Ullrich Stahl

Theoretical and experimental mass-sensitivity analysis of polymer-coated SAW and STW resonators for gas sensing applications

Polymer-coated surface transverse waves (STW) resonators have recently been successfully studied for organic gas sensing applications. The first results indicate increased absolute and even relative sensitivity as compared to similar resonators with surface acoustic waves (SAW). However, the gain in sensitivity is accompanied by the adverse effect of an increased attenuation and the advantage frame is difficult to establish quantitatively. In this paper, a new set of experimental samples with Parylene C-coated quartz substrates are studied. The samples are matched in frequency and wavelength. The results are compared and the obtained features explained using available theoretical algorithms for analyzing layered SAW and Love configurations, and a recently developed STW algorithm. The approximate limits of advantageous applicability of the STW resonator gas sensors are discussed.

Gas analysis with SAW sensor systems

Abstract At the institute of instrumental analysis, a family of three gas analysis instruments based on mass-sensitive gas sensor devices are developed. The SAW Aroma and Gas Analyzing Systems (SAGAS) are used to detect and analyze a large variety of gases and aromas. The common heart of these systems is a sensor head with eight differently polymer-coated surface acoustic wave (SAW) devices. These “semi-selective” sensors are used to generate typical signal pattern (“finger prints”) of analyte gases and gas mixtures. Using chemometrical data analysis, such as PCA, classification and PLS, as well as artificial neural networks, these signal patterns are used for qualitative analyte recognition and for quantitative gas mixture determination. In the following, some sensor properties are discussed and some applications of the systems are presented.

Adhesives: a new class of polymer coatings for surface acoustic wave sensors for fast and reliable process control applications

Fast and reliable on-line detection of organic vapors for control of chemical processes is a challenging application for a new type of analytical instruments: sensor systems based on an array of differently selective chemical sensors. In this work we present the use of mass-sensitive polymer coated surface acoustic wave sensors (SAWs). The sensors were initially coated with a standard set of polymers consisting of a known composition. But this first approach could not meet all requirements. Therefore, a new class of commercially available polymer coating, namely adhesives, was developed. The coating procedure was optimized and the aging process of the adhesives was carefully investigated. As a result the selectivity for ambitious separation problems arising from similar polarity of the components of typical solvent mixtures could be remarkably increased. The system was then applied in a real testing environment application at a chemical plant: the fast on-line control of a preparative reversed phase process HPLC (RP-PHPLC). Data from this industrial application are shown.

Detection of coffee flavour ageing by solid-phase microextraction/surface acoustic wave sensor array technique (SPME/SAW)

The use of polymer coated surface acoustic wave (SAW) sensor arrays is a very promising technique for highly sensitive and selective detection of volatile organic compounds (VOCs). We present new developments to achieve a low cost sensor setup with a sampling method enabling the highly reproducible detection of volatiles even in the ppb range. Since the VOCs of coffee are well known by gas chromatography (GC) research studies, the new sensor array was tested for an easy assessable objective: coffee ageing during storage. As reference method these changes were traced with a standard GC/FID set-up, accompanied by sensory panellists. The evaluation of GC data showed a non-linear characteristic for single compound concentrations as well as for total peak area values, disabling prediction of the coffee age. In contrast, the new SAW sensor array demonstrates a linear dependency, i.e. being capable to show a dependency between volatile concentration and storage time.

Optimization of surface acoustic wave sensor arrays and application to high performance liquid chromatography

Abstract Some issues regarding the optimization of sensor arrays are discussed. The criteria considered here are based on Lorbers figures of merit and nonlinear generalizations thereof. Especially, it is discussed how the averaging over the analyte space should be performed for nonlinear data and what optimization criterion should be used. A genetic algorithm is used as optimization algorithm. These considerations are used for sensor selection and sensor reduction of surface acoustic wave sensor devices applied in preparative process high performance liquid chromatography.

Long-Term Stability of Polymer-Coated Surface Transverse Wave Sensors for the Detection of Organic Solvent Vapors

Arrays with polymer-coated acoustic sensors, such as surface acoustic wave (SAW) and surface transverse wave (STW) sensors, have successfully been applied for a variety of gas sensing applications. However, the stability of the sensors’ polymer coatings over a longer period of use has hardly been investigated. We used an array of eight STW resonator sensors coated with different polymers. This sensor array was used at semi-annual intervals for a three-year period to detect organic solvent vapors of three different chemical classes: a halogenated hydrocarbon (chloroform), an aliphatic hydrocarbon (octane), and an aromatic hydrocarbon (xylene). The sensor signals were evaluated with regard to absolute signal shifts and normalized signal shifts leading to signal patterns characteristic of the respective solvent vapors. No significant time-related changes of sensor signals or signal patterns were observed, i.e., the polymer coatings kept their performance during the course of the study. Therefore, the polymer-coated STW sensors proved to be robust devices which can be used for detecting organic solvent vapors both qualitatively and quantitatively for several years.

Influence of phase position on the chemical response of oscillator driven polymer coated SAW resonators

The use of oscillator operated SAW resonators as chemical sensors is widely accepted to have the best sensing properties. However, some important parameters of influence are not sufficient resolved, yet. In general, these are effects arising from the electronic circuitry such as the phase situation on the chemical response of these sensors. We have found that they can be almost dramatic for several types of sensitive coatings and, thus, are not negligible, especially their influence on the sensitivity. In this work some exemplary results with SAW resonators working at 433,92 MHz coated with different polymeric films such as polyisobutylene (PIB), polyepichlorohydrine (PECH) or polydimethylsiloxane (PDMS) and sampled with toluene as analyte. Depending on thickness and homogeneity of the polymer film serious influences of the set phase positions on the quality of the chemical response were observed such as curve shape, and signal to noise ratio. Also, a simulation using an equivalent circuit model of the transducers including the polymer coating is used for a deeper understanding of these phenomena.

On the mass sensitivity of Rayleigh surface acoustic wave (RSAW) resonators

We provide and discuss experimental data on the mass sensitivity of Rayleigh surface acoustic wave (RSAW) two-port resonators (TPR) as an alternative to the quartz crystal microbalance (QCM) for thickness measurements and characterization of extremely thin dielectric layers — in the sub nm to a few tens of nm range where a QCM does not provide sufficient sensitivity. The calibration process is performed in a CVD reactor with precisely controlled coatings of Parylene C — a polymer that is deposited onto the TPR surface at room temperature in the absence of strong electromagnetic fields. Thus the SAW device and the equipment measuring its frequency response online are protected in the process of deposition. The RSAW TPR demonstrates more than 4000 times higher relative sensitivity and up to 180 times lower detection limit than a classical 5 MHz QCM.