Peter Boeker

Monolithic sensor array based on a quartz microbalance transducer with enhanced sensitivity for monitoring agricultural emissions

Abstract A low-cost sensor array system for agricultural applications is presented. The sensor array is based on a quartz crystal microbalance (QMB) transducer with a novel heating structure. Selective host molecules are used as gas-sensitive layers. The gas-sensitive layers were optimized for the selective detection of ammonia at varying humidity concentrations. A mode of operation is presented which allows on-line monitoring of ammonia without a reference gas (RG). In addition, this temperature modulation (TM) mode improves the signal-to-noise ratio of the sensors compared to the usually applied RG mode. Multivariate regression models are applied on the sensor array data to selectively predict the ammonia concentration in agricultural emissions.

Online measurement of odorous gases close to the odour threshold with a QMB sensor system with an integrated preconcentration unit

Today odour measurement close to the odour threshold can be performed only by classical olfactometry. Classical olfactometry is an expensive and time-consuming method that is not suitable for continuous monitoring as needed by operators of compost facilities. This paper describes a system consisting of a quartz microbalance (QMB) sensor array with an integrated preconcentration unit which is able to measure odorous gases below the odour threshold. The system is developed for use in connection with biofilters in biological waste utilisation (compost facilities). The system is highly sensitive to the main odorous components of waste gas, e.g. limonene, 2-butanone or ethyl acetate. Moreover, the system evinces linear behaviour over a wide range between the odour threshold and some hundred parts per million. It was evaluated both in the laboratory and in field measurements, and it shows very good sensitivity and stability about the tested time in the field measurements (2 weeks).

A miniaturized thermal desorption unit for chemical sensing below odor threshold

We present a thermal desorption unit with features designed for use with chemical sensors to enhance sensitivity and selectivity. A novel method is used to measure and empirically describe influences on breakthrough characteristics. A fast heating technique and low dead volume result in sharp desorption peaks (approx. 0.53 ml). Enrichment factors of more than 100-fold allow for detection of odorous substances below their odor threshold.

A QMB-based temperature-modulated ammonia sensor for humid air

Abstract A temperature-modulated sensor for ammonia based upon two quartz microbalances (QMBs) has been developed. One sensor element is coated with the tris-diethylammonium salt of 4,4′,4″-[benzene-1,3,5-triyl-tri(ethin-2,1-diyl)tribenzoic acid (SPCA) [D. Meinhold, Thesis, Institute of Organic Chemistry, Mining Institute of Technology, Freiberg/Saxony, Germany, 1999.], 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 poly[ethylene imine] (PEI) functions as a humidity sensor.

Mechanistic Model of Diffusion and Reaction in Thin Sensor Layers — The DIRMAS-Model

For a better understanding and prediction of the sensor behaviour the mechanistic model DIRMAS (Diffusion and Reaction Model of Analytes in Sensor) layers was developed. Quartz microbalance sensors measure the mass increase of a sensitive layer caused by the absorption of analytes. In order to detect analytes which are minor components of a complex gas mixture the sensitive layers should be highly selective to these analytes. However, a high selectivity often results in slow kinetics of the absorption/desorption process. A better understanding of the kinetic reaction can help to design optimised layers and to decrease analysis time.

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.

Temperature dependence of an ammonia sensor in humid air based on a cryptophane-coated quartz microbalance

Abstract A cryptophane-coated sensor for ammonia based upon a quartz microbalance has been developed and the influence of the operating temperature and humidity have been evaluated. Whereas the time constant of the sensor reaction decreases at higher temperature, a lower temperature yields a higher sensor signal. To characterize the interference of relative humidity on the determination of the ammonia concentration, we have constructed a scheme of isotherms based upon measurements using pulses of a carrier gas (purge-pulse (PP) method). This scheme of isotherms enables us to explain and estimate the efficiency of the PP and of the alternate temperature-pulse (TP) method.

Fast and solvent-free quantitation of boar taint odorants in pig fat by stable isotope dilution analysis-dynamic headspace-thermal desorption-gas chromatography/time-of-flight mass spectrometry.

Boar taint is a specific off-odour of boar meat products, known to be caused by at least three unpleasant odorants, with very low odour thresholds. Androstenone is a boar pheromone produced in the testes, whereas skatole and indole originate from the microbial breakdown of tryptophan in the intestinal tract. A new procedure, applying stable isotope dilution analysis (SIDA) and dynamic headspace-thermal desorption-gas chromatography/time-of-flight mass spectrometry (dynHS-TD-GC/TOFMS) for the simultaneous quantitation of these boar taint compounds in pig fat was elaborated and validated in this paper. The new method is characterised by a simple and solvent-free dynamic headspace sampling. The deuterated compounds d3-androstenone, d3-skatole and d6-indole were used as internal standards to eliminate matrix effects. The method validation performed revealed low limits of detection (LOD) and quantitation (LOQ) with high accuracy and precision, thus confirming the feasibility of the new dynHS-TD-GC/TOFMS approach for routine analysis.

Near real time detection of hazardous airborne substances.

A fast near real-time monitoring system for hazardous airborne substances, such as chemical warfare agents (CWA) is presented and limits of detection (LOD) for five CW simulants are determined. A tandem thermal desorber (TTD) continuously collects and pre-concentrates air. The pre-concentrated samples are then separated in a fast gas chromatographic (GC) run of 6.9min. and detected by a time-of-flight mass spectrometer (TOFMS). The GC-TOFMS signals are evaluated using chemometric methods for deconvolution and target identification. The high toxicity of nerve agents requires extremely low detection limits; for some as low as 100 ng/m(3) (10 ppt). The combination of TTD, TOFMS and chemometric data evaluation methods enables the system to fulfill this requirement. Calibration measurements for five different CWA simulants show lower limits of detection in the range of 10 ng/m(3)-60 ng/m(3) (1-11 ppt). In addition, the ability to detect trace concentrations of real CWA is demonstrated with a measurement of 30 pg Sarin on column. Several other real CWA measurements are shown, like sulfur mustard in diesel, lewisite under humid conditions and VX. As part of this work the influence of stationary film thickness on peak tailing of organophosphates is investigated for peak shape optimization.

Flow field thermal gradient gas chromatography.

Negative temperature gradients along the gas chromatographic separation column can maximize the separation capabilities for gas chromatography by peak focusing and also lead to lower elution temperatures. Unfortunately, so far a smooth thermal gradient over a several meters long separation column could only be realized by costly and complicated manual setups. Here we describe a simple, yet flexible method for the generation of negative thermal gradients using standard and easily exchangeable separation columns. The measurements made with a first prototype reveal promising new properties of the optimized separation process. The negative thermal gradient and the superposition of temperature programming result in a quasi-parallel separation of components each moving simultaneously near their lowered specific equilibrium temperatures through the column. Therefore, this gradient separation process is better suited for thermally labile molecules such as explosives and natural or aroma components. High-temperature GC methods also benefit from reduced elution temperatures. Even for short columns very high peak capacities can be obtained. In addition, the gradient separation is particularly beneficial for very fast separations below 1 min overall retention time. Very fast measurements of explosives prove the benefits of using negative thermal gradients. The new concept can greatly reduce the cycle time of high-resolution gas chromatography and can be integrated into hyphenated or comprehensive gas chromatography setups.