Mass sensitivity, absolute

The Sauerbrey equation predicts a mass sensitivity per unit area of 0.226 Hz cm 2 ng . For a typical crystal the exposed area is c. 0.25 cm2 and the absolute mass sensitivity is 0.904 Hz ng 1. The resolution of modern frequency counters is easily +0,1 Hz in 10 MHz, giving a theoretical mass resolution of c. 9 x 10 10 g in practice this is usually found to be closer to 2 ng. 

The dispersion effect of the sample volume was discussed in Chapter 4 and little more needs to be said about it. It will be seen later that the sample volume controls both the concentration and the mass sensitivity of the chromatographic system and thus, should be made as large as possible. This means that all other sources of band dispersion must be kept to an absolute minimum to permit the maximum possible sample volume to be used. A better understanding of the causes of band dispersion has resulted in... 

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... 

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 ... 

As advantages, capillary separation techniques demonstrate high separation efficiency. On occasion, the number of theoretical plates available from these approaches has exceeded 1 million [29]. Also, very small sample volumes, on the order of 100 to 0.5 nL, are needed for these techniques. This can be an advantage for sample-limited situations, which are often encountered in bioanalysis. High mass sensitivity (the absolute weight of analyte injected) can be achieved, as the narrow capillary concentrates the sample plug and allows less opportunity for band broadening. 

Figure 3. Increase in absolute sensitivity as a function of number of functionalized holes. FDTD simulation showing the mass sensitivity of the device plotted as a function of the number of functionalized holes. The blue circles indicate the sensitivity values calculated from the simulations.

With bioanalytes, different aspects have to be taken into accoimt in this case even an analyte monolayer covering of the transducer surface leads to highly appreciable sensor effects. But on the other hand it is absolutely useless to design a sensor layer with interaction centres distributed throughout the bulk of the material, for two reasons First, the sensor layers would become very bulky, which can lead to problems with mass-sensitive transducers. Second, such layers would require very long diffusion pathways, thus making real sensor detection in a reasonable time impossible. 

A number of investigators have observed that there are differences in performance of nanospray versus electrospray ionization. The very different conditions (temperature, capillary diameter, use of nebulizer gas) typically employed for conventional and low-flow electrospray make absolute comparisons of sensitivity at widely different flow rates difficult. The current perception, however, is that very low flow rate systems have greater mass sensitivity than higher-fiow-rate systems. 

Because of the importance of mass airflow rate in establishing engine output power, power available is sensitive to ambient conditions. Full-throttle engine power varies approximately inversely with inlet-air absolute temperature, but more significantly, approximately directly with ambient pressure. Mountain passes exist on public roads in the United States that have altitudes of over 12,000 ft. The normal atmospheric pressure at such altitudes results in a one-third loss in power capability in the typical passenger-car engine. 

MARE [32-38], Neutrino oscillation experiments have proved that neutrinos are massive particles, but cannot determine their absolute mass scale. Therefore, the neutrino mass is still an open question in elementary particle physics. An international collaboration is growing around the project of microcalorimeter arrays for a rhenium experiment (MARE) for directly measuring the neutrino mass with a sensitivity of about 0.2eV/c1 2 4. 

DBD experiments with even better sensitivities (of the order of meV) will be essential to fix the absolute neutrino mass scale and possibly to provide information on CP violation. It is therefore evident that next-generation neutrinoless DBD experiments are the next important steps necessary for a more complete understanding of the physics of neutrinos. In the next section, we will describe the CUORE experiment and show how it could reach the required sensitivity. 

Several kinds of detection systems have been applied to CE [1,2,43]. Based on their specificity, they can be divided into bulk property and specific property detectors [43]. Bulk-property detectors measure the difference in a physical property of a solute relative to the background. Examples of such detectors are conductivity, refractive index, indirect methods, etc. The specific-property detectors measure a physico-chemical property, which is inherent to the solutes, e.g. UV absorption, fluorescence emission, mass spectrum, electrochemical, etc. These detectors usually minimize background signals, have wider linear ranges and are more sensitive. In Table 17.3, a general overview is given of the detection methods that are employed in CE with their detection limits (absolute and relative). 

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