Liquid mass-sensitive

Many authors claim that the dilution by the sheath flow would not significantly affect the detection sensitivity, because it is completely evaporated in the spray process. Moreover, it has been discussed that in this layered-flow approach, preferably the inner layer of the spray enters the collector opening. If this were true, the composition of sheath liquid would be less important. Anyhow, it has to be stated that there is a dilution problem in the sheath-flow approach. In addition, it has been proven many times that ESI is a concentration-sensitive, not mass-sensitive, process. Knowing this, it makes sense to reduce the sheath liquid flow rate to the minimum required for stable spray conditions. 

Narrow-bore columns of between 1.0 and 2.5 mm ID are available for use in specially designed liquid chromatographs having an extremely low extracolumn dispersion. For a concentration-sensitive detector such as the absorbance detector, the signal is proportional to the instantaneous concentration of the analytes in the flow cell. Peaks elute from narrow-bore columns in much smaller volumes compared to those from standard-bore columns. Consequently, because of the higher analyte concentrations in the flow cell, the use of narrow-bore columns enhances detector sensitivity. The minimum detectable mass is directly proportional to the square of the column radius (107) therefore, in theory, a 2.1-mm-ID column will provide a mass sensitivity about five times greater than that of a 4.6-mm-ID column of the same length. 

In this chapter, we study various correlations for gas-liquid mass transfer, interfacial area, bubble size, gas hold-up, agitation power consumption, and volumetric mass-transfer coefficient, which are vital tools for the design and operation of fermenter systems. Criteria for the scale-up and shear sensitive mixing are also presented. First of all, let s review basic mass-transfer concepts important in understanding gas-liquid mass transfer in a fermentation system. 

It is reasonable to ask how accurately the mass sensitivity in vacuum reflects the sensitivity when the device has liquid contacting the surface. This was investigated by monitoring the frequency shift of a single device both during vacuum deposition of a metal film and removal of the same film in an etching solution. The sensitivity in the liquid was approximately 6% less than the value measured in vacuum, a discrepancy that lies within our estimates of experimental uncertainty in this case [54]. 

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 ... 

Chemical sensors are by definition small, inexpensive and preferably hand-held devices, capable of continuously monitoring chemical constituents in liquids or gases. MIP sensors usually consist of an imprinted sensitive layer and a transducer to convert the chemical information, in real time, into an electrical or optical signal which is further evaluated electronically [12]. Figure 21.1 shows the set-up of chemosensors and two typical mass-sensitive devices. 

MIP sensor elements are also suitable for the analysis of multicomponent samples. The cost-effective, miniaturised, non-covalent MIP sensor arrays, when combined with computational data evaluation, make weak artificial recognition phenomena highly applicable for smart sensors. In comparison to gas or liquid chromatography, the results with mass-sensitive MIP sensors are faster and cheaper to obtain [32]. For effective on-line monitoring, the ideal MIP sensor or actuator should allow reversible analyte enrichment without dependencies on intermediate washing procedures (with organic solvents, for example). 

In particular, the detection of neutral or inert analytes, such as anaesthetics, odours or hazardous compounds via weak complex formation within the MIPs is the forte of mass-sensitive devices due to the ultra-low detection limits, such as 1 pg for a IGHz SAW. It is important that the sensitive layer is tightly bound to the metal electrode (QCM) or covalently linked to the piezo substrate (SAW) in order to achieve a stable coating in liquids. A stable link is sometimes crucial, for example when using polystyrenes (PSts). 

This method uses the high-performance liquid chromatography (HPLC) equipment for sample handhng and requires molar mass sensitive detectors (such as light scattering and/or viscometry) to obtain a mean property values from each detector (Mw and/or IV, respectively). The FIA result from a concentration detector yields polymer content in a sample, which can also be determined with other well-established methods. The FIA approach requires expensive and well-maintained equipment, and will not save much time or solvent furthermore, no distribution information is available. 

Mechanically stirred hybrid airlift reactors (see Fig. 6) are well suited for use with shear sensitive fermentations that require better oxygen transfer and mixing than is provided by a conventional airlift reactor. Use of a low-power axial flow impeller in the downcomer of an airlift bioreactor can substantially enhance liquid circulation rates, mixing, and gas-liquid mass transfer relative to operation without the agitator. This enhancement increases power consumption disproportionately and also adds other disadvantages of a mechanical agitation system. 

Temperature dependence is a second major issue. It is small for AT-cut crystals however, temperature fluctuations cause fluctuations in Rq inversely proportional to Q [7]. This effect is small compared to temperature effects having their origin in properties of the measurand. In liquid apph-cations, the most temperature-sensitive value is the liquid viscosity. Here, temperature-induced variations in frequency increase with whereas mass sensitivity increases with m. Therefore, an elevated resonance frequency is helpful. 

When the QCM is used as a mass-sensitive device in electrochemical experiments, it is often important to control the electrical potential of the electrode that is facing the liquid. Thus, an additional (reference) electrode is introduced into the QCM chamber in order to provide well-defined electrochemical conditions and to allow for various kinds of electrochemical reactions at the crystal surface. A well-known example is the electrodeposition of metals on the electrode surface that is often used to calibrate the device and calculate its mass sensitivity. When these kind of electrochemical studies are combined with QCM readings, the acronym EQCM is used, abbreviating electrochemical quartz crystal microbalance. 

The detector is another of the critical components of a high pressure liquid chromatograph, and in fact, the practical application of liquid chromatography had to await a good detector system. Many types of detectors are now on the market. The four most common, the ultraviolet absorption (uv), fluorescence, refractive index (RI), and electrochemical (EC) detectors, will be discussed as well as the newer light scattering mass sensitive detector. 

QCM was extensively used as a mass sensitive detector in vacuum applications and has become an important tool for monitoring mass changes occuring at the crystal surface when it is immersed in a liquid, as reviewed recently [148]. Sauerbrey provided a description and experimental proof of the linear mass-frequency relation for... 

The major part of the volumes consists of a careful description of basic sensors in Chapters 7-13. They include liquid electrolyte sensors, solid electrolyte sensors, electronic conductivity and capacitance sensors, field effect sensors, calorimetric sensors, optochemical sensors, and mass sensitive sensors. 

Even though there is a big shift in the resonance frequency, the mass sensitivity to thin film is observed to be the same in a fluid as in a vacuum. In addition to the viscous motion, many other factors still affect the resonance frequency of QCM in a high-density fluid. Roughness of the surface is one important factor. Based on the measurement, resonance frequencies turned out to be dependent on the surface roughness [14,15]. The roughness seems to induce a shift of the resonant frequency by both the inertial contribution from the liquid mass rigidly coupled to the surface, and the viscous contribution from the viscous energy dissipation caused by the nonlaminar motion in the liquid [16]. 

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