Mass-sensitive sensors,

Sensors based on gas sorption Polymer sensors (swelling) Fiber-optic sensors Mass sensitive (quartz crystal microbalance (QMB) surface (SAW) and bulk acoustic wave (BAW), microcantilevers)... 

Chemical sensors may be classified according to the operating principle of the transducer as optical, electrochemical, electrical, mass sensitive, etc. 

The features of the monoHthic integrated sensor systems have not yet been fully exploited. The almost linear relationship between input reference voltage and microhotplate temperature renders the systems suitable for applying any temperature modulation protocol. Due their compatibility with other CMOS-based chemical sensors the microhotplates can be also combined with, e.g., polymer-based mass sensitive, calorimetric or capacitive sensors. The co-integration with such sensors can help to alleviate problems resulting from cross-sensitivities of tin-oxide based sensors to, e.g., volatile compounds such as hydrocarbons. A well-known problem is the crosssensitivity of tin oxide to humidity or ethanol. The co-integration of a capacitive sensor, which does not show any sensitivity to CO, could help to independently assess humidity changes. 

A final example of a mass-sensitive MIP device is a B AW sensor for determination of phenacetin in human serum and urine (Tan, Peng, et al. 2001). A phenacetin imprinted polymer was synthesized and used as the artificial recognition element on a piezoelectric element. 

Altitude Response. Pressure response is an issue that needs to be addressed for every instrument deployed on an aircraft. First, it must be decided how chemical abundances are to be reported. If standard practice is followed and they are reported as mixing ratios, then it must be determined whether the instrument is fundamentally a mass- or a concentration-depen-dent sensor, because this definition determines the first-order means by which instrument response is converted to mixing ratios as a function of pressure. In this context, a mass-sensitive detector is a device with an output signal that is a function of the mass flow of analyte molecules a concentration-sensitive detector is one in which the response is proportional to the absolute concentration, that is, molecules per cubic centimeter. 

For sensors that are truly mass sensitive and for which the mass flow of sample through the sensing element is held constant as a function of pressure (for example, by use of electronic mass-flow controllers), instrument response is proportional to the mixing ratio independent of the pressure. For concentration-sensitive detectors, such as simple spectrophotometric instruments measuring absorbance or fluorescence, instrument response is a function of the absolute concentration, and the response will decrease for a constant mixing ratio as the pressure decreases. For example, the response of a pulsed fluorescence SO instrument sampling air containing a fixed... 

Explosive-based terrorism is an eminent threat to a civilized and free society. Accurate and cost-effective explosive sensors are, therefore, essential for combating the terrorist threat. Some of the main performance characteristics for explosive sensors include sensitivity, selectivity, and real-time fast operation. As the vapor pressures of commonly used explosives are extremely small, highly sensitive sensors are essential for detecting trace quantities of explosives. Moreover, the sensors should have high selectivity to have an acceptable rate of false positives. Also, these sensors should have the capability of mass deployment because of the breadth of terrorist threats involving explosives [1], Finally, these sensors should have fast detection and regeneration time for fast operation. Currently available sensors are unable to satisfy these requirements. 

Detecting Chemical Vapors with the FPW Sensor A practical FPW chemical vapor sensor typically employs a sorptive film on the plate, as do the TSM, SAW and APM devices (Figure 3.43, page 122). When calculating the mass sensitivity of the coated FPW sensor, we simply include the mass per unit area of... 

Figure 3.44a shows FPW sensor response to toluene vapor [68]. The sorptive coating was a l.S-/im-thick layer of poly(dimethylsiloxane), PDMS. The sorptive polymer was in its rubbery state at the measurement temperature of 24 C. Very linear response was observed (Figure 3.44b). A different FPW device tp-erating at 2.8S MHz and having a 0.5-/u.m-thick coating of ethyl cellulose, which was expected to be better suited to toluene detection [70], had a measured mass sensitivity of S = 1064 cm /g. Its estimated minimum detectible concentration was 70 ppb, based on an assumed 3 1 signal-to-noise ratio and die measured short-term frequency instability of 0.04Hz.

The high mass sensitivity of ETSM sensors renders them particularly suited for the analysis of monolayer and submonolayer films. In fact, the earliest applications of the ETSM involved studying the electrochemical deposition of monolayers, including the formation of metal oxides [207], electrosorption of halides [208], and the underpotential deposition of metal atoms [209-213]. In some cases, the electrovalency (i.e., the ratio of moles of electrons transferred at the electrode to moles of adsorbate deposited) was found to vary with adsorbing species the adsorption of iodide onto gold, for example, occurs with complete charge transfer from the halide to the electrode, whereas the adsorption of bro-... 

Surface mass changes can result from sorptive interactions (i.e., adsorption or absorption) or chemical reactions between analyte and coating, and can be used for sensing applications in bodi liquid and gas phases. Although the absolute mass sensitivity of the uncoated sensor depends on the nature of the piezoelectric substrate, device dimensions, frequoicy of operation, and the acoustic mode that is utilized, a linear dependence is predicted in all cases. This allows a very general description of the working relationship between mass-loading and frequency shift, A/ , for AW devices to be written ... 

Yet another significant challenge to the successful use of AW sensors is the isolation of their sensitivities to numerous different perturbations, so that only a single, desired interaction is observed. As an example, AW sensors are sensitive to enviixmmental variables such as temperature, pressure, and gas or liquid flow rate in mass-sensing applications, excessive response to these variables can be a serious problem. Controlling the AW sensor environment is the focus of Section 6.4. 

---