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Eingeladener Vortrag
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Application of electronic nose and ion mobility spectrometer to quality control of spice mixtures
(2009)
Ionische Flüssigkeiten als neuartige Sensorbeschichtung für coulometrische Spurenfeuchtesensoren
(2018)
Die Arbeit präsentiert erste Messungen mit coulometrischen Sensoren, die mit einer ionischen Flüssigkeit beschichtet sind. Diese Sensoren werden in der Industrie verwendet, um die Spurenfeuchte in unterschiedlichen Gasen zu messen. Normalerweise erfolgt eine Beschichtung der Sensoren mit Phosphorpentoxid und dessen Hydrolyseprodukte. In dieser Arbeit wurde stattdessen eine hygroskopische ionische Flüssigkeit benutzt. Die generierte Gasfeuchte für die Untersuchungen betrug bezogen auf die Frostpunkttemperatur -80 °C bis -30 °C, was gleichbedeutend mit einem Wasserdampf-Stoffmengenanteil von 0,5 μmol·mol-1 bis 376 μmol·mol-1 ist. Zusätzlich zu der Messung mit coulometrischen Sensoren erfolgte eine Referenzmessung der Gasfeuchte mittels eines Taupunkthygrometers. Erste Ergebnisse haben gezeigt, dass es möglich ist mit dieser Art der Beschichtung unterschiedliche Gasfeuchten zu messen. Aber es ergaben sich trotz gleicher Feuchte unterschiedliche Signalintensitäten bei den benutzen Sensoren und es zeigte sich eine Drift des Sensorsignales.
Technische Richtlinien enthalten Empfehlungen sowie Anforderungen zum Stand der Technik zur Aufstellung von Bewertungskriterien und Behandlung wissenschaftlich-technischer Fragen. Sie fördern den Erfahrungsaustausch und Technologietransfer zwischen Entwicklern und Anwendern.
In unserem Beitrag stellen wir die Richtlinie VDI/VDE 3518 vor. Diese Richtlinienreihe definiert Begriffe, beschreibt den Aufbau und klassifiziert Multigassensoren (Blatt 1), befasst sich mit den Anforderungen für das Unterscheiden, Erkennen und mengenmäßige Bestimmen von Gemischen gasförmiger Stoffe und die Prüfung von Multigassensoren (Blatt 2) und gibt Anleitung für geruchsbezogene Messungen mit „Elektronischen Nasen“ (Blatt 3). Multigassensoren können als elektronische Nasen in Anlehnung an den biologischen Geruchssinn zur Wahrnehmung und Charakterisierung von Gerüchen eingesetzt werden. Die Spezifik von Multigassensoren besteht darin, dass durch die Wahl und Betriebsweise der Sensoren eine Vielzahl von geeigneten Messsignalen erzeugt werden, aus denen mit mathematischen Verfahren die für die Anwendung gewünschten Informationen extrahiert werden. Dazu ist ein Training der Messeinrichtung mit Proben bekannter Eigenschaften notwendig.
Diese Richtlinie soll Herstellern, Anbietern und Nutzern als ein Leitfaden Informationen und Erläuterungen für die korrekte und sichere Arbeit mit Multigassensoren liefern. Sie soll Prüflaboratorien und Anwendern helfen, bei der Beurteilung von Qualitätsmerkmalen einheitlich vorzugehen und dafür gemeinsame Randbedingungen anzugeben
This work presents a first attempt to use ionic liquids as a new coating for planar coulometric sensors. These sensors are used for the measurement of trace humidity in various gases. Usually, the coating of the sensors is tetraphosphorus decaoxide and its hydrolysis products. Instead, a hygroscopic ionic liquid was used as sensor coating in this work. Generated frost point temperatures tf in the gas ranged from -80 °C to -30 °C, which is equivalent to vapour mole fractions xv from 0.5 μmol·mol-1 to 376 μmol·mol-1. In addition to the coulometric sensors, the generated humidity is determined by a precision dew point hygrometer as reference. First results show that it is possible to measure humidity with ionic liquid based coulometric sensors.
Planar coulometric sensors were investigated in humidified synthetic air at various absolute gas pressures, i. e. 2 bar, 5, bar, 10 bar, and 15 bar. Humidified gas flow at adjusted gas pressure was split into two flows, one passed a coulometric sensor and the other one passed a reference hygrometer after decompression. Both signals were recorded and then compared after calculation of resulting frost point temperature. Calculation is based on a calibration function obtained at ambient pressure. Comparison showed that an increasing pressure resulted in a higher derivation between sensor signal (calculated frost point temperature) and reference frost point temperature. At an absolute pressure of 2 bar the differences were minor in consideration of the uncertainty, however at 15 bar the differences were 6.77 K. Nevertheless, it was possible to measure the gas humidity at higher pressure with coulometric trace humidity sensors.
The chapter describes the application of electronic noses (multigas sensors) for the quality control of spices and spice mixtures. Electronic noses were successfully applied for headspace analysis of spices. It was demonstrated in many investigations that electronic noses can contribute to the characterization of spices and spice mixtures in order to distinguish spices and spice mixtures, differentiate by origin, growth seasons,and processing,indicate adulteration from original, detect mold infestation. Electronic noses can be used as a fast screening method to provide information about the product quality. However, it needs samples and methods for reference, careful training, and complex calibration to consider influencing and disturbing effects as well as the possible limitations of the instrumentation. The correlation to classical chemical analysis methods is always advisable. Machined olfaction methods are capable to support the sensory analysis; however, they cannot yet substitute them.
Because ammonia and its reaction products can cause considerable damage to human health and ecosystems, there is a need for reliably operating and reversibly interacting sensor materials to monitor traces of gaseous ammonia in ambient air, which at best can be used on-site for in-the-field measurements. Herein, the development of a sensor material for gaseous ammonia in the lower ppm to ppb range using optical fluorescence as transduction mechanism is presented. A fluorescent dye, which shows reversible fluorescence enhancement in the presence of ammonia is incorporated into a polymer matrix, the latter to ensure the accumulation of ammonia. The sensor material is integrated into a prototype of a miniaturized sensor device, facilitating long-term operation. To calibrate the optical sensor system a gas standard generator, producing standard gas mixtures, is used, leading to a sensitivity down to lower ppm concentrations of ammonia.
Pollution through emission of toxic gases is an increasing problem for the environment. It affects similarly agricultural, industrial and urban areas. In future, environmental emissions in ambient air must be monitored at even lower concentrations as nowadays. One environmental relevant compound is ammonia and its conversion product ammonium that have strong negative impact on human health and ecosystems. Most ammonia measurements in ambient air are performed in the range below 1000 nmol·mol-1 and thus there is a need for reliable traceable ammonia gas standards and in addition in situ analytical procedures for monitoring (in ambient air to avoid that thresholds are exceeded). Therefore, the use of reference materials is necessary for development accompanying test or for calibration, e. g. of structure-integrated sensors and mobile multi-gas sensors.
The developed gas standard generator produces gas mixtures that comply with the metrological traceability for ammonia gas standards in the desired environmentally relevant measurement range. The method is based on the permeation of ammonia through a membrane at constant temperature and pressure. The resulting ammonia penetrant gas flow is then mixed with a carrier gas flow to generate a gas standard flow of known concentration. The dynamic rage is enlarged by using a two dilution steps. Depending on the permeation rate, generable molar fractions are possible in the range nmol·mol-1 to a few µmol·mol-1. We present the design of an ammonia gas standard generator and first results of the characterisation of its individual components supporting the uncertainty assessment according to GUM for stable gas concentrations in this range. The relative uncertainty of the generated ammonia gas standard is smaller than 4 % (k = 2).
The efficiency of gas sensor application for facilitating the safe use of hydrogen depends considerably on the sensor response to a change in hydrogen concentration. Therefore, the response time has been measured for five different-type commercially available hydrogen sensors. Experiments showed that all these sensors surpass the ISO 26142 standard; for the response times t90 values of 2 s to 16 s were estimated. Results can be fitted with an exponential or sigmoidal function. It can be demonstrated that the results on transient behaviour depend on both the operating parameters of sensors and Investigation methods, as well as on the experimental conditions: gas change rate and concentration jump.