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A QMB-based temperature-modulated ammonia sensor for humid air
Sensors and Actuators B 67 Ž2000. 219–226 www.elsevier.nlrlocatersensorb
A QMB-based temperature-modulated ammonia sensor for humid air a a U. Schramm a , D. Meinhold b, S. Winter b, C. Heil a , J. Muller-Albrecht , L. Wachter , ¨ ¨ H. Hoff a , C.E.O. Roesky b, T. Rechenbach c , P. Boeker c , P. Schulze Lammers c , E. Weber b, J. Bargon a,) a
Institute of Physical and Theoretical Chemistry, UniÕersity of Bonn, Wegelerstrasse 12, Bonn D-53177, Germany b Institute of Organic Chemistry, Institute of Technology, Freibergr Saxony, Germany c Institute of Agricultural Engineering, UniÕersity of Bonn, Bonn Germany Received 14 November 1999; accepted 9 March 2000
Abstract A temperature-modulated sensor for ammonia based upon two quartz microbalances ŽQMBs. has been developed. One sensor element is coated with the tris-diethylammonium salt of 4,4X ,4Y-wbenzene-1,3,5-triyl-triŽethin-2,1-diyl.tribenzoic acid ŽSPCA. wD. Meinhold, Thesis, Institute of Organic Chemistry, Mining Institute of Technology, FreibergrSaxony, Germany, 1999.x, which serves to detect ammonia, but shows a cross-sensitivity to humidity. To compensate for the influence of the latter, a sensor element coated with polywethylene iminex ŽPEI. functions as a humidity sensor. q 2000 Elsevier Science S.A. All rights reserved. Keywords: Ammonia; Humidity; Sensor; Quartz microbalance; Host–guest chemistry
1. Introduction For the selective detection of gases and vapors, various sensor systems exist w1–4x. The detection limit of known ammonia sensors ranges down to 10 ppm, which allows to detect ammonia concentrations below the MAK value of 50 ppm w5x. Our sensor system matches the sensitivity of the previously known sensor systems w6–9x. Most contemporary ammonia detectors show a cross-sensitivity to water vapor; therefore, they give rise to considerable baseline drift effects. To qualify our sensor system as a continuous ammonia monitor, we apply a temperature modulation method to render the system reversible and to compensate for these drifts. Due to selective inclusion of ammonia into appropriate host molecules, such as cryptophane w10x, a correspondingly coated quartz microbalance ŽQMB. can be used as a compact and selective NH 3 sensor, for which a prototype has been developed w11x. This arrangement is sensitive, however, to both ammonia and humidity. Therefore, the interference of the relative humidity with the sensitivity to ammonia has been evaluated via field tests conducted in various animal housings and livestock buildings w12x. For )
Corresponding author.
this purpose, periodic pulses of a virgin carrier gas such as nitrogen have been initially applied — an approach termed the purge gas–pulse ŽPP. method. This PP method allows to modulate the resonance frequency of the QMB, thereby eliminating drift effects. Since this method requires a significant amount of supplemental equipment, a simpler alternative method based upon cycling or programming the temperature of the coated QMB has been exploited: our goal was to reduce the overall cost of this sensor and to assure simple maintenance w13x. This so-called temperature–pulse ŽTP. method is performed using a temperaturecontrolled measurement cell to make sure that the operation occurs within a defined temperature range. Long-term investigations of the cryptophane-coated sensor with the TP method revealed, however, that this sensor-active material is affected by the temperature changes in long time measurements, i.e., it experiences ‘‘aging’’. Therefore, we have explored and found more suitable, alternative materials. 2. Materials and methods 2.1. Preparation of the QMB sensors To prepare the QMB sensors, an electrostatic spray method, originally developed for application in mass spec-
0925-4005r00r$ - see front matter q 2000 Elsevier Science S.A. All rights reserved. PII: S 0 9 2 5 - 4 0 0 5 Ž 0 0 . 0 0 4 2 2 - 6
U. Schramm et al.r Sensors and Actuators B 67 (2000) 219–226
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Fig. 1. The electrostatic spray method for applying the coatings.
troscopy, was modified and adapted for our purpose. As shown in Fig. 1, a solution of the sensor-active material is loaded into a syringe, which is coupled to a step motor and is acting as a pump. Due to a high DC voltage applied, typically corresponding to a potential difference of a few kilovolts between the needle and the gold electrode of the QMB as the target, the solution is transferred onto the target in the form of a finely divided spray, yielding a rather uniform coating. By use of a frequency counter, which monitors the resulting frequency shift of the QMB during the application of the coating, the resulting thickness of the layer can be controlled in situ. This method works both fast and economically, uses the sensor-active material sparingly, and yields reproducible coatings. 2.2. Experimental setup The experimental setup consists of three main components, as shown in Fig. 2: a calibration unit consisting of three mass flow controllers and a humidifying unit adjusts different probe gas mixtures, which are led into the temperature-controlled measurement cell containing the appropriately coated QMB sensors. The electrical setup consists of an array of 12 QMB resonators coupled with tempera-
Fig. 2. Experimental setup consisting of three main components: the probe gas calibration unit, the temperature-controlled measurement cell containing an array of 12 QMB sensors, and the data acquisition and control unit.
ture-controlled double oscillators. The signal detection is carried out by two frequency counter modules simultaneously. Each module is able to count six frequency values. To perform the TP method, the measurement cell and electrical setup are placed into a temperature-controlled box. For data acquisition, a PC reads out the QMB frequencies from the counter modules and controls the calibration unit and the temperature of the measurement cell using the computer software, LabVIEW, in combination with RS232 and RS485 communication. 2.3. 4,4X ,4Y-[Benzene-1,3,5-triyl-tri(ethin-2,1-diyl)tribenzoic acid (SPCA) and poly[ethylene imine] (PEI) as attractiÕe sensor-actiÕe layer materials for the detection of ammonia and humidity in the gas phase The structure of the ammonia-sensitive host compound used is shown in Fig. 3. The temperature dependence of a SPCA-coated and a PEI-coated sensor for ammonia in humid air has been evaluated using the PP method at constant temperature values of 408C and 808C. Thereby, the concentration of the compound to be analyzed is modulated by switching between the pure and inert carrier Žpurge. gas and a mixture thereof and the compound to be investigated w12x. The response characteristics of the SPCA-coated sensor to ammonia at 408C and 808C are presented in Fig. 10. The pre-examination of both host compounds by means of the PP method yielded promising results to qualify them as sensor-active materials for the ammonia detection in humid air. Neither substance shows any detectable crosssensitivity to either dinitrogen oxide, carbon dioxide, methane or dihydrogen. Fig. 10 points out that to boost the sensitivity of this system, it is advisable to operate at a relatively low temperature in order to facilitate the deposition of ammo-
Fig. 3. Triswdiethylammoniumx salt of SPCA — an attractive sensor-active layer material for ammonia detection.
U. Schramm et al.r Sensors and Actuators B 67 (2000) 219–226
nia in the sensor-active layer. By contrast, a fast response sensor for ammonia based on this concept requires a relatively high operating temperature, which reduces the sensitivity of this device due to a shift of the thermodynamic equilibrium.
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Table 2 Humidity value of the different measurement sequences Sequence number Humidity value X w% r.h.x
1 0
2 20
3 40
4 60
2.4. The TP method The TP method instead takes advantage of two aspects simultaneously, namely, by essentially modulating the resonance frequency of the QMB in a rather pronounced fashion, since the periodic switching occurs between an isotherm at 408C associated with a high deposition ratio and a slow regeneration mechanism, on the one hand, and an isotherm at 808C coupled with a low deposition ratio but a quick regeneration mechanism, on the other hand. A comparison of the advantages and disadvantages of the PP and the TP methods is conducted in Section 3. For taking data via the TP method, properly coated QMB sensors are placed into the temperature-controlled measurement cell, upon which the calibration unit prepares predetermined mixtures of various probe gases with the inert carrier gas, nitrogen. Characteristic probe gasses used in this studies were ammonia and water, which — mixed with N2 — were adjusted to a flow rate of 200 mlrmin each and led through the measurement cell. During the exposition of the sensors to each individual probe gas mixture, the measurement chamber is taken through three temperature cycles. Each one consists of a 30-min-lasting cooling-down phase to 408C followed by a 30-min-lasting heating-up phase to a peak temperature of 808C. Any measurement for a specific ammonia concentration Žexcept for mixture no. 1 or 14., consisted of four sequences conducted at four different humidity values as listed in Table 1. The humidity values X chosen within each sequence are listed in Table 2. 2.5. Temperature dependence of the reference sensor and thermal inertia of the system
system, however, stable temperature and frequency values are constant after about 10 min during the heating-up phase. In the cooling-down cycle, a stable temperature and frequency reading adjusts after 15 min. The corresponding t 90 % values are 6 min for the heating-up phase and 12 min for the cooling-down phase. All temperature-dependent frequency shifts of the reference quartz show reproducible values with a mean value for the shift of 234.3 Hz and a standard deviation of 1.7 Hz. 2.6. Temperature dependence of the resonance frequency of the QMB sensors Fig. 5 shows the sensor response of a SPCA-coated QMB sensor to various heating and cooling cycles. During the first three temperature cycles, the probe gas mixture consists of 200 ppm ammonia in nitrogen as the carrier gas. During the following three cycles, only the inert carrier gas is applied. In all cases, the frequency rises during the cooling phase and decreases during the heating phase. After an initial frequency ‘‘glitch’’, the frequency adjusts at the end of each heating and cooling cycle to a stable and reproducible value; likewise does the temperature. The adjusted frequency measured at 408C and under exposure to 200 ppm of ammonia has typically experienced a shift by a value of D F200 ppm,408C s 45 Hz, which is lower than the value under a pure carrier gas atmosphere. This shift, D F200 ppm,408C , corresponds to the quantity of ammonia bound in the sensor-active layer. To regenerate the host, i.e., the sensor-active layer, the coated QMB is heated up to 808C. As a consequence, the thermo-
Fig. 4 shows a plot of the normalized resonance frequency of a blank, i.e., uncoated reference quartz vs. time. During the heating-up phase, the frequency decreases, whereas during the cooling-down phase, the frequency increases. Due to the thermal inertia of the measurement
Table 1 Sequence of gas mixtures with various ammonia and humidity concentrations Mixture number 1 2 3 4 5 6 7 8 9 10 11 12 13 14 Ammonia 0 0 20 40 60 80 100 120 140 160 180 200 0 0 wppmx Humidity 0 X X X X X X X X X X X X 0 X s ŽTable 2.
Fig. 4. Thermal inertia of the sensor system during the heating and the cooling phases.
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this method. The subsequently measured frequency values are again reproducible.
3. Discussion 3.1. Reproducibility of ammonia measurements obtained Õia the TP method
Fig. 5. Sensor reaction of a SPCA-coated sensor to three consecutive heating and cooling cycles each during an exposure to 200 ppm of ammonia, followed by three cycles of exposure to pure carrier gas, i.e., to an atmosphere of nitrogen only.
dynamic equilibrium shifts in favor of the free ammonia in the probe gas atmosphere. The associated frequency shift adjusts to D F200 ppm,808C s 15 Hz, which is again lower than the analogous value measured under the inert carrier gas atmosphere. Therefore, this thermal regeneration procedure does not completely achieve the frequency corresponding to the status prior to the exposure to ammonia, rather the desorption of the ammonia from or out of the sensor-active coating remains incomplete. In turn, the difference of the two frequency shifts, D Fx ppm,408C y D Fx ppm,808C , is a measure for the ammonia concentration given off into the gas phase. The sensor signal obtained via the TP method, STP , can be constructed out of two components, namely, the frequency shift corresponding to each TP cycle, D Fx ppm,TP , which, however, has to be corrected by a constant offset of the frequency shift corresponding to the value when the sensor is exposed to the inert carrier gas exposure only: D F0 ppm ,TP s F0 ppm ,408C y F0 ppm ,808C ,
Fig. 6 shows two response characteristics of a cryptophane-coated QMB sensor to ammonia obtained by means of the TP method. With rising ammonia concentration, the TP signal increases. Unfortunately, a second set of data taken after 3 weeks of performing automated measurements using the TP method reveals that the sensitivity of the sensor to ammonia has dropped. Accordingly, the sensor-active material seems to become affected when performing that many temperature variations, i.e., it ages. Fig. 7 shows the results of three sets of measurements of the response characteristic of a SPCA-coated QMB sensor to ammonia obtained using the TP method. As mentioned above, the TP-derived signal can be constructed out of the frequency difference as derived from the measured values at the end of each cooling phase and the following heating phase, respectively, which has to be corrected by the temperature-dependent frequency shift. The response of the sensor is proportional to the ammonia concentration, whereby a sensor signal corresponding to a shift of 7 Hz amounts to 20 ppm of ammonia. Within the range between 80 and 200 ppm, the response characteristic of the sensor can be approximated by a linear function with a slope of 0.1 Hzrppm. The measurements were taken at a distance of 1 week, during which time the sensor was in continuous use for other investigations using the TP method universally. The measurements yield a reproducible response characteristic to ammonia with an associated error range of 2.2 Hz related to the mean value, which can be described by the following mathematical function.
S x ppm ,TP s D Fx ppm ,408C y D Fx ppm ,808C s D Fx ppm ,TP y D F0 ppm ,TP . During the first heating cycle in pure carrier gas exposure, a systematic error occurs in form of a frequency shift D Frest s 9 Hz. This value is less than the former value D F200 ppm,808C . It is the consequence of the relatively slow time constant of the desorption reaction and its temperature dependence. Immediately following the switch from the ammonia-containing mixture to pure carrier gas, the cooling phase starts, and the desorption rate sinks accordingly. The desorption process at 408C is not fast enough to provide thermodynamic equilibrium because the quantity of ammonia left bound in the sensor-active layer is too big. During the ensuing heating phase, however, the residual ammonia is driven out by the heat such that the ammonia still remaining thereafter is less than the detection limit of
Fig. 6. Initial w-B-x and subsequent w-l-x response characteristic upon aging of a cryptophane-coated QMB sensor to ammonia and reproducibility of the data using the TP method.
U. Schramm et al.r Sensors and Actuators B 67 (2000) 219–226
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3.2.2. Eq. 3: mathematical connection of the indiÕidual response characteristics to dry ammonia and to humidity, respectiÕely S1 Ž c NH 3 ,c H 2 O . s S1 Ž c NH 3 . q S1 Ž c H 2 O . qDS1 Ž c NH 3 . S1 Ž c H 2 O .
Fig. 7. Response characteristic of a SPCA-coated QMB sensor to ammonia and reproducibility of the data obtained via the TP method.
3.1.1. Eq. 1: response characteristic to ammonia of the SPCA-coated QMB sensor 1 operating in the TP mode S1 Ž c NH 3 . s
Ac NH 3 B q c NH 3
q C NH 3 c NH 3
with A NH 3 s 12.576 Hz
Ž 1.
BNH 3 s 28.10852 ppm C NH 3 s 0.08738 Hzrppm. 3.2. Cross-sensitiÕity to humidity The SPCA-coated sensor also shows a reproducible reaction to humidity when operated in the TP mode. Within the range of error, the response characteristic to humidity can be described analogously to the response characteristic to ammonia as the following. 3.2.1. Eq. 2: response characteristic to humidity of a SPCA-coated QMB sensor operating in the TP mode S1 Ž c H 2 O . s
AH 2 O cH 2 O BH 2 O q c H 2 O
Ž 3.
Eq. 3 outlines a formula describing the sensitivity of a SPCA-coated QMB sensor operating in the TP mode to ammonia at different values for the humidity. A cross-term is used to describe the mutual dependence of the sensitivity to ammonia and humidity. In Fig. 8, the calculated ammonia characteristics belonging to the various humidity values are plotted as hollow symbols. The error bars refer to the calculation of the error propagation. The hollow symbols represent the experimentally determined values. 3.3. Compensation of the mutual dependence of the sensitiÕity to ammonia and humidity by use of an additional humidity sensor To determine the ammonia concentration in a humid atmosphere accurately, an additional humidity sensor is necessary. Fig. 9 outlines the linear response characteristic of a humidity detector based upon a QMB coated with PEI and operating in the TP mode. This device shows no significant cross-sensitivity to ammonia. The standard deviation of the measured frequency shifts is 6 Hz. 3.3.1. Eq. 4: function modeling the linear response characteristic of a PEI-coated QMB sensor operating in the TP mode S2 Ž c H 2 O ,c NH 3 . s a q bc H 2 O b s 5.81806
with as y 2.70356
Ž 4.
where c H 2 O represents the relative humidity in percent w% r.h.x at 208C.
q CH 2 O cH 2 O
with A H 2 O s 9.19236 Hz
D s 0.018.
Ž 2.
BH 2 O s 31.78174 ppm C H 2 O s 0.52743 Hzr% r.h. Fig. 8 outlines the dependence of the sensitivity for the detection of ammonia on the relative humidity. The error bars correspond to an uncertainty of 2.2 Hz. A rising humidity in the probe gas atmosphere induces a shift of the frequency response of the coated QMB to higher values coupled with an increasing slope of the response characteristic to ammonia. Therefore, the cross-sensitivity of the sensor reaction cannot be described by a simple linear combination of the individual sensitivities for dry ammonia and humidity. Rather, any such relation has to be extended by a term taking care of the cross-sensitivity, as is attempted in Eq. 3.
Fig. 8. Response characteristic of a SPCA-coated QMB sensor to ammonia and its cross-sensitivity to humidity, as determined using the TP method.
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3.4. Comparison of the TP method and the PP method for using SPCA as the sensor-actiÕe layer
Fig. 9. Response characteristic of a PEI-coated QMB sensor operating in the TP mode to humidity. Within the range of errors, no significant cross-sensitivity to ammonia is observed.
Eq. 5 outlines the strategy to determine the relative humidity and the concentration of ammonia from the sensor signals of a PEI-coated QMB and a SPCA-coated QMB, both operating in the TP mode. Using the PEI-coated sensor to exclusively determine the humidity, the value for the concentration of ammonia in humid air can now be determined via Eq. 4. For this purpose, using Eq. 2, a virtual value for the contribution of the humidity to the sensor signal of the SPCA-coated QMB operating in the TP mode can be calculated. This virtual component of the sensor signal represents the part due to the humidity. In an analogous fashion, Eq. 3 compensates for the cross-sensitivity and yields a virtual contribution of the ammonia to the sensor signal for the SPCA-coated QMB sensor when operating in the TP mode. Eq. 1, an accurate value for the concentration of ammonia in a humid atmosphere, can now be calculated. 3.3.2. Eq. 5: determination of the humidity and the concentration of ammonia from the sensor signals of a PEI-coated and a SPCA-coated QMB sensor operating in the TP mode cH 2 O s
S2 y a
S1,NH 3 ,imag .s
c NH 3 s y with p s
and q s
Ž 5.
b
S1,H 2 O ,imag .s
AH 2 O cH 2 O BH 2 O q c H 2 O
q CH 2 O cH 2 O
S1 y S1,H 2 O ,imag . 1 q DS1,H 2 O ,imag .
p q 2
In Fig. 10, a scheme of isotherms corresponding to the response characteristic of a SPCA-coated QMB sensor to ammonia as obtained via the PP method at 408C and 808C is constructed in comparison to the response characteristic to ammonia as measured by means of the TP method. Since here the difference of the isotherms is such that it overlaps with the data obtained via the TP mode, it follows that in this case, the heating phase as well as the cooling phase of the TP method lasted long enough to achieve thermodynamic equilibrium at the end of each phase, a situation as is typically the case for the PP method anyway. Because the sensitivity of the PP method at 408C is limited due to the slow desorption mechanism, speeding up this process should result in a boost of the response time. However, shorter deposition and regeneration phases will cause decreasing sensor signals. Investigations conducted with the TP method and operating with 15-min-long heating and cooling phases, respectively, caused a decrease of the sensor signal of 60%. However, this value is effected by the slow temperature response of the measurement system, as discussed above in Section 2.5. When accelerating the thermal inertia of the QMB-based sensor, the effective regeneration time of the sensor operated at 808C during the heating phases could be elongated, upon which the sensor system could make use of the then faster regeneration mechanism at this high temperature. At the same time, the intensity of the sensor signal obtained when applying the TP method profits from a longer effective deposition time due to operating the sensor at 408C during the cooling phases. Accordingly, the reduction of the resulting sensor response in the TP mode when using shorter thermal cycles could be compensated by a faster temperature control of the QMB sensors. With this goal, our industrial project partner, FOQ Piezo Technik, has
(
p2 4
yq
A NH 3 q B NH 3 C NH 3 y S1,NH 3 ,imag . C NH 3
S1,NH 3 ,imag . B NH 3 C NH 3
.
Fig. 10. Scheme of isotherms corresponding to the response characteristic of a SPCA-coated QMB sensor to ammonia as obtained via the PP method at 408C and 808C and a comparison to corresponding characteristic as obtained using the TP method.
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developed a sensor array consisting of six QMBs together with an integrated resistive heater on the same quartz blank. This system is currently being evaluated at the Institute of Agricultural Engineering at the University of Bonn.
w5x w6x
w7x
4. Conclusion The response characteristic of a SPCA-coated QMB sensor to ammonia and its cross-sensitivity to humidity has been evaluated using a TP method. PEI has been found to function as a suitable sensor-active material for the detection of humidity. Values for the humidity so obtained can be used to compensate for the cross-sensitivity of the SPCA sensor to ammonia and humidity. Switching the temperature between 408C and 808C allows to compensate undesirable baseline drifts. This method makes use of two effects: the TP method essentially modulates the resonance frequency of the QMB by periodically switching between 408C, thereby achieving a high deposition rate coupled with a slow regeneration mechanism, and 808C, causing a slow deposition rate coupled with a quick regeneration mechanism. Based upon our measurements, our industrial partner, FOQ Piezo Technik, has developed a prototype for an array consisting of six QMBs on one universal quartz chip with an integrated resistive heater. This arrangement essential represents a ideal sensor array to conduct measurements applying the TP method. The results of measurements currently conducted at the Institute of Agricultural Engineering of the University of Bonn will be published elsewhere shortly.
Acknowledgements We gratefully thank the German Ministry of Science and Technology ŽBMBF. and the Fonds der Chemischen Industrie, Frankfurt ŽMain. for financial support, and the companies, HKR Sensor Systems, Munich, Germany, and FOQ Piezo Technik, Bad Rappenau, Germany, for comprehensive technical assistance.
References w1x W. Gopel, Ch. Ziegler, H. Breer, D. Schild, R. Apfelbach, J. ¨ Joerges, R. Malaka, Bioelectronic noses: a status report, Part I, Biosensors and Bioelectronics 13 Ž3–4. Ž1998. 479–493. w2x Ch. Ziegler, W. Gopel, H. Hammerle, H. Hatt, G. Jung, L. Lax¨ ¨ huber, H.-L. Schmidt, S. Schutz, A. Zell, Bioelectronic ¨ F. Vogtle, ¨ noses: a status report, Part II, Biosensors and Bioelectronics 13 Ž1998. 539–571. w3x U. Weimar, K.D. Schierbaum, W. Gopel, Pattern recognition meth¨ ods for gas mixture analysis; application to sensor arrays based upon SnO 2 , Sensors and Actuators, B 1 Ž1990. 93–96. w4x F.L. Dickert, O. Schuster, Piezoelektrische Chemosensoren-von der
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Biographies Udo Schramm received a diploma in physics from the University of Paderborn, Germany, in 1995. In 1999, he finished his PhD thesis at the Institute of Physical Chemistry, University of Bonn, building ammonia sensors. He is now working in the industry.
Carsten Heil has studied chemistry and received his diploma in 1996 at the Institute of Physical Chemistry, University of Bonn. Now he is working towards his PhD at the same institute. His research is focused on host–guest interactions and their application to sensors.
Jens Muller-Albrecht has studied chemistry and received his diploma in ¨ 1997 at the Institute of Physical Chemistry, University of Bonn. Now he is working towards his PhD at the same institute. His research is focused on host–guest interactions and their application to sensors.
Lars Wachter has studied chemistry and received his diploma in 1997 at ¨ the Institute of Physical Chemistry, University of Bonn. Now he is working towards his PhD at the same institute. His research is focused on host–guest interactions and their application to sensors.
Henrik Hoff has studied chemistry and received his diploma in 1996 at the Institute of Physical Chemistry, University of Bonn. Now he is working towards his PhD at the same institute. His research is focused on host–guest interactions, their application to sensors and building sensor prototypes.
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Joachim Bargon is Professor of Physical Chemistry at the University of Bonn and a Director of the Institute of Physical and Theoretical Chemistry there. He received his undergraduate and graduate education at the Technical University of Darmstadt, Germany, from where he graduated in 1968 with a degree in physics. He was a postdoctoral fellow in organic chemistry at the California Institute of Technology in Pasadena, USA, and joined IBM as a research staff member, initially at the Yorktown Heights Research Center, New York. In 1971, he transferred to the IBM San Jose Research Center in California, where he was department manager in basic research. In 1984, he returned to Germany, accepting a chair of Physical Chemistry at the University of Bonn. He has since spent two sabbatical leaves at the IBM Almaden Research Center at San Jose, CA, and one at the University of California, Berkeley. His research interests include aspects of catalysis, spectroscopy, and, not at least, sensors.
Edwin Weber is a Professor of Organic Chemistry at the Mining Institute of Technology, FreibergrSaxony. He received his PhD from the University of Wuerzburg in 1976 and his qualification as a university lecturer from the University of Bonn in 1984. His research interests focus on the design, synthesis, and use of all kinds of host–guest inclusions and molecular recognition systems including macrocycles and clathrates. He is also interested in crystal engineering, solid-state reactivity and chiral resolution.
Dorit Meinhold has studied chemistry and received her diploma at the Institute of Organic Chemistry of the Mining Institute of Technology, FreibergrSaxony, Germany. In 1999, she finished her PhD thesis at the same institute, dealing with the syntheses and host–guest chemistry of polycarbonic acids.
Peter Boeker holds a diploma degree in chemical engineering from the University of Erlangen-Nuernberg and a PhD in physical chemistry from the University of Bonn. He works at the Institute of Agricultural Engineering in the fields of odor emissions of agricultural origin. He is the coordinator of a joint research group for agricultural gas sensor development.
Silke Winter has studied chemistry and received her diploma in 1996 at the Institute of Organic Chemistry, Mining Institute of Technology, FreibergrSaxony, Germany. Currently, she is working towards her PhD at the same institute. Her research is focused on organic chemistry using metalloreceptors. Christian E.O. Roesky has studied chemistry and received his diploma in 1993 from the University of Bonn and a PhD in organic chemistry in 1996 from the Mining Institute of Technology, FreibergrSaxony working with Prof. Weber. He has studied the syntheses and host–guest chemistry of endofunctional cryptophanes. He is now employed by the industry.
Thomas Rechenbach received a diploma in physics from the University of Heidelberg in 1996. He is now working as a researcher on multi-gas sensors at the Institute of Agricultural Engineering at the University of Bonn. He is also a graduate student working towards his PhD at the Institute of Physical and Theoretical Chemistry at the University of Bonn.
Peter Schulze Lammers holds a diploma degree in mechanical engineering from the Technical University of Munich and a PhD in engineering form the same university. After 6 years of industrial employment working in technical calculations, he was appointed as professor at the University of Bonn at the Institute of Agricultural Engineering.
A QMB-based temperature-modulated ammonia sensor for humid air a a U. Schramm a , D. Meinhold b, S. Winter b, C. Heil a , J. Muller-Albrecht , L. Wachter , ¨ ¨ H. Hoff a , C.E.O. Roesky b, T. Rechenbach c , P. Boeker c , P. Schulze Lammers c , E. Weber b, J. Bargon a,) a
Institute of Physical and Theoretical Chemistry, UniÕersity of Bonn, Wegelerstrasse 12, Bonn D-53177, Germany b Institute of Organic Chemistry, Institute of Technology, Freibergr Saxony, Germany c Institute of Agricultural Engineering, UniÕersity of Bonn, Bonn Germany Received 14 November 1999; accepted 9 March 2000
Abstract A temperature-modulated sensor for ammonia based upon two quartz microbalances ŽQMBs. has been developed. One sensor element is coated with the tris-diethylammonium salt of 4,4X ,4Y-wbenzene-1,3,5-triyl-triŽethin-2,1-diyl.tribenzoic acid ŽSPCA. wD. Meinhold, Thesis, Institute of Organic Chemistry, Mining Institute of Technology, FreibergrSaxony, Germany, 1999.x, which serves to detect ammonia, but shows a cross-sensitivity to humidity. To compensate for the influence of the latter, a sensor element coated with polywethylene iminex ŽPEI. functions as a humidity sensor. q 2000 Elsevier Science S.A. All rights reserved. Keywords: Ammonia; Humidity; Sensor; Quartz microbalance; Host–guest chemistry
1. Introduction For the selective detection of gases and vapors, various sensor systems exist w1–4x. The detection limit of known ammonia sensors ranges down to 10 ppm, which allows to detect ammonia concentrations below the MAK value of 50 ppm w5x. Our sensor system matches the sensitivity of the previously known sensor systems w6–9x. Most contemporary ammonia detectors show a cross-sensitivity to water vapor; therefore, they give rise to considerable baseline drift effects. To qualify our sensor system as a continuous ammonia monitor, we apply a temperature modulation method to render the system reversible and to compensate for these drifts. Due to selective inclusion of ammonia into appropriate host molecules, such as cryptophane w10x, a correspondingly coated quartz microbalance ŽQMB. can be used as a compact and selective NH 3 sensor, for which a prototype has been developed w11x. This arrangement is sensitive, however, to both ammonia and humidity. Therefore, the interference of the relative humidity with the sensitivity to ammonia has been evaluated via field tests conducted in various animal housings and livestock buildings w12x. For )
Corresponding author.
this purpose, periodic pulses of a virgin carrier gas such as nitrogen have been initially applied — an approach termed the purge gas–pulse ŽPP. method. This PP method allows to modulate the resonance frequency of the QMB, thereby eliminating drift effects. Since this method requires a significant amount of supplemental equipment, a simpler alternative method based upon cycling or programming the temperature of the coated QMB has been exploited: our goal was to reduce the overall cost of this sensor and to assure simple maintenance w13x. This so-called temperature–pulse ŽTP. method is performed using a temperaturecontrolled measurement cell to make sure that the operation occurs within a defined temperature range. Long-term investigations of the cryptophane-coated sensor with the TP method revealed, however, that this sensor-active material is affected by the temperature changes in long time measurements, i.e., it experiences ‘‘aging’’. Therefore, we have explored and found more suitable, alternative materials. 2. Materials and methods 2.1. Preparation of the QMB sensors To prepare the QMB sensors, an electrostatic spray method, originally developed for application in mass spec-
0925-4005r00r$ - see front matter q 2000 Elsevier Science S.A. All rights reserved. PII: S 0 9 2 5 - 4 0 0 5 Ž 0 0 . 0 0 4 2 2 - 6
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Fig. 1. The electrostatic spray method for applying the coatings.
troscopy, was modified and adapted for our purpose. As shown in Fig. 1, a solution of the sensor-active material is loaded into a syringe, which is coupled to a step motor and is acting as a pump. Due to a high DC voltage applied, typically corresponding to a potential difference of a few kilovolts between the needle and the gold electrode of the QMB as the target, the solution is transferred onto the target in the form of a finely divided spray, yielding a rather uniform coating. By use of a frequency counter, which monitors the resulting frequency shift of the QMB during the application of the coating, the resulting thickness of the layer can be controlled in situ. This method works both fast and economically, uses the sensor-active material sparingly, and yields reproducible coatings. 2.2. Experimental setup The experimental setup consists of three main components, as shown in Fig. 2: a calibration unit consisting of three mass flow controllers and a humidifying unit adjusts different probe gas mixtures, which are led into the temperature-controlled measurement cell containing the appropriately coated QMB sensors. The electrical setup consists of an array of 12 QMB resonators coupled with tempera-
Fig. 2. Experimental setup consisting of three main components: the probe gas calibration unit, the temperature-controlled measurement cell containing an array of 12 QMB sensors, and the data acquisition and control unit.
ture-controlled double oscillators. The signal detection is carried out by two frequency counter modules simultaneously. Each module is able to count six frequency values. To perform the TP method, the measurement cell and electrical setup are placed into a temperature-controlled box. For data acquisition, a PC reads out the QMB frequencies from the counter modules and controls the calibration unit and the temperature of the measurement cell using the computer software, LabVIEW, in combination with RS232 and RS485 communication. 2.3. 4,4X ,4Y-[Benzene-1,3,5-triyl-tri(ethin-2,1-diyl)tribenzoic acid (SPCA) and poly[ethylene imine] (PEI) as attractiÕe sensor-actiÕe layer materials for the detection of ammonia and humidity in the gas phase The structure of the ammonia-sensitive host compound used is shown in Fig. 3. The temperature dependence of a SPCA-coated and a PEI-coated sensor for ammonia in humid air has been evaluated using the PP method at constant temperature values of 408C and 808C. Thereby, the concentration of the compound to be analyzed is modulated by switching between the pure and inert carrier Žpurge. gas and a mixture thereof and the compound to be investigated w12x. The response characteristics of the SPCA-coated sensor to ammonia at 408C and 808C are presented in Fig. 10. The pre-examination of both host compounds by means of the PP method yielded promising results to qualify them as sensor-active materials for the ammonia detection in humid air. Neither substance shows any detectable crosssensitivity to either dinitrogen oxide, carbon dioxide, methane or dihydrogen. Fig. 10 points out that to boost the sensitivity of this system, it is advisable to operate at a relatively low temperature in order to facilitate the deposition of ammo-
Fig. 3. Triswdiethylammoniumx salt of SPCA — an attractive sensor-active layer material for ammonia detection.
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nia in the sensor-active layer. By contrast, a fast response sensor for ammonia based on this concept requires a relatively high operating temperature, which reduces the sensitivity of this device due to a shift of the thermodynamic equilibrium.
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Table 2 Humidity value of the different measurement sequences Sequence number Humidity value X w% r.h.x
1 0
2 20
3 40
4 60
2.4. The TP method The TP method instead takes advantage of two aspects simultaneously, namely, by essentially modulating the resonance frequency of the QMB in a rather pronounced fashion, since the periodic switching occurs between an isotherm at 408C associated with a high deposition ratio and a slow regeneration mechanism, on the one hand, and an isotherm at 808C coupled with a low deposition ratio but a quick regeneration mechanism, on the other hand. A comparison of the advantages and disadvantages of the PP and the TP methods is conducted in Section 3. For taking data via the TP method, properly coated QMB sensors are placed into the temperature-controlled measurement cell, upon which the calibration unit prepares predetermined mixtures of various probe gases with the inert carrier gas, nitrogen. Characteristic probe gasses used in this studies were ammonia and water, which — mixed with N2 — were adjusted to a flow rate of 200 mlrmin each and led through the measurement cell. During the exposition of the sensors to each individual probe gas mixture, the measurement chamber is taken through three temperature cycles. Each one consists of a 30-min-lasting cooling-down phase to 408C followed by a 30-min-lasting heating-up phase to a peak temperature of 808C. Any measurement for a specific ammonia concentration Žexcept for mixture no. 1 or 14., consisted of four sequences conducted at four different humidity values as listed in Table 1. The humidity values X chosen within each sequence are listed in Table 2. 2.5. Temperature dependence of the reference sensor and thermal inertia of the system
system, however, stable temperature and frequency values are constant after about 10 min during the heating-up phase. In the cooling-down cycle, a stable temperature and frequency reading adjusts after 15 min. The corresponding t 90 % values are 6 min for the heating-up phase and 12 min for the cooling-down phase. All temperature-dependent frequency shifts of the reference quartz show reproducible values with a mean value for the shift of 234.3 Hz and a standard deviation of 1.7 Hz. 2.6. Temperature dependence of the resonance frequency of the QMB sensors Fig. 5 shows the sensor response of a SPCA-coated QMB sensor to various heating and cooling cycles. During the first three temperature cycles, the probe gas mixture consists of 200 ppm ammonia in nitrogen as the carrier gas. During the following three cycles, only the inert carrier gas is applied. In all cases, the frequency rises during the cooling phase and decreases during the heating phase. After an initial frequency ‘‘glitch’’, the frequency adjusts at the end of each heating and cooling cycle to a stable and reproducible value; likewise does the temperature. The adjusted frequency measured at 408C and under exposure to 200 ppm of ammonia has typically experienced a shift by a value of D F200 ppm,408C s 45 Hz, which is lower than the value under a pure carrier gas atmosphere. This shift, D F200 ppm,408C , corresponds to the quantity of ammonia bound in the sensor-active layer. To regenerate the host, i.e., the sensor-active layer, the coated QMB is heated up to 808C. As a consequence, the thermo-
Fig. 4 shows a plot of the normalized resonance frequency of a blank, i.e., uncoated reference quartz vs. time. During the heating-up phase, the frequency decreases, whereas during the cooling-down phase, the frequency increases. Due to the thermal inertia of the measurement
Table 1 Sequence of gas mixtures with various ammonia and humidity concentrations Mixture number 1 2 3 4 5 6 7 8 9 10 11 12 13 14 Ammonia 0 0 20 40 60 80 100 120 140 160 180 200 0 0 wppmx Humidity 0 X X X X X X X X X X X X 0 X s ŽTable 2.
Fig. 4. Thermal inertia of the sensor system during the heating and the cooling phases.
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this method. The subsequently measured frequency values are again reproducible.
3. Discussion 3.1. Reproducibility of ammonia measurements obtained Õia the TP method
Fig. 5. Sensor reaction of a SPCA-coated sensor to three consecutive heating and cooling cycles each during an exposure to 200 ppm of ammonia, followed by three cycles of exposure to pure carrier gas, i.e., to an atmosphere of nitrogen only.
dynamic equilibrium shifts in favor of the free ammonia in the probe gas atmosphere. The associated frequency shift adjusts to D F200 ppm,808C s 15 Hz, which is again lower than the analogous value measured under the inert carrier gas atmosphere. Therefore, this thermal regeneration procedure does not completely achieve the frequency corresponding to the status prior to the exposure to ammonia, rather the desorption of the ammonia from or out of the sensor-active coating remains incomplete. In turn, the difference of the two frequency shifts, D Fx ppm,408C y D Fx ppm,808C , is a measure for the ammonia concentration given off into the gas phase. The sensor signal obtained via the TP method, STP , can be constructed out of two components, namely, the frequency shift corresponding to each TP cycle, D Fx ppm,TP , which, however, has to be corrected by a constant offset of the frequency shift corresponding to the value when the sensor is exposed to the inert carrier gas exposure only: D F0 ppm ,TP s F0 ppm ,408C y F0 ppm ,808C ,
Fig. 6 shows two response characteristics of a cryptophane-coated QMB sensor to ammonia obtained by means of the TP method. With rising ammonia concentration, the TP signal increases. Unfortunately, a second set of data taken after 3 weeks of performing automated measurements using the TP method reveals that the sensitivity of the sensor to ammonia has dropped. Accordingly, the sensor-active material seems to become affected when performing that many temperature variations, i.e., it ages. Fig. 7 shows the results of three sets of measurements of the response characteristic of a SPCA-coated QMB sensor to ammonia obtained using the TP method. As mentioned above, the TP-derived signal can be constructed out of the frequency difference as derived from the measured values at the end of each cooling phase and the following heating phase, respectively, which has to be corrected by the temperature-dependent frequency shift. The response of the sensor is proportional to the ammonia concentration, whereby a sensor signal corresponding to a shift of 7 Hz amounts to 20 ppm of ammonia. Within the range between 80 and 200 ppm, the response characteristic of the sensor can be approximated by a linear function with a slope of 0.1 Hzrppm. The measurements were taken at a distance of 1 week, during which time the sensor was in continuous use for other investigations using the TP method universally. The measurements yield a reproducible response characteristic to ammonia with an associated error range of 2.2 Hz related to the mean value, which can be described by the following mathematical function.
S x ppm ,TP s D Fx ppm ,408C y D Fx ppm ,808C s D Fx ppm ,TP y D F0 ppm ,TP . During the first heating cycle in pure carrier gas exposure, a systematic error occurs in form of a frequency shift D Frest s 9 Hz. This value is less than the former value D F200 ppm,808C . It is the consequence of the relatively slow time constant of the desorption reaction and its temperature dependence. Immediately following the switch from the ammonia-containing mixture to pure carrier gas, the cooling phase starts, and the desorption rate sinks accordingly. The desorption process at 408C is not fast enough to provide thermodynamic equilibrium because the quantity of ammonia left bound in the sensor-active layer is too big. During the ensuing heating phase, however, the residual ammonia is driven out by the heat such that the ammonia still remaining thereafter is less than the detection limit of
Fig. 6. Initial w-B-x and subsequent w-l-x response characteristic upon aging of a cryptophane-coated QMB sensor to ammonia and reproducibility of the data using the TP method.
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3.2.2. Eq. 3: mathematical connection of the indiÕidual response characteristics to dry ammonia and to humidity, respectiÕely S1 Ž c NH 3 ,c H 2 O . s S1 Ž c NH 3 . q S1 Ž c H 2 O . qDS1 Ž c NH 3 . S1 Ž c H 2 O .
Fig. 7. Response characteristic of a SPCA-coated QMB sensor to ammonia and reproducibility of the data obtained via the TP method.
3.1.1. Eq. 1: response characteristic to ammonia of the SPCA-coated QMB sensor 1 operating in the TP mode S1 Ž c NH 3 . s
Ac NH 3 B q c NH 3
q C NH 3 c NH 3
with A NH 3 s 12.576 Hz
Ž 1.
BNH 3 s 28.10852 ppm C NH 3 s 0.08738 Hzrppm. 3.2. Cross-sensitiÕity to humidity The SPCA-coated sensor also shows a reproducible reaction to humidity when operated in the TP mode. Within the range of error, the response characteristic to humidity can be described analogously to the response characteristic to ammonia as the following. 3.2.1. Eq. 2: response characteristic to humidity of a SPCA-coated QMB sensor operating in the TP mode S1 Ž c H 2 O . s
AH 2 O cH 2 O BH 2 O q c H 2 O
Ž 3.
Eq. 3 outlines a formula describing the sensitivity of a SPCA-coated QMB sensor operating in the TP mode to ammonia at different values for the humidity. A cross-term is used to describe the mutual dependence of the sensitivity to ammonia and humidity. In Fig. 8, the calculated ammonia characteristics belonging to the various humidity values are plotted as hollow symbols. The error bars refer to the calculation of the error propagation. The hollow symbols represent the experimentally determined values. 3.3. Compensation of the mutual dependence of the sensitiÕity to ammonia and humidity by use of an additional humidity sensor To determine the ammonia concentration in a humid atmosphere accurately, an additional humidity sensor is necessary. Fig. 9 outlines the linear response characteristic of a humidity detector based upon a QMB coated with PEI and operating in the TP mode. This device shows no significant cross-sensitivity to ammonia. The standard deviation of the measured frequency shifts is 6 Hz. 3.3.1. Eq. 4: function modeling the linear response characteristic of a PEI-coated QMB sensor operating in the TP mode S2 Ž c H 2 O ,c NH 3 . s a q bc H 2 O b s 5.81806
with as y 2.70356
Ž 4.
where c H 2 O represents the relative humidity in percent w% r.h.x at 208C.
q CH 2 O cH 2 O
with A H 2 O s 9.19236 Hz
D s 0.018.
Ž 2.
BH 2 O s 31.78174 ppm C H 2 O s 0.52743 Hzr% r.h. Fig. 8 outlines the dependence of the sensitivity for the detection of ammonia on the relative humidity. The error bars correspond to an uncertainty of 2.2 Hz. A rising humidity in the probe gas atmosphere induces a shift of the frequency response of the coated QMB to higher values coupled with an increasing slope of the response characteristic to ammonia. Therefore, the cross-sensitivity of the sensor reaction cannot be described by a simple linear combination of the individual sensitivities for dry ammonia and humidity. Rather, any such relation has to be extended by a term taking care of the cross-sensitivity, as is attempted in Eq. 3.
Fig. 8. Response characteristic of a SPCA-coated QMB sensor to ammonia and its cross-sensitivity to humidity, as determined using the TP method.
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3.4. Comparison of the TP method and the PP method for using SPCA as the sensor-actiÕe layer
Fig. 9. Response characteristic of a PEI-coated QMB sensor operating in the TP mode to humidity. Within the range of errors, no significant cross-sensitivity to ammonia is observed.
Eq. 5 outlines the strategy to determine the relative humidity and the concentration of ammonia from the sensor signals of a PEI-coated QMB and a SPCA-coated QMB, both operating in the TP mode. Using the PEI-coated sensor to exclusively determine the humidity, the value for the concentration of ammonia in humid air can now be determined via Eq. 4. For this purpose, using Eq. 2, a virtual value for the contribution of the humidity to the sensor signal of the SPCA-coated QMB operating in the TP mode can be calculated. This virtual component of the sensor signal represents the part due to the humidity. In an analogous fashion, Eq. 3 compensates for the cross-sensitivity and yields a virtual contribution of the ammonia to the sensor signal for the SPCA-coated QMB sensor when operating in the TP mode. Eq. 1, an accurate value for the concentration of ammonia in a humid atmosphere, can now be calculated. 3.3.2. Eq. 5: determination of the humidity and the concentration of ammonia from the sensor signals of a PEI-coated and a SPCA-coated QMB sensor operating in the TP mode cH 2 O s
S2 y a
S1,NH 3 ,imag .s
c NH 3 s y with p s
and q s
Ž 5.
b
S1,H 2 O ,imag .s
AH 2 O cH 2 O BH 2 O q c H 2 O
q CH 2 O cH 2 O
S1 y S1,H 2 O ,imag . 1 q DS1,H 2 O ,imag .
p q 2
In Fig. 10, a scheme of isotherms corresponding to the response characteristic of a SPCA-coated QMB sensor to ammonia as obtained via the PP method at 408C and 808C is constructed in comparison to the response characteristic to ammonia as measured by means of the TP method. Since here the difference of the isotherms is such that it overlaps with the data obtained via the TP mode, it follows that in this case, the heating phase as well as the cooling phase of the TP method lasted long enough to achieve thermodynamic equilibrium at the end of each phase, a situation as is typically the case for the PP method anyway. Because the sensitivity of the PP method at 408C is limited due to the slow desorption mechanism, speeding up this process should result in a boost of the response time. However, shorter deposition and regeneration phases will cause decreasing sensor signals. Investigations conducted with the TP method and operating with 15-min-long heating and cooling phases, respectively, caused a decrease of the sensor signal of 60%. However, this value is effected by the slow temperature response of the measurement system, as discussed above in Section 2.5. When accelerating the thermal inertia of the QMB-based sensor, the effective regeneration time of the sensor operated at 808C during the heating phases could be elongated, upon which the sensor system could make use of the then faster regeneration mechanism at this high temperature. At the same time, the intensity of the sensor signal obtained when applying the TP method profits from a longer effective deposition time due to operating the sensor at 408C during the cooling phases. Accordingly, the reduction of the resulting sensor response in the TP mode when using shorter thermal cycles could be compensated by a faster temperature control of the QMB sensors. With this goal, our industrial project partner, FOQ Piezo Technik, has
(
p2 4
yq
A NH 3 q B NH 3 C NH 3 y S1,NH 3 ,imag . C NH 3
S1,NH 3 ,imag . B NH 3 C NH 3
.
Fig. 10. Scheme of isotherms corresponding to the response characteristic of a SPCA-coated QMB sensor to ammonia as obtained via the PP method at 408C and 808C and a comparison to corresponding characteristic as obtained using the TP method.
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developed a sensor array consisting of six QMBs together with an integrated resistive heater on the same quartz blank. This system is currently being evaluated at the Institute of Agricultural Engineering at the University of Bonn.
w5x w6x
w7x
4. Conclusion The response characteristic of a SPCA-coated QMB sensor to ammonia and its cross-sensitivity to humidity has been evaluated using a TP method. PEI has been found to function as a suitable sensor-active material for the detection of humidity. Values for the humidity so obtained can be used to compensate for the cross-sensitivity of the SPCA sensor to ammonia and humidity. Switching the temperature between 408C and 808C allows to compensate undesirable baseline drifts. This method makes use of two effects: the TP method essentially modulates the resonance frequency of the QMB by periodically switching between 408C, thereby achieving a high deposition rate coupled with a slow regeneration mechanism, and 808C, causing a slow deposition rate coupled with a quick regeneration mechanism. Based upon our measurements, our industrial partner, FOQ Piezo Technik, has developed a prototype for an array consisting of six QMBs on one universal quartz chip with an integrated resistive heater. This arrangement essential represents a ideal sensor array to conduct measurements applying the TP method. The results of measurements currently conducted at the Institute of Agricultural Engineering of the University of Bonn will be published elsewhere shortly.
Acknowledgements We gratefully thank the German Ministry of Science and Technology ŽBMBF. and the Fonds der Chemischen Industrie, Frankfurt ŽMain. for financial support, and the companies, HKR Sensor Systems, Munich, Germany, and FOQ Piezo Technik, Bad Rappenau, Germany, for comprehensive technical assistance.
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Biographies Udo Schramm received a diploma in physics from the University of Paderborn, Germany, in 1995. In 1999, he finished his PhD thesis at the Institute of Physical Chemistry, University of Bonn, building ammonia sensors. He is now working in the industry.
Carsten Heil has studied chemistry and received his diploma in 1996 at the Institute of Physical Chemistry, University of Bonn. Now he is working towards his PhD at the same institute. His research is focused on host–guest interactions and their application to sensors.
Jens Muller-Albrecht has studied chemistry and received his diploma in ¨ 1997 at the Institute of Physical Chemistry, University of Bonn. Now he is working towards his PhD at the same institute. His research is focused on host–guest interactions and their application to sensors.
Lars Wachter has studied chemistry and received his diploma in 1997 at ¨ the Institute of Physical Chemistry, University of Bonn. Now he is working towards his PhD at the same institute. His research is focused on host–guest interactions and their application to sensors.
Henrik Hoff has studied chemistry and received his diploma in 1996 at the Institute of Physical Chemistry, University of Bonn. Now he is working towards his PhD at the same institute. His research is focused on host–guest interactions, their application to sensors and building sensor prototypes.
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Joachim Bargon is Professor of Physical Chemistry at the University of Bonn and a Director of the Institute of Physical and Theoretical Chemistry there. He received his undergraduate and graduate education at the Technical University of Darmstadt, Germany, from where he graduated in 1968 with a degree in physics. He was a postdoctoral fellow in organic chemistry at the California Institute of Technology in Pasadena, USA, and joined IBM as a research staff member, initially at the Yorktown Heights Research Center, New York. In 1971, he transferred to the IBM San Jose Research Center in California, where he was department manager in basic research. In 1984, he returned to Germany, accepting a chair of Physical Chemistry at the University of Bonn. He has since spent two sabbatical leaves at the IBM Almaden Research Center at San Jose, CA, and one at the University of California, Berkeley. His research interests include aspects of catalysis, spectroscopy, and, not at least, sensors.
Edwin Weber is a Professor of Organic Chemistry at the Mining Institute of Technology, FreibergrSaxony. He received his PhD from the University of Wuerzburg in 1976 and his qualification as a university lecturer from the University of Bonn in 1984. His research interests focus on the design, synthesis, and use of all kinds of host–guest inclusions and molecular recognition systems including macrocycles and clathrates. He is also interested in crystal engineering, solid-state reactivity and chiral resolution.
Dorit Meinhold has studied chemistry and received her diploma at the Institute of Organic Chemistry of the Mining Institute of Technology, FreibergrSaxony, Germany. In 1999, she finished her PhD thesis at the same institute, dealing with the syntheses and host–guest chemistry of polycarbonic acids.
Peter Boeker holds a diploma degree in chemical engineering from the University of Erlangen-Nuernberg and a PhD in physical chemistry from the University of Bonn. He works at the Institute of Agricultural Engineering in the fields of odor emissions of agricultural origin. He is the coordinator of a joint research group for agricultural gas sensor development.
Silke Winter has studied chemistry and received her diploma in 1996 at the Institute of Organic Chemistry, Mining Institute of Technology, FreibergrSaxony, Germany. Currently, she is working towards her PhD at the same institute. Her research is focused on organic chemistry using metalloreceptors. Christian E.O. Roesky has studied chemistry and received his diploma in 1993 from the University of Bonn and a PhD in organic chemistry in 1996 from the Mining Institute of Technology, FreibergrSaxony working with Prof. Weber. He has studied the syntheses and host–guest chemistry of endofunctional cryptophanes. He is now employed by the industry.
Thomas Rechenbach received a diploma in physics from the University of Heidelberg in 1996. He is now working as a researcher on multi-gas sensors at the Institute of Agricultural Engineering at the University of Bonn. He is also a graduate student working towards his PhD at the Institute of Physical and Theoretical Chemistry at the University of Bonn.
Peter Schulze Lammers holds a diploma degree in mechanical engineering from the Technical University of Munich and a PhD in engineering form the same university. After 6 years of industrial employment working in technical calculations, he was appointed as professor at the University of Bonn at the Institute of Agricultural Engineering.