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

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.