconcentrator, and F (l/s) the flow rate of carrier gas. D (l) is the dead volume, the capacity of a preconcentrator. The retention time of each sample depends on its breakthrough volume Table 2 Combination of QCM sensors with sensing films
Ch1 Ch2 Ch3 Ch4 Ch5 Ch6 Ch7 Ch8
Temprature (◦ C) 80
3.1. Sensor selection
Sensing film
Abbreviation
f (kHz)
Divaleroyl phosphatidyl choline Dipalmitoyl phosphatidyl choline Dioleoyl phosphatidyl choline Dioleoyl phosphatidic acid Dioleoyl phosphatidylserine Siponate DS-10 Poly(styrenesulfonic acid-co-maleic acid), sodium salt Poly(allylaminehydrochloride)/ poly(acrylic acid)
DVPC DPPC DOPC PTA PTS DS-10 PSS
21.71 20.60 21.89 21.09 20.74 20.98 20.79
PAH/PAA
65.96
433
Water Methyl mercaption Dimethyl sulfide Hydrogen Sulfide a b
213.7 865.0b 1038.5b 1082.7b
90 152.5 793.6b 751.1b 786.8b
100 121.9 492.6 438.2 485.8
110 103.1 358.2 339.5 341.2
120 92.9a 281.7 257.9 278.3
130 87.8a 222.2 208.6 222.2
Breakthrough volume is less than 100 dm3 g−1 . Breakthrough volume is more than 500 dm3 g−1 .
under the same conditions. We measured the retention time using the preconcentrator with a volume of 1.96 ml packed with 490 mg of active carbon at a flow rate of 50 ml/min. The retention time was the time required for the vapor to pass the preconcentrator tube. The measured breakthrough volume of each sample is shown in Table 3. Volume of water used as a sample is also shown for comparison. Water should be separated since it interferes with the detection of odors from aqueous solutions. The breakthrough volume of water was 213 (l/g) at 80 ◦ C and 121.9 (l/g) near its boiling point. Only a small amount of water was adsorbed onto the adsorbent at a temperature more than 100 ◦ C. The breakthrough volume decreased as the temperature was increased. The breakthrough volume of each VSC was more than 800 l/g at 80 ◦ C, four or five times larger than that of water at the same temperature. Thus, an aqueous sample can be measured without the influence of humidity on the basis of difference in the breakthrough volume at 80 ◦ C. 3.3. Measurement of VSCs using preconcentrator The sample vapor was accumulated in the preconcentrator before its exposure to the QCM sensor array. The thermal desorption of sample vapor is caused by the decrease in the breakthrough volume during the heating process. The sensor responses to methyl mercaptan under high humidity are shown in Figs. 3a–c. Among the eight sensors, only the response of PAH/PAA sensor is shown for easy understanding. The sample vapor was introduced to the preconcentrator between 70 and 100 s. Then, the preconcentrator was dried for 200 s at 80 ◦ C. The temperature was further increased to 170 ◦ C at 300 s. Since halitosis should be measured under the condition of high humidity, aqueous methyl mercaptan solution (0.1% (w/w)) and pure water were used as samples. The sensor responses to aqueous methyl mercaptan solution without switching the flow path had two peaks (Fig. 3a). Since the sequence used for data shown in Fig. 3a did not have a drying phase as shown in Fig. 2, the measurement system and the sequence were simple. Thus, we adopted the sequence without the drying phase at the initial stage. In this sequence, water was exposed to the sensors during the drying time whereas for consistency water was not exposed during time phase in the case of the sequence shown in
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The first peaks at about 200 s were responses to water and the second peaks at about 350 s were the responses mainly to methyl mercaptan. When the aqueous methyl mercaptan solution was used, the methyl mercaptan peak was insufficiently separated from the water peak. Thus, we measured these samples using the sequence shown in Fig. 2. The sensor responses to methyl mercaptan and pure water are shown in Fig. 3b. Only data obtained during the drying and heating phases are shown in this figure. Time scale in this figure is different from that shown in Fig. 3a since the flow rate at the drying phase was maintained at one-half of the initial rate (25 ml/min). When the sequence in Fig. 2 including the drying phase was used, the first peaks due to humidity shown in Fig. 3a were not observed in this figure. Since the water vapor flowed through the bypath without flowing to the sensors, the obtained signals of methyl mercaptan were clearer. Although the humidity was reduced in comparison with the case without the drying phase, the second peaks still included the responses to water to some extent. Therefore, the dehumidification filter was placed immediately prior to a sensor cell for the removal of the remaining humidity. It was markedly reduced as shown in Fig. 3c in comparison with the case without the flow path switch. Thus, the VSC vapors even under the high-humidity condition were separated from the water vapor. The analytical results of the three substances have been previously reported by other researchers [8,9]. One of them has suggested that there is halitosis when the total VSC concentration is more than 250 ppb. In practical application, when VSCs are approximately 250 ppb they are detected according to the subjective method of halitosis assessment described earlier. Measurement using the detector tube suggests that the concentration of hydrogen sulfide was approximately 20 ppm and those of methyl mercaptan and dimethyl sulfide were approximately 12 ppm. Although these concentrations were higher than the concentrations in actual halitosis, the sensor responses we obtained were much higher than the detection limit. Since the detection limit is approximately a few Hz and the current sensor responses were more than a few hundred Hz, samples with concentrations 100 times less than the current concentration are also detectable. Moreover, the adjustment of the amount of the adsorbent and the temperature parameters may enable further improvement. Thus, it is possible to detect a sample with a realistic concentration. 3.4. Experiment on the VSC discrimination Fig. 3. Sensor response (PAH/PAA) to methyl mercaptan during vapor accumulation/desorption process of preconcentrator: (a) without drying phase; (b) with sequence in Fig. 2; (c) with sequence in Fig. 2 and dehumidification filter.
Fig. 2. In this sequence, only one MFC was used to maintain the flow rate at 50 ml/min. The sensor response to methyl mercaptan diluted with nonvolatile liquid ODO and the response to pure water are shown in Fig. 3a for comparison.
An experiment for the discrimination of these VSCs was performed. The headspace vapor of each sample diluted with ODO was supplied to the preconcentrator for 30 s, followed by an exposure of the vapor thermally desorbed at 170 ◦ C to the QCM sensor array. The reason we used samples diluted with ODO is that dimethyl sulfide is not water soluble. In this experimental setup, the odors of the samples are generated by putting carrier gas into the head space over the liquid
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435
Fig. 4. Sensor response patterns of eight sensors to VSCs.
sample in the test tube; the ODO dilutions were used to measure three types of VSC under the same conditions. The 10 measurement average of the maximum frequency change of each sensor is shown in Fig. 4. The data were analyzed using principal component analysis and its scatter diagram using three major principal components is shown in Fig. 5. It was noted that the sensor array output patterns of three VSCs were separated. Various odor substances are present in the breath, such as smells of various foods and odors caused by the physical conditions. In the dental field, it is well known that the VSCs mainly govern halitosis. The assessment of halitosis at dental clinics is currently accomplished by eliminating the dietary or other daily factors [10]. Patients are instructed to
abstain from eating foods with strong smells for at least 48 h, from using oral rinse and breath fresheners for 12 h, and from smoking for 12 h before the assessment. Odor-causing factors other than VSCs are eliminated by this method. Our sensing system may further help dentists in halitosis assessment in such a situation.
4. Conclusion In this study, we measured the VSCs, the halitosis-causing substances, using QCM sensors. It was noted that the sensors of lipids, a polyion complex film and a mutually self-assembly film were sensitive to VSCs. By adjusting the temperature profile of the preconcentrator and the flow path switching sequence in addition to the use of a dehumidification filter, VSC vapors even under the high-humidity condition were separated from the pure water vapor. Furthermore, the principal component analysis revealed that three types of VSC were discriminated.
Acknowledgements
Fig. 5. PCA result using peaks of eight sensor responses to VSCs.
I wish to thank Prof. T. Moriizumi of Tokyo Institute of Technology for helpful advice, Mr. J. Ide of T. Hasegawa Co. Ltd., for kindly providing the sulfide samples and Prof. S. Shiratori of Keio University for kindly providing the PAH/PAA sensors. This research was supported in part by Health Sciences Research Grants (H15-21EBM-018) from the Ministry of Health, Labour and Welfare, Japan.
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References
Biographies
[1] J. Tonzetich, Direct gas chromatographic analysis of sulphur compounds in mouth air in man, Arch. Oral. Biol. 9 (1964) 39–45. [2] J. Tonzetich, K. Yaegaki, J.M. Coil, Collagen metabolism by fibroblasts cultures in presence of methyl mercaptan, J. Dent. Res. 64 (1986) 786. [3] T. Nakamoto, T. Moriizumi, Artificial olfactory system using neural network, in: H. Yamazaki (Ed.), Handbook of Sensors and Actuators, vol. 3, Elsevier, 1996, p. 263. [4] T. Nakamoto, Y. Isaka, T. Ishige, T. Moriizumi, Odor sensing system using preconcentrator with variable temperature, Sens. Actuators B Chem. 69 (2000) 58. [5] T. Nakamoto, K. Sukegawa, E. Sumitomo, Higher-odor sensing using QCM sensor array and preconcentrator with variable temperature, Proc. IEEE Sens. Conf. USA, June 12–14 (2002) 33.3. [6] S. Shiratori, Y. Inami, M. Kikuchi, T. Yamada, M. Yamada, Polym. Adv. Technol. 11 (2000) 766–771. [7] W. Funasaka, N. Ikegawa, Gas Chromatography, Hirokawa Book Mart, vols. 12–20, pp. 76–84 (in Japanese). [8] M. Tanaka, H. Anguri, N. Nishida, M. Ojima, H. Nagata, S. Shizukuishi, Reliability of clinical parameters for predicting the outcome of oral malodor treatment, J. Clin. Res. 28 (2001) 813–819. [9] K. Yaegaki, K. Sanada, Volatile sulfur compounds in mouth air from clinically healthy subjects and patients with periodontal disease, J. Periodontal Res. 27 (1992) 233–238. [10] K. Yaegaki, J.M. Coil, Examination, classification, and treatment of halitosis, clinical perspectives, J. Can. Dent. Assoc. 66 (5) (2000) 257–261.
Junji Ito studied dentistry at Tokyo Medical and Dental University and finished his diploma thesis in 1998. He majored in prosthetics from 1998 to 2000 at Department of Geriatric Dentistry of Tokyo Medical and Dental University. Since April 2000, he has been a PhD candidate of gerodontology of Tokyo Medical and Dental University. Since July 2001, he joined the research team at Department of Physical Electronics of Tokyo Institute of Technology. He is interested in the field of an odor sensing system and diagnosis of Halitosis.
Takamichi Nakamoto received BE, ME and PhD degrees from Tokyo Institute of Technology in 1982, 1984 and 1991, respectively. He worked for Hitachi Ltd. from 1984 to 1987. In 1987, he joined Tokyo Institute of Technology as a research associate. He is currently an associate professor with the Department of Physical Electronics, Tokyo Institute of Technology. From 1996 to 1997, he was a visiting scientist at Pacific Northwest Laboratories, Richland, WA, USA. His research interest covers chemical sensing system, virtual environment with smell and ASIC chip design.
Hiroshi Uematsu studied dentistry at Kanagawa Dental College and finished his diploma thesis in 1972. In 1982, he received a PhD degree in dentistry from Tokyo Medical and Dental University. He worked for Saitama Prefectural Rehabilitation Center from 1986 to 1998. He has been a professor with the Department of Gerodontology of Tokyo Medical and Dental University since 1998.
Discrimination of halitosis substance using QCM sensor array and a preconcentrator Junji Ito a,∗ , Takamichi Nakamoto b , Hiroshi Uematsu a a b
Department of Gerodontology, Tokyo Medical and Dental University, 1-5-45 Yushima, Bunkyo-ku, Tokyo 113-8549, Japan Department of Physical Electronics, Tokyo Institute of Technology, 2-12-1 Ookayama, Meguro-ku, Tokyo 152-8552, Japan Received 19 August 2003; received in revised form 8 December 2003; accepted 12 December 2003
Abstract An organoleptic test for the assessment of halitosis is usually performed by a dental clinician, although it is a subjective test. Thus, a halitosis test for objective evaluation is required. Halitosis is due to volatile sulfur compounds (VSCs) produced by bacterial metabolism. In this study, we proposed a method for discriminating sulfur compounds using a quartz crystal microbalance (QCM) sensor array combined with a preconcentrator. The adjustment of the temperature profile, the optimization of the flow path sequence and the use of a dehumidification filter enabled VSC measurement under a high-humidity condition. Moreover, the result of principal component analysis indicated that three types of VSC were separated using the sensor array output pattern. © 2004 Elsevier B.V. All rights reserved. Keywords: Halitosis; QCM; Preconcentrator; VSCs; Odor sensing system
1. Introduction Recently, an increase in the market sale of mouth washing suggests that the awareness of body odor including halitosis is increasing. In particular, young Japanese tend to become sensitive to their own bad breath as well as to others. In such a social situation, the consultation about halitosis has become more frequent in dental clinics. However, dental clinicians have not yet established a standard assessment of halitosis. Halitosis is due to volatile sulfur compounds (VSCs), such as hydrogen sulfide, methyl mercaptan, and dimethyl sulfide produced by bacterial metabolism. Oral pathologic odor is also caused by disease, pathologic conditions and oral tissue malfunction. These sulfides are mainly derived from the tongue coating, modified by pathologic conditions. In the dental field, it is well known that these substances mainly govern the halitosis [1]. Although hydrogen sulfide is the main compound causing halitosis, the occurrence of halitosis is also correlated with the amount of methyl mercaptan even when present at low concentrations [2]. Nowadays, an organoleptic test is mainly used by a medical practitioner for the evaluation of halitosis although this
∗ Corresponding author. Tel.: +81-358035559; fax: +81-358030208. E-mail address: [email protected] (J. Ito).
0925-4005/$ – see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.snb.2003.12.012
test is not objective. Since the final decision about halitosis treatment is based on the subjective evaluation of the patient, problems often occur due to the difference in the treatment evaluation between the patient and the doctor. Thus, the development of halitosis measurement system for objective and reliable evaluation is required. To date various methods of halitosis measurement have been developed for general use and for use in dental clinics. Gas chromatography is often used to measure halitosis. However, this method is not practical in dental clinics because of the specialized skill required for the operation. Commercially available portable sulfide monitors using a gas sensor have insufficient sensitivity and selectivity to measure the VSCs. In this study, a method for discriminating VSCs using a quartz crystal microbalance (QCM) sensor array combined with a preconcentrator is proposed. The QCM works as a gas sensor when its surface is coated with a sensing film. The identification of the sensor array output pattern enables odor recognition when the array includes several QCMs with different sensing films [3]. Although a QCM sensor coated with an appropriate sensing film is sensitive to VSCs, its sensitivity to humidity should be reduced. Thus, we used the preconcentrator to overcome this problem. Although the preconcentrator is commonly used to increase the sensitivity of sensors, it can also be used to reduce the influence of humidity when the appropriate temperature profile of the preconcentrator during the desorption process is adopted [4,5]. In the present
432
J. Ito et al. / Sensors and Actuators B 99 (2004) 431–436
study, we report the result of the measurement of VSCs using the QCM sensor array combined with the preconcentrator.
2. Experimental setup The experimental setup of the preconcentrator with variable temperature is shown in Fig. 1. It consists of the vapor supply system, the preconcentrator and the QCM sensor array. After the supplied vapor accumulates in the preconcentrator, the thermally desorbed vapor is exposed to the QCM sensor array, followed by the recognition of its output pattern. The shifts in the oscillation frequencies of the QCMs measured using the multichannel frequency counter implemented into a programmable logic LSI and the data are transferred to the computer via a serial interface. The temperature of the liquid samples is maintained using a thermobath. Two mass flow controllers (MFCs) with different flow rates were used in this study. One was used for the carrier gas and the other at a lower flow rate was used for the drying phase. The stainless preconcentrator tube (o.d.: 6.00 mm, i.d.: 5.00 mm, length: 100 mm) packed with 480 mg active carbon (30/60 mesh, GL Sciences) was used. Active carbon was held in place with glass wool. The active carbon with large-grain sizes was used to reduce the pressure difference between the inlet and the outlet of the preconcentrator tube. The tube was heated using an insulated nichrome coil wound around it when VSCs were thermally desorbed. The operation sequence is shown in Fig. 2. An appropriate measurement sequence was selected to suppress the humidity influence. (a) Cleaning: the preconcentrator tube is heated to clean out the tube. (b) Sampling: the sample gas is introduced into the preconcentrator (flow rate: 50 ml/min, time: 30 s).
Fig. 2. Close-up of flow path in each sequence.
(c) Drying: on the basis of the difference in desorption temperature, VSCs trapped in preconcentrator are dried by heating this tube at the intermediate temperature where only water vapor desorbs (temperature: 80 ◦ C, time: 300 s). The water vapor flows through the bypath without going to the sensors. Then, the flow rate in the preconcentrator is kept at one-half of the initial rate (25 ml/min) using the mass flow controller to prolong the drying time. (d) Heating: thermally desorbed vapor from the preconcentrator flows into the sensor cell. The preconcentrator is further heated from 80 to 130 ◦ C. Since the preconcentrator enhances sensitivity and reduces humidity, it is suitable for the measurement of a low-concentration sample. Because humidity effect cannot be completely eliminated, the dehumidification filter is placed immediately prior to a sensor cell to remove the remaining humidity. A Teflon tube packed with 900 mg Na2 SO4 (Wako Pure Chemical Industries Ltd.) was used as the dehumidification filter in this study. Three types of VSC (hydrogen sulfide, methyl mercaptan, and dimethyl sulfide) diluted to 0.1% (w/w) were used as samples. The odor samples were generated by putting carrier gas into the head space over the liquid sample (4 ml) in a test tube (volume: 15 ml). The sample and dry air were alternately introduced to the sensor cell by switching the solenoid valves. In our experimental setup, the test tubes were placed in the thermobath to maintain the sample temperature at 27 ◦ C. The samples are shown in Table 1. The Table 1 Odor samples
Fig. 1. Flow-type odor sensing system with a preconcentrator.
Name
Molecular weight
Boiling point (◦ C)
Hydrogen sulfide Methyl mercaptan Dimethyl sulfide
34.08 48.11 62.10
−60.3 5.95 37.0
J. Ito et al. / Sensors and Actuators B 99 (2004) 431–436
VSCs were diluted with water or nonvolatile liquid such as octyl decyl oil (ODO).
Table 3 Relationship between temperature and breakthrough volume of each sample Sample
3. Result
Various materials, such as lipids, polar GC stationary phase materials, and cellulose were used as sensing films. We first measured the VSCs using QCMs (AT-cut, 20 MHz) with various coatings. The spray-coating method was used to fabricate the sensors. The resonance frequency change of each sensor except the PAH/PAA sensor due to the film coating was about 20 kHz. The frequency change of the PAH/PAA [6] sensor due to the coating was about 66.0 kHz. Eight sensors listed in Table 2 were selected after eliminating the sensors with responses less than 50 Hz or unstable responses. The mutual self-assembly and polyion complex films were found to be sensitive to the VSCs. 3.2. Breakthrough volume and retention time of an adsorbent material Breakthrough volume is the volume of carrier gas per gram of adsorbent required to cause the analyte molecules to migrate from the front of the adsorbent bed to the back. The breakthrough volume may change depending on the sample temperature. The vapor compound with a small molecular size and a low boiling point generally tends to be desorbed easily. On the other hand, a large amount of carrier gas is required to desorb the vapor compound with a large molecular size and a high boiling point [7]. The breakthrough volume (B (l/g)) decreases as temperature increases. Retention time (R (s)) required for the desorption is given as, R=
BW + D . F
Here, W (g) is the weight of the adsorbent in the preconcentrator, and F (l/s) the flow rate of carrier gas. D (l) is the dead volume, the capacity of a preconcentrator. The retention time of each sample depends on its breakthrough volume Table 2 Combination of QCM sensors with sensing films
Ch1 Ch2 Ch3 Ch4 Ch5 Ch6 Ch7 Ch8
Temprature (◦ C) 80
3.1. Sensor selection
Sensing film
Abbreviation
f (kHz)
Divaleroyl phosphatidyl choline Dipalmitoyl phosphatidyl choline Dioleoyl phosphatidyl choline Dioleoyl phosphatidic acid Dioleoyl phosphatidylserine Siponate DS-10 Poly(styrenesulfonic acid-co-maleic acid), sodium salt Poly(allylaminehydrochloride)/ poly(acrylic acid)
DVPC DPPC DOPC PTA PTS DS-10 PSS
21.71 20.60 21.89 21.09 20.74 20.98 20.79
PAH/PAA
65.96
433
Water Methyl mercaption Dimethyl sulfide Hydrogen Sulfide a b
213.7 865.0b 1038.5b 1082.7b
90 152.5 793.6b 751.1b 786.8b
100 121.9 492.6 438.2 485.8
110 103.1 358.2 339.5 341.2
120 92.9a 281.7 257.9 278.3
130 87.8a 222.2 208.6 222.2
Breakthrough volume is less than 100 dm3 g−1 . Breakthrough volume is more than 500 dm3 g−1 .
under the same conditions. We measured the retention time using the preconcentrator with a volume of 1.96 ml packed with 490 mg of active carbon at a flow rate of 50 ml/min. The retention time was the time required for the vapor to pass the preconcentrator tube. The measured breakthrough volume of each sample is shown in Table 3. Volume of water used as a sample is also shown for comparison. Water should be separated since it interferes with the detection of odors from aqueous solutions. The breakthrough volume of water was 213 (l/g) at 80 ◦ C and 121.9 (l/g) near its boiling point. Only a small amount of water was adsorbed onto the adsorbent at a temperature more than 100 ◦ C. The breakthrough volume decreased as the temperature was increased. The breakthrough volume of each VSC was more than 800 l/g at 80 ◦ C, four or five times larger than that of water at the same temperature. Thus, an aqueous sample can be measured without the influence of humidity on the basis of difference in the breakthrough volume at 80 ◦ C. 3.3. Measurement of VSCs using preconcentrator The sample vapor was accumulated in the preconcentrator before its exposure to the QCM sensor array. The thermal desorption of sample vapor is caused by the decrease in the breakthrough volume during the heating process. The sensor responses to methyl mercaptan under high humidity are shown in Figs. 3a–c. Among the eight sensors, only the response of PAH/PAA sensor is shown for easy understanding. The sample vapor was introduced to the preconcentrator between 70 and 100 s. Then, the preconcentrator was dried for 200 s at 80 ◦ C. The temperature was further increased to 170 ◦ C at 300 s. Since halitosis should be measured under the condition of high humidity, aqueous methyl mercaptan solution (0.1% (w/w)) and pure water were used as samples. The sensor responses to aqueous methyl mercaptan solution without switching the flow path had two peaks (Fig. 3a). Since the sequence used for data shown in Fig. 3a did not have a drying phase as shown in Fig. 2, the measurement system and the sequence were simple. Thus, we adopted the sequence without the drying phase at the initial stage. In this sequence, water was exposed to the sensors during the drying time whereas for consistency water was not exposed during time phase in the case of the sequence shown in
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The first peaks at about 200 s were responses to water and the second peaks at about 350 s were the responses mainly to methyl mercaptan. When the aqueous methyl mercaptan solution was used, the methyl mercaptan peak was insufficiently separated from the water peak. Thus, we measured these samples using the sequence shown in Fig. 2. The sensor responses to methyl mercaptan and pure water are shown in Fig. 3b. Only data obtained during the drying and heating phases are shown in this figure. Time scale in this figure is different from that shown in Fig. 3a since the flow rate at the drying phase was maintained at one-half of the initial rate (25 ml/min). When the sequence in Fig. 2 including the drying phase was used, the first peaks due to humidity shown in Fig. 3a were not observed in this figure. Since the water vapor flowed through the bypath without flowing to the sensors, the obtained signals of methyl mercaptan were clearer. Although the humidity was reduced in comparison with the case without the drying phase, the second peaks still included the responses to water to some extent. Therefore, the dehumidification filter was placed immediately prior to a sensor cell for the removal of the remaining humidity. It was markedly reduced as shown in Fig. 3c in comparison with the case without the flow path switch. Thus, the VSC vapors even under the high-humidity condition were separated from the water vapor. The analytical results of the three substances have been previously reported by other researchers [8,9]. One of them has suggested that there is halitosis when the total VSC concentration is more than 250 ppb. In practical application, when VSCs are approximately 250 ppb they are detected according to the subjective method of halitosis assessment described earlier. Measurement using the detector tube suggests that the concentration of hydrogen sulfide was approximately 20 ppm and those of methyl mercaptan and dimethyl sulfide were approximately 12 ppm. Although these concentrations were higher than the concentrations in actual halitosis, the sensor responses we obtained were much higher than the detection limit. Since the detection limit is approximately a few Hz and the current sensor responses were more than a few hundred Hz, samples with concentrations 100 times less than the current concentration are also detectable. Moreover, the adjustment of the amount of the adsorbent and the temperature parameters may enable further improvement. Thus, it is possible to detect a sample with a realistic concentration. 3.4. Experiment on the VSC discrimination Fig. 3. Sensor response (PAH/PAA) to methyl mercaptan during vapor accumulation/desorption process of preconcentrator: (a) without drying phase; (b) with sequence in Fig. 2; (c) with sequence in Fig. 2 and dehumidification filter.
Fig. 2. In this sequence, only one MFC was used to maintain the flow rate at 50 ml/min. The sensor response to methyl mercaptan diluted with nonvolatile liquid ODO and the response to pure water are shown in Fig. 3a for comparison.
An experiment for the discrimination of these VSCs was performed. The headspace vapor of each sample diluted with ODO was supplied to the preconcentrator for 30 s, followed by an exposure of the vapor thermally desorbed at 170 ◦ C to the QCM sensor array. The reason we used samples diluted with ODO is that dimethyl sulfide is not water soluble. In this experimental setup, the odors of the samples are generated by putting carrier gas into the head space over the liquid
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Fig. 4. Sensor response patterns of eight sensors to VSCs.
sample in the test tube; the ODO dilutions were used to measure three types of VSC under the same conditions. The 10 measurement average of the maximum frequency change of each sensor is shown in Fig. 4. The data were analyzed using principal component analysis and its scatter diagram using three major principal components is shown in Fig. 5. It was noted that the sensor array output patterns of three VSCs were separated. Various odor substances are present in the breath, such as smells of various foods and odors caused by the physical conditions. In the dental field, it is well known that the VSCs mainly govern halitosis. The assessment of halitosis at dental clinics is currently accomplished by eliminating the dietary or other daily factors [10]. Patients are instructed to
abstain from eating foods with strong smells for at least 48 h, from using oral rinse and breath fresheners for 12 h, and from smoking for 12 h before the assessment. Odor-causing factors other than VSCs are eliminated by this method. Our sensing system may further help dentists in halitosis assessment in such a situation.
4. Conclusion In this study, we measured the VSCs, the halitosis-causing substances, using QCM sensors. It was noted that the sensors of lipids, a polyion complex film and a mutually self-assembly film were sensitive to VSCs. By adjusting the temperature profile of the preconcentrator and the flow path switching sequence in addition to the use of a dehumidification filter, VSC vapors even under the high-humidity condition were separated from the pure water vapor. Furthermore, the principal component analysis revealed that three types of VSC were discriminated.
Acknowledgements
Fig. 5. PCA result using peaks of eight sensor responses to VSCs.
I wish to thank Prof. T. Moriizumi of Tokyo Institute of Technology for helpful advice, Mr. J. Ide of T. Hasegawa Co. Ltd., for kindly providing the sulfide samples and Prof. S. Shiratori of Keio University for kindly providing the PAH/PAA sensors. This research was supported in part by Health Sciences Research Grants (H15-21EBM-018) from the Ministry of Health, Labour and Welfare, Japan.
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References
Biographies
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Junji Ito studied dentistry at Tokyo Medical and Dental University and finished his diploma thesis in 1998. He majored in prosthetics from 1998 to 2000 at Department of Geriatric Dentistry of Tokyo Medical and Dental University. Since April 2000, he has been a PhD candidate of gerodontology of Tokyo Medical and Dental University. Since July 2001, he joined the research team at Department of Physical Electronics of Tokyo Institute of Technology. He is interested in the field of an odor sensing system and diagnosis of Halitosis.
Takamichi Nakamoto received BE, ME and PhD degrees from Tokyo Institute of Technology in 1982, 1984 and 1991, respectively. He worked for Hitachi Ltd. from 1984 to 1987. In 1987, he joined Tokyo Institute of Technology as a research associate. He is currently an associate professor with the Department of Physical Electronics, Tokyo Institute of Technology. From 1996 to 1997, he was a visiting scientist at Pacific Northwest Laboratories, Richland, WA, USA. His research interest covers chemical sensing system, virtual environment with smell and ASIC chip design.
Hiroshi Uematsu studied dentistry at Kanagawa Dental College and finished his diploma thesis in 1972. In 1982, he received a PhD degree in dentistry from Tokyo Medical and Dental University. He worked for Saitama Prefectural Rehabilitation Center from 1986 to 1998. He has been a professor with the Department of Gerodontology of Tokyo Medical and Dental University since 1998.