A humidity-independent ammonia sensor based on a quartz microbalance: a test under agricultural conditions

A humidity-independent ammonia sensor based on a quartz microbalance: a test under agricultural conditions

Sensors and Actuators B 57 (1999) 255 – 260 A humidity-independent ammonia sensor based on a quartz microbalance: a test under agricultural condition...

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Sensors and Actuators B 57 (1999) 255 – 260

A humidity-independent ammonia sensor based on a quartz microbalance: a test under agricultural conditions T. Rechenbach a,*, U. Schramm b, P. Boeker a, G. Horner c, C.E.O. Roesky e, J. Trepte d, S. Winter e, R. Pollex e, J. Bargon b, E. Weber e, P. Schulze Lammers a b

a Institute of Agricultural Engineering, Nussallee 5, Uni6ersity of Bonn, D-53115 Bonn, Germany Institute of Physical and Theoretical Chemistry, Uni6ersity of Bonn, Wegelerstraße 12, D-53115 Bonn, Germany c HKR Sensor Systems, Gotzinger Straße 56, D-81371 Munich, Germany d Feinchemie Sebnitz GmbH, D-01855 Sebnitz i. S., Germany e Institute of Organic Chemistry, Uni6ersity of Mining Technology, D-09599 Freiberg i. S., Germany

Received 23 October 1998; received in revised form 15 January 1999; accepted 28 January 1999

Abstract Sensor-arrays consisting of multiple quartz microbalances (QMB) each coated with different cryptophanes (CPH), macrocycles (MC) (U. Schramm, T. Rechenbach, P. Boeker, S. Winter, C. Roesky, R. Pollex, E. Weber, P. Schulze Lammers, J. Bargon, Ammonia sensor based on carboxylic-acid functionalized cryptophanes and macrocycles, Proceedings of the EUROSENSORS XII Conference, 1998, pp. 533–536), or heterocalixarenes (E. Weber, J. Trepte, K. Gloe, M. Piel, M. Czugler, V.Ch. Krartsov, Yu.A. Simonov, J. Aiptrowsky, E.V. Ganin, Heterocalixarenes featuring the benzimidazol-2-one subunit synthesis and X-ray structural studies of solvent inclusion, J. Chem. Soc., Perkin Trans. 2 (1996) 2359 – 2364) have been used to detect ammonia under agricultural conditions. As with almost all other QMB-based ammonia sensors, when using only one ammonia-sensitive sensor element on the array, the response of this initial device to ammonia fluctuated significantly due to a pronounced sensitivity to humidity. This cross sensitivity to humidity has been compensated using a heterocalixarene as a coating on one additional element of the array which thus functions as a humidity sensor. Combined with a partial least-square (PLS) analysis, this combination yields a robust humidity-independent ammonia sensor for agricultural applications. © 1999 Elsevier Science S.A. All rights reserved. Keywords: Sensor; Ammonia; Humidity; Quartz microbalance; Host/guest inclusion; Pattern recognition

1. Introduction The agricultural sector needs a low cost sensor system to monitor and control ammonia at various concentrations. At present, the only reliable measurement systems that are available are very expensive. In addition, these instruments — such as spectrometers —are only suitable for laboratory use and require specialists for their operation. Alternatively, gas sensors based on a quartz microbalance (QMB) use a measurement principle which is very promising because of its low cost and its potential of high integration and hence, a compact design. In the first part of this paper, a short explanation of * Corresponding author.

the sensor principle will be given. In the second part the choice of the suitable sensor materials will be explained. The evaluation of different sensor materials was done using a calibration system described in the text. The obtained screening results are presented and discussed. The third part deals with two different practical tests which were performed with quartz sensor arrays coated with suitable sensor materials. After the presentation of the measurement setup for the practical test, the sensor signals are analyzed with a partial least square (PLS) algorithm. A conclusion follows. In the last part an outlook is given on the further development of the presented sensor system. This part focuses on the different modes in which the sensor array can be operated.

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3. Choice of suitable sensor materials

3.1. Test system for sensor materials

Fig. 1. Test system for sensor materials.

2. The sensor principle: gas sensors based upon quartz microbalance arrays The gas to be analyzed, i.e. the analyte, is adsorbed onto a so-called sensor layer consisting of substances displaying the host/guest inclusion phenomenon [1]. Upon chemical equilibrium, the amount of bound analyte is proportional to the concentration of the analyte present in the gas mixture. Due to the adsorption, the effective mass of the sensor layer increases. This mass change can be measured by a QMB operating in the megahertz domain, typically at 10 MHz [2]. To achieve a high selectivity for the sensor, attractive sensor layers consist of compounds with especially designed adsorption sites [3,4]. For molecules with low molecular masses like ammonia and water, it is very difficult to design selective adsorption sites, because both molecules have similar chemical properties and a rather similar volume and shape.

The active sensor materials were dissolved in chloroform and sprayed onto commercially available quartz resonator discs (FOQ-Piezotechnik, Bad Rappenau, Germany). After this preparation the sensor materials were evaluated with a test system which can be seen in Fig. 1. The coated quartz discs were put into a temperature controlled measurement chamber. The measurement chamber (volume= 150 ml) can take up to 12 QMB sensors. It includes a set of ASIC-oscillators (FOQ-Piezotechnik, Bad Rappenau, Germany) and a matching frequency counter (HKR Sensor Systems, Munich, Germany) combined in circuitry which allows the signal from the 12 different quartz oscillators to be read out simultaneously. A calibration unit for generating a multi-component gas mixture was used to evaluate the sensor characteristics [5,6]. The gas mixtures with defined gas concentration was obtained by mixing calibrated premixed gases with defined flow rates. The unit can control the flow rate of the different gas components via eight mass flow controllers. For the humidification of the gas mixture a humidifier was used. The measurement chamber was operated at a continuous gas flow of 200 ml/min. The coated quartz discs were exposed to a given gas mixture for 15 min. After every exposure the coated quartz discs were regenerated by purging the measurement chamber with nitrogen. The frequency shift between the regenerated quartz and the quartz after an exposure time of 15 min was taken as the sensor signal.

Fig. 2. Molecular structure of the cryptophane (CPH) and the macrocycle (MC) used as ammonia sensitive materials in this paper.

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Fig. 3. Molecular structure of 1, heterocalix[4]arene, R1 = OCH3, R2 =C6H5; and 2, heterocalix[8]arene, R1 =R2 =OCH3 used as humidity sensitive materials in this study.

3.2. Screening results Figs. 2 and 3 show the sensor materials, which are both sensitive to ammonia and water. Fig. 4 shows the sensor signals of quartz discs coated either with cryptophane (CPH), macrocycle (MC), heterocalix[4]arene (C4a(2)) or heterocalix[8]arene (C8a(7)) when exposed to 20 000 ppm humidity and 1000 ppm ammonia, respectively. Because of their remarkable ammonia signals, CPH and MC are suitable materials for ammonia detection. The cross sensitivity of these materials to humidity is significant, however. For the heterocalixarenes, the sensor response to 20 000 ppm humidity is sufficiently high, whereas the sensor reaction to 1000 ppm ammonia is almost negligible. Accordingly, the investigated heterocalixarenes are suitable humidity sensitive sensor materials. It must be taken into account that under agricultural conditions other gas components occurring in the ppm range might have an influence on the sensor signal. For this reason all available sensor materials were examined for their cross sensitivity to CO2, N2O and H2S. No

sensor reaction was found when exposing the sensors to 1000 ppm of CO2, N2O or H2S, respectively.

4. Field measurements Two different sensor arrays were prepared for use in the field measurements. Sensor array A was coated with CPH and MC and sensor array B was coated with CPH, MC, C4a(2) and C8a(7).

4.1. Sensor array for field experiments The sensor hardware is based upon a commercially available module from HKR Sensor Systems, Munich, Germany. The sensor module consists of a 4 ml Teflon measurement chamber. In the chamber there are two parallel quartz wafers. On each quartz wafer there are three resonator zones which can individually be coated with different sensor active materials. The measurement chamber contains integrated oscillator circuits and an ASIC frequency counter in a single unit. The coating of the sensor elements was performed as described above.

4.2. Partial least square analysis

Fig. 4. Response of the sensor elements coated with CPH, MC, C4a(2) and C8a(7) to 20 000 ppm humidity and 1000 ppm ammonia, respectively.

To reduce the cross sensitivity of an individual sensor to other gas components, one widely followed approach is the use of a sensor array in combination with pattern recognition methods [7]. In the experiment outlined here, the PLS method is used for the analysis of the data. The PLS analysis is a well known pattern recognition method. The concept of the PLS analysis is to assume that there exists a linear correlation between the concentration of the different gas components ci in the measured gas mixture and the response signals sj of the different sensor elements on the sensor array, i.e.

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The coefficients aij can be calculated from calibration data by the PLS algorithm. The calibration data include the gas concentrations ci of several gas mixtures and the corresponding set of response signals sj of the sensor array. The PLS algorithm searches the coefficients aij in such a way that the deviation of the predicted concentration and the actual concentration is minimized.

plotted as a function of time. During the experiment the ammonia concentration decreased from 60 to 25 ppm, while the humidity concentration varied between 7000 and 9500 ppm. The PLS analysis of the obtained data was performed in two different ways: In the first analysis (PLS 1) the sensor signals from all six sensor elements were taken into account as unique signals for the ammonia concentration. A considerable deviation of the predicted ammonia concentration from the actual ammonia concentration was found, however. Upon a careful analysis, the data revealed that there is a clear correlation between the variation of the humidity and the deviation between the actual and the predicted concentration of ammonia. This correlation can be explained by the high humidity cross sensitivity of CPH and MC. In the second set of experiments using the PLS analysis (PLS 2) the signal from only one sensor element was used but the actual humidity as measured by the FT-IR spectrometer was taken into account. The ammonia concentration as predicted by this PLS 2 analysis corresponds very well with the actual ammonia concentration.

4.3. Setup for field measurements

4.5. Field measurement in a poultry shed

The setup for field measurements is shown in Fig. 5. A membrane pump was used to transport the test gas to the sensor chamber. A needle valve in the flowmeter was used to restrict the gas flow through the sensor module to 20 ml/min. A timer was used to switch a three-way valve every 15 min between the test gas and the purge gas alternatively. The purge gas used was dry and ammonia-free air, generated using a FT-IR purge gas generator, or, alternatively, via a combination of a charcoal filter and a silica gel filter. The purge gas was found to be necessary to compensate for sensor drift [5]. An FT-IR spectrometer [8] (Protege 460, Nicolet Instruments GmbH, Offenbach, Germany) and a photoacoustical gas monitor (Bru¨el & Kjaer 1302, Naerum, Denmark) were used for reference.

Field measurements using sensor array B were performed in a poultry shed. The results of this field measurement are summarized in Fig. 7, wherein the time variation of the actual and predicted ammonia concentrations as well as the prevailing humidity data are shown. The calculated data obtained via PLS predictions and the actual ammonia concentrations are found to be equal within a margin of 10 ppm. A variation of the humidity between 13 000 and 18 000 ppm has no significant influence on the PLS predictions. This can be explained by the fact that the integrated heterocalixarene-coated sensor elements show almost no sensitivity to ammonia. In this fashion an effective separation of the sensor signals due to ammonia and humidity, respectively, has been achieved.

Fig. 5. Experimental setup for the QMB sensor array.

ci = %j aij sj

(1)

4.4. Field measurement in a manure chamber Field measurements with sensor array A were performed in a measurement room with a defined air flow [9]. Approximately 1000 kg of manure were stored in the room. The gas sample for the sensor array and the reference gas monitor was taken from the outgoing air flow. The actual ammonia and humidity concentrations were monitored continuously using FT-IR spectrometer during the whole measurement period for reference. The results of the field measurement obtained with the sensor array A are summarized in Fig. 6. In this figure the measured and calculated concentrations are

Fig. 6. Fluctuations of the measured and calculated ammonia concentrations together with the actual humidity.

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Three different modes of operation are shown in Fig. 8.

Fig. 7. Time variation of the measured and calculated ammonia concentration.

5. Conclusions In practical field tests under agricultural conditions it has been shown that a QMB based sensor array coated with CPH, MC, and heterocalixarenes can be used as an ammonia sensor with an estimated detection limit of less than 10 ppm of ammonia. The humidity cross sensitivity of the CPH and MC coated sensor elements has been compensated using heterocalixarenes as humidity sensitive coating layers on the array in combination with a PLS analysis. To achieve a compact sensor design the sensor elements were integrated onto one and the same quartz wafer.

In the switched-line mode the sensor is stabilized by regularly regenerating the sensor array with purge gas. This mode of operation was used in the field experiments as described in this paper. For the further improvement of the sensitivity of the sensor the purge and trap mode is favored. For this purpose a trap is put in front of the sensor cell. The gas is pumped through the trap into the sensor cell. The trap containing an absorber material can be switched between two temperatures (high and low). At low temperature ammonia is absorbed in the trap and the sensor is purged with clean gas. At high temperature the absorbed ammonia desorbs from the trap into the sensor cell. Preliminary results showed that the detection limit of ammonia can be increased by up to a factor of 100 using this mode of operation. In the heat-pulse mode the regeneration of the sensor is achieved by regularly heating the sensor elements. No purge gas is needed. This mode of operation improves the reaction time of the sensor because the high temperature increases the desorption reaction. A combination of these modes of operation as outlined here is also possible. Such additional improvement of the sensor system would open up a wide field of applications, both in agriculture and in industry.

6. Outlook

Acknowledgements

Agriculture demands a sensor system which is stable, fast, and highly sensitive. Whereas this goal has partly been achieved, the development of the sensor system is still in progress. The search for better sensor materials will be continued and the mode of operation of this sensor system will be fine tuned further. This outlook will focus on the improvements which can boost the performance of the sensor system even further.

This work was supported by a research grant from the German Ministry of Education, Science, Research and Technology (BMBF). U.S. and J.B. thank the Fonds der Chemischen Industrie, Frankfurt-M., Germany, for financial support. We thank FOQ Piezotechnik, Bad Rappenau, for a gift of various quartz arrays and for technical advice.

References

Fig. 8. Modes of operation of the sensor system.

[1] E. Weber, Inclusion compounds, in: J. Ikroschwitz (Ed.), Kirk– Othmer Encyclopedia of Chemical Technology, 4th edn, Wiley, New York, 1995, pp. 122 – 154. [2] G. Sauerbrey, Verwendung von Schwingquarzen zur Wa¨gung du¨nner Schichten und zur Mikrowa¨gung, Z. Phys. 155 (1959) 206 – 222. [3] K.D. Schierbaum, W. Go¨pel, Functional polymers and supramolecular compounds for chemical sensors, Synth. Met. 61 (1993) 37 – 45. [4] E. Weber, Shape and symmetry in the design of new hosts, in: J.J. Atwood, J.E.D. Davies, D.D. MacNicol, F. Vo¨gtle (Eds.), Comprehensive Supramolecular Chemistry, vol. 6, Elsevier, Oxford, 1996, pp. 535 – 592. [5] U. Schramm, T. Rechenbach, P. Boeker, S. Winter, C. Roesky, R. Pollex, E. Weber, P. Schulze Lammers, J. Bargon, Ammonia sensor based on carboxylic-acid functionalized cryptophanes and

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Biographies Thomas Rechenbach received a diploma in physics (1996) from the University of Heidelberg. He is now working as a researcher on multigas sensors in the Department of Agricultural Engineering at the University Bonn. Udo Schramm received a diploma in physics from the University of Paderborn. Currently he is a member of the research group of Professor Bargon. As a graduate student he is completing his doctoral thesis in physics dealing with multigas sensors. Peter Boeker obtained a diploma degree in chemical engineering from the University of Erlangen – Nuernberg and a Ph.D. in physical chemistry from the University of Bonn. He works in the Department of Agricultural Engineering in the field of emissions of biological origin. He is the co-ordinator of a joint research group for agricultural gas-sensor development.

Jo¨rg Trepte received his diploma in chemistry (1994) from the TU Bergakademie Freiberg and Ph.D. in inorganic chemistry (1997) from the TU Dresden collaborating closely with the group of his former supervisor Professor Weber. He has studied the synthesis and host-guest chemistry of heterocalixarenes. Presently he is working in the industry on chemical wood protection methods. Silke Winter received a diploma in chemistry (1996) from the TU Bergakademie Freiberg. Currently she is working in the research group of Professor Weber on her doctoral thesis in organic chemistry dealing with metallo-receptors. Rolf Pollex is a teaching and research assistant of organic chemistry at the TU Bergakademie Freiberg. He received both his diploma in chemistry (1989) and his Ph.D. in organic chemistry (1993) from the University of Bonn. His current research interest include ultrasound-assisted asymmetric synthesis and the development of macrocyclic receptors. Joachim Bargon, a physicist by training, is a full professor of physical chemistry at the university of Bonn, Germany since 1984. He spent 15 years as a member of the IBM Research Division in the USA and has been a postdoctoral fellow at the California Institute of Technology in organic chemistry.

Gerhard Horner studied electrical engineering at the University of Munich and holds a Ph.D. of the same university. He is now managing director of HKR Sensorsysteme, Munich.

Edwin Weber is a full professor of organic chemistry at the TU Bergakademie Freiberg. He received his Ph.D. from the University of Wurzburg 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 inclusion and molecular recognition systems, including macrocycles and clathrates. He is also interested in crystal engineering, solid-state reactivity, and chiral resolution.

Christian E.O. Roesky received a diploma in chemistry (1993) from the University of Bonn and a Ph.D. in organic chemistry (1996) from the TU Bergakademie Freiberg while working with Professor Weber. He has also studied the synthesis and host – guest chemistry of endo-functional cryptophanes. He is now working in industry.

Peter Schulze Lammers holds a diploma degree in mechanical engineering from the Technical University of Munich and a Ph.D. in engineering from the same university. After 6 years of industrial employment and working in technical calculations, he was appointed as professor at the University of Bonn in the Department of Agricultural Engineering.

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